Jump to content
  • Welcome to Recreational Flying!
    A compelling community experience for all aviators
    Intuitive, Social, Engaging...Registration is FREE.
    Register Log in
  • Tutorials

    Aviation and other tutorials to help users in their flying endeavours


    09. Stability

    By Admin, in Flight theory,

    9.1  Concepts of stability and trim
    The aircraft's response to disturbance is associated with the inherent degree of stability; i.e. self-correction  built in by the designer — in each of the three axes — that eventuates without any pilot action.   Another condition  affecting flight  is the aircraft's state of trim —  or equilibrium  where the net sum of all  forces equals zero, i.e. the aerodynamic forces are balanced and the aircraft maintains a steady flight condition when cruising, climbing or descending.  Some aircraft can be trimmed by the pilot to fly 'hands off' for straight and level flight, for climb or for descent. But very light aeroplanes generally have to rely on the state of trim built in by the designer and adjusted by the rigger, although most  have a rather basic  elevator trim device, but no rudder or aileron trim facility.   If  natural trim is poor — and perhaps it flies with one wing low — inherent stability may maintain equilibrium with that wing-low attitude and not restore the aircraft to a proper wings-level attitude.  In which case, the pilot has to maintain a slight but  constant control column deflection to hold the wings level,  which can be quite annoying.
    It is desirable that longitudinal trim doesn't change significantly  with alterations in power, nor does directional trim change significantly  with alterations in airspeed.
    An aircraft's stability is expressed in relation to each axis:  lateral stability  —  stability in roll, directional stability — stability in yaw  and longitudinal stability — stability in  pitch. The latter is the most important stability characteristic. Lateral and directional stability have some inter-dependence.
    Degrees of stability
    An aircraft will have differing degrees of  stability about each axis; here are a few  examples:
    When disturbed a totally stable aircraft will return, more or less immediately, to its trimmed state without pilot intervention; however, such an aircraft is rare — and undesirable. We usually want a sport and recreational aircraft just to be reasonably stable so it is comfortable to fly. If overly stable they tend to be sluggish in manoeuvring and heavy on the controls; i.e. significant control force is required to make it deviate from its trimmed state. If it tends toward instability the pilot has to continually watch the aircraft's attitude and make the restoring inputs, which becomes tiring, particularly when flying by instruments. Some forms of instability make an aircraft unpleasant to fly in a bumpy atmosphere.
      The normally stable or positively stable aircraft, when  disturbed from its trimmed  flight state,  will  —  without pilot intervention  —  commence an  initial movement back towards  the trimmed flight state but overrun it, then start  a series of diminishing damping oscillations about the original flight state. This damping process is usually referred to as dynamic stability (or the tendency over time) and the initial movement back towards  the flight state is called static stability. The magnitude of the oscillation and the time taken for the oscillations to completely damp out is another aspect of stability. Unfortunately a statically stable aircraft can be dynamically unstable in that plane; i.e. the oscillations do not damp out.
      The neutrally dynamically stable aircraft will continue oscillating after disturbance, but the magnitude of those oscillations will neither diminish nor increase. If these were oscillations in pitch, and if there were no other disturbances and the pilot did not intervene, the aircraft would just continue 'porpoising'.
      The negatively stable or fully unstable aircraft may be statically unstable and never attempt to return towards the trimmed state. Or it can be statically stable but dynamically unstable, where it will continue oscillating after disturbance, with the magnitude of those oscillations  getting larger and larger. Significant instability is an undesirable characteristic, except where an extremely manoeuvrable aircraft is needed and the instability can be continually corrected by on-board 'fly-by-wire' computers rather than the pilot — for example, a supersonic air superiority fighter. The best piston-engined WWII day fighters were generally designed to be just stable longitudinally, neutrally stable laterally and positively stable directionally.  
    9.2 Longitudinal stability
    Longitudinal stability is associated with the restoration of aoa to the trimmed aoa after a disturbance changes it; i.e. if a disturbance pushes the nose up the tailplane will counter with a nose-down pitching moment.  In section 6.2 we discussed the provision of a tailplane to act as a horizontal (longitudinal) stabiliser. Before we go any further we need to look at another structural aspect of the airframe.
    Angle of  incidence
    Angle of  incidence is a term that is sometimes mistakenly used as  synonymous with wing  angle of attack; however, the former  cannot be altered in flight except in weight-shift control aircraft (hang gliders and trikes). Angle of incidence, usually just expressed as incidence, is within the province of  the aircraft designer who  calculates the wing aoa to be employed in the main role for which the aircraft is being designed, probably the aoa  in performance cruise mode.  The designer might  then plan the  fuselage-to-wing mounting  so that the fuselage is aligned to produce the least drag when the wing is flying at the cruise aoa. Wings that incorporate washout will have  differing angles of incidence at the wing root and at the outer section.
    A notional horizontal datum line is drawn longitudinally  through the fuselage, and the angle between that fuselage reference line  [FRL] and the wing chord line is the angle of incidence. Incidence should be viewed as the mounting angle of the fuselage rather than the mounting angle of the wings — see  'Stuff you don't need to know'.
    Incidence  may also be called the 'rigger's incidence' or some similar expression carried over from the earlier days of aviation. For ultralight aircraft,  incidence is something that should be checked  at regular inspections by a qualified person
    Longitudinal dihedral
    An angle of incidence is also  calculated for the horizontal stabiliser with reference to the FRL. The angular difference between wing and stabiliser angles is called the longitudinal dihedral, although it is probably more correct to say that the longitudinal dihedral is the angular difference between the two surfaces at their zero lift aoa. The angle of the line of thrust is also expressed relative to the FRL.
    Positive longitudinal dihedral — where the wing incidence is greater than that of the  stabiliser  —  will help control a stall by ensuring that, if the aircraft approaches a stall, the wing will stall before the tail,  giving the tail a chance to drop the nose.   The tailplane of most very light 3-axis control aeroplanes is mounted in a position where the wing downwash may  effect the angle of attack of the tailplane and that downwash angle increases as the wing angle of attack increases.
    It is the  horizontal stabiliser area and moment arm  that provides the restoring moment to return aoa to the trimmed state. However, bear in mind that the moment arm, which supplies the restoring leverage and thus the stability,  is affected by the cg position. If the cg lies outside its limits, the aircraft will be longitudinally unstable.
    We learned in section 2.6 that when flying with level wings, at a particular weight, each aoa is associated with a particular IAS. We might as well take advantage of that by arranging the longitudinal dihedral so that the built-in state of trim produces a particular indicated airspeed. In some ultralights a designer/rigger might pick Vbg — best power-off glide speed  — as the natural airspeed so that, lacking pilot input, the aircraft will naturally attempt to adjust its aoa to the Vbg aoa, whether power is on or off.
    Oscillating motions
    It is possible that an aircraft, properly trimmed  for continuing level flight, may develop a 'phugoid' motion if affected by a sharp disturbance. A phugoid cycle is a pitch increase followed by a pitch decrease without any discernible aoa change, i.e. a short climb during which speed decreases and the nose drops into a short descent during which speed increases and the cycle starts again. The aircraft is trading kinetic energy for an increase in the potential energy of height,  using the latter to return to the trimmed airspeed in the descent; the cycle time for one oscillation in a very light aircraft might be 20 seconds or so. The oscillating motion issometimes described as 'porpoising'.
    If the pilot doesn't intervene and the aircraft is phugoid stable the phugoid cycles will damp out after a few diminishing oscillations. If the aircraft is phugoid unstable the oscillations will diverge and the pilot must intervene.
    The longitudinal dihedral and the tail moment arm  affect  phugoid stability.
    9.3 Directional stability
    Directional stability is associated with the realigning of the longitudinal axis with the flight path (the angle of zero slip)  after a disturbance causes the aircraft to yaw out of alignment and produce slip; remember yaw is a rotation about the normal (vertical) axis. In section 6.3 we discussed the provision of a fin to act as a directional stabiliser. The restoring moment —  the static stability  —  provided by the fin is the product of the fin area and the moment arm. The moment arm leverage will vary according to the cg position — the aircraft's  balance.
    The area required for the fin has some dependency on the net sum of all the restoring moments associated with the aircraft fuselage and undercarriage side surfaces  fore (negative moments)  and aft (positive moments) of the cg. For instance, the Breezy has, except for the pilot's body,  very little lateral moment ahead of the cg because of the open frame fuselage; thus  a small fin provides all the moment necessary for directional stability.  But if the pilot and passenger were enclosed in a cockpit or pod, with a much greater side surface, then the negative moments would be greater and consequently the fin area would have to be greater. If the pilot removes his/her feet from the rudder pedals the rudder, will 'float', aligning itself with the relative airflow and thereby reducing the restoring moment of the fin.
    The directional stability of  very light aircraft with a lot of forward keel area — such as those with a cockpit pod and a 'boom' in place of a rear fuselage —  may be 'conditional'; i.e. it is sensitive both to the position of the cg within its normal range and to the amount of sideslip. This is because the negative lateral forces of the pod are very much greater than the positive lateral forces of the boom and fin. Thus, beyond a certain angle of slip the moments change,  positive stability is changed to neutral stability and  yaw becomes locked in. It might also be associated with the fin stalling at high sideslip angles.
    The most noticeable symptom to the pilot is aerodynamic rudder overbalance (or 'rudder force reversal' or 'rudder lock') — where the rudder moves to full deflection without any additional pilot input, or doesn't return to the neutral position when the rudder pedal pressure is released, or the pedal force has to be reversed as sideslip angle is increased. It may require significant opposite rudder input, and probably an increase in airspeed,  to return to the normal state.
    The areas of side surface above and below the cg also  affect  other aspects of stability.
    The   term 'weathercocking'  refers to the action of an aircraft, moving on the ground,  attempting to swing into wind. It is brought about by the pressure of the wind on the rear keel surfaces, fin and rudder, which cause the aeroplane to pivot about one or both of  its main wheels. It is usually more apparent in tailwheel aircraft because of the longer moment arm between the fin and the main wheels; although if a nosewheel aircraft is 'wheelbarrowing' with much of the weight on the nosewheel, then there will be a dangerously long moment arm between the nose wheel pivot point and the fin.
    9.4 Lateral stability
    Lateral stability refers to roll stability about the longitudinal axis;   in section 4.10 we established that ailerons provide the means whereby the aircraft is rolled in the lateral plane.  However, unlike the  longitudinal and normal planes where the horizontal and vertical stabilisers provide the restoring moments necessary for pitch and yaw stability, no similar restoring moment device exists in the lateral plane.
    But let's imagine that some atmospheric disturbance has prompted the aircraft to roll to the left, thus the left wingtip will be moving forward and down, and the right wingtip will be moving forward and up. Now think about the aoa for each wing — the wing that is moving down will be meeting a relative airflow  coming from forward and below, and consequently has a greater aoa than the rising wing. A greater aoa, with the same airspeed, means more lift generated on the downgoing side and thus the left wing will  stop going further down or perhaps even rise a little, although pilot action is usually needed to get back to a wings level state. This damping of the roll is known as lateral damping.
    So roll stability, except when at or very close to the stall, is intrinsic to practically all single-engined light aircraft. (When the aircraft is flying close to the stall, the aoa of the downgoing wing could exceed the critical aoa and thus stall, which will  exacerbate the wing drop and might  lead to an incipient spin condition. See the stall/spin phenomenon.)
    But — and there always seems to be a 'but'  —  when the aircraft is banked, other forces come into play and affect the process. If you re-examine the turn forces diagram in the manoeuvring forces module, you will see that when an aircraft is banked the lift vector has a substantial sideways component; in fact, for bank angles above 45°,  that sideways force is greater than weight. So we can say that any time the aircraft is banked, with the rudder and elevators in the neutral position, an additional force will initiate a movement in the direction of bank; i.e. creating a slip. We know from the  section 7.3 that the aircraft's directional stability will then yaw the nose to negate the slip and the yaw initiates a turn, which will continue as long as the same bank angle is maintained. 
    There are several design features that stop the slip and level the wings, thus  promoting lateral stability. For instance, placing the wing as high as possible above the cg increases  so-called 'pendular stability', (The stability due to the high wing is not really pendulum stability  such as that applicable to powered parachutes.)  Wing dihedral* is usually employed with low-wing monoplanes (and to a lesser degree of tilt with high wings), where the wings are tilted up from the wing root a few degrees. A swept-back wing format is used with trikes. Another design method  is anhedral, where the wings are angled down from the wing root,  but it is unlikely to be used in light aircraft, although  the powered parachute wing utilises an anhedral arc for stability.
    (*'Dihedral' is a mathematical term  denoting the angle between two intersecting planes.)
    Spiral instability
    An aircraft with positive spiral stability  tends to roll out of a turn by itself if the controls are centred. Some light aircraft with little or no wing dihedral and a large fin tend to have strong static directional stability but are not so stable laterally. If slip is introduced by turbulence or by the pilot, such aircraft —  left to their own devices  — will gradually start to bank and turn — with increasing slip and nose drop — and hence increasing turn rate and rapid increase in  height loss. Neutral spiral stability is the usual aim of the designer.  The turning process starts slowly in aircraft with slight spiral instability  but leads to  spiral divergence which, if allowed to continue  and given sufficient height,   will accelerate into a high-speed spiral dive. This often occurs when a pilot without an instrument flight rating strays into cloud where all visual cues are lost. In that condition it is known as the 'graveyard spiral'.  
    Inadvertent entry into a fatal spiral dive, leading to inflight breakup, can happen even with experienced IFR pilots, see this  Australian Transport Safety Bureau report.
    It is evident that directional stability and lateral stability are coupled (i.e. rotation about one axis prompts rotation about the other) and to produce a balanced turn; i.e. with no slip or skid,  the aileron, rudder and elevator control movements and pressures must be balanced and coordinated.
    Dutch roll
    Induced motion in the lateral plane generally brings about a coupled motion in the directional plane, and vice versa. Dutch roll is a phenomenon in level flight where a disturbance causes a combined yaw and roll followed by a return to the level flight condition then a yaw and roll to the other side: the oscillations continuing until damped out. In a very light aircraft the time for each cycle  might be 5 to 10 seconds. The motion is quite uncomfortable, viewed from the cockpit the wingtips complete a circular motion against the horizon as does the nose. Pilot  intervention is by use of rudder.
    9.5 Trim and thrust
    We have covered above the reaction of the aircraft to changes in relative airflow whether  induced by the pilot or minor atmospheric turbulence. We know from sections 1.8  and 1.9 that if an aircraft is properly trimmed for cruise flight and we increase thrust then it will climb; and if we reduce thrust it will descend. But how this eventuates is not at all straightforward. The reaction to changing power, without the pilot touching the control column, depends on whether the cg is  above,  below  or inline with the line of thrust; in the Breezy, the cg is below the thrust line.  The thrust line is best located so that it passes close to the vertical cg position to minimise the initial pitching moments associated with power changes. 
    The placement of the horizontal and vertical stabilisers, in relation to the propeller slipstream and to the wing downwash, affects flight performance and particularly flight at slow speeds — because then  the total air velocity within the slipstream tube is nearly double that outside the tube; also the slipstream is rotating, and will thus impart a sideways moment to the fuselage and vertical stabiliser.  Effects on individual aircraft types vary according to the designer's inbuilt compensations: for example, if the  horizontal stabiliser  operates in the wing root downwash airflow, then when the wing root stalls and the downwash becomes turbulent the stabiliser might undergo  an abrupt change in aoa (and thus in its stability restoring moment). Or if the horizontal stabiliser operates substantially outside the downwash but if it is in the path of the turbulent flow from the stalled wing, it will then lose part of its aerodynamic force.
    If a modification is made to that design, even a seemingly minor change, the consequential effect on stability may be quite surprising. To illustrate the point, I suggest you read an  "airworthiness report  regarding (among other factors contributing to general stability problems)  a small change made in relocating the exhaust manifold of a Thruster that, at a particular aoa, promoted  turbulent flow over the upper wing surface, which then  extended to the horizontal stabiliser, and reduced the stabilising moment imparted by that surface.

    Stuff you don't need to know The term 'decalage' (French = gap or shift forward/back) relates to the difference in the angles of incidence of the upper and lower mainplanes of a biplane. Decalage is now occasionally used as synonymous with longitudinal dihedral.
      The angle of incidence has some effect on the pilot's view over the nose. A very few naval aircraft designs have included 'variable incidence wings' where the angle of incidence could be changed by the pilot during flight, within a range of say 2–15°, using electric motors. Such aircraft included leading edge slats as a high-lift device.The idea was to take full advantage of the high maximum CL and consequent low speed, during the landing approach, without having the fuselage cocked up at a high angle blocking the view. As aoa increased and the aircraft slowed, the pilot wound the fuselage down, so that it remained more or less level during the approach and thus provided a better view of the flight deck! Variable incidence wings were also used with one of the post-WWII Supermarine amphibian designs.  

    8.1 Airframe basics
    The empennage
    A monoplane has a single wing,  or a left and right  pair of wings — the port and starboard mainplanes. A biplane  has two sets of wings mounted one above the other.  The engine and propeller may be in front of the wing (a 'tractor' configuration) or behind the wing (a 'pusher' configuration). It must have some sort of pilot/passenger seating (usually enclosed in a cockpit or pod, which will have either tandem or side-by-side seating and is referred to as the 'occupant zone'); fuel tank(s); and a  rigid structure mounting the engine/propeller, the wheels or undercarriage, and the cockpit, so that their weight is supported by the wing main spar(s)  and the propeller thrust reaction is transmitted to the body and wings.

    The photograph of  Mick McCann's 'Breezy' shows a basic   high-wing monoplane, pusher-engine configuration with tandem pilot/passenger seating, nose-wheel undercarriage and an open-frame, welded tubular-steel fuselage — the aft part of which is upswept, so that the aircraft's attitude in pitch can be adjusted during take-off and landing without the tail striking the ground. Also the arrangement keeps the rear stabilising and control surfaces within the energetic airflow of  the propeller slipstream. The term fuselage is derived from an old French word meaning a tapered 'spindle' used for manually weaving yarn. The 'Breezy' has no refinements for comfort —  or for drag reduction. The fuel tank is not discernible in the photograph but is small and close to the engine.
    Attached to the rear fuselage are the horizontal stabiliser and elevators, plus the vertical stabiliser or fin and the rudder,  together forming the pitch and yaw stabilising and control mechanisms  — the tail assembly or empennage.   The latter term is derived from a French word meaning to feather an arrow; maybe that is why some people refer to the empennage as the 'tailfeathers'. The horizontal stabiliser and elevators are referred to as the 'tailplane'. 
    Moments and couples
    The moment of a force  or the torque is a measure of   the rotational effect produced by a force acting about  —  or with respect to  —  a  fulcrum, axis, centre of mass (cg) or aerodynamic centre.  Its magnitude is the product (in newton metres) of the force (N) and the length (m) of the arm (the leverage) from the  pivotal point to the line of action of the force. The moment will act in a particular direction, for example, as we saw in the 'Aerofoils and wings' module, the  pitching moment of a cambered wing produces a nose-down torque.
    The forces generated by the tailplane control surfaces are dependent on the stabiliser area, the control surface area, the length of the tail arm to the cg, the control surface deflection  and the airspeed. Only deflection and airspeed are controlled by the pilot.
    Two equal and opposite forces acting parallel to each other, but separated,  form a couple. The rotational effect or moment of a  couple   is the product of  one force and the perpendicular distance between them. The ailerons, for example, form a couple when deflected.
    8.2 Tailplane
    Horizontal stabiliser
    The existence of the wing pitching  moment makes the wing inherently unstable. To overcome this problem, it is necessary  to couple it with another aerodynamic moment  about the lateral or pitch axis — opposing the wing pitching moment — that  will balance that  moment at an airspeed selected by the designer.  The moment of a force is   the arm length multiplied by the force; so the longer the tail arm,  the smaller the aerodynamic force required. The standard solution is to extend the fuselage rearwards so that  a horizontal stabiliser can be mounted at a distance from the cg; note the Breezy's very long tail arm – between the cg and the small horizontal stabiliser.  The horizontal stabiliser is usually a  lift-generating surface — or 'plane' — mounted  so that the aerodynamic force it generates acts in the opposite direction to the lift from the mainplane, i.e. generally downwards.  The plane could incorporate a cambered aerofoil with the cambered surface underneath, or perhaps  a symmetrical aerofoil, or even just a flat plate —  as the Breezy's appears to be. The symmetrical aerofoil and the flat plate would both be mounted at a negative incidence to produce the downward force.  The end result is that the net pitching moment of the mainplane and tailplane couple is zero at a particular geometric aoa of the main wing; that aoa would equate with a speed selected by the designer —  usually the designed cruise speed or perhaps the engine-off glide speed. The fuselage may also produce pitching moments that must be balanced by the stabiliser.
    As the horizontal stabiliser  is usually designed to produce negative lift, then the wing must fly at a slightly greater aoa to provide  additional lift, so that the net aircraft lift balances weight.
    The pilot must be able to initiate and hold aoa changes for airspeed adjustments, manoeuvres (accelerations) in the pitching plane (pull-ups, turns, push-downs) and adjustments of aircraft attitude relative to the airfield or alighting area surface during take-off and landing.   The elevators —  hinged to the trailing edge of the horizontal stabiliser so that they may be deflected up or down — are the control surfaces that enable controlled changes in  wing aoa.  Elevators are aerodynamically similar to the  ailerons, but move  in unison rather than differentially.
    The elevators are linked, via control rods or cables, to forward/backward movement of the control column, so the pilot can, in effect,  increase or decrease the camber of the stabiliser–elevator combination.  Camber changes will alter the magnitude and direction of the aerodynamic reactions generated by the stabiliser–elevator, and the changed forces impart a pitching moment in the longitudinal plane.  This pitching moment rotates the aircraft about its lateral axis, initiating the change in wing aoa. Once the new aoa is established, the pitch moment returns to zero and the aircraft will hold that aoa — provided the elevators are held in the deflected position by the pilot or a trim device — thereby controlling airspeed for a given power setting. Backward movement of the control column raises the elevators and the aircraft's nose  pitches up; forward movement lowers the elevators and the aircraft's nose  pitches down. The force able to be exerted via the elevators is the most significant control force. The  'up' and 'down' terms in pitch are not relative to the horizon but to the original flight path in the aircraft's longitudinal plane.
    A stabilator is an 'all-moving' or 'all-flying' tailplane combining the horizontal stabiliser and elevator  providing similar force with a lesser deflection, thus less drag.  Sometimes used in higher speed light aircraft but rarely in very light aircraft.  There may be some net advantages in mounting the stabiliser and elevators in front of the wing — a canard — but such arrangements are rather rare amongst very light aircraft.
    8.3 Vertical stabiliser and  rudder
    Because of drag and other effects, aircraft perform much better if their longitudinal axis is accurately aligned, in plan view, with the flight path.   If unaligned, the aircraft velocity will have both  a forward component and a  slight lateral component, and the relative airflow — the flight path — will not be aligned with the longitudinal axis. Such bodily sideways (translational) movement along the lateral axis  is called slip or sideslip  or skid. The skid term is generally associated with excess 'bottom' rudder and skidding out in a turn, as a road vehicle might. 
    Thus, some means is required to ensure that if the horizontal direction of the relative airflow  is changed (i.e. the aircraft acquires slip because of a minor disturbance)  then the aircraft will automatically yaw — rotate itself about its normal axis —  to realign its longitudinal axis with the airflow, so that   the sum of all the lateral moments —  fore and aft of the cg —   equals  zero. 
    The long-established means is to use a fin, or vertical stabiliser, mounted at the rear of the aircraft,  that has an  aerofoil section — usually symmetrical — or is just a flat plate.  The fin applies the restoring moment to realign the longitudinal axis with the airflow.  That moment does not realign the aircraft with its original flight path; after restoring alignment with the relative airflow, the aircraft may be aligned with a  different flight path, depending on the amount of original displacement.
    The fin is often angled away from the aircraft's longitudinal axis by a few degrees. This offset creates an  aerodynamic force that compensates for the rotating propeller slipstream applying a force to one side of the fin.
    The rudder is the control surface hinged to the fin and is the lateral plane equivalent of the elevators; though the rudder is operated by the pilot's rudder pedals rather than the control column. Pressure on the left pedal causes the rudder to deflect to the left, so that the fin/rudder act together as  a cambered aerofoil to produce an aerodynamic force that pushes the tail to the right — and consequently the nose swings  left; i.e. the  aircraft  yaws left. (Yaw is an old nautical term associated with the motion of the sea swinging the bow off-course.)  The amount of yaw, at a given airspeed, is dependent on the degree of rudder deflection. (But, of course, it is primarily dependent on the tail moment arm and rudder area.) The aircraft will continue yawing if the rudder deflection is held by the pilot, but as the aircraft turns (i.e. it is rotating about its normal or vertical axis while moving forward), the wing on the outside of the turn must be moving slightly faster than the inner wing and thus generates more lift. The increased lift will raise the outer wing and the aircraft will enter a banked turn, but will tend to  skid out because the bank angle will not be correct for the turn. Only one bank angle will produce the desired radius or rate of turn for a particular airspeed. 
    Note the Breezy's small fin with its relatively  large rudder.  The pilot's feet are on  the pedals  linked to the rudder and he is holding the control column —  linked to the ailerons and the elevators —  with one hand. The other hand is probably holding the engine throttle lever. The rudder initiates yaw about the normal axis; the ailerons initiate roll about the longitudinal axis;  the elevators  initiate  pitch movement about the lateral axis.
    8.4 Control balance
    Aerodynamic balance
    Aircraft designers try to impart a good 'feel' to the controls so that the pilot finds they  are not too 'heavy' or too 'light' to operate through most of the speed range. So, the elevators and rudder are usually fitted with some sort of aerodynamic balance, which puts part of the control surface forward of the hinge line. Such devices might be inset hinge balances, leading-edge balances or control horns that reduce the hinge moments needed to deflect the control surface.
    Mass balance
    Control surfaces  need to be hinged near the leading edge, the centre of their mass will be well aft of the hinge line; i.e. the mass of the control is not statically balanced. That, combined with the necessary elasticity of aircraft structures,   leads to a  control flutter problem.  This might occur with mass unbalanced control surfaces at any speed, but particularly with ailerons at high speed. Flutter has the potential to lead to structural failure. The prime solution to the mechanical unbalance and the flutter problems is for the manufacturer to accurately balance the  mass of the control surface by  inserting weights forward of the hinge line usually within the hinge insets or the control horn. This — known as mass balance — increases the stability of the control surface and ensures that accelerations don't deflect the control surface.
    Notes for scratch-builders You may occasionally come across the terms tail volume and tail volume ratio. The horizontal tail volume is the surface area of the horizontal stabiliser plus elevators multiplied by the length of the moment arm of the horizontal stabiliser measured from the wing MAC quarter chord to the horizontal tail MAC quarter chord. The horizontal tail volume ratio or tail volume coefficient is the tail volume divided by the product of wing area and wing MAC. Tail volume ratio is usually in the range 0.35 to 0.45 for minimum aircraft and 0.45 to 0.55 for aircraft of composite construction — when the units of measurement are feet. The higher the coefficient value, the more stable the aircraft. There is a similar equation for the vertical stabiliser and rudder, but the divisor is the product of wing area and wing span. Such ratios are of interest to an aircraft designer, as there is a linear relationship between tail moment or tail area, and stability — doubling the tail moment or the tail area doubles the static stability and quadruples the dynamic stability. The aspect ratio of the tail affects tailplane pitching moments; for a given area higher aspect ratio produces less induced drag and hence the lift component of the aerodynamic force is higher. STRICT COPYRIGHT JOHN BRANDON AND RECREATIONAL FLYING (.com)

    7.1 Engine power output
    Engine power equals the product of force and speed.   Torque is the  rotational force acting about the engine crankshaft multiplied by the moment arm; i.e. it is the product of the firing stroke in the cylinder and the radius of the crank to which the connecting rod is attached. The bigger the cylinder the bigger the rotational force — the 'bang'. Engine speed  is measured in crankshaft revolutions per minute [rpm].
    In the 'Manoeuvring forces' module we discussed the power required for various flight conditions, and looked at power required/power available curves and the effect of altitude on power output. It may be appropriate to review section 1.7 of that module.
    Normally aspirated aero engines
    The maximum power that can be developed, in the cylinders of a particular piston engine, increases or decreases directly with the density of the air in the intake manifold, and air density decreases as altitude increases — or temperature increases. See the atmospheric density and the International Standard Atmosphere sections  in the 'Airspeed and the properties of air' module. Thus, the full throttle  power output of a normally aspirated engine — one that relies solely on the ambient atmospheric density — decreases as operating altitude increases.  The diagram in section 1.7  shows how maximum brake horse-power [bhp], delivered at full throttle in a normally aspirated engine, decreases with  altitude. A 100 hp engine operating at 65% power will be delivering 65 hp.
    Power produced is proportional to  the air density at the intake manifold, the cylinder displacement and compression ratio, the number of cylinders, and the  rpm. Of those  items, only  the  air density at the intake manifold and the engine  rpm alter, or can be altered, during flight. (With a normally aspirated engine and a propeller whose pitch is not variable in flight, the throttle  controls manifold pressure,  which then determines rpm.) A traditional four-stroke  light aircraft engine, such as the  Lycoming O-235, has an individual cylinder displacement of 950 cc, a compression ratio of 7:1 and a maximum design speed of 2600 rpm, at which its rated 110 bhp is produced — in sea-level ISA conditions. The  Rotax 912, the most common lightweight four-cylinder aero-engine, utilises an individual cylinder displacement of only 300 cc, a compression ratio of 9:1, but doubles the maximum design speed to 5500 rpm to achieve its rated 100 bhp. The lightweight Jabiru 2200 utilises  an individual cylinder displacement of  550 cc, a compression ratio around 8:1 and a maximum design speed of 3300 rpm to achieve its rated 80 hp.  
    The three engines mentioned are all  horizontally opposed, four-stroke and four-cylinder; a popular  configuration providing a fully balanced engine that doesn't require crankshaft balance weights.  Engines are often described in terms of 'total capacity' (cylinder displacement by number of cylinders) in litres or  cubic centimetres. Thus, the Lycoming O-235 is 3.8 litres or 3800 cc (235 cubic inches), the Rotax 912 is  1.2 litres and the Jabiru 2200 is 2.2 litres.    Most engines used in ultralights tend to be around 30% lighter (in terms of weight per rated hp) than the ubiquitous Lycoming and Continental piston engines used in general aviation aircraft. Thus, they are cheaper to manufacture but less robust, with a consequent shorter time between overhaul [TBO].
    Although aero-engines can quite happily operate continually at their rated power, doing so is not  good practice. It is uneconomical in terms of fuel efficiency, but  — more importantly  — it may shorten engine life, if engine operating temperatures and pressures are exceeded. Normally the maximum  — and optimum  — power setting for continuous cruise operation is 75% of rated power.
    Turbo-charged aero-engines
    The volumetric efficiency (i.e. the cylinder-filling capability) of an engine can be improved by increasing the density  of the fuel/air charge delivered to the cylinders by compressing the air in the atmospheric intake manifold. This process is  supercharging and develops more torque at all engine speeds. The compressor is usually a lightweight  centrifugal impeller driven by a gas turbine that utilises the otherwise wasted energy of the engine exhaust gases. Such a system is a turbine-powered supercharger, usually described as a turbocharger.   An oil pressure-driven   butterfly valve or waste gate is  incorporated within the exhaust manifold system,  automatically adjusting — according to the pressure within the intake manifold — to allow all, or a portion, of the exhaust gases to bypass the turbine; thus continually maintaining the system within the designed operating limits. There is a slight  penalty in that turbocharging also increases the temperature of the charge. This consequently decreases the achievable density and possibly leads to detonation, unless a charge cooling device — an intercooler — is incorporated between the compressor and the cylinders. 
    For some information on mechanically powered  supercharging,  read this magazine article.
    Turbocharging may be used to increase the sea-level rated power of the engine, but the use of that full throttle power at low altitudes would normally be  limited to short periods because of engine temperature limitations.  The big advantage is the increase in power available at altitude. The diagram plots the power achieved (percentage of rated power) at full throttle, in ISA standard conditions,  for a normally aspirated engine and the turbocharged version. The turbocharged engine can maintain its rated power from sea-level up to the 'critical altitude', probably around 6000 or 7000 feet, after which it will decrease. The waste gate would probably be fully open at sea-level and then start closing as altitude increases — so that it would be fully closed at, and above, the critical altitude.
    Turbocharging raises the service ceiling of the aircraft. The service ceiling is the ISA altitude at which the aircraft's best rate of climb (from an extended climb starting at MTOW and unassisted by any atmospheric phenomena) drops below 100 feet per minute — regarded as the minimum useful climb rate. This should be the aircraft's ceiling quoted by the  manufacturer.
    The Rotax  914 series 115 hp turbocharged engines are often regarded as just being suitable for ultralight aircraft. However, those engines power the Predator RQ1/MQ1, unmanned aerial reconnaissance and surveillance vehicles, used so successfully in the  Afghanistan and Iraq campaigns of recent years. The Predators  have a maximum take-off weight around 1000 kg, cruise around 90 knots,  normal mission duration around 20 hours — but could operate for 40 hours — and service ceiling of 25 000 feet. They often   carried two 50 kg Hellfire missiles for attacking acquired targets — they also need  5000 feet of paved runway for take-off.
    Two-stroke aero-engines
    The lower power (say, up to 65 hp) engines used in ultralight aircraft are usually two-stroke  engines, although the half-VW four-stroke auto engine conversions are around  40 hp. Two-strokes don't have very good volumetric efficiency, and the engine is  generally efficient only in the upper 30% of its rpm and throttle opening range. In fact, ultralight two-strokes tend to run very roughly at speeds below 2500 rpm and achieve their rated power at rotational speeds in the 5500 to 6500 rpm range.  The three most common two-strokes are two-cylinder models with individual cylinder displacements around 250 cc;  they achieve their rated power at 6500 rpm and 75% power at around 5300 rpm. Fuel efficiency drops off very quickly as rpm is reduced below the 75 % power figure; see the Rotax two-stroke engine operator's manual section 10.2.  Gearing or belt reduction is used to improve the torque delivered to the propeller shaft while also reducing the rpm to something more suitable for the propeller. The torque increases because of the larger rotational radius of the driven gear.  
    The big advantage with two-stroke engines is their mechanical simplicity, and consequent weight and cost saving, because they lack the camshaft and associated valve train of the four-strokes. Some very small (15 hp) two-strokes are used to power self-launching powered hang-gliders.  Between 1999 and 2003, there were 98 engine failures reported to RA-Aus; 39 were two-stroke engines and 59 were four-stroke. It is estimated at that time about 65% of the ultralight fleet, of some 1800–2000 aircraft, were equipped with two-strokes. It would appear during that period the two-strokes were more reliable than the lightweight four-stroke aero-engines, though the development of lightweight four-strokes was then  not as far along the learning curve as two-stroke development.
    7.2 Propeller power output
    Propeller efficiency
    An aircraft  engine supplies energy, in the form of rotational power,  to the propeller shaft. The propeller converts the rotational power to thrust power,  either pulling the aircraft along behind it (a tractor installation) or pushing the aircraft in front of it (a pusher installation). The pusher installations are usually the only options when the engine is mounted on a carriage or cart rather than a fuselage structure. That option is the standard for trikes, nanolights and gyroplanes. The 10 hp or so engines attached to the 'backpack' harnesses of powered hang gliders and powered paragliders are, of course, pushers. The major problem with pusher propellers in very light aircraft is the avoidance of something  from the occupant zone — just in front of the engine  —  moving through the propeller disc or being entangled in it.  
    The propeller accelerates a tube of air, with much the same diameter as the propeller disc;   i.e. it adds momentum to the tube of air and the reaction force propels the aircraft forward. The velocity of this accelerated airstream (the slipstream) has both  rotational  and rearward components.  Momentum = mass × velocity, so if the mass of  air passing through per second is increased by increasing the diameter of the propeller,  the rearward velocity  imparted can be decreased but still produce the same  rearward or axial  momentum. The rate at which axial momentum  is imparted to the air  equates with thrust. 
    Propeller efficiency  is the ratio of the thrust power (thrust × aircraft forward speed) output to the engine power input.
    The work done (the energy expended) by the propeller is the  kinetic energy  imparted to the slipstream = ½mv² joules (if mass is in kilograms and v in metres per second), so less energy is expended if the mass is increased and the velocity decreased.
    Using a simplified static thrust example,  if m = 10 kg and v = 100 m/s, then the momentum is 1000 kg·m/s and energy expended is ½ × 10 × 100² = 50 kJ. But if the values for m and v are interchanged (i.e. m = 100 kg and v = 10 m/s) the momentum will still be the same but the energy expended will be decreased substantially; i.e. ½ × 100 × 10² = 5 kJ.  
    Thus, the most  efficient system is to utilise the greatest propeller diameter possible — limited by: 
    the stress effects on the engine (the gyroscopic moments  increase exponentially with diameter; see below) ground clearance requirements in worst conditions (e.g. heavy landing and deflated tyre) propeller blade strength – centrifugal forces are extremely high, much greater than aerodynamic forces, even a lightweight blade would be experiencing forces around 2500g. blade tip speed.   When a propeller is rotating, the speed at any point on a blade is the product of the rpm and  the distance of that point from the hub, and thus the speed at the propeller tip is the greatest. Compressibility  constraints dictate that the speed at the blade tips should not exceed about Mach 0.85 — 560 knots or 290 m/s at sea-level. But significant compressibility effects become evident at 250 m/s and, if the propeller is close to the pilot, the noise may be extremely uncomfortable.  So, for comfort, tip speed is usually in the range 200–240 m/s. it is not only the aircraft occupants who must be comfortable with the noise, there are restrictions on engine and propeller noise in the vicinity of aerodromes – see the aircraft noise regulations.  
    For light aircraft engine/propeller systems, it is usual to restrict propeller speed to less than 3500 rpm; so, the high rpm engines must incorporate a gear-driven or  belt-driven propeller speed reduction unit [PSRU] between the crankshaft and the propeller shaft. The rotational speed of the fixed-pitch propeller depends on the pitch of the blades, the power supplied to the propeller and the aircraft velocity.
    Propeller blade area is an important consideration in propeller design and choice.  Blade aspect ratio is usually maintained around 6–8; so, with a limited propeller diameter,  blade area can only be increased by increasing the number of blades. 
    Matching engine and propeller
    Propellers must be carefully matched with the  characteristics of the airframe, engine and reduction gear to which they are mated. The engine must be neither underloaded nor overloaded. At best, a mismatch could make the engine and aircraft incapable of  delivering its designed performance, or  create the situation where the engine cannot be opened up to full throttle because the lack of load (see the following  paragraph) would take the rpm beyond the red-line limit, or it could result in crankshaft or crankcase fracture. At worst, a mismatch  could lead to torsional vibration or propeller blade destruction induced by  centrifugal force. This  can readily cause  the engine to dismount from the airframe and lead to consequent total loss of the aircraft. When discussing the power required curve it was noted that power required is proportional to aircraft velocity cubed. Similarly, the power delivered by a propeller varies in accordance with rpm cubed (if everything else is kept constant). Thus, the load on the propeller may be substantially increased just with a relatively minor further increase in rpm when operating at high rpm, which can lead to loss of the blades. Note that centrifugal forces on the blades change  in accordance with the rpm squared.
    Note: The  load on the engine is the propeller  torque. When the aircraft is stationary, with the engine throttle wide open, the propeller torque and the static thrust generated (i.e. the efficiency of the engine and the propeller combination) depend on the propeller pitch. If the pitch is zero or slightly negative, the static thrust will be zero and the propeller torque will be very low so that the engine will race — overspeed — and lose power because of inefficient cylinder charging, etc. 

    On the other hand, if the pilot is able to set the prop to a more negative pitch, then reverse thrust will be generated together with sufficient torque to maintain constant engine rpm and the aircraft will move backward.

    If the pitch is 'fine' (low aoa), the propeller will generate near maximum static thrust and sufficient torque to maintain high engine rpm, thus delivering ample power to the propeller shaft. This is the ideal situation to get the aircraft rolling for take-off and climb-out.

    If the pitch is very 'coarse' (high aoa), then static thrust is low but propeller torque is very high, which will slow the engine. This is the worst situation for take-off — the aircraft will move forward sluggishly and, hopefully, never reach take-off speed. For an interesting article on ground testing of aircraft engines for power output, read "Testing one, two three" in the July-August 2002 issue of  'Flight Safety Australia'  magazine.
    When an aircraft with a fixed-pitch propeller is flying the  back of the power curve (i.e. an increasing thrust power output is needed as the airspeed decreases), the propeller efficiency will  decrease as airspeed decreases, while the increasing propeller torque will be slowing the engine power. Thus, it may be difficult to arrest any sink that develops at low speeds — as might be experienced on the approach to a short-field landing.
    However, even with an apparently well-matched engine/propeller combination, there may be a certain rpm range (or ranges) where the frequency of a particular  engine vibration resonates, with some natural frequency of the propeller, to produce an intrusive vibration and a potentially damaging stress cycle. In such aircraft, that rpm range or ranges is (or should be) indicated as a yellow, perhaps red, arc on the face of the engine tachometer. Rpm settings within those ranges should not be used.
    Any gyroscopic moment induced   depends on the rate of change in aircraft pitch or yaw, and the rotational speed and moment of inertia of the propeller. Its mass moment of inertia depends on propeller mass and diameter.  The gyroscopic loads are transferred to the airframe via the engine crankshaft, crankcase and mountings. Under some conditions, gyroscopic loads may lead to crankshaft/crankcase failures. See 'The Fox story'.
    The failure conditions usually identified are the use of a propeller of excessive diameter (the moment of inertia increases exponentially with diameter) possibly combined with  an excessive 'overhung' moment — the distance from the propeller cg to the engine. Excessive gyroscopic loads may also be placed on the crankshaft/crankcase by using brake, rudder and a burst of throttle to swing an aircraft rapidly when taxiing.
    The flight conditions that follow propeller blade failure cannot be simulated in training, but an extreme out-of-balance condition (loss of one blade for example) can very quickly shake the engine from its mountings.
    7.3 Propeller types
    The following is a copy of a document authored by Marcus Graney and published on the web site of the New Zealand manufacturer of Airmaster propellers. I have added the notes presented in italic.  ...  JB 
    The most common type of propeller in sport aviation is the fixed-pitch propeller.  Although cheap, this is one of the crudest propulsion  devices you could use, and has  been  superseded by a variety of more advanced options, now  readily available on the market. But, how do you know  how each type of propeller operates and what advantages the different  types offer? How are you going to choose between the different types available for your aircraft, especially considering that a more capable propeller  is also more expensive?
    There are four common  families of propeller, which I will introduce to you. They are fixed-pitch, ground-adjustable, inflight-adjustable and constant-speed. The last two  are both examples of variable-pitch propellers.
    In order to appreciate  the advantages which are characteristic of the different families of propeller, we must first consider the most fundamental characteristic of a propeller — the pitch. Pitch is important, as it is the manner in which pitch is  controlled that allows us to differentiate between one family of propeller and another.
    A useful analogy when  considering the affect of pitch is that of an automobile gearbox. By comparing a propeller's pitch to a gear ratio, and considering the function of a  gearbox, we will gain an appreciation of the different families of propellers.
    What  is pitch?
    Propeller theory includes  a variety of concepts that may at times be called pitch. Pitch can refer  to the blade angle with respect to a flat plane, the distance that a propeller  will advance through the air for each rotation or the amount of "bite"  that the blade has on the air. Essentially these concepts all describe  the same thing. To use our automobile analogy, pitch is like the gear  ratio of the gearbox. The important thing to note with pitch, is that it is available in a wide variety of degrees, or 'amounts', much like different gear ratios. To demonstrate, consider the following examples:
    A fine pitch propeller  has a low blade angle, will try to move forward a small distance through  the air with each rotation, and will take a 'small' bite of the air.   It requires relatively low power to rotate, allowing high propeller  speed to be developed, but achieving only limited airspeed. This is like having a low gear in your automobile.
      A coarse pitch propeller has a high blade angle, will try to advance a long distance through the air with each rotation, and will take a big 'bite' of the  air. It requires greater power to rotate, limiting the propeller speed that can be developed, but achieving high airspeeds. This is like having a high gear in your automobile.  
    Pitch  and the different families of propellers
    As we saw above, pitch is a key element in the description of propellers (along with other factors such as diameter and blade area). When considering the four families of   propellers it is useful to start with the simple fixed-pitch propeller,  and look at the enhancements in pitch control that are gained as we progress  through each family to the most advanced, the constant-speed propeller.
    Fixed-pitch propeller
    With a fixed-pitch propeller, the pitch of the propeller is fixed from manufacture. The performance of your aircraft is determined on the day your propeller is fitted, and  is going to be limited within the constraints of the propeller. An analogy  with an automobile is as though you had only one gear. Often when choosing a fixed-pitch propeller for your aircraft, manufacturers give you a choice  of either a climb or a cruise prop. A climb propeller has a relatively  fine pitch and a cruise propeller has a relatively coarse pitch. This  is like a car manufacturer giving you a choice of a low or a high gear.  Either you will be really slow off the mark, or your engine is going to  have to be red-lined to get anywhere at a reasonable speed.  
    Ground-adjustable propeller
    Many propellers manufactured and sold for ultralight and experimental aircraft are ground-adjustable. These propellers have the advantage of being able to have their pitch set before each flight if required, taking into account the type of flying  you intend to do. More usually however they are used as a low cost way  to try out various pitches and settle on the propeller pitch that best suits your aircraft and your style of flying. This can be compared to  having a gearbox in your car that you can only change before you set out on your journey.
    Variable-pitch propeller
    With a variable-pitch propeller, you really have choices. To use the automobile analogy again, your car now has a real gearbox that you can change gear with on the go.  (I hope that your car can do this at least!) In addition, rather than  being limited to 4 or 5 gears, you can utilise any pitch along the continuum  from maximum to minimum. The pitch of the propeller may be controlled  in flight to provide improved performance in each phase of flight. Typically  you would take-off in a fine pitch (low gear) allowing your engine to  develop reasonable revs, before increasing the pitch (change up gears)  as you accelerated to your cruising speed. You'll end up with the propeller  at a relatively coarse pitch, (high gear) allowing the miles to pass beneath   you at a rapid rate, while your engine is gently ticking over at a comfortable  speed.
    This feature of a variable-pitch propeller will provide you with performance advantages, including:
    Reduced take-off roll and improved climb performance. Fine pitch allows the engine to  reach maximum speed and hence maximum power at low airspeeds. Vital  for take-off, climb, and for a go-around on landing.
      Improved fuel  efficiency and greater range. Coarse pitch allows the desired aircraft  speed to be maintained with a lower throttle setting and slower propeller speed, so maintaining efficiency and improving range.
      Higher top speed.  Coarse pitch will ensure your engine does not overspeed while the propeller absorbs high power, producing a higher top speed.
      Steeper descent  and shorter landing roll. With a fine pitch and low throttle setting,  a slow turning propeller is able to add to the aircraft's drag, so slowing  the aircraft quicker on landing.  
    Variable-pitch propellers actually come in a variety of versions. These different versions refer to the different ways that they are controlled, and include:
    Two-position propeller. Inflight-adjustable  propeller. Automatic propeller. Constant-speed propeller.  
    A couple of these are now of historic interest only, so lets concentrate on the two most  common options these days; the inflight-adjustable operation and the constant-speed propeller.
    The inflight-adjustable propeller allows the pilot to directly vary the pitch of the propeller   to the desired setting. Combined with the throttle control, this control allows a wide variety of power settings to be achieved. A range of airspeeds   can be maintained while keeping the engine speed within limits. While  rare in larger aircraft, the inflight-adjustable propeller is the most  common type of variable-pitch propeller that is encountered in sport aviation.
    When operated in manual  mode, the Airmaster propeller is an example of an inflight-adjustable propeller.
    Constant-speed propeller
    The constant-speed propeller is a special case of variable pitch, which is considered in  a family of its own, and offers particular operating benefits.
    With constant speed  control, the pitch of the variable-pitch propeller is changed automatically by a governor. After the pilot sets the desired engine/propeller speed  with the propeller speed control, the governor acts to keep the propeller  speed at the same value. If the governor detects the propeller speed increasing,  it increases the pitch a little to bring the speed back within limits.  If the governor detects the propeller speed decreasing, it decreases the  pitch a little to bring the speed again back within limits. This operation may be compared to an automatic gearbox in an automobile, where the gears are changed automatically to keep the engine operating at a reasonable speed.
    (The governor or constant speed unit [CSU] may be an electronic device that detects the rotational speed of a slip-ring incorporated in the propeller hub, and controls operation of a servomotor/leadscrew pitch change actuator in the hub assembly. Or, it may be an hydraulic fly-ball governor attached to the engine, using engine oil to operate a hydraulic pitch change piston in the hub assembly. In the first case, the cockpit control device is likely to be  knobs and switches. In the hydraulic system, the governor is  likely to be  cable operated from  a cockpit  lever — JB.)
    A constant-speed propeller  will automatically deliver you the advantages outlined above for variable-pitch propellers, with almost no control required from the pilot. Once  a propeller/engine speed is selected, the pilot is able to control the  power purely with the throttle (actually controlling the absolute pressure of the fuel/air mix in the intake manifold [MAP] which then determines power output) and the controller will act to keep  the propeller/engine speed at the selected setting.
    While allowing the pilot to ignore the propeller for most of the time, the pilot must still  choose the most appropriate engine/propeller speed for the different phases of flight:
    Take-off, go-around and landing. A high speed setting is used when maximum power is needed  for a short time such as on take-off. The high speed setting may also  be used to keep the propeller pitch low during approach and landing,  to provide the desired drag and be ready for a go-around should it be required.
      Climb and high  speed cruise. A medium speed setting is used when high power is needed  on a continuous basis, such as during an extended climb, or high speed cruise.
      Economic cruise. A low speed setting is used for a comfortable cruise with a low engine  speed. This operation produces low fuel consumption and longer range,  while the advantages of low noise and low engine wear are also enjoyed.  
    When operated in automatic  mode, the Airmaster propeller is an example of a constant-speed propeller.
    Special pitch modes 
    As well as the ability to vary the pitch of the propeller to optimise the aircraft performance, some variable-pitch propellers have some other special modes of operation  that can be very useful in certain circumstances:
    Feather. A feathering  propeller can alter the pitch of the blades up to almost 90 degrees. That is, the blade pitch is changed so that they have their leading  edge pointing right into the direction of flight, offering minimum resistance to the airflow. This mode allows the propeller rotation to be stopped, without adding excessive drag to the aircraft. Feather may be used to  improve the performance of the aircraft after the failure of an engine,  but more usually in light aircraft it is used in motor glider applications.  Here the engine is used to gain altitude, before the engine is switched  off, the propeller feathered, and then gliding flight commenced.
      Reverse. A reversing  pitch propeller can alter the pitch of the blades to a negative angle. That is, the blade pitch is changed so that they have their leading  edge pointing slightly opposite to the direction of flight. This mode allows reverse thrust to be developed by the propeller. In larger commuter  and transport aircraft this feature is often used to slow the aircraft rapidly after landing, but in sport aircraft it is more usually used  to enhance manoeuvring on the ground. A popular application is in seaplanes,  where the ability to manoeuvre backwards, and sometimes to reduce the  thrust to nothing, is especially useful.  
    This overview was designed to assist the understanding of how the ability to control propeller pitch is used to categorise the different families of propeller design.  More importantly it has illustrated that as we progress from one design  family to another, we realise significant improvements in performance, effectiveness and efficiency.
    While a family of  propellers that offers better performance is likely to be more expensive to purchase, you can expect that over time the efficiency of a higher performance propeller will produce savings that will offset the initial  cost. In addition your flying will be a more relaxed and enjoyable experience! 
    When deciding what   type of propeller to buy for your aircraft, you have to weigh up the relative advantages and costs.  To help, we can summarise the most common families  of propellers, and make a simple comparison of their respective advantages in cost and capability.
    Marcus Graney
     Aeronautical Engineer 
     November 2000
    There is another type of propeller that is quite rare; the single-blade propeller and, more particularly, the single-blade folding propeller  associated with low-power engines in motor-gliders, see 'Single-blade propellers in very light aircraft'. 
    7.4  Propeller theory
    The forces
    Propeller blades  are constructed using aerofoil sections to produce an aerodynamic force, in a similar manner to a wing. Consequently, the blades are subject to the same  aerodynamics  — induced drag, parasite drag, wingtip vortices, lift/drag ratios at varying aoa,  pressure distribution changing with aoa, etc. There is a difference in application because, in flight,  the propeller has rotational velocity added to the forward velocity. Thus, the flight path of any blade section is a spiral — a helical flight path.
    The diagram at left represents a blade section in flight and rotating about the shaft axis.  Because of the different application, it  doesn't serve much purpose to express the resultant aerodynamic force  as we would for a wing; i.e. with  the components acting perpendicular (lift) and parallel (drag) to that helical flight path, as in the upper figure.  So, we resolve  the  aerodynamic force into the component  acting forward and aligned with  the aircraft's longitudinal axis as the thrust force, and that  acting parallel to the direction of rotation as the propeller torque force.
    As you see  in the lower  figure the component of the 'lift' acting in the rotational plane has now been added to the 'drag' to produce the 'propeller torque force' vector. The remaining forward-acting portion of 'lift' is then the thrust. That is why propeller efficiency is usually no greater than 80–85%; not all the 'lift' can be used as thrust, and the propeller torque force consumes quite a bit of the shaft horsepower. The propeller torque  and the engine torque will be in balance when the engine is operating at constant rpm in flight.
    Centrifugal force imposes considerable tensile stress while  trying to pull the blades from the hub. Torque reaction applies bending stress to the blades in the reverse direction of rotation while the thrust force tends to bend the outer sections of the blades forward. The centrifugal twisting moment tends to twist the blades to a decreased (finer)  pitch and the aerodynamic twisting moment (similar to the wing pitching moment) tends to twist the blades to a coarser pitch.  The air inflow at the face of the propeller disc also affects propeller dynamics.
    Blade angle and pitch
    Although all parts of the propeller, from the hub to the blade tips, have the same forward velocity, the rotational velocity — and thus the helical path of any blade  station — will depend on its distance from the hub centre. Consequently, unless adjusted, the angle of attack will vary along the length of the blade.  Propellers  operate most efficiently when the aoa at each blade station is  consistent (and,  for propeller efficiency, that giving the best lift/drag ratio)  over most of the blade, so a twist is built into the blades to achieve a more or less uniform aoa.
    The blade angle is the angle the chord line of the aerofoil makes with the propeller's  rotational plane and is expressed in degrees. Because of the twist, the blade angle will vary throughout its length. So, normally the standard blade angle is measured at the blade station, 75% of the distance from the hub centre to the blade tip. The angle between the aerofoil chord line and the helical flight path (the relative airflow)  at the blade  station is the angle of attack and the angle between the helical flight path and the rotational plane is the angle of advance or helix angle. The aoa and helix angle vary with rotational and forward velocity.
    The basic dimensions of propellers for light aircraft are usually stated in the form of number of blades, and diameter and pitch with  values  in inches; e.g. 3-blade 64" × 38". The pitch referred to is the geometric pitch that is calculated for any blade station, but usually the station at 75% radius.
    Geometric pitch = the   circumference (2πr) of the propeller disc at the blade station multiplied by the tangent of the blade angle. Thus, it is the distance the propeller — and aircraft — would advance during one revolution of the propeller if the blade section followed a path extrapolated along the blade angle.
    e.g. For a blade station 24 inches from the hub centre (0.75r)  and a 14° blade angle, the circumference = 2 × 3.14 × 24 = 150 inches, and tangent 14° = 0.25. Thus, the geometric pitch is 150 × 0.25 = 38 inches. Propellers are usually designed so that all blade stations have much the same geometric pitch.
    Designers may establish the ideal pitch of a propeller, which is the theoretical advance per revolution that would cause the blade aerofoil to be at the zero lift aoa; thus, it would generate no thrust and, ignoring drag, is the theoretical maximum achievable aircraft speed.
    The velocity that the propeller imparts to the air flowing through its disc is the slipstream. Slip used to be described as the difference between the velocity of the air behind the propeller (i.e. accelerated by the propeller) and that of the aircraft. Nowadays, slip has several interpretations, most being aerodynamically unsatisfactory, but  you might consider it to be the difference, expressed as a percentage, between the ideal pitch and the advance per revolution when the the propeller is working at maximum efficiency in converting engine power to thrust power. Slip in itself  is not a measure of propeller efficiency; as stated previously, propeller efficiency  is the ratio of the thrust power (thrust × aircraft velocity) output to the engine power input.
    Pitch and velocity
    The performance of aircraft fitted with fixed-pitch or ground-adjustable propellers is very much dependent on the chosen blade angle.   Fixed-pitch propellers limit the rpm developed by the engine at low forward velocity, such as occurs during the take-off ground roll; they may also allow the engine rpm to exceed red-line maximum when the load on the engine is reduced, such as occurs in a shallow dive.  Fixed-pitch propellers operate at best efficiency at one combination of shaft power and airspeed. Blade angle is usually chosen to produce maximum performance at a particular flight condition, for example:
             •   Vy climb; i.e. a climb propeller
             •   Vc  cruise; i.e. a cruise propeller.
    The climb propeller is usually chosen when the aircraft normally operates from a restricted airfield or in high density altitude conditions.  The climb propeller will produce  maximum efficiency at full throttle around the best rate of climb airspeed and will perform fairly well at take-off. But during the initial take-off acceleration,  even the  climb propeller may restrict the engine rpm to less than 75% power. The cruise propeller will achieve maximum efficiency at 75% power at airspeeds around the design cruising speed but aircraft take-off and climb performance will not be the optimum.  The cruise propeller usually has a little more pitch than the standard propeller fitted to the aircraft. A high-speed propeller might be fitted when the aircraft is intended to be operating at, or above, rated power for short periods — in speed competition, for example.
    A variable-pitch constant-speed propeller allows the engine to develop maximum rated power and rpm during the ground roll, and to develop full power throughout its normal rpm range. With a constant-speed propeller, the pilot controls the inlet manifold absolute pressure [MAP] with the throttle lever and the engine rpm with the rpm control lever or knob/switches. (MAP is the pressure of the air/fuel mixture being delivered to the cylinders and  is usually measured in inches of mercury [in/Hg] rather than hectopascals. Standard sea-level barometric pressure is 29.92 in/Hg or 1013.2 hPa.) The aircraft flight manual usually provides the pilot with several combinations of rpm/MAP to achieve a particular power setting. For example, in one particular aircraft, the recommended combinations  for 65% power at sea-level are 2100 rpm + 26 in/Hg MAP, or 2200 rpm + 25 in/Hg, or 2300 rpm + 24 in/Hg, or 2400 rpm + 23 in/Hg. So, you can use low rpm and high MAP, or high rpm and low MAP, to achieve exactly the same power output. The 2100 rpm/26 in/Hg low rpm/high MAP combination probably gives more efficient cylinder charging and better combustion plus  less friction. The high MAP also acts as a cushion in the cylinders, reducing engine stress. Obviously, if a constant-speed propeller is fitted to an aircraft then an intake manifold pressure gauge —  marked with the allowable engine operating ranges — must be fitted, otherwise excessive manifold pressure (which raises the cylinder compression pressure) may overstress the engine. Variable-pitch in-flight adjustable propellers also necessitate fitment of a manifold absolute pressure gauge.
    7.5 The windmilling propeller
    The angle of attack of a fixed-pitch propeller, and thus its thrust, depends on its pitch, the forward speed of the aircraft and the rotational velocity.  Following a non-catastrophic engine failure, the pilot tends to lower the nose so that forward airspeed is maintained while at the same time the rotational velocity of the engine/propeller is winding down. As the forward velocity remains more or less unchanged while the rotational velocity is decreasing, the angle of attack must be continually decreasing. It is possible (depending on the particular PSRU, blade angle etc.) that at some particular rpm, the angle of attack will become negative to the point where the lift component becomes negative (reverses) and the propeller may autorotate; in effect,  driving the dead engine as an air pump. This acts as greatly increased aerodynamic drag, which adversely affects the aircraft's L/D ratio and thus glide angles. The parasitic drag (including the 'reversed thrust') is  greater than that of a stationary propeller.  The engine rotation may cause additional mechanical problems if oil supply is affected.
    If the forward speed is increased, windmilling  will increase. If forward speed is decreased, windmilling will decrease. Thus, the windmilling might  be stopped by temporarily reducing airspeed  possibly to near stall — so that the reversed thrust is decreased to the point where the engine airpump torque and  friction will stop rotation. This is not something that should be attempted without ample height.
    Should the PSRU fail in flight, the propeller is thereby disconnected from the engine and may 'freewheel' rather than 'windmill'.
    In the diagram, the  upper figure shows the forces associated with a section of a propeller blade operating normally.  The lower figure shows the forces and the negative aoa associated with the propeller now windmilling at the same forward velocity.
    A variable-pitch propeller may have a feathering facility, which turns the blades to the minimum drag position (i.e. the blades are more or less aligned fore and aft) and thus stops windmilling when the engine is no longer producing power. Such a feature is not usually fitted to a single-engine aircraft, but a few powered recreational aircraft are designed with very low parasitic  drag plus wide span, high aspect ratio wings  that provide L/D ratios around 30:1, and thus have excellent soaring capability. Propeller parasitic drag will have a relatively high effect on the  performance of such aircraft so they are usually fitted with a feathering propeller.
    The image at left is from a FAA Special Airworthiness Information Bulletin (please read) and shows the change in equivalent parasite drag for both a windmilling propeller and a stationary propeller at blade angles from fully flat to feathered. It can be seen that, in this particular case, the windmilling propeller produces  more drag than the stationary propeller up to blade angles of 18 degrees or so.
    It can be inferred from the preceding material  that the windmilling vs stationary drag characteristics for  aircraft/propeller combinations will be subject to considerable variation.
    Some motor-gliders are designed with the engine/propeller unit mounted on a retractable pylon, so that when good atmospheric lift conditions exist the engine plus propeller can be stopped and stowed within the fuselage. 
    7.6 The runaway propeller
    As a propeller system increases in complexity, then the possibilities for malfunction increase. A problem associated with constant-speed propellers is  governor failure during flight which, in most installations, will cause the propeller blades to default  to their fine pitch limit. This greatly reduces the load on the power plant, and the engine will immediately overspeed, particularly if in a shallow dive. Depending on the fine pitch limit setting, the rpm of an overspeeding engine — sometimes referred to as a 'runaway prop' —  may quickly go way past red-line rpm and, unless immediate corrective action is taken, the engine is likely to self-destruct and/or the propeller blades break away from the hub due to the increased centrifugal force.   
    The corrective action is to immediately close the throttle and reduce to minimum flight speed by pulling the nose up. (But see 'Recovery from flight at excessive speed'.)  Once everything is settled down, fly slowly, consistent with the fine pitch setting,  to a suitable airfield using minimum throttle movements.  (The constant-speed propeller fitted to a competition aerobatic aircraft  usually defaults to their coarse pitch limit to prevent overspeeding, but an immediate landing is required.)
    Propeller theory is  complex and not appropriate to this Flight Theory guide, but the  outline  above at least introduces some of the everyday terms encountered. 
    Things that are handy to know The term 'brake horsepower' is a measure of the power delivered at the engine output shaft; measured by means of a dynamometer or similar braking device. The term 'shaft horsepower' [shp] is a measure of the engine power available at the propeller shaft. Generally it is the same as bhp but if the coupling is not direct drive — a propeller speed reduction unit [PSRU] is interposed between the crankshaft output and the propeller shaft as in the Rotax 912 — the shp will be a little less than bhp because of the power loss in driving the belt or gear driven PSRU.
      The use of the horsepower term for piston aero engines has successfully withstood metrication. To convert horsepower to watts multiply by 745.7 or by 0.75 to convert to kilowatts. When torque is expressed in newton metres, and engine speed in radians per second, power will be in watts.
      The stoichiometric (chemically correct) air/fuel mixture produces complete combustion of all the fuel and all the oxygen in the cylinder charge — and also the highest temperatures, which may be detrimental to the engine metallurgy. The stoichiometric air/fuel ratio for gasoline fuels is 14.7:1 by weight.

    Spark ignition engines provide best power with an air deficiency of 5–15% from stoichiometric — i.e. about 12–13:1 (rich) — and provide minimum fuel consumption with around 10% excess air; i.e. about 16:1 (lean).
    This indicates that the engine, at sea-level and using a stoichiometric mixture, would process about 8500 litres of air per litre of fuel. (Avgas weighs 0.71 kg per litre, and air (at standard sea-level conditions) weighs 1.225 kg per 1000 litres.) The leaned mixture for best economy cruise is around 16:1 (9000 litres of air), and for maximum engine rich mixture performance, around 12:1 (7000 litres of air).
    The Rotax 912 1.2 litre engine produces 75% power at 5000 rpm, and with a firing cycle every second revolution it would process 1.2 x 5000/2 = 3000 litres of air/fuel mixture per minute. The fuel used would be 3000/9000 = 0.33 litres/minute or around 20 litres/hour, at sea-level.
      Most four-stroke, normally aspirated, aero-engines between 80 and 400 hp have a specific fuel consumption close to 0.19 kg or 0.27 litres, per horsepower per hour (or 0.42 lbs/hp/hr). Then the Jabiru's engine, rated at 80 hp, but using only 65% for the 97 knot cruise, would consume 80 × 0.65 × 0.27 = 14 litres over 100 air nautical miles, or 7 air nautical miles per litre. Note that you can create a little rule of thumb here that is applicable to most four-stroke engines — "the fuel burn, at 'performance cruise speed', is about one-fifth of the rated engine horsepower — in litres per hour." Thus, fuel burn for the Jabiru cruising at 75% power is 80/5 = 16 litres/hour. Two-stroke engines have to use a richer mixture to run cooler so, for such engines, add about 10% to the calculated result.  

    6.1 Lift generation
    In the 'Basic forces' module it was stated that when an aircraft is moving through the air, the consequent pressure changes or aerodynamic reactions to its motion will be acting at every location on its  surface. We had a look at the formula for calculation of lift from the wings:
    (Equation #1.1)   Lift [ newtons] = CL × ½rV² × S 
    It is usual to substitute the symbol 'Q' to represent dynamic pressure [½rV²] so the expression above may be more simply presented as:
    (Equation #4.1)  Lift [newtons] = CL × Q × S 
    where Q × S is a force.
    It is appropriate to state here that the formula is an approximation of the average lift from the wings. At any one time, the aerodynamic reactions will vary over the span of the wing and with the position at which  the wing control surfaces are set.
    Aerofoils and the aerodynamic force
    An aerofoil (airfoil, parafoil, wing section or wing profile) is an object  — with the shape of the cross-section of the wing —  having the function of producing a controllable net aerodynamic force by its motion through the air. To be useful this aerodynamic force must have a lifting component that is much greater than the resistance or drag component. In a powered aircraft,  motion through the air is provided by the thrust; so in effect, the aerofoil is a device that converts  thrust into lift; in a glider the aerofoil converts much of the gravitational force (the potential energy of height) into lift.
    The aerodynamic force  has two sources: the frictional shear stress, or skin friction, that acts tangential to the surface at every point  around the lifting body; and the pressure exerted perpendicular to the surface at every point. (At speeds over about 250 knots,  flow compressibility introduces other factors.) The resultant net aerodynamic force is the sum of all those forces as distributed around the body. For wings, it is conventional to show the resultant force as acting from an aerodynamic centre  and resolved into two components: that acting perpendicular to  the  flight path is the lift, and that acting parallel to the flight path is the drag. For propeller blades, the aerodynamic reaction is resolved into the thrust component and the propeller torque component. For rotor blades, a  more complex resolution is necessary.
    Note: normally the aerofoil is incorporated into a wing with  upper and lower surfaces enclosing the load bearing structure. However, when designing a low speed  minimum aircraft such as the Wheeler Scout there are advantages in using a 'single surface' cambered aerofoil wing, very similar to a hang glider wing. Such wings incorporate a rounded leading edge (formed by the aluminium tubing  leading edge main spar) that directs the airflow into the upper and lower streams at all angles of attack. The slight camber is formed by battens sewn into sleeves in the  'sails'. Such wings are somewhere between a thin curved  plate and a full aerofoil, and are similar in cross-section to a bird's wing. A parachute wing uses the ram air principle to form the  aerofoil shape — see 'The  ram-air parachute wing'. 
    Now we need to establish how that  airflow actually produces the lifting force.  John S Denker has published a web book 'See How it Flies'  that has a particularly good section on lift generation with excellent illustrations. You should carefully read through  section 3 'Airfoils and airflow'  and particularly acquaint yourself with the  Eulerian  approach  of 'streamlines' to visualise airflow. In the illustrative diagram at left, narrowing (A) of  streamlines  indicates accelerating local speed and decreasing local pressure — a favourable pressure gradient. Opening up (D) of streamlines  indicates flow deceleration and increasing pressure — an adverse pressure gradient. The term 'free stream' is usually substituted for 'flight path' when discussing aerofoil characteristics because the aerofoil is presumed stationary, as in a wind-tunnel, and the airstream flows around it.
    The following  summarises the content of  section 3 of 'See How it Flies':
    •   A flat plate, held at a small aoa, will generate an aerodynamic force — lift and drag —  and indeed, some low momentum  aircraft do use basically flat plates as  their tailplane surfaces. As mentioned above, the shape of sail-type wings is somewhere between a  plate and the more usual wing.  However, for  aircraft that cruise in the 50–150 knot range,  a wing with a rounded leading edge, a sharp or square-cut trailing edge, a cambered upper surface and a flat or slightly cambered  bottom surface — i.e. a full  aerofoil section — will be far more efficient —  aerodynamically and structurally — and more effective in performance. (The faster the aircraft, the more the aerofoil section tends to flatten out. So, for supersonic aircraft we are nearly back to the sharp-edged flat plate.)
    Aerofoil characteristics

    The straight line joining the leading edge (left) and trailing edge (right) is the chord line. The curved mean camber line is drawn equidistant between the top and bottom surfaces, and the light coloured gap between the chord and mean camber lines represents the camber — which, in this particular aerofoil [a NACA 4415], equates to 4% of the length of the chord at its  maximum point  which occurs at  40% of chord length from the leading edge. Aerofoil  thickness is the distance between upper and lower surfaces. The maximum thickness  of this aerofoil equals 15% of the chord; that is called the 'thickness ratio'.  At the trailing edge the included angle between the upper and lower surfaces is significant in wake generation — a lower  angle is better, and if the trailing edge is square-cut the thickness there  should not exceed 0.5% of the chord.  In flight, the angle the wing chord line subtends with the flight path is the geometric angle of attack.
    •   A cambered wing will still produce lift at  zero, and slightly negative, geometric angles of attack, as  shown in the lift coefficient diagram.  The aoa where no lift — only drag — is produced is called the zero-lift aoa which, in the diagram, is nearly  –2°. From that diagram you can infer  that camber contributes a lift coefficient of about 0.2 and anything greater must be provided by  aoa. Of course, this will vary with the amount of camber in a particular aerofoil. If the aoa was reduced below the zero-lift value, for example –4°, then the direction of lift would be reversed. The only time you would need such a negative aoa is when you are flying inverted, or performing aerobatics, neither of which are currently allowable in aircraft registered with the RA-Aus.
    At the zero-lift aoa,  all the aerodynamic force is acting parallel to the free stream and is mostly skin friction drag, with a less significant  amount of pressure drag but the latter will increase as the aoa is increased. Pressure drag is explained in section 4.7 'Parasite drag'.
    Cambered wings perform quite well in inverted flight, but are not as efficient as in normal flight because a higher aoa is needed to make up for the lower wing surface having the maximum camber when inverted. For this reason, aerobatic aircraft tend to use symmetrically shaped aerofoils — i.e. the 'camber' of the bottom surface balances the 'camber' of the top surface and aerodynamically the result is zero camber  — thus such wings rely purely on the geometric aoa to produce lift.
    •   At positive angles of attack there is a stagnation point, or line, just under the leading edge of the aerofoil where some of the airflow has been brought to a standstill. The air molecules reaching that line, in the incoming stream,  are equally likely to go under or over the wing. Stagnation pressure, the highest in the system, exists along the stagnation line.  The  location moves down and under the leading edge as aoa increases, up to the stalling aoa. Another more confined  stagnation point exists at the trailing edge. If an imaginary line is drawn between the two stagnation points, the  cross-sectional view of the division of the aerofoil into upper and lower flow areas becomes apparent.
    •   The behaviour of the airstream flowing around such a wing accords with   Bernoulli's principle. As the air accelerates away from the stagnation line, the local airflow over the upper surface  gains a greater speed than the lower. Consequently, to retain constancy, the  static pressure on the upper surface will decrease, and on the lower surface it may decrease very slightly  at low aoa but will increase as aoa increases.
    There is another concept for explaining the  pressure differential between upper and lower wing surfaces. Leonhard Euler was a  mathematician who was a contemporary of, and collaborator with,  Daniel Bernoulli. The Euler  Equations (a special case of Newton's Third Law of Motion) express the relationship between flow velocity and the pressure fields in frictionless flow. Because the air particles follow the curved streamlines above the upper surface, there must be a centripetal force across the streamlines that accelerates the flow towards the centre of curvature. That force must be associated with a pressure gradient across the streamlines; i.e.  ambient atmospheric pressure at some distance from the surface, grading to a lower pressure on the upper wing surface. For more information enter the terms 'Euler curvature airfoil OR aerofoil' into a search engine.
    •   The  usual  way of looking  at the lift force is that the wing produces an upflow in the air in front of it and a downwash behind it. That downwash continuously imparts  momentum —  with  a downward velocity component  —  to the air affected by the passage of the aircraft. As you will recall from the 'Basic forces' module  the action of adding downward momentum will have an equal and opposite reaction, which in this case is an  upward force applied to the wing. And, of course, the energy provided to impart momentum to the air comes from engine power; in a glider it would come from the gravitational potential energy of height. There is   a distinction  between the  'downflow' produced by the aerofoil and the additional 'downwash' produced by wing vortices (see below), the deflection of which increases with angle of attack. However, for our purposes we can treat all the momentum imparted to the airstream as 'downwash'.  
    You will also recall, from the 'Basic forces'  module,  that thrust is the reaction from the momentum imparted to  a tube of air with the diameter of the propeller. The associated slipstream or 'prop wash' is the  added momentum —  quite apparent if you stand behind a stationary aircraft when 'running-up' the engine. Helicopter rotor blades are long, slender rotating wings  — somewhere between variable pitch  propeller blades and normal wings — and the momentum applied to the air — the 'rotor wash' —  can  be  seen clearly by its effect on dust, vegetation  and other objects (like parked ultralights) beneath a hovering helicopter. Similarly,  a wing producing lift continuously accelerates  a  flattened  tube of air with diameter approximating  the wing span;  the  longitudinal downward inclination to the flight path of that flat tube  increases as aoa increases. Some liken that concept to the wing acting as an airscoop.    
    •   Another  concept associated with the aerodynamic force — circulation theory — is a mathematical description of a 'bound vortex',   which also fits in with the generation of the physical wing-tip vortices. Vorticity is rotary motion in a fluid, and you could regard  'circulation' as referring to the apparent flow rotation — upwash then downwash — around the upper/lower surfaces.
    Note: there is a long-held and still-continuing argument, particularly in newsgroups and other internet venues, about the pros and cons of the various lift generation theories. None of the  arguments put forward (often ill-informed) affect in any way  how a light aircraft flies, how it should be safely and economically operated, or how it should be built; so it is best to ignore them unless you are particularly interested in the science of aerodynamics and skilled in mathematics.
    Pressure differential
    At any aoa between the zero lift and stalling angles, the total pressure pushing down on the wing upper  surface will always be less than the total pressure pushing up on the lower surface. The absolute pressure difference between the upper and lower  surfaces will increase as aoa increases up to the stalling aoa.
    Although it is still  small in comparison with the ambient  atmospheric  pressure, it is this pressure differential resulting from the wing deflecting the air that initiates the lifting force; and this is true however lift theory may be expounded. Much work has been done in designing aerofoils that will maintain the required pressure difference in the targeted flight conditions.
    We can calculate the net pressure difference for the Jabiru using the scenario in the 'Basic forces' module  section 1.4;    i.e.  cruising at 6500 feet, airspeed 97 knots or 50 m/s, air density 1.0 kg/m³.  The ISA atmospheric pressure at 6500 feet is about 800 hPa:
    static pressure = 800 hPa dynamic pressure = Q = ½rV² = ½ × 1.0 × 50 × 50 = 1250 N/m² = 12.5 hPa  
    Multiplying the  dynamic pressure of 1250  N/m² by the lift coefficient of 0.4 gives the pressure differential of 500 N/m².   That  pressure differential of 500 N/m² (5 hPa)  is less than 1% of the ambient static pressure, but applying that over the 8 m² of wing area  gives the lift force of 4000 newtons that we calculated in section 1.4.
    Lift coefficient 
    The lift coefficient CL is a dimensionless (or nondimensional) quantity (it has no units of measure)   relating mostly to aoa. It increases as the aoa increases from the normal aoa used in cruise flight,  and also to the form of the wing and the aerofoil section.   CL represents the proportion of  total dynamic pressure  converted to lift force.
    When the aircraft designer calculates the CL curve for an aircraft it must be  related to a particular wing reference area. This may be the visible plan area of the wings but it could also include that area of the wings conceptually enclosed within the fuselage.
    Note that the CL for an aerofoil will have a  value perhaps 10–20% higher than the CL for any wing incorporating that aerofoil;  this is discussed in the spanwise pressure gradient section. (The convention is to use a lower case 'L' [thus Cl ] when referring to the lift coefficient for an aerofoil  to distinguish it from the lift coefficient for a wing, but I have retained CL for both.)
    In level, non-manoeuvring flight, lift equals weight, so  equation 4.1 can be restated as:
    (Equation #4.2)  CL = W /  (Q × S)
    The usable value of CL in a very light aircraft with low-aspect ratio wings  without  lift-enhancing devices  might range between 0.1 and 1.6. (Unless it is a symmetrical aerofoil — same camber  top and bottom — the lift coefficient range will  be different for the same wing when in  inverted flight.)
    However, a very low CL value  can be obtained momentarily if the wings are 'unloaded' in flight. This can be achieved by applying sufficient continuous forward pressure on the control column to attain a near-zero aoa such that the net pressure differential between the upper and lower wing surfaces is very low.  This would imply low lift  generation and reduced drag, so the thrust  will accelerate the aircraft a little faster than normal.
    Furthermore, a negative CL can be obtained by maintaining so much forward pressure on the control column that the aerodynamic force is reversed. If initially  flying straight and level, the aircraft will 'bunt';  i.e. enter the first few degrees of an outside loop with the centripetal force for the turn being supplied by the reversed lift. (This reverses the direction of the wing loading and should never be attempted in weight-shift aircraft nor three-axis aircraft unless the three-axis  manufacturer's flight manual allows such a manoeuvre.) And, of course a suitably equipped aircraft can be flown in inverted level flight — in which case the under-wing surface becomes the upper and a completely different CL range applies, because the cambered surface is now underneath and a higher aoa is necessary to maintain the  lift required for level flight.
    Incidentally  many pilots utilise the low   CL technique when landing a taildragger. The application of forward pressure on the control column after touchdown 'pegs' the aircraft down by reducing the aoa and thus generated lift, and thereby puts increased pressure on the tyres, and amplifies friction and  any braking force applied. The same technique was used to bring military DC3 aircraft to a quick stop.
    4.2 Aerofoil simulation
    Whichever way lift theory is expounded, this simple equation is applicable:
      Lift  = CL × Q × S 
    I suggest you try out what you have learned so far in an aerofoil flight test simulation program. You need a Java-enabled  browser.  Read the instructions carefully and reset the measurement units from pounds to newtons. In this case, airspeed will be shown in km/h but just mentally divide by two (and add 10%) to get knots — halve it again if you want m/s.
    You can try this simple model out with a popular aerofoil, the NACA 2412, which is one of a series dimensioned by the U.S. National Advisory Committee for Aeronautics (the forerunner of NASA) in the 1920s and 1930s. The 2-4-12 (twenty-four twelve) has  a camber of  2% [2] of chord with maximum camber occurring at 40% [4] of chord from the leading edge and a thickness/chord ratio of  12% [12].
    Note that all dimensions are proportional to the chord so the same aerofoil section shape is retained throughout a  wing even if it is tapered in plan form. The wing is  thickest at the root and thinnest at the tip; i.e. it must also be tapered in thickness.  Most aerofoils suitable for light aircraft have a camber of 2–4%, thickness ratio of 12–15% and the maximum thickness (not camber) occurring at around 30% of chord.
    Now type the following data into the FoilSim boxes using the 'enter' key or use the sliders:
    Size: chord 1 m, span 8 m  (area 8 m²)
    Shape: angle (of attack) 2°, camber 2%, thickness 12%
    Flight test: speed 166 km/h (90 knots), altitude 1947 m (6400 feet)
    Check the results displayed in the black boxes and in the plots. The static air pressure should be 80.0 kPa (800 hPa) and the lift is 4233 N.  If you select  'surface pressure'  from the output plots, you will see a plot of the pressure distribution across the chord for the upper (white line) and lower (yellow line) surfaces. Anything appearing above the green line (the atmospheric static pressure) can be regarded as a positive pressure pushing that surface at that point. Anything below the green line is a negative pressure pulling that surface at that point. The area between the two curves represents the magnitude of the differential pressure distribution.  The horizontal axis indicates the percentage distance from the mid-chord position.
    The pressure gradient plot for the upper surface  shows a maximum decrease of around 1.5 kPa (15 hPa) close to the leading edge but changing to a slight positive increase in  pressure at the trailing edge. The pressure gradient plot for the lower surface  shows an increase in  pressure under the leading edge, quickly changing to a decreased pressure of a few hPa then back to a positive pressure from mid-chord back. If you press the 'Save Geom' button, a data table will be displayed showing  the pressure and local velocity readings at 19 X-Y coordinate positions on both the upper and lower surfaces.
    If you now select  'surface velocity'  for the output plot, you will see a plot of the local velocity distribution across the chord for the upper (white line) and lower (yellow line) surfaces. You can see that the local velocity increases to about 40% above the free stream velocity a very short distance downstream from the leading edge, then it gradually slows until local velocity is less than free stream velocity at the trailing edge.
    Now change  the airspeed to 110 km/h (60 knots) and the aoa to 12°, and look at the surface pressure and surface velocity plots again. Note the big increase in local velocity that is now some 2.5 times the free stream velocity a very short distance downstream from the leading edge. Also note the big increase in the pressure differential and that most (about 70%) is occurring within the first 25% of the chord.  
    You should do a little exploration starting with the aerofoil design,  changing just one value at a time and noting the changes in the upper and lower pressure gradients.  For instance change the camber from 2 to 4% (i.e. the NACA 4412 aerofoil) and see the lift generated increase to 6369 N with a  CL now 0.74.  You can do the same with the flight performance items under pilot control —  aoa, altitude and airspeed. Of course, FoilSim doesn't provide any information concerning drag generation or pitching moment.
    4.3 Boundary layer airflow
    In the following section I  use the concept of the airstream flowing over a stationary wing (as in a wind tunnel experiment) rather than the reality of the aircraft moving through stationary air, for easier explanation. 
    The  innermost  molecules of the moving air come into contact with the solid surface of  the  wing (and other parts of the aircraft) and  are entrapped by the surface structure of the airframe materials. This is called the 'no-slip condition' and is common to all fluid flows. The interaction between those air  molecules  and the molecules of the solid   surface transfers energy and momentum from the air molecules to the solid surface molecules — producing skin friction drag and shear stress that act tangentially to the surface. Those surface-interacting air molecules retreating from the surface consequently carry less momentum than they did on approach. In the very thin viscous sublayer adjacent to the solid surface, these molecules with reduced momentum move randomly into the fluid a small distance  from the surface. The streamwise momentum per unit volume of the molecules that have interacted with the surface is less than the momentum a small distance from the surface. The random mixing of the two groups of molecules reduces the streamwise momentum of the molecules that have not directly interacted with the surface. This exchange of momentum between slower and faster molecules is the physical origin of air viscosity (the resistance to flow when a fluid is subject to shear stress) and of that viscous sublayer or boundary layer comprising  the region between the wing surface and the unrestrained or inviscid outer stream. The diagram shows the velocity gradient within the boundary layer; the more turbulent the flow, the steeper the gradient and the greater the shear stress and friction.
    The atmospheric boundary layer is similar but, of course, on a grander scale.
    Laminar and turbulent flow
    The thickness of the boundary layer starts at zero at the wing leading edge stagnation point, but will increase (as an increasing number of molecules lose momentum) until a maximum thickness  is reached near the trailing edge.   The friction between air layers moving at different velocities within the boundary layer is generally weak, so the flow from the stagnation point is initially made up of smooth-flowing stream lines or laminae — laminar boundary layer flow. But on both the  wing upper and lower surfaces not far downstream from the leading edge, the laminar flow, less than 1 mm in thickness, usually transitions to a flow with small irregular fluctuations — turbulent boundary layer flow — and continues to increase in thickness by around 1% of the distance travelled  to a maximum near the trailing edge of  perhaps 10–15 mm for a 1200 mm wing chord. Drag increases as the boundary layer thickens.
    The extent of laminar flow and thus the location of the transition zone   — where boundary flow is a mix of laminar and turbulent —   depends on the designed aerofoil shape in profile, the angle of attack, contour variations (ripples, waviness) formed during construction and service,  the flexibility of the wing's skin, surface roughness/cleanliness, porosity,  and  the pressure gradient  along the wing chord. In the area where the pressure gradient is favourable  (i.e. decreasing, thus the flow is accelerating), laminar flow will tend to continue, though becoming thicker, unless something  trips it into the more irregular  turbulent boundary layer flow — even paint stripes can trip laminar flow.
    The laminae nearest the skin move slowly and cohesively, thus minimising skin friction drag.  In the turbulent flow boundary layer, the  air nearer the wing is moving faster and somewhat chaotically, thus greatly increasing skin friction drag. The transition zone tends to occur a particular distance downstream (for a  combination of the preceding factors) rather than a percentage of chord even though the aerofoil might be designed for laminar flow for a particular percentage of chord.    
    The aerofoils used for light aircraft wings have very little laminar flow. But  specialised high-speed aerofoils are designed to promote  laminar flow  over perhaps the first 30–40%  of the wing chord by providing a favourable pressure gradient for at least that distance (i.e. maximum thickness at 40–50% of chord) and a properly contoured, very smooth, clean, non-flexing, seamless skin. The latter conditions are also important for minimising  the thickness of the turbulent boundary layer flow with consequent reduction in skin friction drag and are  achievable in composite construction.
    Flow separation
    Generally at lower angles of attack, the  boundary layer and the outer stream will  separate (break away  or detach) from the wing upper surface at the trailing edge or perhaps slightly upstream from the trailing edge, causing  a thin trailing wake to form between the outer streams. As aoa increases past perhaps 12°, the  boundary layer  separation  on the wing upper surface might tend to move upstream a little.  But at the stalling aoa,  separation will suddenly move much further upstream, and  a thick  turbulent wake  will form between the two remnant boundary or shear layers and will be dragged along by the aircraft. The reaction to the wing accelerating and energising that previously stationary air  is a sudden deceleration of the aircraft, accompanied by a sudden increase in the magnitude of the nose-down pitching moment.  Downwash disappears and the rate of loss of lift will increase rapidly as the aircraft slows.
    Aerodynamicists  devote much effort to controlling and energising the boundary layer flow to delay separation and thus allow flight at lower speeds; for example, see vortex generators. More lift and much less pressure drag is generated in attached  turbulent boundary layer flow than in  partially separated flow.
    4.4 Aspect ratio
    Aspect ratio is the wing span divided by the mean wing chord.  An aircraft with a rectangular  wing of area  12 m² might have a wing span of 8 m and constant wing chord of 1.5 m. In this case the  aspect ratio is 5.33. If the span was 12 m and the chord 1 m, then the aspect ratio would be 12.  However because wings  have varied plan forms, it is usual to express aspect ratio as:
    Aspect ratio =  wing span² / wing area
    It is conventional to use the symbol 'b' to represent span, so the equation above is written as:
     (Equation #4.3)      A =  b² / S
    The Jabiru's aspect ratio (span 7.9 m,  area 8.0 m²) = 7.9 × 7.9 / 8 = 7.8, whereas an aircraft like the Thruster would have an aspect ratio around 6.  Consequently you would expect such an aircraft to induce much more drag at high angles of attack, and thus slow much more rapidly than the Jabiru.
    And incidently, the mean chord (not the mean aerodynamic chord) of a wing is span/aspect ratio. A high-performance sailplane wing designed for minimum induced drag over the CL range  might have a wingspan of 22 m and an aspect ratio of 30, thus a mean chord of 0.7 m.   There are a few ultralight aeroplanes, designed to have reasonable soaring capability, that have aspect ratios around 16–18, but most ultralights would have an aspect ratio between  5.5 and 8, and averaging 6.5. General aviation aircraft have an aspect ratio between 7 and 9,  probably averaging around 7.5. Note that the higher the aspect ratio in powered aircraft, the more likely is wingtip damage on landing.
    Note that 'wing area' includes the nominal extension of the wing shape into and through the fuselage. This would appear quite apt for a parasol wing or a high-wing aircraft, but will no doubt seem odd for a mid or low wing. It is just a means for consistent application/comparison between aircraft designs.
    The span loading is the aircraft weight divided by the wingspan = W/b. The term  sometimes refers to the loads applying at specified stations along the span.
    4.5 Spanwise pressure gradient
    There is a positive spanwise pressure gradient (the rate of pressure change with distance) on the upper wing surface from the wing tip to the wing root,  imparting an inward acceleration to the  airflow close to and above the wing.  Conversely, at other than a very small aoa, there is a positive underwing pressure gradient from the wing root to the wingtip, and airflow under the wing acquires an outward acceleration. These spanwise (or more correctly semi-spanwise) pressure gradients on the upper and lower surfaces are caused by the higher pressure air from the undersurface revolving around the wingtip into the lower pressure upper surface. This tip effect results in a near total loss of lift at the wingtip because of the reduced pressure differential, with the loss of pressure differential  progressively decreasing with distance inboard.
    Where these two surface airflows with different spanwise velocities recombine past the  trailing edge, they initiate a sheet of trailing vortices. These are  weakest near the fuselage and strongest at the wingtips, and roll up into two large vortices, centred just inboard and aft of each wingtip.  The vortices  increase in magnitude as aoa and lift increase, and so increase the vertical component of, and the momentum imparted to, the downwash. As the centre of each vortex is a little inboard of the wingtip, the vortices also have the effect of reducing the effective wing span, the effective wing area and probably the effective aspect ratio.
    The vortices also affect the air ahead of the aircraft by reducing the magnitude of the upflow in front of the wing and thus  modifying (decreasing) the effective wing aoa, with the greatest effect near the wing tip and little effect near the wing root.  When a wing is at a low CL aoa the airstream affected by the wing has a slight downward flow. When it is at maximum CL aoa, that airstream has a more substantial downward flow contributed by the vortices.  
    Because of the reduction in the effective aoa, the wing must fly at a greater aoa to achieve the same lift coefficient  that a two-dimensional aerofoil will achieve in the laboratory. Also, the wing tip vortices have a decreasing effect with increasing aspect ratio. This is demonstrated in the diagram where there are three (exaggerated) CL and aoa curves plotted. On the left is the laboratory curve for an aerofoil, in the middle the curve for a high aspect ratio wing utilising the same aerofoil and the  curve on the right is for a low aspect ratio version. The red horizontal line connects with a particular CL value, say 1.2. The vertical red lines indicate a different aoa for each curve at the same CL, thus the high aspect ratio wing must fly at a higher aoa and the low aspect ratio wing must fly at a still higher aoa for either to achieve CL 1.2. Or to put it another way, at any aoa the wings produce less lift than the laboratory aerofoil.
    Also apparent from the diagram is that a higher aspect ratio  has the effect of a higher rate of lift increase, as aoa increases, than lower aspect ratio wings.  A high aspect ratio wing will have a higher CLmax but a lower stalling aoa than a low aspect ratio wing utilising the same aerofoil. Induced drag has a direct relationship to aspect ratio; see section 4.6.
    Wing-tip vortices  make up most of the wake turbulence created by an aircraft in flight and are certainly the most hazardous to following aircraft. They are usually referred to as wake vortices in the context of air traffic and are the same as other atmospheric vortices in that there is a central low pressure core that is often visible as condensation trails when an aircraft pulls higher g in a humid atmosphere.  Read the New Zealand Civil Aviation Authorities booklet 'Wake Turbulence'.
    4.6 Induced drag
    As explained in  section 4.5 the effect of the vortices is to reduce the effective aoa of the wing compared to that of the laboratory aerofoil, which has the further effect of giving a more rearward inclination to the resultant aerodynamic force for the wing, compared to the aerofoil, at a particular geometric aoa. When that aerodynamic force is resolved into lift and drag components, the additional inclination will produce a reduced lift vector (apparent in the preceding CL/aoa diagram) and an increased drag vector. That increase in the drag vector is the induced drag.
    Induced drag is least at minimum aoa and greatest at maximum aoa. It is often said  that the induced drag is the energy dissipated to induce lift; i.e. if CL is increased, induced drag  increases, so thrust must be increased to provide  additional energy  —  if the aircraft's flight path is to continue as before. For example, if the pilot wants to  increase aoa and maintain the same  airspeed (as in a constant rate level turn), then thrust must be increased to counter the increase in induced drag.
    There is a point in an aircraft's flight envelope where, because of the increasing  induced drag,  the  slower you want to fly the greater the power you must apply  —   known as 'flying the back of the power curve'  — which is opposite to the norm of applying power to fly faster.
    Elliptical lift force distribution
    As stated in section 4.5, with most wings —  particularly rectangular wings — the higher pressure air underneath the wing flows around  the wing tip into the lower pressure area above, thus reducing the pressure differential and the lift; the effect of this  decreases as span and/or aspect ratio increase.  
    Induced drag is minimised if the spanwise distribution of the lift forces can be made to present an elliptically shaped pattern, as shown in the diagram, and that aerodynamic load is equally distributed over the wing so that all areas of the wing contribute to load sharing. (This idealised lift force distribution diagram presents a head-on view of the whole wing without any representation of — or distortion by —  the fuselage.) .    
    Elliptical spanwise lift distribution will provide a desirable  uniform downwash along the span, and can be achieved  by choice of wing plan form and/or by twisting the wing to provide something near an elliptical distribution in a  speed band selected by the designer.
    High aspect ratio elliptically shaped (in plan form)  wings generally achieve spanwise elliptical lift distribution; however, because of the compound skin curvatures they are the most difficult and  time-consuming to construct.  Low aspect ratio constant chord (i.e. rectangular) wings without twist are the easiest to construct but generate the most induced drag; however, the introduction of twist makes such a wing much more efficient. Medium aspect ratio wings  with a medium taper ratio plus twist are probably the most used shape.
    Taper ratio is the ratio of the tip chord to the wing root chord.  'Medium taper' would indicate that the tip chord is greater than 50% of the root chord.
    Sailplane designers have demonstrated that the most effective high aspect ratio wing is one that has a straight (i.e. non-tapered) trailing edge with a leading edge that is increasingly tapered in sections from root to tip.
    Wing twist or washout
    The terms 'wing twist' and 'washout' refer to wings designed so that the outboard sections have a lower incidence, 3–4° or so,  and thus lower aoa than the inboard sections in all flight conditions. The main reason for wing twist is to reduce induced drag (see section 'Elliptical lift force distribution') and particularly so at a cruising angle of attack or perhaps the climb speed angle of attack. Another  reason is to improve the stall characteristics of the wing so that flow separation begins near the wing roots and moves out towards the wingtips.
    With twist, the  sections near the wing root reach the stalling aoa first, thus  allowing effective aileron control even as the stall progresses from inboard to outboard. This is usually achieved by building geometric twist into the structure by rotating the trailing edge,  so providing a gradual decrease in aoa from root to tip. Washout  reduces the total lift capability a little but this disadvantage is more than offset by the wing twist improving elliptical lift distribution and thus decreasing induced drag.
    Another form of washout — aerodynamic twist — might be  attained by using an  aerofoil  with a higher stalling aoa in the outboard wing sections.
    Aircraft incorporating washout tend to not drop a wing during an unaccelerated  stall.  Instead, there is a tendency to just 'mush' down sedately then drop the nose and regain flying speed. The turbulent wake from airflow separation  starting at the wing root buffets the tailplane, thus providing some warning of the oncoming stall before it is fully developed.  Also, washout is usually applied, for aerodynamic balance,  to the swept wings utilised in weight-shift ultralights. However, geometric  washout can cause problems at excessive speed.
    Effect of wing span/aspect ratio on induced drag
    The equation for calculating induced drag for a wing  is: 
    Induced drag = (k × CL² / A)  × Q × S   where A is the wing aspect ratio [b²/S] and k is related to a span effectiveness ratio.
    So, induced drag is directly proportional to CL² and inversely proportional  to dynamic pressure [Q], and might comprise 50% of total drag at maximum angle of climb  speeds.  The lower the span loading [W/b](i.e. the greater the physical span or the 'effective' span), the lesser the induced drag at all angles of attack. This results in a  decrease in the thrust needed, particularly for climb — or an increase in the potential energy of height  for a sailplane. Various wingtip designs,  such as  Hoerner wingtips, have the effect of moving the  vortices slightly  further outboard, thereby increasing the effective span and thus  reducing the span loading and induced drag.
    The information in the following box may only be of interest to aircraft homebuilders, so skip it if you wish and go  to the next part .
    Aspect ratio equals b²/S (equation #4.2), so the equation above can be rewritten as:

    (Equation #4.4)     Induced drag = (k × CL² × S / b²)  × Q × S 

    The factor k  equals 1/Pe where  P [pi] equals 3.14 and e is the span effectiveness factor that might vary between 0.7 and 0.9 for the aircraft as a whole. For an elliptic plan form wing, something like  that of the near-elliptical wing of the Seafire 46 at left, with (theoretically) no fuselage interference, then e=1.0 and k =1/3.14 × 1.0 = 0.32. A non-twisted tapered wing will have a span effectiveness factor of perhaps 0.9, so induced drag will be 10% greater and greater still (+20%?) for a non-twisted rectangular wing. However, fuselage and fuselage junction interference will reduce the span effectiveness of the wing.

    Equation #4.2 states that CL = W /  (Q × S). Substituting that for CL² in Equation #4.4:

    Induced drag = k × [W²/ (Q² × S²)] × (S / b²) × Q × S

    Some of the terms cancel out, leaving:

    (Equation #4.5) Induced drag = k × W² /  (b² × Q)  

    Equation #4.5 shows that induced drag is  proportional to span loading squared [W²/b²] and inversely proportional to dynamic pressure [Q], so that two aircraft with quite different aspect ratios but having an identical span effectiveness factor, wing span and weight would produce the same induced drag at the same dynamic pressure (e.g. same density and TAS or lower density and higher TAS, etc). Anything done that gives a small increase in effective wing span will provide a proportionately higher reduction in induced drag.  
    Jabiru induced drag calculation
    If we  guess that  the Jabiru aircraft span effectiveness factor is about 0.8,  we  have enough information to do a rough calculation of the induced drag on our Jabiru cruising at 97 knots at 6500 feet (as in the pressure differential calculation above). We will use a more practical form of induced drag  equation for those who skipped  the preceding box:
    Induced drag = k × CL² / A  × ½rV² × S  
    For the Jabiru, k = 1/(3.14 × 0.8)= 0.4,  aspect ratio [A] is 7.8 and the  CL at that speed is 0.4.
    = 0.4 × (0.4 × 0.4  / 7.8)  × (0.5 × 1.0 × 50 × 50) × 8.0
    = 0.4 × 0.02 × 1250 × 8 = 80 newtons
    If you repeat the  CL calculation in section 1.4 using the  Jabiru's stall speed at 6500 feet, say a TAS of 25 m/s, you will find that CLmax is 1.6. Now if you repeat the induced drag calculations,  you will find it has increased fourfold:
    Induced drag =  0.4 × (1.6 × 1.6  / 7.8)  × (0.5 × 1.0 × 25 × 25) × 8.0  
    = 0.4 × 0.33 × 312.5 × 8 = 330 newtons  
    4.7 Parasite drag
    Parasite drag is all the air resistance to a light aircraft in flight  that is not considered as 'induced',  and consists solely of pressure drag and skin friction drag; the latter is due to viscous flow  and has been covered in the boundary layer air flow section above. The parasite drag  constitutes much of the total aircraft drag at minimum aoa (i.e. high speed) but comparatively little at maximum aoa (minimum speed).  Refer to the diagram in section 1.6.  When associated with airflow around an aerofoil, the parasite drag is termed profile drag.    
    Pressure drag or form drag is the net pressure differential of those  points on the wing; for example, where a component of the pressure acts in the fore and aft direction, and that pressure differential tends to retard the aircraft. Pressure drag, like skin friction, applies to all parts of the aircraft 'wetted' by the airflow. It is greatest for any part of the airframe that presents a flat surface perpendicular to the flow and least for a streamlined shape that has a fineness ratio (i.e. length to breadth) between 3:1 and 4:1.

    The illustration — a cross-section of a 3:1 fineness ratio wing strut — shows the flow streamlines detaching from the surface close to the trailing edge, with the characteristic wake associated with pressure drag. What is not apparent from the illustration is that, in this instance, the skin friction drag would be significantly greater than the pressure drag
      There are two specially named classes of parasite drag: interference and cooling drag. Interference  drag occurs at the junctions of airframe structures; for example, the junction of the wings and fuselage or the junction of the undercarriage legs and fuselage. The boundary and outer streamflows interfere with each other at the intersections and cause considerable turbulent drag. Interference drag for a well-designed composite aircraft might be 5–10% of total parasite drag but can be very much higher. The cross-flow associated with unbalanced flight (slip/skid) exacerbates interference drag.
    If interference drag potential is ignored by the designer, vortex development can occur at the wing/fuselage junctions,  effectively splitting the spanwise lift distribution into two separate elliptical patterns; this is particularly so with low-wing configurations but not so much with high wings. The problem is minimised, and total parasite drag considerably decreased, by careful design to reduce the number of junctions, and  to use fillets and fairing to direct a smooth airflow around the remainder. Usually the most visible evidence of an interference drag reduction program is the large wing root fillet used in low wing aircraft as seen in the AR-5 photograph.
    Engine cooling  drag is normally associated with the cooling airflow for engines enclosed in a drag reducing cowling. The cooling airflow is designed to be efficiently directed from an air intake through a system of baffles for optimum engine cooling, and  perhaps to utilise the energy of the added heat to provide a little thrust at the cowling exit point. Where the engine is not cowled, there is a great deal of parasite drag  that certainly cools the engine but would not be specially classed as  cooling drag.
    4.8 Aircraft lift/drag ratio
    In unaccelerated straight and  level flight, lift equals weight, and thus will be a constant value. If you look at the total  drag  diagram in section 1.6  you will see that the drag varies  with the airspeed which means, of course, that it varies with angle of attack. The diagram on the left is a  plot of the fixed lift value divided by the  total drag value; i.e. the L/D ratio, at varying aoa for a reasonably efficient aircraft. It can be seen that L/D [L over D] improves rapidly between zero or negative aoa up to 4–5° then drops off until the stall angle, where the deterioration rate accelerates. Note that a non-aerobatic light aircraft in normal flight would not experience these low L/D values at aoa between  0° and 2°.
    The maximum L/D   for light aeroplanes  —  a measure of the aerodynamic efficiency of the aircraft  — is  possibly between 8  and 12.  Some of the ultralights designed with wide span, high aspect ratio wings to provide some  soaring capability have a maximum L/D around 30.    High-performance sailplanes that are built with very wide span, slender, high aspect ratio wings have the greatest L/D, at  40 –50, and thus the greatest efficiency. Powered parachutes have a L/D ratio around 3.
    There is a limit to the thrust that the engine/propeller can provide (i.e. the drag that it can match) thus there is also a minimum L/D at which maximum engine power is required to maintain constant altitude. Consequently, there will be a minimum aoa (maximum airspeed) and a maximum aoa (minimum airspeed) at which an aircraft can maintain level flight. As there may not be much range between minimum and maximum L/D, the minimum L/D can be quite significant for ultralight aircraft, where a range of engines, some with rather low power, may be utilised in the same model. An under-powered aircraft will perform very badly at the back of the power curve.
    Glide ratio
    Maximum L/D usually occurs at an angle of attack between 4° and 5°,  or where the  CL is around 0.6. This L/D  ratio is also termed the glide ratio because it is just about the same ratio as distance covered/height lost in an engine-off glide. For example, if maximum L/D =12 then the glide ratio is 12:1, meaning the aircraft will  glide a distance of 12 000 feet for each 1000 feet of height lost, in still air.
    We can use the '1-in-60' rule to calculate the angle of the glide path relative to the ground; for example:
    L/D = 12, then 60/12 = 5° glide path angle. 
    If the aircraft is maintained in a glide at a degraded L/D, then the glide path will be steeper: L/D = 8, then 60/8 = 7.5° glide path angle. This is one effect of using flaps (see section 4.11).
    Be aware that quoted L/D ratios  rarely take into account the considerable drag generated by a windmilling propeller.
    The aoa associated with maximum L/D decides the best engine-off glide speed  [Vbg] for distance and the best speed for range [Vbr] according to the operating weight of the aircraft.  But because of the flat shape of the curve around maximum L/D, these speeds are more akin to a small range of speeds rather than one particular speed.
    4.9 Pitching moment
    When using the FoilSim  aerofoil flight test simulation program, the  static pressures around the aerofoil are given in the output plot that shows the pressure distribution pattern changing with the aoa.  It is  convenient to sum that distribution and represent it as one lift force vector acting from the centre of pressure [cp] of the aerofoil or wing for each aoa; much the same way as we sum the distribution of aircraft mass and represent it as one force acting through the centre of gravity.  The plot on the left is a representation of the changing wing centre of pressure position with aoa. The cp position is  measured as the distance from the leading edge expressed as a percentage of the chord. (Please note the diagram is not a representation of the pitching moment.) 
    At  small aoa (high cruise speed) the cp is located around 50% chord. As aoa increases (speed decreases) cp moves forward reaching its furthest forward position around 30% chord  at 10–12° aoa, which is usually around  the aoa for Vx, the best angle of climb speed.    With further aoa increases, the cp now moves rearward; the rate of movement accelerates as the stalling aoa, about 16°, is passed. Most normal flight operations are conducted at angles between 3° and 12°, thus the cp is normally positioned between 30% and 40% of chord.
    The movement of the cp of the lift force  changes the pitching moment of the wing, a rotational force applied about some reference point — the leading or trailing edges  for example —  which, in isolation, would result in a rotation about the aircraft's lateral axis. The consequence of the rotation is a further change in aoa and cp movement that, depending on the cp starting position may increase or decrease the rotation. Thus a wing by itself is inherently unstable and will change  the aircraft's attitude in pitch — i.e. the aircraft's nose will  rotate up or down about its lateral axis, which may be reinforced or countered by the action of the lift/weight couple — so there must be a reacting moment/balancing force built into the system  provided by the horizontal stabiliser and its adjustable control surfaces.  This will be discussed further in the Stability and Control modules.
    Aerodynamic centre
    There is a point on the wing's mean aerodynamic chord (see below) called the aerodynamic centre [ac] where the pitching moment  coefficient   [ Cmac ] about that point is small — for the NACA 2412 aerofoil Cmac is –0.1. The negative value indicates the moment produces a nose-down torque, which is the norm for cambered wings. Cmac remains more or less constant with aoa changes but becomes more nose-down at the stall.  For the cambered aerofoils used in most light aircraft wings, that aerodynamic centre will be located  in a position between 23% and 27% of the chord length aft of  the leading edge, but for standardisation, aerodynamicists generally establish the lift, drag and pitching moment coefficients at the 25% (quarter) chord position. The notation for the pitching moment at  quarter chord is  Mc/4.
    The  pitching moment is consistently nose-down,  changing in magnitude as airspeed changes. When plotted on an aerofoil wind tunnel data graph, the moment coefficient Cmc/4 is a roughly horizontal line for most of the angle of attack range, but the straight line may have a slight slope if the actual aerodynamic centre varies a little from the 25% chord location.  
    Pitching moment equation:
    (Equation #4.6)   Pitching moment [ Mc/4 ] = Cmc/4 × ½rV² × S × c  
    The  pitching moment equation is much the same as the lift and drag equations with the addition of the mean aerodynamic chord [c] for the moment arm; using SI units the result is in N·m.  As the coefficient is always negative and nearly constant (up to the stall), then V² is the significant contributor to the nose-down pitching torque, which must be offset  by tailplane forces to keep the aircraft in balanced flight. However, high torsion loads may still exist within the wing structure; see  aerodynamic effects of flight at excessive speed.
    The concept of the aerodynamic centre is useful to designer/builders, because it means the centre of application of lift can be assumed fixed at 25% chord and only the lift force changes. For non-rectangular wings, a mean aerodynamic chord [MAC] for the wing  has to be calculated; see ascertaining mean aerodynamic chord graphically — in that diagram the aerodynamic centre position [ac] is  shown on the root chord line.
    Neutral point
    It is not just the wings that produce lift, the tailplane surfaces  also  produce lift (which is discussed in module 6), and so do parts of a well-designed fuselage. Consequently the aerodynamic centre for the aircraft as a whole, known as the neutral point, will not be in the same location as the wing aerodynamic centre but —  for a tailplane aircraft — behind it and on the fuselage centreline. This is the fixed point from which net lift, drag and  aircraft pitching moment are assumed to act.
    4.10  Ailerons
    We mentioned in section 1.4 that the pilot cannot change the shape of the wing aerofoil. But this, like many statements made regarding aeronautics, needs qualification.  In fact, the pilot manoeuvres the aircraft in the lateral plane by altering the effective camber of the outboard sections of the wings.  And remember in  the last paragraphs of section 4.1 above, using FoilSim, we found that altering camber from 2% to 4% produced a substantial increase in  CL and lift.
    If you examine the Seafire photograph,  in section 4.6,  you will see that each wing has a separated section at  the outboard trailing edge. These are ailerons, hinged to the main wing so that they can move down or up and linked, via control rods or cables, to left/right movement of the pilot's control column. The control column is a simple lever which amplifies  forces applied by the pilot. Thus the pilot can, in effect,  increase or decrease the camber of the outer portion  of each wing; as shown by the effective chord lines in figures A and B at left.  The ailerons are  interconnected  so that downward movement —  a camber increase  —   in one is combined with an upward movement  —  a camber 'reflex' —   in the other. The aileron movement then increases the lift generated by the outer section of one wing whilst decreasing that from the other, thus the changed lift forces (at a distance from the aircraft's longitudinal axis) impart a rolling moment in the lateral plane about that axis. This rolling moment is primarily used to initiate a turn but other manoeuvres depend on the amount and timing of aileron movement; more about this in the 'Control' module; see 'Control in a turn'.
    Ailerons span perhaps  the outer 35% of each wing and occupy perhaps the aft  20% of the wing chord at that location. High-speed aircraft may have two sets: a normal outer wing set used only for low-speed flight (because of the moment of force they are capable of applying at high speed) and a second, high-speed set of spoiler-type ailerons located at the inboard end of the wing.
    Aileron drag
    Increasing camber and thus CL also increases induced drag (in proportion to CL²) so that the wing that is producing greater lift will also be producing greater induced drag,  tending to rotate (yaw) the aircraft's nose in the direction of the lowered aileron. Parasite drag will be increased on the wing with the lowered aileron. This induced plus parasite drag reaction is called aileron drag and particularly complicates aileron effects at low speeds  when CL is high, the aerodynamic pressure on control surfaces is low, and it is easy to impart an excessive control movement.  Because the yaw is towards the lowered aileron and thus opposite to the required direction of turn, the effect is called adverse yaw and is particularly evident in aircraft that have long-span wings where the ailerons have a much longer moment arm.  
    Aileron drag can have an opposite yaw effect. When an aircraft is turning at low speed and the pilot applies aileron to roll upright, the downwards movement of the aileron on the lower wing  might take the aoa, on that part of the wing, past the critical aoa. Thus that section of wing — rather than increasing lift and making the wing rise  — will stall and  lose lift.  The aircraft, instead of straightening up, will roll into a steeper bank.  Although the wing section may be stalled,  CL and thus induced drag will still be fairly high, so there will be a substantial  yaw toward the lower wing which pulls the nose down and  increases the rate of descent. There is potential for other aileron-induced problems when turning at low speeds; see 'Control in a turn'.
    There are a number of  configurations which, used singly or jointly, reduce aileron drag. For example, differential ailerons, where the down-going aileron moves through a smaller angle  than the up-going aileron or Frise ailerons, where the leading edge of the up-going aileron protrudes below the wing undersurface, increasing parasite drag on the down-going wing.
    4.11 Flaps
    The other camber increasing devices, forming part of the inboard wing trailing edge in the Seafire photo, are the flaps. Plain flaps are also a hinged section of the wing — as in figures C and D in the aileron diagram above  —  but  move only (and  jointly) downward  usually to fixed predetermined positions,  each position providing varying degrees of increased lift coefficient and increased drag coefficient that the designer thought appropriate. For instance, for one particular aircraft, at 5° deflection there is a good increase in CL with only slight increase in drag. At 15° the drag increase starts to equate with the increase in the CL, whereas at 25° or 30° the increase in drag  is much greater than the increase in CL; at 45° the flap is starting to act as an airbrake.
    The change in camber (over perhaps 50–60% of the wing span and  20–25% of the wing chord) caused  by lowering flaps in flight, without changing other control positions, has  effects which will vary according to the amount of deflection employed:
    The aircraft's nose will pitch down a few degrees  about its  lateral axis (i.e. its attitude in pitch is altered) because of  the nose-down pitching moment associated with flaps. The position of the aircraft's line of drag  will  change and this also tends to change the aircraft's attitude in pitch. Depending on the relative mounting of the aircraft's wings and tailplane, the change of direction (and the increase) of  downwash may affect the trim of the aircraft  —  nose up or down. The lift increases and the aircraft will initially tend to rise. The drag increases and the aircraft slows below its trimmed airspeed,  lift reduces, and the aircraft sinks unless power is increased. The pilot  has to take appropriate control action depending on the reason for lowering flaps.  
    The effects of trim  associated with lowering  or raising  flaps for a particular aircraft type will be noted in the Pilot's Operating Handbook.
    As we saw in FoilSim, the effect of increasing camber is an increase in CL (the ratio of lift to dynamic pressure or airspeed) at all aoa. This is shown in the plot at the left. At an aoa of 6° CL is about 1.0 with flaps lowered — about 50% greater than the CL of 0.65 with flaps raised. What this means is that the  minimum controllable flight speed is lower with flaps deployed.
    So, returning to the equation:
    lift = CL × ½rV² × S 
    thus  for lift to remain constant if CL  increases then V² must decrease.   Consequently, the stall speed is also lower with flaps deployed.
    (Incidently, this diagram shows that the zero lift aoa for this  wing occurs at –2°.)
    Note that the flapped section will  stall  at a lower aoa than the unflapped section. Generally the flapped wing area, being the inboard section of the wing, represents a very large proportion of the total wing area — check  the Seafire photo. Also, even if the flapped section has passed its stalling angle, it is still producing lots of lift. Providing there is sufficient thrust available to overcome the big increase in drag, the aircraft can still maintain height and stability because the wing outboard section and ailerons are not stalled.
    Bear in mind that to maintain the same airspeed and altitude after lowering flaps, that thrust, if available,  must be increased to counter the additional drag from the lowered flaps. Similarly, when  flaps are  raised, the aircraft will initially sink due to the loss of lift unless the pilot takes compensating control action; this is particularly important when a landing approach is discontinued and a go-around initiated.
    Now what aoa are we measuring? If you look at figure C (in the  drawing in section 4.10) which represents the unflapped part of the wing, you can see that it has an aoa of about 5° or so whereas, at the same time, the flap extended section of wing (figure D) has a considerably greater aoa. As the flapped section  will still have a stalling aoa around 16°  we can surmise that this flapped wing section is going to stall when the unflapped section is only at 13° or so. (The horizontal axis of the plot shows only the aoa of the unflapped wing.) However, we also have to take into account the increased downwash and thus the change in effective aoa associated with it, so the effect of flaps is not as straight-forward as implied in the preceding.
    Flap systems
    There are a many types  of  flap systems, but if flaps are used at all in ultralights or other very light aircraft,  then  only the simpler  devices shown at left are needed.
    The most common (because of its simplicity) is the plain flap, which might provide a 0.5 increase in CLmax with a large increase in drag when fully deflected. The split flap  provides slightly more increase in lift but a larger increase in drag, and is more difficult to construct and thus probably not worth the effort.
    The slot  incorporated into the junction between the main wing and the plain flap in the slotted flap arrangement allows airflow from under the wing to energise (i.e. accelerate and smooth) the turbulent boundary layer flow over the upper surface of the lowered flap. This provides better downstream  boundary layer adherence, and thus allows a larger angle of attack to be achieved before stall, with higher CL and lower drag than the plain flap. Ailerons may also be 'slotted' for improved performance.
    The rearward extension of the Fowler flap as it is deflected  increases wing area as well as camber, so it provides the best increase in lift of all the simpler systems — although perhaps even a single-element Fowler flap like that shown is not that simple to construct.
    Summary — flap effect on coefficient of lift
    In the diagram above, it can be seen that the deflection of flaps provides an increase in  CL of about 0.4 at all angles of attack. This is probably representative of plain flaps extending along 50% of the wing trailing edge with chord equivalent to about 20% of the wing chord, and deflected 25°. The attainable CL increase depends on flap span, chord and degrees deflected, plus the complexity of the flap system —  CL increase of 0.8 might be achieved with long-span Fowler flaps deflected to   35°. Incorporating slots into plain or Fowler flaps increases CL.
    Advantages of using  flaps
    If flaps are fitted, a small flap deflection —  say 10°  —  might be used for safer take-off, due to the lower lift-off speed available. But  half to full  flap deflection is always used for landing to provide:
    lower safe approach  and touch-down speeds a nose-down attitude for a better view of the landing area a steeper approach path (because of the degraded L/D) for better obstacle clearance,   which can be controlled at will a shorter 'float' after rounding out because of increased drag a shorter  ground roll, if flaps are left fully extended until the aircraft has exited the runway.  
    And flaps enable the approach to be made with engine power well above idle, which is beneficial to the engine, allows power changes to either increase or decrease the rate of sink  and provides better engine response in case of a go-around.
    In some light aircraft designs, particularly those with short take-off and landing [STOL]  capability, it has been found expedient to incorporate the aileron and a plain flap into one control surface that extends the full length of the wing trailing edge. The different functional movements are sorted out by a control mixer mechanism. Usually, the flaperon is not integral with the wing but  bracketed to the underwing to provide a slotted flap —  acting like an external aerofoil flying in close formation with the main wing. Although the CL increase attainable might be 1.0, there are drawbacks to this arrangement, which particularly exacerbate low speed aileron drag.  
    Reflex flaps
    Some aircraft  are fitted with flaps that also can be deflected upward 5° or 10° above the normal neutral or stowed position in addition to the normal downward deflection positions described above.  Upward deflection of flaps is done at cruising speed, and increases the maximum cruise speed  perhaps 5% by reflexing  camber and reducing drag, and is often associated with aerofoils that have good laminar flow.  
    4.12 High-lift devices
    Another short take-off and landing [STOL] device used in light aircraft is an aerofoil section —  a slat — fixed to the leading edge of the wing, with a slot between the  slat and the wing. The slat/slot works in much the same way as the slotted flap except that leading edge slats induce a nose-up pitching moment. At low aoa, the fixed slat has no value; it just increases drag and thus degrades cruise performance. At high aoa, the higher pressure on the underside of the slat is channelled through the slot, gaining velocity and energising the boundary layer flow over the upper surface of the wing — thus delaying boundary layer separation, adding perhaps a 0.6 CL increase and increasing  the stalling aoa to perhaps 20°.  The usual increase in CL  and the stalling aoa is illustrated with the green curves in the CL/aoa diagram above.
    Some slat/slot systems also have the effect of increasing wing area thus reducing W/S and stall speed.
    Leading edge slots combined with long-span slotted flaps, as used in STOL aircraft, allow a critical aoa much greater than the usual 16°.  They can perhaps double the maximum CL of the basic wing, which  allows much lower landing speeds but requires flight at the back of the power curve. Fixed leading edge slots work particularly well with a  tailwheel configuration in a 'utility' aircraft such as the Slepcev Storch, but in a touring aircraft they have no value unless the pilot intends operating into very small, rough airstrips. There are  simple automatic slat/slot systems where the slat is stowed when flying at lower angles of attack but pops out to form the slot when a particular angle of attack is reached. There are also retractable slat/slot systems that provide STOL capability when required without sacrificing cruise performance, except for the weight increase due to the more complex operating system.
    I suggest now you have a look at the diagrams in Anatomy of a STOL aircraft.
    4.13 Lift spoilers and airbrakes
    The converse of the high-lift devices is  the light aircraft spoiler, common in gliders but occasionally seen in high L/D ratio ultralights. The  usual spoiler is a flush-mounted front-hinged spring-loaded flat plate incorporated into the upper wing surface, which can be elevated by lever operation to varying degrees of opening. When activated,  it induces  separation over part of the wing, thereby acting as a lift-dumper. But it is not speed limiting; the nose will pitch down and the pilot must use elevator to maintain the required approach speed; thus the spoiler is used to increase the sink rate on the  approach path.
    Airbrakes or speedbrakes  have a similar but more effective function. They are often vertically mounted plates, pairs of which are incorporated into  the wing  structure and which protrude from the upper and lower wing surfaces when activated.  They create a lot of drag but little or no change in pitch, so the pilot must lower the nose to maintain approach speed. Airbrake or spoiler  configurations are sometimes associated with flap systems that are primarily directed to lift generation, rather than lift generation plus drag creation. Such flap systems would have maximum downward deflection of perhaps 20°.
    Military aircraft utilise very complex flaperon/spoileron systems.
    Things that are handy to know The aerofoil is often referred to as a 'two-dimensional' object. This means that  that the spanwise — thus 'third-dimensional' — pressure gradient effects associated with a normal wing, and varying significantly with the wing form rather than the aerofoil shape, are ignored when considering aerofoil characteristics.
      Wing upflow: all the air disturbances caused by the passage of an aircraft are propagated as pressure pulses moving outward (from molecule to molecule) in all directions at the speed of sound. Thus, in subsonic flight, the pressure variances (compression then relaxation) contribute to the air upflow occurring in front of the wings.
      In sport and recreational aviation the term aircraft is a generic covering all types of aerial (airborne) vehicles; it includes 'lighter-than-air' (aerostats) and 'heavier-than-air' (aerodynes) but not vehicles that derive their lift from air reaction with the surface, e.g. hovercraft. The aerostats include hot-air balloons and power-driven hot-air airships, both deriving lift from buoyancy. The aerodynes derive their lift from the aerodynamic reactions described above and are in two classes — rotary-wing (rotorcraft) and fixed-wing. Rotorcraft are represented by helicopters, gyroplanes and the towed gyrogliders or rotor-kites.

    The fixed-wing aerodynes may be power-driven or unpowered, the latter represented by the various glider classes — sailplanes, hang gliders, paragliders and the towed parasails or para-kites.

    The power-driven aerodynes are represented by three groups: the weight-shift controlled trikes, powered parachutes, powered hang gliders and  powered paragliders. the 3-axis controlled power-assisted sailplanes and motor-gliders and finally the ubiquitous 3-axis controlled aeroplanes.
      For more information see sport and recreational aircraft categories.
    Notes for homebuilders
    • The parasite drag coefficient. The equation for calculation of the total parasite drag for an aircraft is:

    Parasite drag [newtons] = CDp × ½rV² × S

    Unlike the lift coefficient, the parasite drag coefficient CDp is more or less a constant — the ratio of drag to dynamic pressure — and thus provides a means for comparing the relative aerodynamic 'cleanness' of two aircraft. The coefficient is usually in the range 0.03 to 0.08 for fixed-undercarriage aircraft.

    • There is another value,  the 'equivalent flat plate area' [FPA] used by aircraft, motor vehicle and structural engineers who are concerned with the calculation of air resistance. FPA is often quoted in aviation magazines when comparing the parasite drag efficiency of an aircraft with other similar aircraft, and it is usually stated in terms of square feet.

    FPA is calculated as CDp times the wing area divided by the CDp for a flat plate. However, it is  assumed that the CDp for a flat plate held at 90° to the airstream = 1  (in fact it is about 20% greater, but that is of no real consequence) so the flat plate CDp is omitted from the calculation, thus:

    FPA = CDp × S ft²

    For example, the FPA for the run-of-the-mill two or four-seater fixed-undercarriage general aviation aircraft would be around 6 ft² with CDp of 0.03 to 0.05, and the retractables around 4–5 ft² with CDp of 0.02 to 0.03. FPA of a very clean, high-performance general aviation aircraft like a Mooney model, is around 3  ft² with CDp about 0.015. Some very clean, high-performance GA kit-built aircraft have FPA less than 2. Note that FPA does not represent the frontal cross-section area of the aircraft.

    One of the smallest known FPA is not associated with a general aviation aircraft but with an owner-designed and built ultralight!  Californian Mike Arnold's  65 hp two-stroke Rotax 582  powered AR-5 held the world speed record, in the under 300 kg FAI efficiency Class C1-A/0  of 213 mph in August 1992. This handsome little glass-epoxy aircraft has an FPA of 0.88  ft² with CDp about 0.016. It demonstrates the efficiency that can be achieved  — an unmatchable 3.3 mph per hp —  in an ultralight design  when the home designer/builder pays the utmost attention to detail. Note the drag reduction achieved by the beautifully shaped engine cowling, the wing root fillet and the minimisation of the junctions of  undercarriage leg fairing and wheel cover. The choice of a fibreglass/foam lay-up composite structure also facilitates the drag reduction program.

    Don't let anyone tell you ultralights have to be slow and draggy!

    •  'Separation bubbles' or  'laminar flow separation bubbles'. In laminar flow,  sometimes the laminar flow boundary layer  separates from the wing surface then reattaches itself a short distance downstream. This forms a 'bubble' of stagnant  air with a significant spanwise dimension  that changes the aerodynamic thickness of the wing which, in effect, increases pressure drag. Bubbles may also cause  increased turbulent flow  to be generated downstream of the reattachment point. Aircraft designers avoid laminar separation at cruising speeds by inducing a turbulent — but attached — boundary layer where necessary. Separation bubbles that increase drag may also occur on the fuselage and tailplane. See vortex generators below.

    • Reynolds number. Occasionally, in reference to boundary layer laminar to turbulent flow and flow separation characteristics you may see mention of the critical Reynolds number [Re]. Re is a measure of the relative influence of viscous and inertia effects on boundary layer behaviour. Like aspect ratio Re is a dimensionless quantity, i.e. it has no units of measurement.

    For rough estimates Re in airstreams = air density/air viscosity × airflow velocity × flow distance.

    ISA sea-level density is 1.225 kg/m³ and standard viscosity is 0.0000179 kg/m/s, so standard air density/air viscosity = 68 459 (say 70 000).

    If the Re is estimated for an average flow across the entire wing at a particular airspeed, then the equation can be simplified to:

    Re = velocity (m/s) × the mean aerodynamic chord (m) × 70 000.

    Thus, the wing chord Reynolds number for an aircraft with a MAC of 1.2 m flying at 97 knots (50 m/s) is roughly 50 × 1.2 × 70 000 = 4 200 000. When that same aircraft is cruising at 78 knots (40 m/s), Re would be about 3 360 000.

    For a particular wing and wing surface condition, there is a critical boundary layer Re, above which the laminar flow will transition to turbulent flow. In slow flight speed, the critical boundary layer Re will be attained a particular distance downstream from the leading edge stagnation point; as airspeed increases (and in accordance with the equation above), that distance must shorten.

    •  The vortex generators [VGs] used in a few light aircraft designs, particularly short take-off and landing [STOL] aircraft —  or as post-delivery 'add-ons' — are small boundary layer control devices  with a swept-back leading edge and a vertical trailing edge, the chord at the base of the device is two to four times its height. VGs are machined from an aluminium 'T' extrusion or formed in polycarbonate, a row of 10 or 15 of which are usually spaced along the upper surface of each  wing, probably close to  the transition zone.  Each VG  is carefully sited,  with a specific  angle of attack (perhaps 15° or more) to the local airflow and with sufficient height to just intrude into the free-stream flow. So situated, they induce fast-rotating,  highly organised, downstream vortices (much the same principle as  wingtip vortices) which mix the high-speed free-stream airflow into the slow-moving, surface boundary layer flow; entraining and re-energising that flow so that the  chordwise pressure gradient profile on the upper surface is decreased (see boundary layer flow). Consequently, surface pressure is decreased, so the pressure differential  —  and thus the lift coefficient — is  increased at all wing aoas. VGs are often paired, to produce counter-rotating vortices. The VGs also  delay boundary layer flow separation at high aoa; i.e. VGs lower the stall speed while improving the aircraft's low-speed behaviour. But there is likely to be minimum warning of onset of the stall, and stall behaviour may be more violent.

    Appropriately sized and sited wing vortex generators can be effective at providing good manoeuvring control of the aircraft when operating low and slow, and provide a greater CLmax, and improve aileron performance and aircraft climb performance. They are sometimes also used on the horizontal and vertical stabilisers mounted just forward of the rudder/elevator hinge lines where they have the effect of allowing greater control surface deflection before separation occurs. VGs are also useful in locations where interference drag is a problem. The use of VGs in light aircraft may slightly degrade performance at the upper end of the speed range, probably depending on the amount of additional turbulence they generate outside the normal turbulent boundary flow.

    • The term burble is sometimes used to describe a turbulent stream. For example a disturbance emanating from something on the fuselage can induce a turbulent streamflow that affects the tailplane. There may also be a separation of flow at the junctions of structural components, which causes interference drag.  

    5.1 Altitude sensing instruments
    The sensitive altimeter
    The altimeter is the cockpit instrument that indicates the aircraft's altitude. The instrument is a refined aneroid barometer with a dial indicating height above mean sea level rather than atmospheric pressure. The main component of such an instrument is a small, flexible, corrugated metal capsule from which the air has been partially evacuated — fitted with a metal closure or diaphragm. There is a spring within the capsule that applies a constant force to the bottom of the diaphragm, while atmospheric static pressure applies a counter force to the top, so that the diaphragm moves as atmospheric pressure changes. The movement of the pressure-sensing capsule is transferred and magnified — via a mechanical linkage or piezo-quartz component — to a dial pointer or pointers, or a digital display, which indicate the altitude reading. The static pressure is drawn from the aircraft's static vent/s, which may induce slight position errors due to aerodynamic effects around a vent. There may be two static vents in different locations on the airframe and the pilot may be provided with the ability to select either or both.

    The level in the atmosphere at which any particular pressure occurs is also dependent on temperature — as we saw in the 'Airspeed and the properties of air' module — but the altimeter does not sense the air temperature. Consequently, all altimeters are calibrated in accordance with the International Standard Atmosphere [ISA] model, which utilises a standard temperature lapse rate with height of 6.5 °C per km (2 °C per 1000 feet). The atmosphere in any region rarely corresponds to the ISA model, so aneroid altimeters do not indicate totally accurate height. This is not that important, as true altitude can be calculated, in the rare circumstance that it is needed for terrain clearance purposes by an aircraft operating under the visual flight rules. There is no problem with air traffic management, in that all aircraft in the same region, with properly set (and functioning) altimeters, will be out by the same amount.

    It is, of course, desirable to set the current local surface pressure into the altimeter by setting that reference pressure into a baro-setting scale or 'sub-scale' (known since the 1930s as the 'Kollsman* window'), which in turn resets the position of the height-indicating pointers against the dial. Or, if the aircraft is on the ground, the same result is achieved by turning the baro-setting knob until the altimeter indicates the known airfield elevation. The sensitive altimeter in the image indicates an altitude of about 1410 feet with the baro-scale setting at 29.9 inches of mercury [in.Hg] — equivalent to 1012.5 hPa. If the altitude was 11400 feet, the pointer with the inverted triangle on the end would be past the figure 1 on the image, indicating +10 000 feet.

    *Paul Kollsman invented his 'sensitive altimeter' in 1929 which was a far superior instrument to those existing at the time but it didn't gain widespread use until 'instrument flying' became common later in the 1930s.

    In Australia, all barometric pressures are reported in hectopascals (equivalent to millibars); and in the USA in units of inches of mercury (one in.Hg = 33.865 hPa so 29.92 in.Hg = 1013.25 hPa). The baro-scale setting range provided in modern altimeters may be from 800 to 1050 hPa.
    Electronic altimeter
    Electronic flight instrument systems [EFIS] use solid-state electronic componentry plus software to display the usual flight instrument readings on a liquid crystal, or similar, screen. In such systems, the atmospheric static pressure is fed to a pressure transducer, which senses and convert pressures to voltages. See the screen display of the Dynon D10A  light aircraft EFIS. Note that the EFIS has an outside air temperature probe and the software can calculate density altitude (see section 'Altitude and Q-code definitions') when needed.

    Electronic altimeters are also available as single instruments or possibly combined with an ASI function.
    Altitude encoding devices
    Altitude encoding devices continually supply pressure altitude data (in Gillham 'Gray' code format) to aircraft transponders and/or GNSS receivers – 'baro-aiding'. There are two types;  encoding altimeters and blind encoders; the latter are stand-alone digital devices with no display (hence 'blind') probably with a pressure transducer  connected to the aircraft's static pressure system.  Standard pressure (1013.25) is  factory pre-set as the scale basis in all altitude encoding devices so both types send pressure altitude not altimeter-indicated altitude. This pressure setting within the device cannot be altered by pilots, such devices being primarily an air traffic management aid.

    A user's manual for the Australian Microair EC2002 low power encoder may be downloaded from the  Microair website.

    5.2 Altitude and Q-code definitions
    Altitude - the third positional dimension
    An aircraft's 3-dimensionsal position may be very accurately defined by its latitude, longitude and altitude; and the latter is normally the most safety-critical dimension. Contour lines and spot points on WACs and VNCs provide an indication of terrain elevation, i.e. height above the reference datum, which is the Australian Height Datum (AHD). The aircraft's altimeter reading provides the aircraft's vertical position and thus an indication of the current height above the terrain indicated on the chart — height above ground level (agl) or airfield level (afl) and the terrain clearance — may be determined. However, in aviation, that altitude reading and the altitude term itself, have many connotations; particularly important is the concept of density altitude.

    Altimeter indicated altitude: is the approximate height of the aircraft above the AHD or above mean sea-level [amsl], calculated in accordance with the ISA but using a local or area QNH as the pressure setting rather than the ISA Standard Pressure of 1013.2 hPa. In Australia the AHD represents mean sea level.

    However an aircraft maintaining a constant altitude — with 1013 hPa or a local/area QNH set in the baro-setting window — is following an isobaric  or contour surface whose height above the AHD will vary according to atmospheric temperature and pressure conditions.
    In the Australian summer temperatures the 'thickness' of the atmosphere is greater than the ISA standard and consequently the rate of pressure decrease with height is less than ISA and the altimeter indicated altitude will be lower than the true altitude.
      If the atmosphere is colder than the ISA the thickness of the atmosphere is less than the ISA standard and consequently the rate of pressure decrease with height is greater than ISA and the altimeter indicated altitude will be higher than the true altitude.
      Also note that;   if you fly from an area of higher pressure to lower and do not obtain and reset the new area/local  QNH the altimeter will be  over-reading (the aircraft is lower than indicated) conversely flying from an area of lower pressure to higher the altimeter will be  under-reading (the aircraft is higher than indicated) but if you fly from an area that is warmer to a cooler one, the altimeter  will be over-reading and conversely, flying from an area that is cooler to one that is warmer the altimeter will be  under-reading and the aircraft is higher than indicated.
    So the adage "From high to low, look out below" is incomplete and the adage "From high to low, hot to cold, look out below" doesn't really apply in  Australia where the continental  low pressure systems (rather than those emanating from the Southern Ocean) are not 'cold core lows' but  'surface heat lows' and troughs. See 'Height contours and thickness charts'.

    These altimeter indicated altitude variations should not be a concern to pilots of aircraft flying under the day visual flight rules and maintaining visual meteorological conditions, particularly so if en route area/local QNH baro-setting information is acquired and properly applied. What should be a particular concern is density altitude rather than true altitude.

    Calibrated altitude: is the altimeter indicated altitude, corrected for internal instrument error and static vent position error by means of reference data for that aircraft installation.

    Pressure altitude: is the altimeter reading when the baro-setting scale is set to 1013.2 hPa; usually termed pressure height in reference to an airfield reading. It is the ISA standard pressure setting.  Standard pressure is also the standard factory setting for altitude encoding devices. All aircraft cruising in the Standard Pressure Region —  above a transition altitude that (in Australia) commences at 10 000 feet  —  use the standard pressure setting, and the subsequent altimeter reading is normally referred to as flight level [FL].

    True altitude: the calibrated/indicated altitude corrected for the outside air temperature conditions. However, there are still problems in the determination of the true height above the AHD, as demonstrated in the following paragraphs. True altitude as calculated in flight from an altimeter reading is of little value to recreational aircraft operating in VMC.

    GPS altitude: the global positioning system uses the WGS84 ellipsoid as its basis for GPS altitude, whereas the AHD (a 'geoid') is the basis for elevations on Australian navigation charts.  The  difference in elevation of a particular point on the Earth's surface — when measured against both the ellipsoid and a national geoid — can be quite considerable, as much as ±250 feet ; this is known as the geoid-ellipsoid separation. In Australia the degree of geoid-ellipsoid separation is quite unusual, in the south-west corner  the AHD geoid is  about 102 feet below the WGS84 ellipsoid while at the north-east corner it is 237 feet above it, so the value of the geoid-ellipsoid separation at all locations must be available to derive true altitude. See 'Geoid-ellipsoid separation and GPS altitude'.

    Density altitude: a calculation used to determine possible  aircraft performance — see section 'High density altitude'  below.   This is the pressure altitude adjusted for variation from standard temperature, or  the height in ISA having a density corresponding to the location density, then called density height.

    Declared density altitude: see  'Method 3: use the CASA declared density altitude charts' below.

    Pivotal altitude: is not associated with altimeter setting; it is a term used by the proponents of 'ground reference' manoeuvres such as 'eights on pylons'. It is a particular height above ground at which, from the pilot's viewpoint, the extended lateral axis line of an aircraft doing a 360° level turn (in nil wind conditions) would appear to be fixed to one ground point, and the aircraft's wingtip thus pivoting on that point. The pivotal altitude in nil wind conditions is easily calculated by squaring the TAS in knots and dividing by 11.3. So an aircraft circling at 80 knots would have a pivotal altitude around 550 feet, no matter what the bank angle.

    When an aircraft is turning at a height greater than the pivotal altitude, the wingtip appears to move backwards over the landscape. When an aircraft is turning at a height less than pivotal altitude (i.e. usually close to the ground) the wingtip appears to move forward over the landscape. For more information see 'pivotal altitude and reversal height'.
    Note: the letters in the Q-code nomenclature have no literal significance; these are remnants of an extensive notation system from the days of wireless-telegraphy and particularly used in marine and aerial navigation/communication. There were some 200 three-letter Q-codes, each representing a sentence, a phrase or a question. For instance, QRM "I am being interfered with"!. Some 30 Q-codes are still used by amateur radio/morse code enthusiasts and the four below, plus QDM (the magnetic bearing to a station), still survive in aviation. For a full listing of Q-codes google 'all Q codes'. The following four codes relate to altimeter settings.

    QFE: the barometric pressure at the station location or aerodrome elevation datum point. If QFE is set on the altimeter pressure-setting scale while parked at an airfield, the instrument should read close to zero altitude — if the local pressure is close to the ISA standard for that elevation. However, the use of QFE is deprecated and anyway, if the airfield elevation is higher than perhaps 3000 feet, older/cheaper altimeters may not be provided with sufficient sub-scale range to set QFE.

    QFF:  the mean sea-level [msl] pressure derived from the barometric pressure at the station location. This is derived by calculating the weight of an imaginary air column extending from the location to sea-level — assuming the temperature and relative humidity at the location are the long-term monthly mean, the temperature lapse rate is ISA, and the relative humidity lapse rate is zero. This is the method used by the Australian Bureau of Meteorology; QFF calculations differ among meteorological organisations. QFF is the location value plotted on surface synoptic charts and is closer to reality than QNH, though it is only indirectly used in aviation.

    QNH: the msl pressure derived from the barometric pressure at the station location by calculating the weight of an imaginary air column extending from the location to sea-level — assuming the temperature at the location is the ISA temperature for that elevation, the temperature lapse rate is ISA and the air is dry throughout the column.

    The Australian aviation regulations state that when an 'accurate' QNH is set on the pressure-setting scale at an airfield, the VFR altimeter indication should read within 100 feet of the published airfield elevation, or 110 feet if elevation exceeds 3300 feet; otherwise the altimeter should be considered unserviceable. However, due to the inherent inaccuracy possible in QNH, this may not be so. The difference between QFF and QNH when calculated on a hot day at a high airfield in Australia can be as much as 4 hPa, equivalent to about 120 feet. The advantage to aviation in using the less realistic QNH is that all aircraft altimeters in the area will be out by about the same amount, and thus maintain height interval separation.

    The local QNH at an airfield is normally derived from an actual pressure reading. But the area QNH used outside the airfield zone is a forecast value, valid for three hours, and may vary by up to 5 hPa from any local QNH in the same area.  Either local QNH or area QNH may be set on the altimeter pressure-setting scale of all aircraft cruising in the Altimeter Setting Region,  which (in Australia) extends from the surface to the Transition Altitude of 10 000 feet. The cruising levels within the Altimeter Setting Region are prefixed by 'A'; e.g. A065 = 6500 feet amsl.

    When there is no official Local QNH available at an airfield and the site elevation is known, the local QNH can be derived by setting the sub-scale (when the aircraft is on the ground at the location) so that the altimeter indicates the known airfield elevation. The use of local QNH is important when conducting operations at an airfield, as the circuit and approach pattern is based on determining height above ground level [agl].

    Note that it is not mandatory for VFR aircraft to use the area QNH whilst enroute. You may substitute the current local QNH of any aerodrome within 100 nm of the aircraft or the local QNH at the departure airfield. See 'Acquiring weather and QNH information  in-flight'.

    The purpose of the Transition Layer is to maintain a separation zone between the aircraft using QNH and those using the standard pressure setting. Cruising within the Transition Layer is not permitted.  If Area QNH was 1030 hPa, there would be about 500 feet difference displayed between setting that value and setting standard pressure. The Transition Layer extends from the Transition Altitude to the  Transition Level  which, in Australia, is usually at FL110 but it may extend to FL125  —  depending on Area QNH. More detail is available in 'Aeronautical Information Publication (AIP) Australia' section ENR 1.7; downloadable from Airservices Australia.

    QNE:  common usage accepts QNE as  the ISA Standard Pressure  setting of 1013.2 hPa. However another definition of QNE is the 'altitude displayed on the altimeter at touchdown with 1013 set on the altimeter sub-scale' i.e. 'pressure height'. It is also referred to as  the 'landing altimeter setting'.

    Within the latter meaning, the term is only likely to be used when an extremely low QNH is outside an aircraft's altimeter sub-scale range, and the pilot requests aerodrome QNE from air traffic services. In Australia, such extreme atmospheric conditions are only likely to occur near the core of a tropical depression/cyclone and as QNE is not listed in the ICAO "Procedures for Air Navigation Services", air traffic services would not provide QNE on request.

    However, QNE can be calculated by deducting the QNH from 1013, multiplying the result by 28 (the appropriate pressure lapse rate per hPa) and adding the airfield elevation.

    For example: QNH 960 hPa, airfield elevation 500 feet, pressure setting 1013.
    QNE = 1013 –960 = 53 × 28 = 1484 + 500 = 1984 feet (the reading at touchdown).
    5.3 High density altitude: effect on take-off/landing performance
    High 'density altitude' conditions at an Australian airfield can provide a severely hazardous environment for any aircraft where the difference between power required and power available is small. This concerns most general aviation and all sport and recreational aircraft engaged in take-off or landing at that airfield. It is the density of the air that provides engine power, propeller performance and lift.

    What we are really doing when calculating density altitude is estimating the density of the air. In ISA conditions, at a  density altitude of 6000 feet amsl,  the air density will be about 1.0 kg/m³, about 20% less than sea-level standard density. The maximum lift possible to be generated will be reduced by 10% (lift = CL × ½rV² × S ) and the ground roll speed related to IAS/CAS prior to take-off will be greater; i.e. during take-off at msl in ISA sea-level conditions TAS = IAS/CAS, but in high density altitude conditions  TAS is greater than IAS/CAS. Remember that V² in the lift equation refers to TAS not CAS. So, at 6000 feet density altitude the TAS at lift-off would be about 10% higher (see rule of thumb) than msl conditions thus the aircraft has to accelerate to a 10% higher ground roll speed before reaching lift-off IAS, and that is before taking into account the effect of the engine and propeller performance reductions on the aircraft's ground roll acceleration performance and its climb-out capability.

    The weight of the charge delivered to the cylinders, in a normally aspirated engine, will be only 80% of the standard sea-level value. Thus, only 80% of the engine's rated power can be supplied at the propeller shaft for take-off and climb-out, or for a go-around. The lower air density ( ½r in the ½rV² term of the lift equation) directly reduces the thrust performance of the fixed-pitch propeller by 10% in which case the thrust performance will be 90% of 80%, or about 72% of the rated sea-level performance. So both the time and the distance needed to acquire take-off lift — and to clear obstacles at the end of the strip — must be increased, the aircraft's rate of climb, and thus angle of climb, will be less than it is near sea level.

    There are many conditions that exist, or might exist, at high density altitude which, though they may be individually slight, all affect the airframe and engine performance adversely. For instance, attempting take-off with a combination of some of the following conditions may cause some difficulty; attempting take-off when most conditions exist may well be disastrous:
    at an elevated airfield with moderate to high surface temperature on a short, soft strip with unslashed,  wet grass at maximum weight incorrect flap setting light and variable winds departing into rising terrain and a sinking air environment.
    The same conditions apply when landing; the TAS at Vref will be  higher and the consequent ground roll will be longer. The thrust available for a go-around, in the event of an aborted approach, might be very much less than the rated msl thrust, which would probably preclude any late go-around.

    In addition, under high density altitude conditions, the mixture may be excessively over-rich. The recommendation for normally aspirated  engines with cockpit mixture control is  that the mixture should be leaned to maximum rpm before taxying, take-off   or landing  if the density altitude is 5000 feet or greater.

    Density altitude at a particular location can vary considerably from day to day, and also according to time of day. For instance, the table below shows a mid-afternoon and an early morning reading at Alice Springs, in central Australia, on different days. The airfield elevation is 1900 feet.
    QFE Temperature Air density Pressure altitude Density altitude 941 hPa 43 °C 1.037 2020 feet 5600 feet 957 hPa –2 °C 1.230 1580 feet –100 feet
    The isotherms plus  colour in-fills on the following Australian Bureau of Meteorology map indicate the mid-afternoon surface screen temperatures on a late-spring day. Note that, except for the mountain area near the south-east coast, the surface temperatures greatly exceed the 15 °C ISA standard.

    5.4 Calculating the dry air density altitude
    The density of dry air (r) varies according to ambient air pressure and ambient air temperature, this is reflected in the equation density = pressure divided by 2.87 times the temperature(K). Pressure (or pressure altitude) is readily obtained from the altimeter, and temperature can be obtained from various sources.
    Method 1: use the temperature differential
    Density altitude is roughly 120 feet greater than pressure altitude for each 1 °C that the temperature exceeds ISA for that level, and 120 feet less for each 1 °C that the outside air temperature is less than ISA. In the ISA table sea-level temperature is 15 °C and the ISA temperature lapse rate is 2 °C per 1000 feet.

    For example: Armidale, New South Wales, airport (elevation 3550 feet) on a warm summer day, temperature 30 °C. Altimeter, with 1013.2 standard sea-level pressure sub-scale setting, reads 3400 feet pressure height/altitude.
    So, ISA standard temperature for an elevation of 3550 feet = [15 –(3.55 x 2)] = 8 °C. The Armidale  temperature then exceeds standard by 22 °C, thus adjustment to be added= 22 × 120 = 2640 feet Pressure altitude = 3400 feet Then the approximate density altitude = 2640 + 3400 = 6040 feet.  
    Method 2: calculate using the air density equation
    The density of dry air at altitude can be calculated using the equation:
    r = P / (2.87 T), where:
    r = rho — the density of dry air  [kg/m³] P = the pressure [hPa] 2.87 = the gas constant for dry air T = the air temperature in kelvin units [K].  
    Using the Armidale example, with the altimeter set so that altitude shows the elevation of 3550 feet, the pressure-setting sub-scale will display  895 hPa (i.e. QFE). The temperature is 303 K  (30 °C + 273) thus density = 895 / (2.87 × 303) = 1.029 kg/m³. The height in ISA having a corresponding density is about 5850 feet. This gives a slightly more accurate calculation of density altitude than method 1.
    Method 3: use the CASA declared density altitude charts
    The ICAO International Standard Atmosphere model, used for flight instrument calibration, is based on average climatic conditions at 40° to 45° N latitudes and as such does not reflect conditions over much of Australia in all seasons, with the discrepancy peaking in summer. The Civil Aviation Safety Authority recognises this and publishes seasonal 'declared density altitude' charts with isopleths delineating regional values to be added to airfield elevation to give declared density altitude.

    The three seasonal charts (summer, winter and autumn/spring) are published as appendices to Civil Aviation Order 20.7.0. For example the summer chart shows regional values of 2000 feet in some south-east and 3600 feet in central areas. These regional values are to be used only if there are no other means of calculating current density altitude at the departure and destination airfields.

    Armidale is located at 30° S and 151° E between the 2800 and 3000 feet isopleths of the summer chart, so adding 2900 to the airfield elevation of 3550 feet gives a declared density altitude of 6450 feet.
    Method 4: use a density altitude computational chart
    First determine current pressure altitude with 1013 hPa standard pressure setting on the altimeter sub-scale, for example 3400 feet. Also determine outside air temperature, for example 30 °C. Draw a horizontal line from the 3400 feet position to the 30 °C vertical line. Determine the density altitude scale at which the line terminates, for this example density altitude = 6000 feet.

    Click the image for a larger scale  printable  computational chart.

    Method 5: use an E6-B type circular scale computer
    The plastic circular slide rule flight planning computers have a density altitude facility that just entails placing the pressure altitude opposite temperature and reading off density altitude from a third scale. Because the scales are close to the centre of the instrument they are small and difficult to set accurately, but the Jeppesen Model CR-3 computer indicates about 6000 feet density altitude for the Armidale example.

    It can be seen that the five methods listed provide much the same result — about 6000 feet density altitude — so use the method which is most convenient; but do the estimate and then calculate the effects on aircraft performance at estimated weight while also including the effects of runway conditions and the wind velocity.
    Method 6: use the information in the Pilot's Operating Handbook
    Charts in the Pilot's Operating Handbook or Flight Manual should provide density altitude plus aircraft performance and maximum weight figures from the input of pressure altitude and temperature.

    Method 7: use one of the density altitude calculators available on the internet
    Google "density altitude calculator". It's probably advisable to do a number of trial runs before choosing a particular DA calculator.

    Know the normal take-off distance required
    Before you can start to estimate the take-off distance required under high density altitude conditions, you must know the take-off distance required under standard ISA mean sea-level conditions.

    CAO 101-28, an airworthiness certification requirements for commercially supplied amateur-built kit ultralights, states in part:
    "The take-off distance shall be established and shall be the distance required to reach a screen height of 50 feet from a standing start, with ... short, dry grass surface ... the aeroplane reaching the screen height at a take-off safety speed not less than 1.2 Vs1 ... take-off charts ... shall schedule distances established in accordance with the provisions of this paragraph, factored by 1.15."

    Sea-level ISA and nil wind conditions are implied.

    CAO 101.55 has much the same wording, but specifies 1.3 Vs1 as the take-off safety speed. FAR Part 23 is similar.

    CAO 101-28 also requires that the landing distance stated will be that to come to a full stop from a screen height of 50 feet at the threshold, with the screen being crossed at 1.3 Vso and the same conditions as specified for the take-off distance. Refer to Vref.

    If buying an aircraft or kit, you should require that the standard take-off and landing distance chart information for the airframe/engine/propeller combination be supplied. Statements such as "Take-off ground roll 10 m to 40 m" have no value. You must insist, particularly with imported aircraft, that the distances should be stated clearly in one form only and for nil wind conditions "Take-off distance to clear 50 feet (15 m) screen" or "Landing distance over 50 feet (15 m) screen". You have to know without doubt, having done the necessary calculations,  that you can clear obstacles at the end of the unslashed paddock on a hot, bumpy day without risk to you or your passenger, and that if it is necessary to abort a landing, the aircraft will have the ability to go-around safely.

    5.5 Calculating and adding the effect of humidity on density altitude
    The ISA is based on dry air and though air density (mass per unit volume) is chiefly dependent on temperature and pressure, humidity — the presence of water vapour — does decrease the density of the air a little and so has a small effect on lift. Humidity also has a small adverse affect on engine performance as combustion performance is dependent on the oxygen intake during each engine cycle, and that amount of oxygen is dependent on the air density.

    The molecular mass ratio of water vapour to dry air is 0.62:1 and the water vapour molecule occupies about the same space as any oxygen or nitrogen molecule it displaces, so air density decreases as the relative humidity of the air increases, and this should be considered when calculating density altitude.

    *Note: see atmospheric moisture for more information.

    Effect on density altitude. The table below gives the density in grams per cubic metre of the water vapour at the saturation point (i.e. relative humidity (RH) = 100%), for air temperatures between zero and 45 °C. As can be seen, at 35 °C and 100% RH the water vapour density is 40 grams/m³ which is 3.5% of the dry air mass at that temperature. However as the mass ratio of water vapour to dry air is 0.62:1, the 40 grams of vapour in the moist air would displace 65 grams of dry air (13g of oxygen). The effect on air density is a net reduction of 25 grams or 2% and, below 5000 feet elevation, equivalent to a density altitude increase of about 750 feet. At 25 °C and 100% RH the water vapour density is 25 grams/m³ which would displace 40 grams of dry air (8g of oxygen) and the effect on air density is a net reduction of 15 grams or 1.2%; equivalent to a density altitude increase of about 500 feet. It can be seen that high humidity has an additional detrimental effect on aircraft take-off and landing performance under high density altitude conditions.
    Water vapour saturation partial pressure, density at sea level
    and effect on density altitude (values rounded) Air
    temperature Saturation
    hPa Vapour
    grams/m³ Dry air
    grams/m³ Net density
    grams/m³ Net density
    % Density
    increase 0 6 5 1290 3 0.2% +100 ft 10 °C 12 10 1250 6 0.5% +200 ft 15 °C 17 15 1225 9 0.8% +250 ft 20 °C 23 20 1200 12 1.0% +350 ft 25 °C 30 25 1180 15 1.3% +450 ft 30 °C 42 30 1160 19 1.6% +550 ft 35 °C 56 40 1150 25 2.2% 750 ft 40 °C 73 50 1130 31 2.7% +900 ft 45 °C 97 65 1110 40 3.6% +1200 ft  
    The Australian Bureau of Meteorology publishes maps of the average monthly relative humidity observations and these might be used as a basis for estimation if you are unable to find the current RH at the location. If the dry-bulb and wet-bulb temperatures are known the following table will provide a reasonable estimate of the current relative humidity. Wet-bulb temperatures are always lower than dry-bulb temperatures.
    Calculation of relative humidity, for dry-bulb temperatures from 20°C to 45°C, knowing the difference between the dry-bulb and wet-bulb temperatures Difference -1° -2° -3° -4° -5° -6° -7° -8° -9° -10° Relative humidity 95% 90% 85% 80% 75% 70% 65% 60% 55% 50%
    A handy rule of thumb to allow for the effect of humidity — after completing the dry air density altitude calculation:
    for air temperatures below 30 °C add 50 feet to the dry air result for each 10% that the local relative humidity exceeds 10% e.g. air temperature 25 °C, RH 60%, altitude to be added is 5×50=250 feet
      for air temperatures 30 °C and above, add 100 feet to the calculated density altitude for each 10% RH that RH exceeds 20%; e.g. air temperature 35 °C, RH 65%, altitude to be added is 5×100=500 feet.  
    The rule is based on the following calculations providing the increase in density altitude for air temperatures from 15 °C to 40 °C and relative humidities between 10% and 100%:
      100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 15 °C +260 ft +230 ft +210 ft +180 ft +160 ft +130 ft +100 ft - - - 20 °C +350 ft +320 ft +280 ft +250 ft +210 ft +180 ft +140 ft +100 ft - - 25 °C +450 ft +400 ft +350 ft +300 ft +260 ft +220 ft +180 ft +130 ft - - 30 °C +560 ft +500 ft +450 ft +390 ft +340 ft +280 ft +220 ft +170 ft +110 ft - 35 °C +740 ft +670 ft +600 ft +530 ft +450 ft +380 ft +300 ft +220 ft +150 ft - 40 °C +900 ft +810 ft +720 ft +630 ft +540 ft +450 ft +360 ft +270 ft +180 ft -
    For more information on take-off and climb performance in high density altitude conditions, see take-off considerations.
    5.6 Physiological effects of altitude
    The tissues and organs of the human body need a constant and adequate supply of oxygen to function at maximum efficiency; insufficient oxygen in those tissue and organs is called hypoxia. There are many causes for the condition, but the one of most interest to sports and recreational aviators is the hypobaric form of hypoxia  caused by continuing flight at an altitude where the partial pressure of the atmospheric oxygen is less than that required for proper functioning of the brain. The body utilises the oxygen partial pressure to pass it through the membrane of the lung alveoli into the bloodstream.

    (The 'stagnant' forms of hypoxia — greyout and blackout —   caused by reduced blood flow to the eyes and brain at aircraft accelerations exceeding +3g to +4g is also, of course, of interest to aerobatic pilots.  For a pilot of average fitness, greyout (dimness of vision) will start between +3.5g and +4.5g, reaching  blackout (complete loss of vision)  between +4g and +5.5g and g-induced loss of consciousness [GLOC]  between +4.5g and +6g.) The application of perhaps –2g or –3g causes increased blood flow to the eyes, resulting in leakage from the blood vessels –redout. Prolonged application of high negative g may severely damage the optic nerves.

    Atmospheric oxygen partial pressure declines as altitude increases; see the atmospheric oxygen section in the Aviation Meteorology Guide. The table in that section shows the time a reasonably fit person will remain conscious at those altitudes without using supplemental oxygen. However, the effects of hypoxia commence at much lower altitudes, probably around 8000 feet for a fit person, less if unfit though much lower for a heavy smoker. These effects include a gradual deterioration in thinking, calculating and reacting; inability to make appropriate judgements; light headedness and a poor memory recall. Unfortunately, the afflicted person is usually quite unaware of the symptoms occurring and may enjoy a feeling of well-being even, perhaps, euphoria. For more information read the article 'Hypoxia' from Flight Safety Australia magazine.

    In Australia recreational aircraft may only be flown at or above 10 000 feet amsl if the pilot has applied to and received written permission for that flight from the Civil Aviation Safety Authority.  The aircraft must be equipped with an operating Mode A/C or S transponder. Also the Australian Civil Aviation Order Part 20.4 paragraph 6 which applies to all Australian aircraft, requires that:  "A flight crew member who is on flight deck duty in an unpressurised aircraft must be provided with, and continuously use, supplemental oxygen at all times during which the aircraft flies above 10 000 feet altitude." Note that   an aircraft may not cruise within the transition layer and that layer could extend to FL125.
    Things that are handy to know
    Altimeter rules of thumb
    • For each 10 °C that the outside air temperature is warmer than ISA standard, increase the indicated altitude by 4% to give true altitude. Conversely, for each 10 °C cooler, decrease indicated altitude by 4% — 10/273 approximates to 4%; refer to Charles' law.

    • When flying from higher to lower pressure conditions, without altering QNH, the altimeter will — if below 10 000 feet — overread (indicate higher than actual altitude) by about 30 feet for each one hPa pressure change.

    • When flying from lower to higher pressure conditions, without altering QNH, the altimeter will — if below 10 000 feet — underread (indicate lower than actual altitude) by about 30 feet for each one hPa pressure change.

    • If the altimeter sub-scale setting is less than QNH the altimeter will overread. Conversely, if the setting is greater than QNH, the altimeter will underread.

    • Air density decreases by about 1% for each:
    — 10 hPa fall in pressure, or
    — 300 feet increase in height, or
    — 3 °C increase in temperature, from the msl standard.

    Stuff you don't need to know
    • There is a semi-diurnal atmospheric tide, similar to the oceanic tide, which is most apparent in the lower latitudes. The tide peaks at 1000 hrs and 2200 hrs local solar time, with the minima at 0400 hrs and 1600 hrs. At Cairns, 17° S latitude, the daily minima and maxima are 2 hPa either side of the mean pressure; e.g. 0400 hrs — 1014 hPa; 1000 hrs — 1018 hPa; 1600 hrs — 1014 hPa; 2200 hrs — 1018 hPa. The runway elevation at Cairns is 10 feet amsl, so that if you left a parked aircraft at 1600 hrs with the altimeter reading 10 feet, six hours later it would be reading 110 feet below mean sea-level. When making their regular pressure reading reports, weather observation stations adjust the reported QFF according to a 'time of day' table.

    • There is also a semi-diurnal gravity variation at the Earth's solid surface, also peaking at 1000 hrs and 2200 hrs. A movement of 50 cm from the low to high earth tide has been ascertained in central Australia.

    •  Perhaps the highest surface pressure recorded is 1083.3 hPa at Agata, Siberia on 31 December 1968.  Agata is 850 feet amsl.  

    4.1 Variometer
    A variometer is a fast response vertical speed indicator usually scaled to match typical glider rates of climb and descent (+/-10 knots or +/- 5 metres per second). The variometer makes soaring possible by displaying the glider rate of climb to the pilot in near real time, enabling the pilot to manoeuvre the glider so as to remain in rising air.
    Variometers come in many types, some sense the airflow from a capacity bottle or chamber (as the outside static pressure increases or decreases due to altitude changes, air flows in or out of the chamber to equalise the pressure) either mechanically or electrically; others measure the air pressure directly using silicon pressure transducers and compute rate of climb electronically from the changes measured. All Borgelt variometers since 1982 use this last method.
    Audio signals which vary with rate of climb/descent are also possible when electronic sensing is used and this is a great help in keeping pilots looking outside the cockpit for other traffic, gliders or birds climbing better in nearby thermals and interesting and/or useful meteorological phenomena.
    4.2 Vario averagers
    When circling in turbulent thermals – while our variometer will help us find the best lift – it is sometimes difficult to know how fast we are really climbing (or whether we really are climbing) as there may be sink in part of the circle and lift in others. In this case we can use an averager which is really just a slow response variometer. Most vario averagers integrate or average the variometer readings so that the running average rate of climb of the last circle or so is shown. This requires averaging over 20 to 30 seconds. The averager display may be "on demand" or continuously displayed in digital form. 
    4.3 Total energy variometer
    The basic variometer described above suffers from the effect that as the glider changes airspeed in response to pilot inputs, large transient rates of climb and descent are induced until the airspeed is stabilised at a new value. These may easily exceed and swamp the rates of climb due to airmass rising and sinking (a 30 degree pullup from 100 knots gives an initial rate of climb of 50 knots, causing the +/- 10 knot scale variometer to peg uselessly at the top of its scale). If, instead of sensing the outside air pressure or static pressure with the variometer, we connect the variometer to a venturi of the correct dimensions we find that as long as the airspeed is constant the pressure in the venturi decreases and increases as in the basic variometer case. If the airspeed decreases due to a pullup the suction produced by the venturi will decrease and will compensate for the reduced static pressure from the climb resulting in no net change of pressure and hence no change in variometer reading.
    The suction produced by this venturi (pressure below static pressure) is the same as the pressure increase above static pressure measured by a pitot tube at the same airspeed.
    For the last 25 years the most common and best "venturi" in fact doesn't look like a venturi at all. A 6 mm tube usually extends from the fin leading edge  and is bent up or down 70°  or so, so that the last 80 mm of the tube is at 20° forward inclination to the airflow. The end of the tube is sealed and the end is cut off square to the tube (NOT parallel to the airflow) and two small holes are drilled in the rear half of the tube as pressure ports. The suction of the device depends on the distance of the holes from the end of the tube. This design is relatively insensitive to yaw (sideslip) and pitch and unlike a real venturi doesn't provide a home for spiders and insects. It is also easy to keep clean (required for correct functioning). As it was invented by Frank Irving it is known as an Irving tube. The generic term for all venturis, probes etc is Total Energy Probe or TE Probe. The total energy referred to is kinetic plus gravitational potential energy.
    A total energy variometer as described above can be further improved. As described the vario will, in still air, show the glider sink rate at the speed being flown.
    Let us take a good modern glider such as an 18 meter racer. The unballasted polar curve for this glider will show a minimum sink rate of around 1 knot at around 45 knots IAS, a best L/D of about 50 at around 55 knots and sink rates of about 2 knots at 75 knots and 4 knots at 100 knots. The sink rate through the airmass at typical thermalling airspeed and bank angle will be about 1.6 knots. Now suppose you are cruising between thermals and you encounter rising and sinking air and you vary the airspeed according to Macready 'speed to fly' theory between 60 knots and 110 knots as you encounter rising and sinking air. The vario is TE compensated so changes in airspeed don't cause large transient indications on the vario. However at 110 knots the glider might be sinking through the airmass at 5 knots and at 60 knots at just over 1 knot.
    These sink rate changes can mask small vertical speed changes in the airmass and make it difficult to pick the best path through the air (which is the path with the most and fastest rising air and the least and slowest sinking air). If encountering a thermal at high speed you may even reject the thermal that is really acceptable. A thermal rising relative to the ground at 8 knots will show as 3 knots up on the vario if you are sinking at 5 knots at 110 knots IAS. In fact after you reduce speed and turn in this air you would climb at 8 – 1.6 = 6.4 knots so it is difficult to use the vario indication to decide whether to accept or reject the thermal being flown through.
    4.4 Netto variometer
    A Netto or airmass variometer adds to the raw TE vario reading an upward deflection to counteract the sink due to the glider polar at that airspeed. Now, in still air, the vario will read zero at any airspeed if the polar we have assumed is correct. Even if it isn't the differences are likely to be small fractions of a knot and it is much easier to use the vario to pick the best path through the air.
    There is one disadvantage and that is if we fly through our thermal rising at 8 knots relative to the ground we see 8 knots on the Netto vario regardless of the airspeed we are flying at. When we slow to circle we climb at 8 – 1.6 = 6.4 knots. So we mentally must subtract our circling sink rate of 1.6 knots to see what rate of climb we will get if we circle now. This is much easier than with our raw TE vario but still adds to workload (for most gliders 2 knots is in fact close enough for practical purposes).
    4.5 Relative Netto variometer
    A further refinement of the Netto vario and is sometimes called just relative or super Netto.
    If we superimpose a downward deflection of 1.6 knots on our Netto variometer we can see that at any airspeed the variometer will show the rate of climb we will get if we circle.
    This now makes it very easy to use the vario to decide if the thermal is good enough (it isn't the only criterion – you may not have flown through the center – you may already be low etc) but at least the vario is easy to interpret.
    For picking the best path through the air the relative vario is almost as good as the Netto. Any time that the vario is heading in the upward direction the air is getting better, downwards worse. To get the best of both the Netto and relative vario just mark the 2 knots down position on the outside of the scale with a sliver of white tape. This serves as the "still air" reference point. Any time the vario is above this point the airmass is rising.
    Do we care about this? Surely we just want the best air?
    Sometimes it is important in interpreting the meteorology to know if the air is rising or sinking. Also our estimate of how well the glider will go on final glide depends on knowing the absolute rise or fall of the air we fly through. We may be flying in the best available air but if that airmass is sinking at only a fraction of a knot on average we must allow extra altitude for final glide.
    4.6 Speed command variometer
    Earlier in the article I mentioned Macready 'speed to fly' theory. Depending on the anticipated strength of the next thermal and the glider polar and the air you are currently flying through there is an optimum indicated air speed to be flying at.
    A table of speeds is one way to do this, a moveable scale (Macready ring) around the vario is another.
    If we have the electronics to measure our airspeed to provide a Netto or relative vario we can also use a little more processing to provide a zero reader for the optimum speed to fly. Pointer above zero - pull up, reduce speed. Below - push and gain speed. This is known as a speed command variometer or speed command.  Audio signals for "fly faster" and "fly slower" may also be generated.
    This combines the TE variometer reading with a downward offset controlled by the MaCready or STF (speed to fly) selector (may be a rotary knob or controlled by accessing a menu on screen) and an upward deflection that gets larger as airspeed is increased. If done correctly the result is that you fly at the optimum speed at all times. The speed changes should be made gently so the zero reader indications are usually filtered to slow down the commanded changes.
    4.7 Effects of altitude
    It should be noted that the pressure change for any given altitude increment reduces with altitude so variometer calibrations require correction for the effects of altitude.
    The TE probe will automatically compensate for the effects of altitude.
    Because the air is less dense at altitude which causes the glider polar to change, Netto, relative and speed command indications also require correction for altitude.
    The glider polar also changes with the weight of the glider and with the contamination of the airfoil by insects so both the glider weight and the degradation of the polar due to bugs must be accounted for in the Netto, relative and speed command indications.
    The processing in the B40 Vario and B50 Super Vario takes all the relevant effects into account.

    3.1 The atmospheric pressure gradient
    The random molecular activity or internal kinetic energy within a parcel of air is known as the static pressure and is proportional to the absolute temperature.

    Static pressure exerts a force on an object (for example, an aircraft wing) at right angles to all the exposed non-porous surfaces, and is measured in newtons per square metre [pascals] of surface. In Australia air pressure is reported as hectopascals [hPa] for meteorological purposes; one hectopascal equals 100 N/m² [or one millibar].

    Atmospheric pressure reflects the average density (i.e. mass per cubic metre), and thus the weight, of the column of air above a given level. So, the pressure at a point on the Earth's surface must be greater than the pressure at any height above it, in that column. An increase in surface pressure denotes an increase in mass — not thickness — of the blanket of air above the surface location. Similarly, a decrease in surface pressure denotes a decrease in the mass of air. The air throughout the column is compressed by the weight of the atmosphere above it, thus the density of a column of air is greatest at the surface and decreases with increasing altitude.

    However, a warmer air column will be thicker — i.e. extend further upwards — than a cooler air column with the same surface pressure. Thus a particular pressure level will be at a higher elevation in the warmer column. This means that the level in the atmosphere at which any particular pressure occurs is also dependent on the temperature (or thickness) of the air column. Meteorological offices produce 'height contour and thickness charts' to determine the locations of upper level troughs and ridges.

    3.2 Atmospheric density
    The average density of dry air in mid-latitude, temperate climates, is about 1.225 kg/m³ at mean sea-level; the density decreases with increasing altitude.

    There are several gas laws that relate the temperature, pressure, density and volume of air. The equation most pertinent to aeronautical needs is the equation of state:

    r = P/RT where:
    r* = air density kg/m³
    P = static air pressure in hectopascals [hPa]
    R = the specific gas constant for dry air = 2.87
    T = air temperature in kelvins [K] = °C + 273

    *r is the Greek letter rho, pronounced 'row', as in 'row your boat'.

    By restating the equation of state as: P = 2.87rT , it can be seen that if density remains constant; pressure increases if temperature increases.
    We can calculate the ISA standard sea-level air density, knowing that standard sea-level pressure = 1013 hPa and temperature = 15 °C or 288 K

    i.e. Air density = 1013 / (2.87 × 288) = 1.225 kg/m³ = ro*

    If the air temperature happened to be 30 °C or 303 K at the same pressure, then density = 1013 / (2.87 × 303) = 1.165 kg/m³, or a 5% reduction.

    *ro — the symbol for the standard sea level density is pronounced 'rho zero', or 'rho nought' if you prefer.
    3.3 The ICAO International Standard Atmosphere
    The International Civil Aviation Organisation's [ICAO] International Standard Atmosphere [ISA] provides a fixed standard atmospheric model that is used for many purposes, among which are the uniform assessment of aircraft performance and the calibration of some aircraft instruments. The model is based on average climatic conditions in 40–45° North latitudes, but contains the following assumptions:
    dry air (no water vapour present) is assumed throughout the atmosphere so the effects of humidity on air density are ignored the mean sea-level [msl] pressure = 1013.25 hPa the msl temperature = 15 °C [288 K] the tropopause is at 36 090 feet [11 km] and the pressure at the tropopause = 226.3 hPa the temperature lapse rate to 36 090 feet = 6.5 °C per km, or very close to 2 °C per 1000 feet the temperature between 36 090 and 65 600 feet [20 km] remains constant at −56.5 °C.  
    The table below shows a few values derived from the ISA. Those pressure levels noted with a flight level designator (FL) are standard pressure levels, rounded to the nearest increment of 500 feet, used for aircraft operating above the altitude transition layer and also for aviation weather purposes, particularly thickness charts.
    hPa   °C kg/m³ feet Pressure Flight level Temperature Air density Altitude 1013   15.0 1.225 msl 1000   14.3 1.212 364 977   13.0 1.190 1000 950   11.5 1.163 1773 942   11.0 1.155 2000 908   9.0 1.121 3000 900   8.6 1.113 3243 875   7.0 1.088 4000 850 A050 5.5 1.063 4781 843   5.1 1.056 5000 812   3.1 1.024 6000 800   2.3 1.012 6394 782   1.1 0.993 7000 753   – 0.9 0.963 8000 750   –1.0 0.960 8091 724   – 2.8 0.933 9000 700 A100 – 4.6 0.908 9882 696   – 4.8 0.905 10 000 650   – 8.3 0.855 11 780 600 FL140 –12.3 0.802 13 801 550   –16.6 0.747 15 962 500 FL185 – 21.2 0.692 18 289 450   – 26.2 0.635 20 812 400 FL235 – 31.7 0.577 23 574 350   – 37.7 0.518 26 631 300 FL300 – 44.5 0.457 30 065 250 FL340 – 52.3 0.395 33 999 200 FL385 – 56.5 0.322 38 662 150 FL445 – 56.5 0.241 44 647 100   – 56.5 0.161 53 083
    It can be seen that (from sea level to 10 000 feet) air density decreases by about 33 grams/m³ per 1000 feet. Also, not immediately apparent from the ISA table, is that the pressure lapse rate (the rate of change with height) is exponential. It starts at about one hPa drop per 28 feet height increase, then slowing to 31 feet per hPa at 6000 feet (averaging 30 feet height increase per hPa drop up to 6500 feet), 36 feet per hPa at 10 000 feet, 50 feet at 20 000 feet and so on.

    However, the following provides a useful rule of thumb:
    Rule of thumb #1
    "For operations below 10 000 feet, an altitude increase (decrease) of 30 feet can be assumed for each one hPa pressure decrease (increase) and for an estimate of air density multiply the altimeter indicated altitude in 1000's of feet by 30 to find the value to deduct from 1.225 kg, e.g. at 6500 feet; 6.5 × 30 = 195 and 1.225 kg minus 195 grams = 1.03 kg." But bear in mind that the ISA model is unlikely to reflect current atmospheric conditions at a particular location, see high density altitude.
    3.4 Bernoulli's principle and the continuity equation
    Bernoulli's principle
    Daniel Bernoulli (1700-1782) was a Swiss mathematician who propounded the principle that for a given parcel of freely flowing fluid, the sum of gravitational potential energy, kinetic energy and static pressure energy always remains constant.

    For our aerodynamic purposes we can ignore the gravitational potential energy. Dynamic pressure = ½rv² and kinetic energy = ½mv² where m = mass. Air density is mass per unit volume; i.e. kg/m³ so dynamic pressure is the kinetic energy per unit volume. Static pressure is internal kinetic energy per unit volume, or pressure potential energy.

    So, for our purposes (in a parcel of freely flowing air), Bernoulli's principle can be reduced to:
    ½rv² [dynamic pressure] + P [static pressure] = constant

    The statement doesn't take into account viscosity, heat transfer or compressibility effects, but for operations below 10 000 feet and airflow velocities below 250 knots, compressibility effects can be ignored — thus no change in flow density [r] is assumed.

    The statement then indicates that, in a free stream flow, if speed [v] increases static pressure [P] must decrease to maintain constant mechanical energy per unit volume; and the converse — if speed decreases, static pressure must increase. Or, turning it around, a free stream airflow will accelerate in a favourable pressure gradient and decelerate in an adverse pressure gradient.

    Bernoulli's principle doesn't apply in boundary layer flow because the viscosity effects introduce loss of mechanical and thermal energy (transferred to the aircraft skin) due to the skin friction.

    (Incidently Daniel Bernoulli's father Johann [born 1667] was the mathematician who first adopted the symbol 'g' for the acceleration due to gravity.)

    Stagnation pressure
    Another aspect of Bernoulli's principle is that the constant is the stagnation pressure — the pressure energy needed to halt the airflow — thus it can be written ½rv² + P = stagnation pressure. The stagnation pressure is the highest pressure in the system. This application of the principle is apparent in the air speed indicator, as demonstrated below.

    Stagnation pressure is the basis of the ram-air parachute wing used in sport parachutes, paragliders and powered parachutes, see 'The ram-air parachute wing'. Also aircraft fuel-tank vents face into the airstream and rely on stagnation pressure to prevent inflight siphoning of fuel from the tank.

    The continuity equation
    There is another principle of aerodynamic interest to us — the fluid flow continuity equation — which states that, in a steadily moving airstream, the product of density, cross sectional area [s] and speed must always be a constant:

    r × s × v = constant

    If there is no change in density within the flow (which is the norm in the airspeed range of light aircraft; see compressibility effects) then we can state that:

    s × v = constant

    Thus, if air flows into a smaller cross-sectional area speed must increase to maintain the constant. Bernoulli's principle states that if speed increases, static pressure must decrease; so the velocity of a constricted airstream increases and its static pressure decreases.

    Both the above principles are related to the conservation laws; Bernoulli's principle to the conservation of energy, and the continuity equation to the conservation of mass. We will examine these properties of air further in the 'Aerofoils and wings' module.

    The venturi effect — used in carburettors, the total energy variometer and the airframe-mounted venturi that provides suction for some flight instruments — is an application of the principles stated above.

    3.5 Measuring airspeed
    The dynamic pressure of the airflow, in N/m², is represented by the expression ½rV², where:
    r is the ambient density of the air [kg/m³ ]
    ( ro is the symbol for the standard sea-level density of 1.225 kg/m³ ) V² is the aircraft (or free airstream) speed [m/s²]  
    and we can deduce that the apparent speed of the airstream is related to air density and dynamic pressure.
    *Note: a lower case v is the symbol for speed in physics while an upper case V is generally the symbol for the free stream speed in aerodynamics, which is why I have used the lower case v in section 2.4 but an upper case V in this section and most of this flight theory guide.)

    We can measure the dynamic pressure with a simple mechanical pressure gauge. Imagine a 6 mm internal diameter aluminium tube positioned under the wing of a moving aircraft, outside the slipstream, so that the open end points forward into undisturbed airflow and the other end of the tube terminates within a spring-loaded, flexible capsule — similar to that in an aneroid barometer — thus the capsule stops the airflow within the tube.

    The back pressure, applied by the capsule to stop the airflow, must be equal to the stagnation pressure. The capsule is contained within a casing which, in turn, is connected to a static vent that supplies the casing with the ambient atmospheric pressure; or, in a lower-quality system, the casing may just be open to the atmospheric pressure within the fuselage.

    So, if we have stagnation (or impact pressure or ram-air pressure) — which is dynamic pressure plus static pressure — within the capsule and static pressure surrounding it, the capsule will expand or contract to reflect the changes in dynamic pressure at the mouth of the tube. (The system is a 'pitot tube' devised by Henri Pitot (1695-1771). During World War I, the airspeed indicating instruments themselves were called 'pitots'.) The capsule movement is mechanically or electrically linked to rotate a pointer on a dial. Although the dial is calibrated and marked to indicate airspeed in knots or mph rather than hPa, it is still basically a simple pressure gauge and an imperfect airspeed gauge. Because the instrument cannot determine the density component of the dynamic pressure, the calibration assumes a constant air density of 1.225 kg/m³. This cockpit instrument is then an airspeed indicator [ASI] and it displays the indicated airspeed [IAS], based on ISA conditions. The indicated airspeed is not the actual aircraft speed through the air, 'V' in the equations. A bit confusing — but brace yourself, for it gets worse!
    We can calculate the dynamic pressure for the Jabiru using the scenario in section 1.4 for calculating CL; i.e. cruising at 6500 feet, true airspeed 97 knots or 50 m/s, and air density 1.0 kg/m³. The ISA atmospheric pressure at 6500 feet is about 800 hPa.

    • static pressure = 800 hPa
    • dynamic pressure = ½rV² = ½ × 1.0 × 50 × 50 = 1250 N/m² = 12.5 hPa
    Note that the dynamic pressure at 1250 N/m², or 12.5 hPa, is less than 2% of the static pressure, but applying that dynamic pressure over the 8 m² of wing area and the lift coefficient of 0.4; i.e. 1250 × 8 × 0.4, still gives the lift force of 4000 newtons that we calculated in the 'Lift' section.

    The airspeed 'V' in the equations is the true airspeed [TAS] — the free stream speed or the air distance flown over time. We know that the ASI dial is calibrated assuming a fixed air density of 1.225 kg/m³ [ro ], so a perfect ASI will only indicate the real airspeed (the true airspeed) when the actual environment density is 1.225 kg/m³; that could only occur when the aircraft is operating at low altitude.
    What will be the IAS (Vis) of our example in the preceding box?

    IAS = Vis = V / √(ro / r)

    = 97 divided by the square root of 1.225 divided by 1.0 = 97 / 1.1 = 88 knots
    From this we can deduce that a perfect ASI will generally underread. The IAS will always be less than the TAS, except in very cold conditions at low altitude where the air density may be greater than 1.225 kg/m³. For instance, using the equation of state above, if temperature was –3 °C and pressure was 1030 hPa, the density would be 1.33 kg/m³.

    Density is about 1% less than (greater than) the ISA value for each 3 °C that the temperature is above (below) the ISA value.

    3.6 Indicated and calibrated airspeed
    The ASI as an angle of attack indicator
    So, you might ask, what's the point of an ASI that really is indicating just dynamic pressure, and is unlikely to indicate your real airspeed — air distance flown over time — accurately? Well, admittedly it does mean a little more calculation to be done in navigation, but there are very significant advantages with an instrument that displays IAS rather than TAS.

    This will be covered in the 'Aerofoils and wings' module. Generally, for all angles of attack in unaccelerated flight at a particular weight, there is a corresponding IAS; though the relationship between aoa and IAS does get a bit fuzzy near CLmax. So, in the absence of an angle of attack instrument, the ASI can generally* be regarded as an indication of angle of attack if the lift being produced matches aircraft mass. Also, all the performance parameters (the 'numbers') for an aircraft — best rate of climb, best angle of climb, best glide angle, etc. — require it to be flown at a particular aoa for that weight, and thus a particular IAS. Or more accurately, a particular calibrated airspeed [CAS] and that particular CAS does not change with altitude (as TAS does), but changes only with weight.

    *The reason why CAS does not always correlate to aoa in level flight is that when inertia and random displacement forces — atmospheric turbulence — come into play, aoa may change momentarily without a noticeable change in CAS.

    (Note: there are means of audibly conveying the angle of attack to the pilot. The simplest is a stall warning horn operated by a basic two-position vane switch incorporated in the leading edge of the wing and switched on by the airstream pressing the vane up if the aoa comes close to the stalling aoa. There are other airstream direction detectors (ADDs) that provide a range of warning tones in the pilot's headset.)

    Calibration of the aircraft's airspeed indicator system
    An ASI is an imperfect mechanical instrument which is subject to instrument errors; the poorer the quality of the instrument the greater the instrument error; the permissable limit for a certified ASI is ±one knot. The associated pitot/static and tubing system is also prone to pressure sensing errors due to the positioning (and design) of the the static air vent —and possibly the pitot head — relative to the airstream. Relative airflows change as aoa changes and when slipping or yawing. It is difficult to find a site on the fuselage for a static vent where the static pressure reflects the ambient static pressure. So after construction is complete the aircraft's airspeed indicating system must be 'measured' to determine the rectifications to be made to the airspeed reading indicated on the ASI.

    CAS is the airspeed after you have applied corrections to the IAS for those instrument and position errors occurring at that aoa in that particular aircraft. The measured corrections should be stated on a card placed near the ASI. You should also be aware that position errors may be quite significant, possibly under-reading by 10 knots or so — particularly at high aoa or when the pilot is maintaining a significant sideslipping manoeuvre. CAS may also be known as the rectified airspeed [RAS].

    Below is an airspeed correction card for a particular aircraft in balanced level flight; i.e. not slipping or skidding. The normal cruise speed for this aircraft is around 95 knots. In this particular installation the ASI significantly underreads at low speeds and overreads at high speeds.
    IAS knots 42 52 61 69 73 87 96 104 113 122 130 CAS knots 49 57 64 71 73 86 94 102 110 117 125
    Converting CAS to TAS
    TAS = CAS × √(ro / r)

    Using our example, from section 2.5, of the Jabiru cruising at 88 knots IAS at 6500 feet, where the air density is 1.0 kg/m²:
    TAS= 88 × √(1.225 / 1.0) = 88 × 1.107 = 97 knots

    We need TAS for navigation and as the density lapse rate just about follows a straight line below 10 000 feet – there is a simple mental calculation to determine TAS from CAS:
    Rule of thumb #2
    "To convert CAS to TAS, multiply the (density) altitude, in 1000s of feet, by a factor of 1.5 to get the percentage increase to apply."
    (For calculation of density altitude see 'High density altitude: effect on take-off/landing performance'.)

    e.g. CAS = 88 knots at 6500 feet = 6.5 x 1.5 = 10% = 97 knots. The multiplication factor increases slightly with increasing altitude, reaching 2 at 30 000 feet.
    The airspeed indicator
    You will note the green and white peripheral arcs, and other colour marks, on the face of this instrument. These are standard markings, some of which should appear on the face of every light aircraft ASI, as they display the speed constraints applicable to the aircraft operations.
    The white arc indicates the flaps operating range starting, at the lower end, from the indicated airspeed, Vso [55 knots], at which the aircraft will stall in the landing configuration with flaps fully extended, and with the throttle closed. The top end of the white arc indicates the maximum speed, Vfe [108 knots], at which the aircraft's flaps can be extended, or remain extended, without causing strain. The bottom end of the green arc indicates the stalling speed of the aircraft, Vs1 [62 knots], with flaps (and landing gear if applicable) up, throttle closed and 1g load factor. The top end of the green arc indicates the maximum structural cruise speed, Vno [173 knots]. The green arc indicates the designed range of speeds for normal operations. The yellow arc indicates a speed range in which the aircraft may be flown, but with caution and only in smooth atmospheric conditions. The red line at the top end of the yellow arc indicates the speed, Vne, that should never be exceeded because of risk of structural damage.

    The other red mark at 70 knots, and the blue mark at 88 knots, are of no interest for single-engine aircraft; these markings only appear on a twin-engine aircraft ASI and relate to operations with one engine shut down.

    A properly functioning ASI responds rapidly to pressure changes because there is no instrument lag. A slow response attributed to instrument lag is most likely only due to the inertia of the aircraft — when attitude in pitch is changed, an aircraft takes a little time to accelerate/decelerate to the appropriate airspeed.

    Airspeed summary
    True airspeed [TAS] = V in the dynamic pressure equation and other expressions = air distance flown over time.
      Indicated airspeed [IAS] = Vis = airspeed displayed on the cockpit airspeed indicator [ASI] — based on a fixed air density ( ro ) of 1.225 kg/m³. The ASI only indicates true airspeed when ambient atmospheric density is actually 1.225 kg/m³ and the system error corrections are applied.
      Calibrated airspeed [CAS] = IAS adjusted (mentally from an airspeed correction card) for known system errors occurring within the normal speed range.  
    Electronic ASI
    Electronic flight instrument systems [EFIS] use solid state electronic componentry as sensors plus software to display flight data on a single screen. In such systems, the static and dynamic pressures are fed to pressure transducers which sense and convert pressures to voltages that the electronic circuitry converts to an airspeed display. See the liquid crystal primary flight display of the Dynon D10A light aircraft EFIS. The EFIS has an outside air temperature probe and, with static pressure, the software can calculate air density and thus display TAS when needed.

    Electronic ASIs are also available as single panel instruments or possibly combined with an altimeter function. The electronic systems are still subject to much the same errors as a mechanical system, and the IAS has to be corrected for CAS unless there is a means for incorporating some form of compensating table into the software.

    3.7 Measuring rate of ascent/descent
    The vertical speed indicator
    In flight it is important for a pilot to know the rate at which the aircraft may be ascending or descending. A simple vertical speed indicator [VSI] is a pressure gauge that measures the rate of pressure change as an aircraft is ascending or descending. The instrument display is usually calibrated in feet per minute but it may be in metres per second.
    There are two pressure inputs, both from the static vent system — one to each side of a flexible diaphragm or capsule. On the open side there is a normal input that reflects the static pressure change as it occurs. On the closed side the input/output is a fine capillary tube that slows the equalising pressure change — and also the response time of the instrument. The resultant deflection of the diaphragm is magnified via a geared mechanical linkage to a dial pointer, which indicates whether the aircraft is maintaining altitude (in which case, the pressure on both sides of the diaphragm is equal), climbing or descending, and the rate, usually graduated in feet [×100] of altitude per minute. Some form of vibration damping and thermal change compensation is included within the VSI and ASI instruments.

    The pressure change has two components, the most significant component is that brought about by the aircraft's rate of height change, as in climbing or descending. The other part is any vertical movement of that part of the air mass in which the aircraft is operating — rising air or sinking air. When an aircraft is climbing, rising air adds to the rate of change, sinking air reduces it. When descending, rising air decreases the rate of change, sinking air adds to it.

    The variometer
    In soaring flight, paraglider, hang glider, sailplane and power-assisted sailplane pilots are totally reliant on finding sources of atmospheric uplift to gain the gravitational potential energy that enables the aircraft to stay airborne for sufficient time to complete the flight plan. A variometer (usually abbreviated to vario) is a specialised vertical speed indicator that enables a pilot to derive the vertical speed of the parcel of air in which the aircraft is soaring.

    For more information on varios and their uses see the article 'Basic sailplane instruments. The article only refers to fixed installation varios but very light-weight hand-held varios are available for hang glider/paraglider pilots.

    3.8 Stalling airspeeds
    The normal 1g stall
    One of the first questions a pilot might consider, when converting to a new aircraft type, is "What's the stall speed?" The reason for considering this is that usually, but not always, the approach speed chosen for landing is 1.3 to 1.5 times Vso — the minimum steady flight speed in the landing configuration, below which speed the aircraft will stall or at which speed the aircraft will stall if any manoeuvring is attempted.

    In aerodynamic terms, the 'stall' is the sudden widespread separation of the boundary layer from the upper wing surface that occurs when the wing exceeds a particular angle of attack. For light aircraft without high-lift devices, this is usually around 15–16° although minimum aircraft with single-surface fabric wings may have a stall aoa 2–3° lower. This critical angle of attack has no relationship with either the aircraft attitude relative to the horizon or the airspeed — it can readily be reached in a high-speed dive. But it does have a direct relationship with elevator position and thus the control column position.

    The separation of the boundary layer starts at the wing trailing edge, generally near the wing root for approximately rectangular wings (and particularly for wings with 'washout'), spreading forward and outward over the upper surface until there is a significant detachment of boundary layer flow over the upper surface. There is probably little change to the under-surface boundary layer flow. Between the two remnant boundary or shear layers, a thick turbulent wake will attach to the wing and be dragged along by the aircraft. The reaction to the acceleration and energising of that wake is a sudden deceleration of the aircraft accompanied by a large increase in the nose-down pitching moment plus some loss of lift. The initial wake turbulence ('burbles') near the wing root may initiate unsteady flow over the tailplane, shaking the tailplane and thus providing a few warning buffets felt in the airframe or smaller 'nibbles' felt in the controls — aerodynamic warnings of an impending stall. There also may be 'oil-canning' noises from pressure changes on metal-skinned fuselages and wings as the thin metal flexes in response to pressure changes. On the other hand, there may be no pre-stall warning whatsoever.

    Some aircraft exhibit undesirable characteristics even before boundary layer separation occurs; for example, the aircraft starts sinking excessively with increasing induced drag or wing rocking occurs. So, from a pilot's point of view, a stall is "the point following deceleration at which the pilot ceases to have full control over the aeroplane"; which adds the concept of the defined stall speed being a minimum controllable steady flight speed at which no undesirable characteristics are exhibited.

    The next comments are specifically aimed at stalls induced when:
    • flying straight and level at slower speeds
    • in a low speed descent — such as the approach to landing
    • in a climb — such as the initial climb after take-off
    • in a go-around following an aborted landing approach.

    The last two circumstances are sometimes referred to as full-power stalls or 'departure stalls'. In non-turbulent atmospheric conditions, and if the aircraft is in balance, all of the circumstances above can only induce a stall if the control column position is placed in, trimmed into or allowed to move into, the last half of its rearward travel. Many aircraft are designed so that the control column must be at or near the limit of its rearward travel to reach the stalling aoa. (The rearward travel range commences from the neutral position, as does the forward travel range.)

    Because of the airflow turbulence and increasing induced drag as the aoa is increasing, total drag increases and the aircraft slows as it approaches CL max. The rapid reduction in airspeed after passing the critical aoa means the wing is now unable to provide sufficient lift to totally balance weight and, in a normal stall, the aircraft starts to sink. The (possibly pronounced) nose-down pitch will occur even though the control column is near its rearward travel limit. However, some aircraft may not assume that nose-down attitude but just sink (mush down) at quite a high rate and at an extreme angle of attack. Because of the nose-up attitude, the high rate of descent may not be apparent unless the aircraft is close to the ground.

    The aircraft is instantly recovered from the stall by smoothly reducing the aoa so that it is below the critical aoa; i.e. easing the control column forward and generally no further than the neutral position. If one wing stalls before the other, that wing will drop. In this case, the control column must be firmly moved sufficiently forward to unstall the dropping wing, the wings levelled with aileron then sufficient rudder applied to stop further yaw. An increase in speed is also needed (by increasing power or holding a lower nose attitude for several seconds) so that a safe flight speed is achieved quickly without wasting much altitude and the aircraft is returned to the intended flight path. See standard recovery procedure for all stall types.

    If the control column movement for stall recovery is both excessive and abrupt, the result could be an aoa movement below the zero lift aoa — in which case there will be a reversed lift force on the wings, which hinders recovery. Weight-shift controlled trikes do not react well to negative g; if this is excessive, the wing spars may buckle at an outer position.

    Many aircraft are designed so that the nose will drop at the stall, but the aircraft will self-recover (i.e. without pilot intervention) in a stable descent or with some oscillations which, if the control column is still held back, will result in another stall. Some aircraft may be designed so that the wing is usually not able to reach the stalling angle, but the aircraft will enter a semi-stable mushing descent — which sounds fine but can be disastrous if the pilot doesn't notice when close to the surface. A normal stall occurs when the load factor is close to normal; i.e. near 1g.

    The cg position will also affect the manner of stall. If the cg is at the extreme forward limit, some aircraft may not fully stall — just mush down. If it is too far aft, the stall aoa can be reached with a much smaller rearward movement of the control column. Another factor affecting the manner of stall is the use of power. Generally, when flying slowly, the longitudinal axis of the aircraft is pitched up relative to the flight path. Consequently the thrust vector will include a vertical component — a lifting force — and the amount of lifting force provided depends on the amount of thrust. Also, for aircraft with the propeller mounted in front of the wings, the energy in the slipstream tube in slow flight increases the velocity of the airstream over part of the wing (depending also on the mounting of the wing in relation to the thrust centre line) and reduces the aoa of that part. Thus the completely stalled wing may occur at a lower speed, depending on the amount of power in use. When it occurs, the stall will be much more pronounced, possibly with a fast-acting wing drop. There are other complications because the slipstream also affects parasite drag and induced drag.

    Many pilots, in suitable aircraft and atmospheric conditions, prefer to land by approaching at 1.3 to 1.5 times normal stall speed — Vso — and, after flaring with the throttle closed, holding the aircraft just above the surface; preventing it touching down by smoothly increasing CL as drag decreases V², thus maintaining constant lift until CLmax is reached. At this point, the aircraft can no longer be 'held off' and it gently sinks the short distance onto the runway, touching down in a nose-high attitude.

    The acceleration or accelerated stall
    It is misleading to talk about stalling speed without further definition. The stall occurs at a particular aoa, not a particular speed. The speed — Vs — below which the stall will occur depends on the load factor. If the aircraft reaches the critical aoa under a load higher than 1g, the stalling speed will be higher than the normal 1g stall speed, at that mass. This latter stall is called an acceleration stall or accelerated stall and is usually more pronounced than a normal stall. The load factor normally increases in a turn— as we saw in section 1.10 where we calculated that, in a 45° banked turn, the load factor was 1.41 times normal. Thus, when turning, the stalling speed is higher than normal and the pilot must maintain a reasonable airspeed margin above that accelerated stall speed throughout the turn. See the table below.

    Be aware that the airspeed at which an acceleration stall in a turn occurs is only indirectly associated with the angle of bank; it is directly brought about by the increase in load factor. Indeed, it is possible to have the aircraft banked at 60° with a stall speed less than Vs1 if the wings are 'unloaded': slight forward pressure on the control column, and the aircraft allowed to sink, produces a load less than 1g — maybe 0.8g — with a stall speed less than Vs1, even though the aircraft is steeply banked. However, once the 'unloaded' condition ceases — if the stalling angle of attack has been passed (either by the rearward movement of the control column or a gust momentarily changes the relative airflow) — the probability of a stall returns immediately.

    The speed at which an accelerated stall occurs is proportional to the square root of the load factor — in the lift equation the airspeed is squared. If that load factor is expressed relative to the normal load, e.g. 2g, then the stall speed at that load factor — Vs 2g — equals the square root of the load factor × normal 1g stall speed; e.g. square root of 2 = 1.41 × Vs.

    The aircraft's momentum may also contribute to an accelerated stall, particularly when the aircraft is diving at speed and the pilot applies a harsh rearward control column movement. This will have the initial effect of rotating the aircraft about its lateral axis while inertia momentarily maintains the aircraft on its pre-existing flight path; thus the aoa may exceed the stalling aoa (even though the control column has not been pulled back to the normal stall position) with a consequent, and rather violent, high-speed stall.

    An acceleration stall can also be produced when:
    • the control column is jerked back whilst the aircraft is climbing or in level flight; see the flick roll
    • an aircraft in level cruising flight encounters a strong vertical gust
    • an abrupt change in flight path is made, which applies acceleration loads
    • an excessive bank angle, coupled with excessive control column back pressure, is applied during a level, climbing or descending turn.
    Note: The US Federal Aviation Regulations Section 23.203 airworthiness standards define accelerated stalls somewhat differently from the above, only referring to 'turning flight stalls' and 'accelerated turning stalls' for airworthiness demonstrations.

    "Turning flight and accelerated turning stalls must be demonstrated in tests as follows:
    (a) Establish and maintain a coordinated turn in a 30 degree bank. Reduce speed by steadily and progressively tightening the turn with the elevator until the airplane is stalled. The rate of speed reduction must be constant, and--
    (1) For a turning flight stall, may not exceed one knot per second; and
    (2) For an accelerated turning stall, be 3 to 5 knots per second with steadily increasing normal acceleration*."

    * 'Normal acceleration' refers to the aerodynamic force parallel to the aircraft's normal axis.

    Load factor in a turn
    The table below shows the increase in stall speed at various bank angles in correctly executed level turns. The load factor or 'g' = 1/cosine of the bank angle and the Vs multiplier = the square root of the load factor. The table shows that once you reach bank angles of 30° or more, the aircraft stall speed increases rapidly; there is a 7% increase at 30°, 19% at 45° and 41% at 60°.

    Thus, level turns involving bank angles exceeding 20–30° should not be made at low levels, including take-off and landing operations. Even so, the airspeed should be increased to allow an appropriate safety margin — for gentle turns, a safe speed near the ground is 1.5 × Vs.

    The stall speed in a turn = Vsturn = Vs × Vs multiplier. A minimum turning speed at a safe height might be 1.2 × Vsturn. For example, if Vs is 50 knots and the bank angle is 45° then Vsturn is 50 × 1.19 = 60 knots and the minimum safe turning speed at height is 1.2 × 60 = 72 knots, or about 1.45 × Vs.
    Bank angle Cosine Load factor [g] Vs multiplier 10° 0.98 1.02 1.01 [+1%] 20° 0.94 1.06 1.03 [+3%] 30° 0.87 1.15 1.07 [+7%] 40° 0.77 1.30 1.14 [+14%] 45° 0.71 1.41 1.19 [+19%] 50° 0.64 1.56 1.25 [+25%] 54° 0.59 1.7 1.3 [+30%] 60° 0.50 2.00 1.41 [+41%] 70° 0.34 2.94 1.71 [+71%] 75° 0.25 4.00 2.00 [+100%]
    Note that the stall speed increases exponentially with bank angle; i.e. the 10° increase in the bank angles between 20° and 30° increases stall speed by another 4%, while the 10° increase in the bank angle between 50° and 60° increases stall speed by a further 16% (i.e. four times as much), while between 60° and 70° the stall speed is increased by a further 30%. Consequently aircraft certificated in the normal category are limited to a turning angle of bank of not more than 60°.

    Note that at an approach speed of 1.3 × Vs the aircraft will stall if turning with a 54° bank. The limits on climbing and descending turns are discussed in the 'Safety: control loss in turns' module.

    The torque stall
    For high-performance aircraft, with a very high power-to-weight ratio, the possibility of a torque stall exists. The most likely scenario is a sudden application of full power in a 'go-around' following an aborted landing, where the airspeed has been allowed to decay below the safety speed. The torque of the engine and inertia of the heavy propeller tends to twist the aircraft around the propeller shaft, and the consequent roll may increase the aoa of the downgoing wing past the critical aoa. If that happens, the wing loses lift, which accelerates the roll and the aircraft loses height very rapidly. However, torque stalls are probably not applicable to light aircraft, although the torque effect may influence the characteristics of a stall in a climbing turn.

    Effect of weight
    If the aircraft is below its MTOW, the operating wing loading will be less than the design W/S and the stall will occur at a lower speed than that marked on the ASI.
    For example, if we refer to the Jabiru, the wing area is 7.9 m², MTOW is 4200 N, Vso is 40 knots CAS and we can calculate that CL with flaps fully extended is 2.0.

    We saw above in the section 'The acceleration or accelerated stall' that W/S at the stall = CL × ½rV².
    We will rearrange that and say Vs² = (W/S) / (CLmax × ½r). Substituting the values, including 1.225 for density, we get:

    Vs² = (4200/7.9) /(2.0 × 0.5 × 1.225) = 532/1.225 = 434 m/s and Vs = 20.8 m/s = 40 knots CAS

    Now what will Vs be when the Jabiru with no passenger on board is at the low weight of 3400 N? Well, substituting that weight we get:
    Vs² = (3400/7.9) /(2.0 × 0.5 × 1.225) = 430/1.225 = 351 m/s and Vs = 18.7 m/s = 36 knots CAS.
    There are other, somewhat simpler, ways to calculate the reduction in Vs corresponding to a reduction in weight but what we see above is that a reduction in weight of 800 N, or about 19%, reduces Vs by 4 knots, or about 10%. This brings us to the mathematical rule of thumb that when two values are not that far apart in percentage terms, say up to 40%, their square roots are about half that distance apart in percentage; and because aerodynamic pressure is proportional to V², there are many occasions where the square root of a value is relevant. This allows a simple, but reasonably accurate, mental calculation:
    Rule of thumb #3
    "The percentage reduction in Vs is half the percentage reduction in weight."

    i.e. If weight is reduced by 10% from MTOW then Vs will be reduced by 5%, and conversely, if weight is 10% over MTOW then Vs will be 5% higher — one of several reasons to avoid overloading an aircraft. (There is further discussion on weight control throughout these notes.)
    Thus in the section 'The acceleration or accelerated stall' above, where we referred to unloading the wings with the aircraft banked at 60°, the load reduction from 1g down to 0.8g is 20% so the unloaded stall speed would be about 90% of Vs1. You can also see the same relationship in the preceding table; for bank angles up to 45° the percentage increase in Vs is about half the percentage increase in W/S.

    It is appropriate to mention here that it is not only aircraft weight/wing loading that affects the stall speed. Some of the other critical performance values are also achieved at a particular aoa, and the associated airspeeds are also changed by a change in weight. The same rule of thumb applies to them. These critical performance values (the 'numbers') are: best rate of climb speed, best angle of climb speed, lowest power-off sink rate speed, best glide ratio speed and manoeuvring speed.

    Another aspect we will look at in the 'Aerofoils and wings' module is the effect of flaps, but we will just state here that flaps provide an increased CL at all angles of attack consequently allowing a reduction in V² and the stalling speed. In some aircraft extending flaps also increases wing area, thus W/S is reduced, a handy technique for high-performance military aircraft, manoeuvring at maximum allowed wing loading — they can tighten the turn even further without breaking the aircraft.

    The final aspect of the stall is the effect of atmospheric turbulence on aoa and this affects 'manoeuvring speed'. We will look at it in the 'Wind shear and turbulence" tutorial.

    3.9 V-speeds
    Airspeed codes
    It is important to have a simple, easily understood and universally accepted identification method for the various airspeeds at which an aircraft may be operated, but currently it's a bit messy and there is no complete, unambiguous, and universally recognised, airspeed designation system published by any regulatory authority.

    Current nomenclatures are generally made up of two to six letters/numbers, with the first being V. Some of these V-speed codes — applicable to single-engine aircraft — with alternatives and definitions are shown below. These are relevant to sport and recreational aircraft including low momentum ultralight aircraft, and might appear in flight manuals, pilot's operating handbooks and even sales literature but those indicated with open bulleting º are probably only applicable to a few very light aircraft types.

    There are two classes of airspeed codes. One is the structural design speeds (specified in national airworthiness requirements) used in determining the airframe and control surfaces strength requirements for type certification. Such speeds include the term 'design' in their description. The other class are the designer recommended operating speeds.

    Please be aware that the various 'best' performance speeds mentioned below — rate of climb, angle of climb, cruising range, gliding range, etc. — merely indicate the midpoint in an airspeed range extending perhaps 1–2% either side of that point. Also, the performance speeds are very much affected by the horsepower of the particular engine fitted, plus the type of propeller and its pitch setting. If there is no pilot's operating handbook for the particular airframe/engine/propeller configuration, then the pilot must calculate the performance speeds by trial and measurement.

    Critical limiting speeds
    • Va — design manoeuvring speed. Design rules state that the minimum acceptable design manoeuvring speed is a fixed calculation relative to Vs1 for all aircraft within the same category. For a 'normal' category light aircraft (whose certificated load limit factor in the pitching plane is +3.8g), minimum Va = Ö3.8 Vs1, or 1.95 × Vs1. For a 'LSA' category aircraft (whose certificated vertical load limit factor is +4g), minimum Va = Ö4 Vs1, or 2 × Vs1. If the designer has opted for a design manoeuvring speed that is greater than the minimum acceptable speed then a Vo operating manoeuvring speed must also be specified.

    Va is also known as the 'optimum manoeuvre speed', or the 'corner speed', to military pilots as it's at the intersection of the structural limit load factor and the maximum aerodynamic force curve (the 'A' corner) in the aircraft's manoeuvring flight envelope, i.e. Va is the speed at which an aircraft can make the tightest possible turn (the minimum radius turn) and the fastest rate of turn by applying the aerodynamic limit (maximum aoa [CL max]) and the structural load limit simultaneously. (Of course, in the military context, in such a turn the aircraft would be comparatively low in kinetic energy.) In the sailplane context the symbol Vm is used for a manoeuvring speed which is the product of the square root of the design load limit factor and the minimum flight speed.

    Va is sometimes referred to as the 'speed for maximum control deflection' which has been the cause of much confusion. It is unwise to make full or abrupt applications of any one primary flight control if you are flying at a speed greater than Va, because at higher speeds it is easy to apply (see the stick force gradient) aerodynamic forces that could exceed the aircraft's structural limitations. But, even when flying below Va, it is unwise to make rapid control reversals or 'checks' such as alternating heavy applications of rudder or suddenly apply heavy asymmetric loads, e.g. heavy application of elevator and rudder or aileron; see the flick roll.

    (That misleading term, 'speed for maximum control deflection', was subject to much debate in 2001 following the horrific crash of American Airlines Flight 587, an Airbus A300 which, shortly after take-off while in a climbing turn at an airspeed 20 knots below standard Va, ran into wake vortices from a Boeing 747 four miles ahead. To counter sideslip it appears the pilot flying employed four nearly full-rudder reversal movements within a seven-second period. Those pilot-commanded side forces induced a tail fin load twice the design load limit and 1.3 times the ultimate load limit, at which point the complete fin and rudder broke away. In 2010 the U.S. Federal Aviation Administation issued a 'Maneuvering Speed Limitation Statement' namely:

    (i) Full application of pitch, roll, or yaw controls should be confined to speeds below the maneuvering speed; (ii) Rapid and large alternating control inputs, especially in combination with large changes in pitch, roll, or yaw, and full control inputs in more than one axis at the same time, should be avoided as they may result in structural failures at any speed, including below the maneuvering speed. )

    Va is usually not marked on the ASI but there should be a placard indicating the MTOW manoeuvring speed on the instrument panel near the ASI or in the Pilot Operating Handbook or Aircraft Flight Manual; if not available, you can assume it's twice maximum weight Vs1 for non-aerobatic light aircraft and reduces as aircraft inertia (i.e. weight or, more properly, mass) reduces and thus Vs1 decreases.

    Note: if a recreational pilot is foolish enough to operate an aircraft at a weight exceeding MTOW then the overloaded aircraft's stall speed will be higher; consequently Va in that condition will be higher.

    So Va is not a fixed documented value, it decreases as the aircraft's weight decreases from MTOW, because the effects of the atmospheric forces become more pronounced as its inertia decreases. Sometimes the aircraft's documentation will specify the Va for weights below MTOW but it may be left up to the pilot to calculate the Va for the current aircraft weight. Using rule of thumb #3 above, the reduction in Va will be half the percentage reduction in aircraft weight; for example if, with only the pilot on board, weight is 16% below MTOW then Va is reduced by 8%. Flying at speeds below that estimated Va in turbulent conditions also enhances the possibility of stalls induced by vertical gusts — and also may reduce aileron and rudder effectiveness, so the pilot in command must be careful to select an optimum speed for the atmospheric conditions. Also note that the documented Va is calculated for the aircraft in a clean configuration — it does not apply to flight with the flaps extended, see Vfe maximum flaps extended speed.

    Misuse of controls in light aircraft can generate structural loads greater than those encountered in turbulence, so Va is also useful as a 'turbulent air operating speed' and most recreational aircraft operating handbooks recommend that airspeed be reduced to Va in turbulent conditions. When flying above this speed, gust-induced loads can exceed the design limit of many structures within the aircraft, including the pilot's seat and the engine mount, possibly even the crankshaft. Gust loads in the high temperature conditions of the Australian tropical continental air mass can be extremely high. Va is the recommended indicated cruising speed (CAS) when flying in moderate turbulence — intermittent, uncomfortable jolts. At this compromise speed, the aircraft will generally produce an accelerated stall and thus alleviate the aerodynamic force (including any manoeuvring forces) on the wings and other structures, if it encounters a vertical current that imparts an acceleration sufficient to exceed the load limit factor. Read 'The speed to fly in turbulence', in the 'Decreasing your exposure to risk' guide.

    • Vo — operating manoeuvring speed. If the aircraft designer has specified a design manoeuvring speed that is greater than the regulatory minimum (Ön × Vs1 where 'n' is the category limit load factor) then, when flying at Va and if a substantial nose-up pitching manoeuvre is applied, the aircraft may exceed the limit load factor before stalling. So, an operating manoeuvring speed Vo should also be established as an operating limitation speed, which is a selected speed that is not greater than Ön × Vs1 and is a speed where the aircraft will stall in a nose-up pitching manoeuvre before exceeding the structural load limits. Thus Va must be equal to or greater than Ön × Vs1 while Vo must be equal to or less than Ön × Vs1. The load limit factor is 3.8g for normal category sport and recreational aeroplanes and 4g for the LSA category. The square roots of 3.8 and 4 are very close — 1.95 and 2 respectively — so we can rephrase the preceding statement as 'Va must be equal to, or greater than, twice Vs1 while Vo must be equal to, or less than, twice Vs1. Remember that Vs1 stall speed reduces when aircraft weight is less than MTOW so the appropriate Va indicated air speed will decrease in proportion to the decrease in aircraft weight. Flying at speeds much below Va in turbulent conditions also enhances the possibility of stalls induced by vertical gusts — and also may reduce aileron and rudder effectiveness.

    • Vb — design speed for maximum gust intensity. The applicable vertical gust intensities range from 25 fps (7.5 m/s) to 50 fps (15 m/s). Also known as the maximum rough air speed. It is not required to be specified for normal, utility and LSA aeroplanes (in those categories Vb would generally only differ by a few knots from Va). Vb is specified in the European Joint Airworthiness Requirements JAR-22 for sailplanes and powered sailplanes in the utility and acrobatic categories but in this case Vb is the speed at which the sailplane is able to withstand a strong vertical gust of 50 fps (15 m/s or 30 knots) without exceeding the load limit factor, i.e. it is the speed at which an encounter with a gust of the specified value produces CL max.

    • Vfe — maximum flaps extended speed. It is indicated by the top end of the ASI white arc. Flight with flaps extended — or extending flaps — above this speed may result in distortion of the flaps or their supporting structure and extension mechanism. Various Vfe speeds may be specified according to the available flap settings. Generally speaking, the flight load limit factor is reduced by about 50% when flaps are fully extended. For example, the aircraft flight manual of a 'normal' category light aircraft will probably note that the load limit factor is reduced from 3.8g to 2g. So, extending some flap in turbulent conditions will decrease the stall speed but will reduce the load limit factor; thus it's very much a 'pilot-in-command' decision of 'when' and 'how much' to use, but certainly full flap is most unlikely in any condition.

    • Vno — maximum structural cruise speed or 'normal operating limit', indicated by the top end of the ASI green arc. Flight above Vno should only be conducted cautiously and in smooth air, while the pilot should not apply any abnormal control inputs when cruising at, or above, Vno. Vno must be equal to or greater than Vc (below, in section 'Cruise speeds'), but in most light aircraft Vno and Vc are assumed synonymous. When cruising at, or below, Vno, the aircraft should not be damaged by a 30 feet/second vertical gust — which is at the bottom end of the moderate to strong vertical gust scale of 25–50 feet/second vertical gusts; read 'The speed to fly in turbulence'.

    • Vne — never exceed speed, which is the IAS that should never be intentionally exceeded in a dive, or other manoeuvre in smooth air. FAR 23 requires that Vne be not more than 90% of the design diving speed Vd or the flight-demonstrated diving speed Vdf. Vd and Vdf are not included in pilot operating documentation — they are the realm of the test pilot. Vne is indicated by the red line at the top end of the ASI yellow arc. For light aircraft operating below 10 000 feet, it can usually be assumed that Vne is a fixed IAS. If aircraft have high altitude capability or particular airframe vibration characteristics, it is possible that the designer will specify Vne as a TAS, above a particular altitude and for various altitude bands of perhaps 3000 feet. If Vne varies with altitude, then FAR Part 23.1545 (c) requires a placard next to the ASI indicating the appropriate IAS limitations throughout the aircraft's operating altitude range. This particularly applies to sailplanes and powered sailplanes whose very high aspect ratio wings are candidates for flutter problems and such low drag aircraft can build up airspeed at altitude very quickly in quite shallow dives. For expanded information see 'Don't fly real fast' in the 'Decreasing your exposure to risk' guide.

    • Vs1 (sometimes incorrectly shown as Vsi) — stalling speed, or the minimum steady flight speed, in a specified flight configuration. For a simple aircraft, Vs1 is normally measured in level flight with flaps up, at MTOW and 1g wing loading, with engine idling following a gradual deceleration (one knot per second) — accompanied by increasing rearward movement of the control column — to that minimum flight speed. It is indicated by the bottom end of the ASI green arc, but it should be documented as both IAS and CAS; if CAS is not mentioned the quoted stall speed is probably inaccurate. Vs1 decreases as the aircraft weight decreases from MTOW, which also means that if the pilot can reduce the wing loading below 1g — by an 'unloading' manoeuvre — Vs1 is decreased. Stalling speed under a 2g wing loading, for instance, might be referred to as Vs2g.

    • Vso — stalling speed, or the minimum steady flight speed, in the landing configuration of flaps down and engine at low or idle power as it would be just prior to touchdown. This is measured using the same method as Vs1 but with the cg at the most extreme position allowed — usually the most forward position where backward movement of the control column may be limited. It is indicated by the bottom end of the ASI white arc.In the documentation both IAS and CAS should be shown. Like Vs1, Vso decreases as the aircraft weight decreases from MTOW. The designation Vs is used as a general reference to the design stall speed.

    • Vmin — minimum airspeed. Vs is generally specified in powered, rigid-wing recreational aircraft as the minimum speed but for other aircraft categories a 'Vmin' may be specified instead. For example for gyroplanes Vmin is the minimum controllable level flight airspeed below which there is insufficient power available to maintain altitude. For paragliders it is the minimum speed, within the wing's available trimmer range, below which the parawing loses its lift.

    Cruise speeds
    A cruising aircraft is normally flying at a moderate, fuel efficient speed and maintaining the appropriate cruising altitude. The Australian Civil Aviation Regulations hold this definition:

    "cruise phase of flight" means the part of an aircraft's flight:
    (a) that starts when the aircraft reaches its first planned cruise level, ... and
    (b) that ends when the aircraft reaches the point at which the aircraft first starts its descent for the purpose of landing; and includes flight level changes made during that part of the flight.

    • Vbr — best range, or Vmd — minimum drag, is the speed that provides maximum L/D by producing minimum drag and thus the best power-to-speed ratio. This speed might utilise about 55% power and is usually flown at the lowest altitude where the throttle is fully open to obtain that speed. Vbr/Vmd decreases as the aircraft weight decreases from MTOW. It's rather boring to fly at that speed, wind conditions have to be taken into account, and the fuel saving may not be that significant compared to flying at a speed 10% faster. Also, the engine manufacturer's operating recommendations should be followed, but mixture is usually leaned, and minimum rpm set if a constant speed propeller is fitted. Vbr/Vmd has the same basic airspeed range as Vy and Vbg [below].
    There is a difference in concept between Vbr and Vmd. Pilots of low-powered aircraft are generally not interested in the best power to airspeed ratio in cruise; ground speed achieved per litre is far more significant, so in some conditions Vbr equals Vmd but in headwind conditions Vbr is increased. Look at the diagram from section 1.7 at left and note the pink line that has been drawn from the junction of the vertical and horizontal axes tangential to the power required curve. That line just touches the curve at a position corresponding to the minimum drag airspeed Vmd. Now imagine a 30 knot headwind and start the tangential line from a point along the horizontal axis that is equivalent to 30 knots; that (the blue line) will be tangent to the power required curve at a position corresponding to a higher speed — Vbr for a 30 knot headwind. The rule of thumb is to add half the head wind to the basic Vbr, which, in this case, indicates a Vbr that is about 15 knots greater than Vmd. This is the same principle used by sailplane pilots to establish their best penetration speed — see the speed polar curves for optimum glide speed in the 'Coping with emergencies guide'.
    • Vbe — best endurance, or Vmp — minimum power, is the CAS that gives the greatest airborne time per litre (i.e. least fuel flow per hour and, of course, power is proportional to fuel flow), possibly around 80% of Vbr/Vmd, and decreases as the aircraft weight decreases from MTOW. Flight at lowest safe altitude provides best engine performance. Might utilise about 45% power at MTOW. It is the speed for minimum power required for level flight, as shown in the power required curve above.

    Vbe/Vmp is the speed that might be used when flying a search pattern to allow a proper area survey, or when waiting for ground fog to disperse, but it is possibly uncomfortable to fly for long periods at such a low speed. Also the very low power setting may be inconsistent with good engine handling practice. Carburettor icing may be aggravated. The Vmp designation and speed is also used as a power-off glide speed, providing the best endurance — least rate of sink — in the glide; see 'Power-off descent speeds' below. Vbe/Vmp is in the same speed range as Vx — the best angle of climb airspeed.

    • Vc — the design cruising speed or the optimum cruise speed — the latter being the speed that gives the most velocity (i.e. greatest distance/time) from a litre of fuel, usually utilises 75% power at MTOW and is about 20–30% greater than the maximum L/D speed — Vbr. The speed and power required both decrease as the aircraft weight decreases from MTOW. Refer to rule of thumb #3 in section 2.8 'Stalling airspeeds'.

    For normal category aircraft, FAR Part 23 specifies a minimum design cruising speed (in knots) = 33 ÖW/S. For this calculation the wing loading W/S is expressed in pounds per square foot. Many minimum ultralights are unable to comply with the FAR Part 23 design requirement for a minimum design cruising speed.

    For most light aircraft, Vc is synonymous with Vno. FAR 23 Appendix A provides simplified design load criteria and allows designers of many conventional single-engine monoplanes weighing less than 2700 kg to take advantage of the simplification. That same appendix is generally duplicated in the design regulations of most other countries. One advantage is that it is not necessary to specify Vno; instead, Vc is designated in the flight manual as the maximum structural cruise speed (i.e. Vno = Vc) and that Vc may be set at 90% of Vh.

    • Vh — the maximum level flight indicated speed (CAS) attainable at sea-level, utilising maximum continuous engine power. For most engines maximum continuous engine power at sea-level will be less than full throttle power.

    Take-off and landing speeds

    • Vle — for retractable undercarriage aircraft — the maximum indicated speed at which the landing gear can remain extended without risking gear door damage.

    • Vlo — the maximum indicated speed at which the landing gear system can be operated. Vle and Vlo are unlikely to be applicable to most ultralights.

    • Vlof — the lift-off indicated speed for normal take-off. Vlof is about 10% above Vmu.

    • Vmu — minimum unstick speed. This is an indicated speed used in take-off conditions where it is advisable to lift off at the lowest possible airspeed to get the tyres off the surface (e.g. soft field or wet grass ) and safely fly in ground effect until a Vtoss is attained to allow climb-out. Acceleration after lift-off at Vmu is slow, due to the drag at the high aoa, and should not be used as an obstacle clearance technique.

    • Vref — the threshold speed or the reference indicated approach speed. Usually about 1.3 to 1.5 times Vso plus 50% of the wind gust speed in excess of the mean wind speed; e.g. Vso = 30 knots, wind speed 10 knots gusting to 20 knots, Vref = 1.3 x 30 + 5 knots = 44 knots. Faster, heavier aircraft would tend towards the 1.3 times Vso end; lighter, slower aircraft would tend towards the 1.5 times Vso end. Normal landing procedure is to set up the approach so that an imaginary 15 metre (50 ft) high screen placed before the runway threshold is crossed at Vref and the airspeed is reduced to maybe 1.2 to 1.3 × Vso — plus the gust allowance — when rounding out prior to touchdown. The ground distance from the screen to the touch-down point can be roughly estimated, using the 1-in-60 rule, from the approach slope. For example, with a 6° slope — which is around the norm for most light 3-axis aircraft — the distance will be 60/6 × 15 = 150 m. To this must be added any float period plus the ground roll distance with normal braking, to give the total landing distance over the standard 15 m screen — in nil wind conditions.

    • Vtoss — minimum take-off safety speed. This is an indicated speed chosen to ensure that adequate control will still exist during initial climb after lift-off if power is lost or turbulence encountered. After lift-off, the aircraft should be held down and not allowed to climb away until Vtoss is attained.

    CAO 101.28, an airworthiness certification requirement for commercially supplied amateur-built kit ultralights, states in part:
    "The take-off distance shall be established and shall be the distance required to reach a screen height of 50 feet from a standing start, with ... short, dry grass surface ... the aeroplane reaching the screen height at a take-off safety speed not less than 1.2 Vs1 ... take-off charts ... shall schedule distances established in accordance with the provisions of this paragraph, factored by 1.15." Sea-level ISA and nil wind conditions are implied. CAO 95.55 has much the same wording but specifies 1.3 Vs1 as the take-off safety speed.

    (Similarly, CAO 101.28 states that the landing distance will be that to come to a full stop from a screen height of 50 feet, with the screen being crossed at 1.3 Vso and the same conditions as specified for the take-off distance.)

    In normal take-off conditions Vtoss should be somewhere between 1.3 and 1.5 times Vs1, with 'draggy' aircraft tending to the higher value. If power is lost in the initial climb, a draggy aircraft will lose airspeed very rapidly and take some time to regain it even though the pilot reacts quickly and pushes the control column forward. See 'Engine failure after take-off'.

    There is a similar code used for multi-engine aircraft — Vtos — which refers to the minimum speed for climb-out with one engine inoperative.

    Climb speeds
    • Vx — indicated speed provides best angle of climb for obstacle clearance; i.e. to attain height over the shortest ground distance using maximum thrust available. This is probably better described as the precautionary climb speed — the initial climb speed used when there are obstructions off the end of a marginal airstrip or when climbing out of an obstructed valley. Vx decreases as the aircraft weight decreases from MTOW (refer rule of thumb #3 above), but the angle of attack is maintained at around 8–10º with very high induced drag. It is the climb airspeed where the ratio of vertical speed to horizontal (ground) speed is the highest. Vx may be less than or equal to Vtoss. The aircraft's power-to-weight ratio (i.e. excess power) and L/D ratio affect the angle of climb at the designated airspeed.

    However, be aware that the angle of climb will also depend on the low-level wind conditions at the airfield. In a headwind, the climb angle is increased and reduced in a tailwind. Also note that aoa during climb may be only 5 or 6° below the critical aoa, thus care must be taken not to induce a 'departure stall', particularly in turbulent conditions. And remember that Vs1 increases in a turn, so that the small safety gap between Vx and Vs1 will be eroded if a climbing turn is attempted; see 'Safety: loss of control in low level turns'.

    Climbing at Vx should always be regarded as a short-term precautionary procedure, and once clear of obstacles, airspeed should be increased to Vy — or any appropriate 'enroute climb speed'. The latter reduces the rate of climb but has the benefit of reducing total sector time, increasing forward visibility and increasing engine cooling — which may be beneficial to engine operation but, more importantly, provides a little more airspeed in hand should the engine falter or fail. The airspeed for Vx increases with (density) altitude and is much the same airspeed as Vbe, although engine cooling needs might require a higher airspeed.

    • Vy — indicated speed for best rate of climb. This speed is used to attain height in the shortest time using maximum power, or possibly maximum continuous climb power. Vy decreases as the aircraft weight decreases from MTOW (refer to rule of thumb #3 above), but the angle of attack is maintained at around 6–8º. After reaching a safe height airspeed may be increased to an appropriate enroute climb speed. The CAS for Vy decreases with (density) altitude — i.e. as TAS increases — and also is usually fairly close to the maximum L/D speed Vbr, taking engine cooling flows into account. Vx and Vy converge as (density) altitude increases.

    Power-off descent speeds
    • Vbg — best power-off glide This is the airspeed that provides minimum drag thus maximum L/D, or glide ratio, and thus the greatest still air glide range from the potential energy of height. It is much the same basic airspeed as Vbr/Vmd and Vy, though it may be a bit lower and decreases as the aircraft weight decreases from MTOW. However, like Vbr, wind direction and speed have to be taken into account before you can choose the Vbg speed when in a forced glide; for more information on the power-off glide speeds read the 'Know the best glide and minimum descent airspeeds' and 'Know the practical glide ratio and terrain footprint' sections in the 'Coping with emergencies guide'. In lower wind conditions, Vbg is increased in a headwind by around one quarter of the windspeed, but is decreased in a tailwind by a similar amount. In higher wind conditions, say above 25 knots, the speed changes required would be around one half of the windspeed.

    • Vmp — minimum power. This is the speed that results in the lowest rate of sink in a power-off glide, and provides the longest duration of flight from the potential energy of height. The lowest rate of sink occurs at the minimum value of drag × velocity. It is probably around 80–85% of Vbg, and may be a similar speed to Vbe and Vx.

    Vbg for an average sailplane with a wing loading of 32 kg/m² could be 50 knots, providing a glide ratio of 38:1, while Vmp would be 41 knots providing a sink rate of 0.6 m/s or 120 feet/minute. If you want further explanation of sink rates, etc. (with excellent diagrams) read this article on glider performance airspeeds.

    (Note: the term Vmd meaning minimum descent, rather than minimum drag, is in common usage to designate the speed for lowest rate of sink in a power-off glide.)

    Both Vbg distance and Vmp time are adversely affected by the extra drag of a windmilling propeller, which creates more drag than a stopped (but unfeathered) propeller following engine shut-down or failure. A windmilling propeller has a negative aoa and the 'thrust' direction is reversed, in effect adding to drag. Something similar might happen with some engine/propeller configurations, when simulating a glide at the specified Vbg/Vmp speed with the throttle closed but the engine still firing; the propeller drag might increase the rate of sink beyond that expected, and perhaps lead to the erroneous conclusion that the best glide speeds in the handbook are understated.

    3.10 The design manoeuvring flight envelope
    The structural design manoeuvring flight envelope of a recreational aircraft describes its structural strength limitations and its maximum aerodynamic capabilities. The various envelopes have been described as "the parameters within which an aircraft can be safely operated, with average pilot ability, at varying density altitudes, airframe states, power outputs, wing loadings and atmospheric turbulence". 'Airframe states' refer to flap and spoiler extensions, undercarriage position, the gross weight and the fore-and-aft position of the centre of gravity. The flight envelope is only relevant for an aircraft within the required weight and balance conditions.

    The boundary of the envelope shown below is formed by combinations of the applicable limiting positive and negative load factors and dynamic pressures (indicated airspeeds). It is a two-dimensional model that has indicated airspeed (i.e. CAS) along the horizontal axis and flight load factors expressed in 'g' accelerations along the vertical. The loads are those parallel to the aircraft's normal axis, i.e. perpendicular to the longitudinal axis — the aerodynamic loads in the aircraft's pitching plane. The symbol 'n' is generally used to identify such loads.

    The parameters for a light aircraft usually are the limiting critical airspeeds — Vs, Va and Vne and the certificated limit load factors. There are other flight limitations which are not displayed in this flight envelope diagram, e.g. an angle of bank limitation of 60° for some aircraft categories. For weight-shift aircraft in particular, there are also pitch limitations; — e.g. 45° nose up or nose down from the horizontal. The manufacturers of high performance non-recreational aircraft would also provide other charts, altitude performance or turning performance for example.

    The V-n [or V-g] diagram below is a simplified representation of a few aspects of the manoeuvring flight envelope for an LSA category aircraft at MTOW and low altitudes. An indicated (CAS) airspeed scale would normally be displayed along the horizontal axis and load factors (in units of 'g') along the vertical axis, between the certificated load limits for light sport aircraft of +4g to –2g. This diagram does not reflect any flaps-extended conditions. The positive and negative 'aircraft normal force coefficient curves'* are for the aircraft as a whole but can be assumed to approximate the accelerated stall speeds at loads of Vs1g × Öload factor.
    (*Normal force: pilots consider the aerodynamic force acting on the aircraft in terms of lift and drag, lift being perpendicular to the relative free-stream airflow and drag being parallel with it. Aerodynamicists may also consider the total aerodynamic force in terms of a 'normal force' which is parallel to the aircraft's normal axis and an 'axial force' which is parallel to the longitudinal axis plus a 'side force' parallel to the lateral axis. The symbol for the aircraft normal force coefficient is Cna, however, the stated aircraft limit load factor (e.g. 3g or 4g) also applies to axial and side forces, not just the normal force.)

    The stall speed at a 4g load limit factor would be Vs1g × Ö4 = Vs1g × 2, thus if the Vs1 stall speed was 45 knots then the stall speed where the positive curve intersects the 4g load limit factor line is 90 knots, i.e. that speed is the lowest possible speed at which the pilot can pull maximum g, i.e. 4g. That corner of the envelope is usually the position of Va — the design manoeuvring speed. You can see from the curve that at the Va airspeed the aircraft will stall when the wing loading exceeds 4g. Sustained flight is not possible in the white region to the left of the accelerated stall curves because the wings will be stalled.

    (Note: the light blue area between +1g and –1g is the realm of reduced gravity, or microgravity. NASA and other organisations use C135 and DC-9 aircraft flying a parabolic trajectory to produce reduced or near-zero gravity conditions, for the aircraft occupants, for periods of 20–30 seconds. A light aircraft can be flown in that area for a brief period by 'unloading' the wings — 'bunting'.)

    Except for transient turbulence loads the negative flight envelope below the 0g line mainly relates to aerobatic aircraft.

    (Note: in section 2.8 we determined that a 60° banked level turn doubled the normal wing load. With Vs1 at 45 knots and Va 90 knots then visualise a horizontal line from the 2g point; the interception with the curve will equate with about 60 knots. So that would be the lowest possible speed for a 60° banked level turn.)
    The white areas above and below the red lines represent structural loads beyond the positive and negative limit loads. Flight loads caused by control misuse and/or atmospheric turbulence that exceed +4g or –2g may cause temporary pilot incapacitation (greyout/blackout/redout) and airframe distortion. Flight loads 50% greater than +4g or –2g (i.e. +6g or –3g) will very likely cause airframe breakup.

    In the aircraft design process the design maximum dive speed Vd is a calculated speed, but in the flight test stage the aircraft may be tested up to a speed where it still demonstrates no flutter, or other, problems. This is the flight-demonstrated dive speed Vdf which is lower than the design Vd but, possibly, it could be equal to it. The dynamic pressure that a pilot must not exceed is represented by the Vne airspeed limit, and that is required to be no more than 90% of Vd or Vdf. At Vne the aircraft is flying at a very small angle of attack, deriving most of the aerodynamic force from the dynamic pressure. If the pilot — or turbulence — suddenly increases the aoa the consequent increase in the lift coefficient CL (amplified by the aircraft's inertia momentarily maintaining the original flight path) could place an extreme load on the airframe, enough to break it. See 'Wind shear and turbulence'.

    Some manoeuvring flight envelopes might have the top right corner cut off, from the Vno line intersection with the limit load factor line, to some less-than-maximum load factor along the Vne line.

    For more information concerning the risks of flight at excessive speed read 'Don't fly real fast!' in the 'Decreasing your exposure to risk' tutorial.

    Vne is the maximum airspeed, but full and rapid control applications are restricted to speeds at or below Va. Vno is the maximum structural cruise speed or 'normal operating limit' for flight in light to medium turbulence. Above the Vno/Vc speed flight should only be conducted cautiously and in smooth air.

    So, the aircraft can be flown in the light green area without limits on smooth control use and it can be operated within the olive-green area in light to moderate turbulence, but it should not be operated in the yellow area except in a reasonably smooth atmosphere. If it is inadvertently operated in the area outside the certificated load limits, or at velocities greater than Vne, structural distortion then failure may result. The more the wings are loaded while the aircraft is operating in the region above Vne, the greater the possibility of structural failure. The potential exists to exceed both Vd and maximum load in the pullout from a spiral dive.

    Vertical gusts impose loads on the wing structure by inducing rapid, but momentary, changes in aoa with consequent changes in the aerodynamic forces. The faster the aircraft is moving, the greater the gust-induced load. FAR Part 23 has requirements for designers to consider unexpected gust loads. The resulting gust envelope is often represented as the flight manoeuvring envelope with overlaid gust lines. Vb is developed by the aircraft designer as a recommended turbulence penetration speed in severe turbulence, with varying vertical gust components — up to 50 feet/second considered for a light aircraft at cruise speed. However, Vb is not specified for most light aircraft because, for such aircraft, there is probably not much difference between Va and Vb.

    The flight envelope is considerably reduced if asymmetric manoeuvring loads are applied to the airframe. Such loads might be applied by an aircraft yawing (side force) or rolling (lateral force) while recovering (normal force) from a high-speed descent. The same applies to the use of flaps.

    There are other attributes that define the envelope – resistance to spin and spin recovery, for example. Note that the term 'average pilot ability' doesn't imply that those who consider themselves 'above average' can push the envelope without losing control or stressing the airframe.

    There is more information on the flight envelope in the safety brief document 'Don't fly real fast'.

    Things that are handy to know
    • Absolute temperature is expressed in kelvins [K], one K equals 1 °C. The base temperature is zero kelvin — equivalent to minus 273 °C — so 0 °C is equivalent to 273 K.

    • In a free stream airflow, a favourable pressure gradient is one where static pressure decreases with distance downstream. An adverse pressure gradient is one where static pressure increases with distance downstream.

    • ASI position error. The static vent is an opening, best placed at a position on the aircraft's fuselage, where atmospheric static pressure is not influenced by the shape of the fuselage or other aerodynamic disturbances. (Some aircraft may be fitted with a static vent on each side of the fuselage to counteract static pressure disturbances caused when the aircraft is slipping/skidding, and/or a switchable alternative static source within the cockpit.) The opening is a tube connected to the cockpit and supplies the ambient atmospheric pressure, or static pressure, to the three pressure sensing instruments — ASI, VSI and altimeter. The static vent is usually subject to some pressure disturbances at particular aircraft attitudes, as is the pitot tube, but probably to a lesser degree.

    In addition if the airflow is not squarely aligned with the entry of the pitot head there will be a reduction in the indicated airspeed which increases as aoa increases. These disturbances result in position error: for a wing-mounted vent, the ASI may underread by 10 knots at stalling aoa. In a sideslip, a single fuselage-mounted static vent may be subject to dynamic pressure and ASI and VSI readings will consequently be completely misleading. Also, the instrument movements will have inbuilt errors, usually caused by excessive friction. Obstructions in the tubes — such as water or wasp's nests — will cause misreadings or no reading.

    Position error corrections plus the instrument error corrections for the system should be noted in the Pilot's Operating Handbook and placarded on the instrument panel. The IAS corrected for instrument and position errors is called the calibrated airspeed [CAS]. Either CAS or IAS may be the reference speed in the Pilot's Operating Handbook for aircraft operations, but if the position error corrections are not shown then the ASI system has not been assessed for accuracy. In some poor ASI installations, IAS may be 20% less than CAS at low speeds, but they are usually much the same at normal cruising speeds.

    Regulations for type-certificated aircraft require that the complete airspeed indicating system of pitot head, static vent, connecting tubes and instrument be tuned so that the IAS reading is within 3% of the true reading over the normal airspeed range from Vs to Vc. However, you should suspect that any non-certificated ultralight ASI system will be inaccurate at all speeds, and particularly so at high aoa. When comparing published stall speeds between different aircraft types, it is wise to determine CAS, as published IAS stall speeds may be downright misleading.

    • Compressibility effects. The compressibility of air within the pitot tube has little effect on the accuracy of the ASI reading for aircraft operating below 10 000 feet and 200 knots; at an airspeed of 200 knots, compressibility will cause CAS to overread by only 0.5 knots or so. However, for aircraft operating at high speed or high altitude, compressibility will cause the ASI to overread significantly, so there is a need to correct CAS using a compressibility correction chart. The correction value is deducted from CAS to give the compressibility corrected CAS — otherwise known as equivalent airspeed [EAS].

    For most medium-speed aircraft, it is probable that the compressibility correction value has been built into the IAS–CAS airspeed correction table.

    There is no practical application for recreational pilots, but aerodynamicists use the EAS term — rather than IAS or CAS — assuming an ASI, that has no errors caused by mechanical, position, aoa or compressibility effects, would display the ISA standard condition sea-level true airspeed, which is equivalent to the dynamic pressure in the instrument at any altitude.

    For more information see 'Notes: compressibility of airflow and Mach number'.

    • Checking validity of claimed stall speeds. There is a simple method to check the validity of published stall speeds. Practically all very light aircraft (except those with single surface wings like the Wheeler Scout or weight-shift aircraft) use simple, long proven, standard camber aerofoils to form the wings. The lift coefficient attainable at maximum aoa with such wings without flaps is about 1.2 or 1.3 for faster-cruising aircraft, and 1.5 or 1.6 for the slower, higher-lift sections. If equipped with flaps over, say, half the trailing edge, then CLmax might be increased by 0.5 when the flaps are extended to at least 35°. When other high-lift devices (for example, full length leading edge slats/slots) are added to the wing, then CLmax might increase 0.6. Thus, a specialised short take-off and landing aircraft fitted with a high-lift aerofoil, full-length leading edge slats and large extended flaps would have a CLmax of (at least) 1.6 + 0.5 + 0.6 = 2.7.

    The lift equation at normal stall speed is:

    Lift = CLmax × ½rV² × S = weight

    or re-arranged:

    CLmax = weight / (½rV² × S)

    We can use that equation to check the validity of stall speed claims if we know the maximum take-off weight [MTOW] and the wing area . Let's say a supplier claims that an aircraft, lacking any high-lift devices, has a stall speed of 30 knots. The MTOW is 450 kg and the wing area is 12 m².

    In the equation, the weight must be expressed in newtons — so multiply kg × 10 = 4500 N; and the stall speed must be expressed in metres per second — so just halve the velocity in knots = 15 m/s: the air density used must be the ISA msl density = 1.225 kg/m³.

    Thus CLmax = 4500 / (0.5 × 1.225 × 15 × 15 × 12) = 2.7

    A lift coefficient of 2.7 is very much higher than that achievable without high-lift devices, so you would conclude that the claimed stall speed is nonsense; a figure of 38 knots is probably closer to the mark.

    • Estimating stall speeds. Conversely you can do a rough approximation of stall speeds using the following simplified formulae if you know the wing loading in kilograms per square metre or in pounds per square foot, and can estimate CLmax with flaps stowed or fully extended.

    Stall speed [knots] = 7.8 × square root (wing loading in kg/m² divided by CLmax)
    Stall speed [knots] = 17.2 × square root (wing loading in lb/ft² divided by CLmax)

    Using our previous example of a lightly-loaded Jabiru with a mass of 340 kg (748 lb), wing area of 7.9 m² (85 ft²) thus wing loading = 43 kg/m² (8.8 lb/ft²) and estimating CLmax with flaps fully extended as 2.0 then:

    estimated stall speed = 7.8 × square root (43/2) = 7.8 × 4.64 = 36 knots

    or estimated stall speed = 17.2 × square root (8.8/2) = 17.2 × 2.1 = 36 knots

    Stuff you don't need to know
    • The tropopause marks the boundary between the two lower layers of the atmosphere — the troposphere, and above it, the stratosphere. The height of the tropopause varies daily, seasonally, and latitudinally — it is about 28 000 feet at the poles and perhaps 55 000 feet at the equator. The significant difference between the troposphere and the stratosphere is that air temperature decreases steadily with height in the troposphere, but initially remains constant then increases steadily with height in the stratosphere, until the stratopause at about 50 km. The stratosphere contains very little water vapour and is much more stable than the troposphere. The ozone layer is within the stratosphere.

    • Boyle's law: at a constant temperature, the volume (V) of a given mass of gas is inversely proportional to the pressure (P) upon the gas; i.e. PV = constant.

    • The pressure law: at a constant volume, the pressure is directly proportional to temperature (T) in kelvins.

    • Charles' law: at a constant pressure, gases expand by about 1/273 of their volume, at 273 K, for each one kelvin rise in temperature; i.e. the volume of a given mass of gas at constant pressure is directly proportional to the absolute temperature.

    • For one mole of gas, the preceding laws are combined in the gas equation PV = RT, where R = the gas constant = 2.87 when P is expressed in hectopascals. Ordinary gases do not behave exactly in accordance with the gas laws.

    • The change in altitude for each one hPa change in pressure can be roughly calculated from the absolute temperature and the pressure at the level using the equation:=96T/P feet.

    • The term 'burble' also refers to the atmospheric wake of an object. Skydivers refer to their wake as 'the burble' while the disturbed airflow and exhaust gases behind a the island structure of a fast-moving aircraft carrier was (and probably still is) known to pilots as 'the burble'.

    2.1 Cruise performance
    Cruise speed options
    When an aircraft is cruising, flying from point A to point B, the pilot has several options for setting cruise speed:
    One choice might be to get there as soon as possible, in which case the pilot would operate the engine at the maximum continuous power allowed by the engine designer. The recommended maximum continuous power is usually around 75% of the rated power of the engine and provides performance cruise.
      Another choice might be to get there using as little fuel as possible but in a reasonable time, in which case the pilot might choose a 55% power setting to provide an economy cruise airspeed. Or the pilot might choose any power setting, in the usual engine design range, between 55% and 75%; refer to cruise speeds in the 'Airspeed and properties of air' module.  
    The power required curve
    In level flight at constant speed thrust power is required to balance induced and parasite drag. Power is the rate of doing work, so power (in watts) is force (newtons) × distance (metres) / time (seconds). Distance/time is speed so power required is drag force (N) × aircraft speed (m/s). Thus, if we use the expression for total drag from section 1.6 and multiply it by V we get:

    (Equation #1.3) Power required for level flight [watts] = CD × ½rV³ × S (note V cubed).
    The total drag curve can be converted into a 'power required' diagram — usually called the power curve — if you know the total drag at each airspeed between the minimum controllable speed and the maximum level flight speed. It is a different curve from that for total drag, because the power required is proportional to speed cubed rather than speed squared. This means that (ignoring the related CD change) if speed is doubled, drag is increased four-fold but power must be increased eight times — which indicates why increasing power output from, say, 75% power to full rated power, while holding level flight, doesn't provide a corresponding increase in airspeed.
    The diagram above is a typical level-flight power curve for a light aircraft. The part of the curve to the left of the minimum power airspeed is known as the back of the power curve — where the slower you want to fly, the more power is needed, because of induced drag at a high angle of attack. The lowest possible speed for controlled flight is the stall speed, which is discussed in the 'Airspeed and properties of air' module. Two aerodynamic cruise speeds are indicated — the speed associated with minimum drag (the point on the curve where the drag force factor has the lowest value) and the speed associated with minimum power (the point on the curve where drag force × speed has the lowest value). To maintain level flight at speeds less than or greater than the minimum power airspeed, power must be increased.

    Power available
    The engine provides power to the propeller. The propellers used in most light aircraft have a maximum efficiency factor, in the conversion of engine power to thrust power, of no more than 80%. (Thrust power = thrust × forward speed.) The pitch of the blades, the speed of rotation of the propeller and the forward speed of the aircraft all establish the angle of attack of the blades and the thrust delivered. The in-flight pitch of ultralight and light aircraft propeller blades is usually fixed (though many such types are adjustable on the ground) so that the maximum efficiency will occur at one combination of rpm and forward speed — this is usually in the mid-range between best rate of climb and the performance cruise airspeeds. Propeller blades are sometimes pitched to give the best efficiency near the best rate of climb speed (climb prop), or pitched for best efficiency at the performance cruise airspeed (cruise prop). The efficiency of all types of propellers falls off either side of the optimum; one with a too high pitch angle may have a very poor take-off performance, while one with a too low pitch may allow the engine to overspeed at any time.

    With the advent of higher-powered four-stroke light engines, such as the Jabiru 3300, there has been a corresponding increase in the availability of more advanced light-weight propeller systems, providing maximum effective power utilisation during all stages of flight. For more information refer to the 'Engine and propeller performance' module.

    Speed, power and altitude
    At sea-level, an aero-engine will deliver its rated power — provided it is in near-perfect ex-factory condition, properly warmed up and using fuel in appropriate condition. However, because air density decreases with increasing altitude, and an engine's performance depends on the weight of the charge delivered to the cylinders, then the full throttle power of a non-supercharged four-stroke engine will decrease with height. So, at about 6000–7000 feet above mean sea-level the maximum power available at full throttle may drop below 75% of rated power. At 12 000 feet full throttle power may be less than 55% of rated power. Thus, as altitude increases, the range of cruise power airspeeds decreases. For best engine performance, select a cruise altitude where the throttle is fully open and the engine is delivering 65% to 75% power.  
    A couple of points to note from the speed-power diagram above:
    As air density, and consequently drag, decreases with height, then airspeed, from a particular power level, will increase with height; e.g. the airspeed attained with 65% power at sea-level is 90 knots increasing to 100 knots at 10 000 feet.
      At sea-level, an increase in power from 75% to 100% only results in an increase in airspeed from 100 to 110 knots. This is the norm with most light aircraft — that last 33% power increase to rated power only provides a 10% increase in airspeed.  
    Power required vs power available
    In the 'power available' diagram at left, power available curves have been added to the earlier 'power required' diagram. The dashed red curve indicates the rated power — that is, the full throttle engine power delivered to the propeller over the range of level flight speeds at sea-level. The upper green curve — maximum thrust power, is that engine power converted by the propeller after allowing for 80% maximum propeller efficiency. The lower green curve is the propeller thrust power available with the engine throttled back to 75% power at sea-level, or if flying at an altitude such that full throttle opening will only deliver 75% of rated power. The intersection of those power available curves with the power required curve indicates the maximum cruise speed in each condition.
    The region between the maximum thrust power curve and the power required (to maintain level flight) curve indicates the excess power available at various cruise speeds — this excess power is available for various manoeuvres if the throttle is fully opened. The simplest use would be a straight unaccelerated climb, in which case the maximum rate of climb would be achieved at the airspeed where the two curves are furthest apart. It can be seen that the best rate of climb speed is around the same airspeed as the minimum drag airspeed shown in the earlier powered required diagram.

    The rate of climb will decrease at any speed either side of the best rate of climb speed because the power available for climb decreases. The rate of climb (metres/second) = excess power available (watts)/aircraft weight (N).
    For example, lets assume the preceding diagram is representative of an aircraft fitted with a 100 hp engine, and at the best rate of climb speed the engine/propeller has 25 hp (18 600 watts) of excess thrust power available. The aircraft weight is 4000 N so the rate of climb = 18 600/4000 = 4.65 m/s. To convert metres/second to feet/minute, multiply by 200 = 930 feet/minute as the maximum rate of climb.
    One thing to bear in mind is that we have assumed the aircraft's aerodynamic shape — its configuration is constant. However if the aircraft is fitted with flaps, high lift devices or spoilers the pilot is able to change its configuration and consequently its performance.

    Thus performance is dependent on power, plus attitude (pitch, bank, sideslip and aoa) plus configuration.

    2.2 Forces in a climb
    When cruising, the difference between the current power requirement and power available — the excess power — can be used to accelerate the aircraft or climb, to accelerate and climb, or perform any manoeuvre that requires additional power. For instance if the aircraft has potential power available and the pilot opens the throttle, the thrust will exceed drag and the pilot can utilise that extra thrust to accelerate to a higher speed while maintaining level flight. Alternatively the pilot can opt to maintain the existing speed but use the extra thrust to climb to a higher altitude. The rate of climb (altitude gained per minute) depends on the amount of available power utilised for climbing, which depends in part on the airspeed chosen for the climb. There are other choices than the best rate of climb speed available for the climb — for example, the best angle of climb speed (which is around the same as the speed for minimum power) or a combination enroute cruise/climb speed. The climb speed chosen depends on terrain, weather, cloud cover and other operating variables.

    If an aircraft is maintained in a continuous full-throttle climb, at the best rate of climb airspeed, the rate of climb will be highest at sea-level; it will decrease with altitude, as engine power decreases. The aircraft will eventually arrive at an altitude where there is no excess power available for climb, then all the available power is needed to balance the drag in level flight and there will be only one airspeed at which level flight can be maintained. Below this airspeed the aircraft will stall. This altitude is the aircraft's absolute ceiling. However, unless trying for an altitude record, there is no point in attempting to climb to the absolute ceiling so the aircraft's service ceiling should appear in the aircraft's performance specification. The service ceiling is the altitude at which the rate of climb falls below 100 feet per minute; this is generally considered the minimum useful rate of climb.
    This diagram of forces in a climb and the subsequent mathematical expressions, have been simplified, aligning the angle of climb with the line of thrust. In fact the line of thrust will usually be 4 to 10° greater than the climb angle. The climb angle (c) is the angle between the flight path and the horizontal plane.
    The relationships in the triangle of forces shown are:
    Lift = weight × cosine c
    Thrust = drag + (weight × sine c)

    In a constant climb the forces are again in equilibrium, but now thrust + lift = drag + weight.

    Probably the most surprising thing about the triangle of forces in a straight climb is that lift is less than weight. For example, let's put the Jabiru into a 10° climb with weight = 4000 N. (There is an abridged trig. table at the end of this page.)

    Then, Lift = W cos c = 4000 × 0.985 = 3940 N

    It is power that provides a continuous rate of climb, but momentum may also be used to temporarily provide energy for climbing; see 'Conserving aircraft energy' below. It is evident from the above that in a steady climb, the rate of climb (and descent) is controlled with power, and the airspeed and angle of climb is controlled with the attitude and particularly the included angle of attack. This is somewhat of a simplification, as the pilot employs both power and attitude in unison to achieve a particular angle and rate of climb or descent.

    The angle of attack in a climb is the pitch attitude minus the angle of climb being achieved plus the wing incidence.

    A very important consideration, particularly when manoeuvring at low level at normal speeds, is that the steeper the climb angle the more thrust is required to counter weight. For example, if you pulled the Jabiru up into a 30° 'zoom' climb the thrust required = drag + weight × sine 30° (= 0.5) so the engine has to provide sufficient thrust to pull up half the weight plus overcome the increased drag due to the increased aoa in the climb. Clearly, this is not possible, so the airspeed will fall off very rapidly and will lead to a dangerous situation if the pilot is slow in getting the nose down to an achievable attitude. Never be tempted to indulge in zoom climbs — they are killers at low levels.

    2.3 Forces in a descent
    If an aircraft is cruising at, for instance, the maximum 75% power speed and the pilot reduces the throttle to 65% power, the drag now exceeds thrust and the pilot has two options — maintain height, allowing the excess drag to slow the aircraft to the level flight speed appropriate to 65% power; or maintain the existing speed and allow the aircraft to enter a steady descent or sink. The rate of sink (a negative rate of climb, or altitude lost per minute) depends on the difference between the 75% power required for level flight at that airspeed and the 65% power utilised. This sink rate will remain constant as long as the thrust plus weight, which are together acting forward and downward, are exactly balanced by the lift plus drag, which are together acting upward and rearward. At a constant airspeed, the sink rate and the angle of descent will vary if thrust is varied. For example, if the pilot increased thrust but maintained constant airspeed, the rate of sink will decrease — even becoming positive; i.e. a rate of climb.
    If the pilot pushed forward on the control column to a much steeper angle of descent, while maintaining the same throttle opening, the thrust plus weight resultant vector becomes greater, the aircraft accelerates with consequent increase in thrust power and the acceleration continues until the forces are again in equilibrium. Actually, it is difficult to hold a stable aircraft in such a fixed angle 'power dive' as the aircraft will want to climb — but an unstable aircraft might want to 'tuck under'; i.e. increase the angle of dive, even past the vertical. We discuss the need for stability in the 'Stability' module.
    When the pilot closes the throttle completely, there is no thrust, the aircraft enters a gliding descent and the forces are then as shown in the diagram on the left. In the case of descent at a constant rate, the weight is exactly balanced by the resultant force of lift and drag.

    From the dashed parallelogram of forces shown, it can be seen that the tangent of the angle of glide equals drag/lift. For example, assuming a glide angle of 10° (from the abridged trigonometrical table below, the tangent of 10° is 0.176), the ratio of drag/lift in this case is then 1:5.7 (1/0.176 =5.7).
    Conversely, we can say that the angle of glide depends on the ratio of lift/drag [L/D]. The higher that ratio is, then the smaller the glide angle and consequently the further the aircraft will glide from a given height.

    For example, to calculate the optimum glide angle for an aircraft with a L/D of 12:1.
    Drag/lift equals 1/12, thus tangent = 0.08 and, from the trigonometrical table, the glide angle = 5°.

    Although there is no thrust associated with the power-off glide, the power required curve is still relevant. The minimum drag airspeed shown in that diagram is roughly the airspeed for best glide angle and the speed for minimum power is roughly the airspeed for minimum rate of sink in a glide. This is examined further in the 'Airspeed and the properties of air' module.

    It may be useful to know that in a glide, lift = weight × cosine glide angle and drag = weight × sine glide angle. There is further information on glide angles and airspeeds in the lift/drag ratio section of module 4.

    2.4 Turning forces
    Centripetal force
    When an aircraft turns in any plane, an additional force must be continuously applied to overcome inertia, particularly as an aircraft's normal tendency is to continue in a straight line. This is achieved by applying a force towards the centre of the curve or arc — the centripetal force — which is the product of the aircraft mass and the acceleration required. Remember that acceleration is the rate of change of velocity — either speed or direction, or both.

    The acceleration, as you know from driving a car through an S curve, depends on the speed at which the vehicle is moving around the arc and the radius of the turn. Slow speed and a sweeping turn involves very little acceleration. But high speed and holding a small radius involves high acceleration, with consequent high radial g or centripetal force and difficulty in holding the turn. Even when an aircraft enters a straight climb from cruising flight, there is a short transition period between the straight and level path and the straight and climbing path, during which the aircraft must follow a curved path — a partial turn in the vertical plane.

    An aircraft turning at a constant rate turn is continuously accelerating towards the centre of the turn. The acceleration towards the centre of the turn is V²/r m/s². The centripetal force required to produce the turn is m × V²/r newtons, where m is the aircraft mass in kilograms and r is the turn radius in metres. Note this is aircraft mass, not weight.

    Turn forces and bank angle
    The diagram below shows the relationships between centripetal force, weight, lift and bank angle.

    In a level turn, the vertical component of the lift (Lvc) balances the aircraft weight and the horizontal component of lift (Lhc) provides the centripetal force.

    (Note: in a right-angle triangle the tangent of an angle is the ratio of the side opposite the angle to that adjacent to the angle. Thus, the tangent of the bank angle is equal to the centripetal force [cf] divided by the weight — or tan ø = cf/W. Or, it can be expressed as tan ø = V²/gr . In the diagram, I have created a parallelogram of forces so that all horizontal lines represent the centripetal force or Lhc and all vertical lines represent the weight or Lvc.)
    Let's look at the Jabiru, of mass 400 kg, in a 250 m radius horizontal turn at a constant speed of 97 knots or 50 m/s:

    Centripetal acceleration = V² / r = 50 × 50 / 250 = 10 m/s²
    Centripetal force required = mass × V² / r = mass × 10 = 400 × 10 = 4000 N
    The centripetal force of 4000 N is provided by the horizontal component of the lift force produced by the wings when banked at an angle from the horizontal. The correct bank angle depends on the airspeed and radius; think about a motorbike taking a curve in the road. During the level turn, the lift force must also have a vertical component to balance the aircraft's weight, in this case it is also 4000 N. But the total required force is not the sum of 4000 N + 4000 N = 8000 N; it is less and we have to find the one — and only one — bank angle where Lvc is equal to the weight and Lhc is equal to the required centripetal force.

    What then will be the correct bank angle (ø) for a balanced turn? Well, we can calculate it easily if you have access to trigonometrical tables. If you haven't then refer to the abridged version below.
    So, in a level turn requiring 4000 N centripetal force with weight 4000 N, the tangent of the bank angle = cf/W = 4000/4000 = 1.0, and thus (from the table) the angle = 45°. Actually, the bank angle would be 45° for any aircraft of any weight moving at 97 knots in a turn radius of 250 metres — provided the aircraft can fly at that speed, of course. (Do the sums with an aircraft of mass 2500 kg, thus weight = 25 000 N.).

    Now, what total lift force will the wings need to provide in a level turn if the actual weight component (aircraft plus contents) is 4000 N and the radial component also 4000 N?

    Resultant total lift force = actual weight divided by the cosine of the bank angle or L = W / cos ø. Weight is 4000 N, cosine of 45° is 0.707 = 4000/0.707 = 5660 N.

    The load on the structure in the turn is 5660/4000 = 1.41 times normal, or 1.41g. Alternatively the 'load factor' = 1/cosine (bank angle); so, cosine 45° is 0.707 = 1/0.707 = 1.41g.
    Manoeuvring loads
    In aviation usage, the lowercase 'g' denotes the acceleration caused by the force of gravity. When an aircraft is airborne maintaining a constant velocity and altitude — the total lift produced equals the aircraft's weight and that lift force is expressed as being equivalent to a '1g' load. Similarly, when the aircraft is parked on the ground, the load on the aircraft wheels (its weight) is a 1g load.

    Any time an aircraft's velocity is changed, there are positive or negative acceleration forces applied to the aircraft and felt by its occupants. The resultant manoeuvring load is normally expressed in terms of g load, which is the ratio of all the aerodynamic forces experienced during the acceleration to the aerodynamic forces existing at the normal 1g level flight state.

    You will come across terms such as '2g turn' or 'pulling 2g'. What is being implied is that during a particular manoeuvre the lift force is doubled and a radial acceleration is applied to the airframe — for the Jabiru a 2g load = 400 kg × 20 m/s² = 8000 N. The occupants will also feel they weigh twice as much. This is centripetal force and 'radial g'; it applies whether the aircraft is changing direction in the horizontal plane, the vertical plane or anything between.

    You may also come across mention of 'negative g'. It is conventional to describe g as positive when the lift produced is in the normal direction relative to the aircraft. When the lift direction is reversed, it is described as negative g. Reduced g and negative g can occur momentarily in turbulence. An aircraft experiencing a sustained 1g negative loading is flying in equilibrium, but upside down. It is also possible for some high-powered aerobatic aircraft to fly an 'outside' loop; i.e. the pilot's head is on the outside of the loop rather than the inside, and the aircraft (and its very uncomfortable occupants), will be experiencing various negative g values all the way around the manoeuvre.

    It can be a little misleading when using terms such as 2g. For instance, let's say that a lightly loaded Jabiru has a mass of 340 kg, and if you again do the preceding centripetal force calculation in a 45° banked turn using 340 kg mass you will find that the centripetal acceleration is 10 m/s², centripetal force is 3400 N, weight is 3400 N and total lift = 4800 N. The total lifting force is 15% less than in the 400 kg mass calculation but it is still a 1.41g turn; i.e. the ratio 4800/3400 = 1.41.

    Rather than thinking in terms of ratios, it may be appropriate to consider the actual loads being applied to the aircraft structures. The norm is to use the lift load produced by the wing as a primary structural load reference. In the 400 kg mass calculation the load produced is 5660/8 = 707 N/m², compared to the 500 N/m² load in normal cruise. However, even if the total weight of the aircraft changes, the forces experienced individual structural items — the engine mountings for example — will vary according to the g force produced by the wings.

    Increasing the lift force in a turn
    You might wonder how does the Jabiru increase the lift (or more correctly, the aerodynamic force) if it maintains the same cruise speed in the level turn? Well, the only value in the equation — lift = CL × ½rV² × S — that can then be changed is the lift coefficient. This must be increased by the pilot increasing the angle of attack. (Conversely if CL — the angle of attack — is increased during a constant speed manoeuvre the lift — and consequently the aerodynamic load factor — must increase.) Increasing aoa will also increase induced drag, so that the pilot must also increase thrust to maintain the same airspeed. Thus, the maximum rate of turn for an aircraft will also be limited by the amount of additional power available to overcome induced drag.

    The radius of turn = V²/g tan ø metres. For a level turn, the slowest possible speed and the steepest possible bank angle will provide both the smallest radius and the fastest rate of turn. However there are several limitations:
    When the steepest bank angle and slowest speed is applied the necessary centripetal force for the turn is provided by the extra aerodynamic force gained by increasing the angle of attack ( or CL ) to a very high value. Also due to the lower airspeed a larger portion of the total lift is provided by CL rather than V². Consequently the induced drag will increase substantially — requiring increased thrust power and there will be a bank angle beyond which the engine/propeller will not be able to supply sufficient thrust to maintain the required lift, and thus height in the turn.
      All aircraft that are not certificated under the utility or aerobatic categories are limited to bank angles not exceeding 60°. A bank angle of 75° in a level turn would induce a 3.8g load factor — the load limit for a normal category certification. Similarly a level turn bank angle of 77° would induce the 4.4g load limit for an utility category aircraft.
      The stall speed increases with bank angle, or more correctly with load factor, thus the lowest possible flight speed increases as bank in a level turn increases.
      Turns at high bank angles, near the accelerated stall speed, with maximum power applied, leaves the aircraft with nothing in reserve. Any mishandling or turbulence may result in a violent wing and nose drop with substantial loss of height. (For more information on turn physics see 'Turning back — procedure and dynamics'.)

    If you consider an aerobatic aircraft weighing 10 000 N and making a turn in the vertical plane —such as a loop — and imagine that the centripetal acceleration is 2g; what will be the load factor at various points of the turn? Actually, the centripetal acceleration varies all the way around because the airspeed and radius must vary. For simplicity we will ignore this and say that it is 2g all around. If the acceleration is 2g then the centripetal force must be 20 000 N all the way around.

    A turn in the vertical plane differs from a horizontal turn in that, at both sides of the loop, the wings do not have to provide any lift component to counter weight, only lift for the centripetal force — so the total load at those points is 20 000 N or 2g. At the top, with the aircraft inverted, the weight is directed towards the centre of the turn and provides 10 000 N of the centripetal force while the wings need to provide only 10 000 N. Thus, the total load is only 10 000 N or 1g, whereas at the bottom of a continuing turn the wings provide all the centripetal force plus counter the weight — so the load there is 30 000 N or 3g.

    This highlights an important point: when acceleration loads are reinforced by the acceleration of gravity, the total load can be very high.

    If you have difficulty in conceiving the centripetal force loading on the wings, think about it in terms of the reaction momentum, centrifugal force which, from within the aircraft, is seen as a force pushing the vehicle and its occupants to the outside of the turn and the lift (centripetal force) is counteracting it. Centrifugal force is always expressed as g multiples.

    Wing loading — W/S
    The term 'wing loading' has three connotations. The prime connotation is the standard expression — design W/S (usually just 'W/S', pronounced 'w-over-s') — which is the ratio of the aircraft designer's maximum allowable take-off weight [W] to the gross wing area . (There are some complications when national regulations specify a maximum allowable weight for an aircraft category that is lower than the design weight of a particular aircraft type; see the 'Weight and balance' module.) Aircraft with low W/S have lower stall speeds than aircraft with higher W/S — so consequently have shorter take-off and landing distances. High W/S aircraft are less affected by atmospheric turbulence. W/S is expressed in pounds per square foot [psf] or kilograms per square metre [kg/m²].

    The second wing loading connotation is as the operating wing loading; if the aircraft takes off at a gross weight lower than the designer's maximum, then the operating wing loading — in level unaccelerated flight — will also be lower than the design W/S, as will its stall speed.

    The third is the load applied by the pilot in manoeuvring flight. As we saw above, pulling 2g in a steep turn will produce a manoeuvring wing loading that is double the operating wing loading. So, if a pilot takes off in an overloaded aircraft (i.e. the aircraft's weight exceeds the design MTOW) and conducts a 2g steep turn, then that manoeuvring wing loading will be greater than the designer's expectations.

    2.5 Limiting loads and ultimate loads
    Manoeuvring loads and gust induced loads
    To receive type certification the design of a general or recreational aviation factory-built aircraft must conform to certain airworthiness standards among which the in-flight manoeuvring loads and the loads induced by atmospheric turbulence, that the structure must be able to withstand, are specified. The turbulence loads are called the gust-induced loads. The U.S. Federal airworthiness standards FAR Part 23 are the recognised world standards for light aircraft certification and the following are extracts [emphasis added]:
    Three seconds is not much time, so any inflight excursion above the ultimate load will probably result in rapid structural failure. The safety factor of 1.5 applies to fairly new aircraft in good condition; as very light aircraft age aerodynamic stresses, corrosion, hard landings and inadequate maintenance contribute to reduction of that safety factor.

    Airworthiness certification categories
    Light aeroplanes can be certificated in one or more of four airworthiness categories — 'normal', 'utility', 'acrobatic' and 'light sport aircraft' (LSA). The minimum positive limit flight load factor that an aircraft in the normal certification category (at maximum gross weight) must be designed to withstand is 3.8g positive. The LSA category minimum positive limit load is 4g. The negative limit flight load factor is –1.5g for the normal category and –2g for the LSA category. Recreational aviation aeroplanes, which are limited to banked turns not exceeding 60°, generally fit into either the normal category or the LSA category. The ultimate loads for the normal category are +5.7g and –2.25g and, for the LSA category, +6g and –4g. Amateur builders should aim to meet the same minimum values for limiting load and ultimate load factors.

    The 'utility' category (which includes training aircraft with spin certification) limit loads are +4.4g and –2.2g while the 'acrobatic' category (i.e. aircraft designed to perform aerobatics) limit loads are +6g and –3g. Sailplanes and powered sailplanes are generally certificated in the utility or acrobatic categories of the European Joint Airworthiness Requirements JAR-22, which is the world standard for sailplanes; aerobatic sailplanes have limit loads of +7g and -5g.

    The 'flight load factor' calculation is defined as the component of the aerodynamic force acting normal (i.e. at right angles) to the aircraft's longitudinal axis, divided by the aircraft weight. A positive load factor is one in which the force acts upward, with respect to the aircraft; a negative load factor acts downward. The inflight load factor is a function of wing loading, dynamic pressure and the aoa, i.e. lift coefficient, but see the flight envelope.

    It should not be thought that aircraft structures are significantly weaker in the negative g direction. The normal level flight load is +1g so with a +3.8g limit then an additional positive 2.8g acceleration can be applied while with a –1.5g limit an additional negative 2.5g acceleration can be applied.

    The manufacturer of a particular aircraft type may opt to have the aircraft certificated within more than one category, in which case there will be different maximum take-off weights and centre of gravity limitations for each operational category. See weight/cg position limitations.

    The sustainable load factors only relate to a new factory-built aircraft. The repairs, ageing and poor maintenance that any aircraft has been exposed to since leaving the factory may decrease the strength of individual structural members considerably. Read the current airworthiness notices issued by the RA-Aus technical manager.

    2.6 Conserving aircraft energy
    Energy available
    An aircraft in straight and level flight has:
    linear momentum — m × v [kg·m/s] kinetic energy (the energy of a body due to its motion) — ½mv² [joules or newton metres (N·m)]; remembering that 'm' in the ½mv² term represents mass
    (Note: normally, the newton metre — the SI unit of moment of force — is not used as the measure of work or energy; however throughout this guide, it is more helpful to express the kinetic energy in the N·m form rather than joules — the N·m and the joule are dimensionally equivalent) gravitational potential energy — in this case, the product of weight in newtons and height gained in metres chemical potential energy in the form of fuel in the tanks air resistance that dissipates some kinetic energy as heat or atmospheric turbulence. To simplify the text from here on, we will refer to 'gravitational potential energy' as potential energy and 'chemical potential energy' as chemical energy.
    We can calculate the energy available to the Jabiru cruising:

    • at a height of 6500 feet (2000 m)
    • and (air distance flown over time)= 97 knots (50 m/s)
    • with mass = 400 kg, thus weight = 4000 N
    • fuel = 50 litres.


    • potential energy = weight × height = 4000 × 2000 = 8 million N·m
    • kinetic energy = ½mv² = ½ × 400 × 50 × 50 = 500 000 N·m
    • momentum = mass × v = 400 × 50 = 20 000 kg·m/s
    • chemical energy = 50 litres @ 7.5 million joules = 375 million joules.

    Because it is the accumulation of the work done to raise the aircraft 6500 feet, the potential energy is 16 times the kinetic energy, and is obviously an asset that you don't want to dissipate. It is equivalent to 2% of your fuel.

    It is always wise to balance a shortage of potential energy with an excess of kinetic energy, and vice versa. For example, if you don't have much height then have some extra speed up your sleeve for manoeuvring or to provide extra time for action in case of engine or wind shear problems. Or if kinetic energy is low (because of flying at lower speeds than normal) make sure you have ample height or, if approaching to land, hold height for as long as possible. The only time to be 'low and slow' is when you are about to touch down.

    However, during take-off it is not possible to have an excess of either potential or kinetic energy; thus, take-off is the most critical phase of flight, closely followed by the go-around following an aborted landing approach. Ensure that a safe climb speed is achieved as quickly as possible after becoming airborne — or commencing a go-around — and before the climb-out is actually commenced; see take-off procedure.
    Kinetic energy measurement
    Kinetic energy is a scalar quantity equal to ½mv² joules if the aircraft is not turning. The velocity must be measured in relation to some frame of reference, and when we discuss in-flight energy management, the aircraft velocity chosen is that which is relative to the air; i.e. the true airspeed. For a landborne (or about to be landborne) aircraft we are generally concerned with either the work to be done to get the aircraft airborne or the (impact) energy involved in bringing the aircraft to a halt. So, the velocity used is that which is relative to the ground. Ground speed represents the horizontal component of that velocity, and rate of climb/sink represents the vertical component.

    Kinetic energy, gravitational potential energy and energy conservation are complex subjects. If you wish to go further, google the search terms 'kinetic energy' and 'reference frame'.

    Momentum conversion
    Let's look at momentum conversion. Consider the Jabiru, weighing 4000 N and cruising at 97 knots (50 m/s) and the pilot decides to reduce the cruise speed to 88 knots (45 m/s). This could be accomplished by reducing thrust — below that needed for 88 knots — allowing drag to dissipate the excess kinetic energy then increasing power for 88 knots. However, if traffic conditions allow, the excess kinetic energy can be converted to potential energy by reducing power, but only to that needed to maintain 88 knots cruise, and at the same time pulling up — thus reducing airspeed but still utilising momentum — then pushing over into level flight just before the 88 knot airspeed is acquired.
    How much height would be gained?

    Consider this:
    • kinetic energy at 97 knots = ½mv² = ½ × 400 × 50 × 50 = 500 000 N·m
    • kinetic energy at 88 knots = ½mv² = ½ × 400 × 45 × 45 = 405 000 N·m
    • kinetic energy available = 95 000 N·m
    • but potential energy [N·m] = weight × height
    • thus height (gained) = energy available divided by weight
    • = 95 000 N·m / 4000 N = 24 metres = 78 feet, or 9 feet gained per knot of speed converted.
    If we recalculate the preceding figures — doubling the initial (100 m/s) and final velocities (90 m/s) — the height gained will increase fourfold to 96 metres, or about 18 feet per knot. Conversely, if we halve the initial velocity to about 50 knots, the height gained per knot converted is halved, to about 4 feet. Note that as mass appears in both the kinetic energy and the weight expressions, it can be ignored; thus the figures are the same for any mass. Sometimes momentum (mass × velocity) is confused with inertia (a particular quality of mass).

    You will come across the expression 'low inertia / high drag' applied to some recreational light aircraft. This means that although all recreational light aircraft are low-inertia aircraft, compared to other recreational light aircraft this minimum aircraft has a relatively low inertial mass combined with a relatively high parasite drag profile; thus if the thrust is reduced or fails, the drag reduces the airspeed very rapidly. This is exacerbated if the aircraft is climbing. An aluminium tube and sailcloth aircraft at one end of the spectrum may be termed 'low momentum' or 'draggy', while an epoxy composite aircraft at the other end may be termed 'slippery'; some are very slippery indeed. The standing world speed record for an aircraft under 300 kg is 213 miles per hour; that amateur-designed and amateur-built aircraft was powered by only a 65 hp two-stroke Rotax. The handling characteristics for a low inertia/low drag aircraft differ considerably from those of a low inertia/high drag (low momentum) aircraft.

    Abridged trigonometrical table
    Relationship between an angle within a right angle triangle and the sides:
    Tangent of angle=opposite side/adjacent
    Sine of angle=opposite/hypotenuse
    Cosine of angle=adjacent/hypotenuse
    Degrees Sine Cosine Tangent   Degrees Sine Cosine Tangent 1 0.017 0.999 0.017   50 0.766 0.643 1.192 5 0.087 0.996 0.087   55 0.819 0.574 1.428 10 0.173 0.985 0.176   60 0.866 0.500 1.732 15 0.259 0.966 0.268   65 0.910 0.423 2.145 20 0.342 0.939 0.364   70 0.939 0.342 2.747 30 0.500 0.866 0.577   75 0.966 0.259 3.732 40 0.643 0.766 0.839   80 0.985 0.173 5.672 45 0.707 0.707 1.000   90 1.000 0 infinity  
    Things that are handy to know Rated power is the brake horsepower delivered at the propeller shaft of a direct drive engine, operating at maximum design rpm and best power fuel/air mixture, in standard sea-level air density conditions. (In a regulatory sense the definition is a little more complex.) An engine is only operated at its rated capacity for short periods during flight, usually during take-off and the initial climb. Rated power for small aero-engines is usually expressed as brake horsepower rather than the SI unit of kilowatts. Further discussion is provided in the 'Engine and propeller performance' module.
      To convert horsepower to watts multiply by 745.7; or to calculate kilowatts, multiply by 0.75.
      Design W/S is usually between 11 and 22 psf for GA aircraft, and 4 and 12 psf for ultralights. Gross wing area includes a notional extension of each monoplane wing up to the fuselage centreline but excludes any fairings at the wing/fuselage junction. For multi-engined aircraft, with the engines enclosed in wing nacelles, the wing area would also include the area occupied by the nacelles.
    Stuff you don't need to know High-performance military aircraft can achieve an aoa exceeding 45°.
      Aerobatic pilots — and combat pilots — use a value termed specific energy, E or energy height, He. It is the potential energy plus the kinetic energy per kg of aircraft weight; i.e.
    He = mgh/W + ½mv²/W
    As W = mg, then the equation can be re-arranged as He = h + V²/2g
    where h = height.

    What it expresses is the height that could be achieved if all kinetic energy were transferred to potential energy, but it is of little interest to recreational aviation.
      The thermal energy content of one litre of avgas is 30 million joules. With good engine handling by the pilot, that litre can provide 10 million joules of mechanical energy to the propeller shaft of most engines. The propeller of the Jabiru is maybe 70% efficient at cruise speed and provides 7.0 million joules, or N·m, of energy from the litre of fuel. Roughly how far will that take the Jabiru cruising at 97 knots? Easy! Drag is 540 N, so 7 000 000 / 540 = 12 965 m or 7.0 air nautical miles. We specify air nautical miles because wind will affect the distance travelled over the ground.

    1.1 Introduction
    The four forces
    When a well-trimmed aircraft is cruising (i.e. flying at a constant speed, and maintaining a constant heading and a constant altitude) in non-turbulent air, there are two sets, or couples, of basic forces acting on it. The two forces in each couple are equal and approximately opposite to each other otherwise the aircraft would not continue to fly straight and level at a constant speed; i.e. the aircraft is in a state of equilibrium where all forces balance each other out so there is no change in its motion.

    The couple that acts vertically is the lift, generated by the energy of the airflow past the wings and acting upward, and the weight acting downward. So, being equal and approximately opposite, the lifting force being generated must exactly match the total weight of the aircraft.

    The couple that acts horizontally is the thrust, generated by the engine-driven propeller, and the air resistance, caused by the friction and pressure of the airflow, or drag, trying to slow the moving aircraft. The thrust, acting forward along the flight path, exactly equals the drag. The thrust provides energy to the aircraft and the drag dissipates that same energy into the atmosphere. The forces are not all equal to each other. In fact, an aircraft in cruising flight might generate ten times more lift than thrust.

    When all forces are in equilibrium a moving aircraft will tend to keep moving along the same flight path at the same speed — whether it is flying straight and level, descending or climbing — until an applied force or a displacement force changes that state of motion. For instance, if the pilot opens the engine throttle fully, and maintains level flight, the thrust force is initially greater than drag and the aircraft accelerates. However, as the speed of airflow over the aircraft increases, the air resistance also increases and the aircraft will soon reach the speed — its maximum — where the forces are again balanced.

    Inertia, momentum and energy
    This property of resisting any change in motion, or continuing in the same state of motion or state of rest, is inertia. The mass of a body is a measure of its inertia; i.e. its resistance to being accelerated by an applied force increases with mass. The unit of mass we will be using is the kilogram [kg]. Some older texts refer to the 'slug', which is the unit of mass in the old British gravitational system of measurement units and equals 32.174 lb. A slug accelerates by one foot per second per second when a force of one pound-force is applied to it.

    Air also has mass and thus inertia, and will resist being pushed aside by the passage of an aircraft. That resistance will be felt both as drag and as pressure changes on the aircraft surfaces.

    A moving aircraft has momentum, which is mass × velocity and is a measure of the effort needed to stop it moving. (Momentum and inertia are not synonymous). The same aircraft also has the energy of motion — kinetic energy — which is related to mass × velocity squared. Also, because it has climbed above the Earth's surface, it has acquired additional gravitational potential energy which, in this case, is weight × height gained. Energy is discussed further in the section on conserving energy in the next module.

    An aircraft in flight is 'airborne' and its velocity is relative to the surrounding air, not the Earth's surface. (A ground-based observer sees the aircraft movement resulting from the sum of aircraft velocity and the ambient air velocity — horizontal motion [the wind] plus vertical motion [updrafts, downdrafts and wave action].) However, when the aircraft encounters a sudden change in the ambient air velocity — a gust — inertia comes into play and momentarily maintains the aircraft velocity relative to the Earth or, more correctly, relative to space. This momentarily changes airspeed and imparts other forces to the aircraft. (The fact that inertia over-rides the physics of aerodynamics is sometimes a cause of confusion.) A more massive (heavier) aircraft has more inertia than a less massive (lighter) one, so is more resistant to random displacement forces — wind shear and turbulence. Ultralights — whose mass is less than 750 kg — are all regarded as 'low inertia' aircraft and particularly affected by acceleration loads produced by turbulence.

    Freedoms of movement
    If the aircraft's control system allows, it can rotate about each of three axes — longitudinal, lateral and normal (or vertical) — and move forward along the longitudinal axis. An aerobatic aircraft can also move backward along the longitudinal axis — in a tail slide. Rotation about the longitudinal axis is roll, about the lateral axis is pitch — as in a ship pitching in a heavy sea — and about the normal axis is yaw — again, like much in aviation, a nautical term.
    Other movements can include a bodily movement along the lateral axis (sideslipping, slipping or skidding) or the normal axis (rising or sinking). Thus, an aircraft has six degrees of freedom of movement — three rotational and three translational. The three axes are relevant to the aircraft and each other (not to the horizon) so that when an aircraft is steeply banked its normal axis is closer to horizontal, rather than vertical to the Earth's surface. The axes are orthogonal (at right angles to each other) and, by convention, all are represented as passing through the aircraft's centre of gravity.

    When manoeuvring, an aircraft may experience any combination of the rotational and translational movements; for example, it may be rolling, pitching, yawing, slipping and sinking all at the same time.

    Direction of forces relative to the flight path
    When an aircraft is in straight and level flight lift acts vertically upward with thrust and drag acting horizontally. In fact, lift acts perpendicular to both the flight path and the lateral axis of the aircraft, drag acts parallel to the flight path, and thrust usually acts parallel to the longitudinal axis of the aircraft.

    So if you imagine an aircraft doing a loop, as in the diagram below, you can see that at one point, when it is going up, thrust will be acting vertically upward, drag vertically downward and lift acting horizontally towards the centre of the loop. At a point on the other side of the loop the thrust acts downward, drag upward and lift again horizontally. Weight, of course, always acts from the centre of mass of the aircraft towards the centre of mass of the Earth, so on the downside of the loop, weight and thrust are acting together and the aircraft will accelerate rapidly unless thrust is reduced.

    You might ask yourself this: if the aircraft is using its maximum thrust when it starts the loop, how can it climb vertically when lift no longer counters weight, and there is no extra thrust available to also counter the drag plus the weight, which are both now acting downwards? The answer is extra momentum which enables the aircraft to accomplish a fast pull-up, and usually provided by the pilot, of a lower-powered aircraft, accelerating the aircraft in a shallow dive before beginning the manoeuvre.

    The lift only matches the weight when the aircraft is flying straight and level. When the aircraft is in a steady descent or in a steady climb the lift is a bit less than the weight. We will explore this in the climb/descent modules but just be aware that when the line of thrust is inclined above the horizon the thrust will have a vertical component; i.e. it will provide a lifting force. When the aircraft is turning in the horizontal plane or in the vertical plane as in the loop, or anywhere in between, the lift is greater than the weight. In high-performance military aircraft it can be seven or eight times greater, because the lift provides the centripetal force to make the turn.

    Note: it's not always true that lift and drag act relative to the flight path. Imagine an aircraft flying straight and level, which encounters a substantial atmospheric updraught. Due to inertia the aircraft will, for the first milliseconds anyway, maintain its flight path relative to the Earth. During that time the 'effective airflow' passing by the wings will no longer be directly aligned with the flight path but will have acquired a vertical component. The lift will now act at 90° to this new 'effective airflow' rather than the actual flight path, and have a significant effect. Also, the wing itself modifies the effective airflow so just for now, until we look at aerofoils and wings, it is simpler to ignore the 'effective airflow' and other concepts and stay with the flight path.

    1.2 Vector quantities
    Velocities and accelerations
    Vector quantities have both magnitude and direction. Velocity is a vector quantity having both a magnitude (the airspeed) and a 3-dimensional direction. A force has both a direction in which it is pushing or pulling and a magnitude (in newtons [N]), thus it is a vector quantity. Momentum, having mass and velocity, is also a vector quantity, but inertia is not.

    Ignoring weight and friction for now; when only one force is applied to a stationary object, the object will accelerate in the same direction as the force applied. Acceleration is the rate of change of velocity, the change being either in speed or three-dimensional direction, or both.

    If an aircraft accelerates in a straight line from an airspeed of 25 metres/second [m/s] to 75 m/s in 10 seconds then the average change in airspeed per second is 75 –25 / 10 = 5 m/s, thus the acceleration is 5 metres per second per second [5 m/s²].

    The common usage term 'deceleration', referring to a reduction in linear speed only, is generally not used in physics as, in that science, 'acceleration' has both positive and negative connotations.

    Resultant forces
    When more than one force is being applied to an object there will be a resultant force, probably imparting an initial acceleration until all forces are again balanced at a new velocity. It is common practice to estimate resultant forces non-mathematically by drawing scaled, arrowed lines to represent each vector quantity, producing the resultant of two vector quantities in a vector parallelogram. The lengths of the lines represent the magnitude of each force and the placements indicate the application points and directions. The diagram is an exaggerated representation of an unpowered aircraft in a constant rate descent, showing that the lift/drag resultant is equal and opposite to the weight vector. In the diagram the resultant shown is the net aggregation of the aerodynamic forces generated by the wings, and it is conventional to then resolve that into its lift and drag components. We will be looking at aerodynamic forces in later modules.
    1.3 Weight
    Weight as a body force
    There is a common, if not universal, tendency to equate the mass of a body with its weight. This is not surprising, as both are usually expressed using the same unit — kilograms [kg]. You need to appreciate that weight is a 'body force', the product of the body mass and the acceleration due to gravity.

    The force due to gravity — or weight — of an aircraft on the ground or in flight is expressed as W = m × g, where — somewhat confusingly — m is the symbol for mass (rather than metres) and g is a gravity constant applied to objects on, or near, the Earth's surface. That constant is not a force but an acceleration of 9.806 m/s² — also known as the acceleration of free fall.
    In coming calculations we will use the performance of an early version of the Australian designed Jabiru aircraft as representative of the general aviation/ultralight four cylinder, two-seat, fixed-pitch and fixed-undercarriage recreational aircraft. All forces, including weight, are measured in newtons. The maximum allowed take-off weight [MTOW], of the Jabiru of mass 430 kg sitting on the runway is m = 430 × g= 9.806 = 4216 N. The Jabiru with an 80 kg pilot on board and with half the maximum fuel load would have a mass of only 340 kg which makes some difference to performance. We shall explore this in other modules. To simplify calculations, we will load full fuel, plus a lightweight passenger, into the Jabiru giving it a loaded mass of 400 kg and use g=10 m/s², thus weight = 4000 N.
    Centre of gravity
    The position of the centre of mass or centre of gravity [cg] within the aircraft will vary according to the seating of passengers and stowage of luggage. The knowledge of the total mass of the loaded aircraft and the cg position — the weight and balance — is very important, as we will see in the 'Weight and balance' module. Weight is always presumed to act from the cg position to the centre of the Earth. We will also see, in the 'Altitude and altimeters' module, that atmospheric conditions affect aircraft performance, and subsequently the appropriate MTOW.

    Vector quantities are sometimes very easy to calculate; for example if a Jabiru, weighing 4000 N, is cruising straight and level, then the lift force must be 4000 N pushing vertically upward.

    1.4 Lift
    The lift equation
    When an aircraft is cruising in straight and level flight, at low altitudes, the wings are set at a small angle — 3 to 5° — to the 'flight path' (or the 'line of flight' or the 'effective airflow' or the 'free stream airflow' or the 'relative wind', all of which mean much the same thing in cruising flight in a non-turbulent atmosphere). The net sum of the aerodynamic reaction on the wing is a resultant force directed upwards and backwards. Aerodynamicists have found it convenient to resolve that resultant force into just two components: that part acting backward along the flight path is the wing drag, and that acting perpendicular to the flight path is the lift. The amount of lift, and drag, generated by the wings depends chiefly on:

    (a) the angle at which the wing meet the airflow or flight path — the angle of attack
    (b) the shape of the wing, particularly in cross-section — the aerofoil
    (c) the density (i.e. mass per unit volume) of the air
    (d) the speed of the free stream airflow; i.e. the airspeed
    (e) and the wing plan-form surface area.

    There is a standard equation to calculate lift from the wings, which will be often referred to in these notes:

    (Equation #1.1) Lift [newtons] = CL × ½rV² × S

    The expression ½rV² (pronounced half roe vee squared) represents the dynamic pressure of the airflow in newtons per square metre [N/m²]. (Please note — if a 'Symbolic' font is not available, your browser will not display the Greek letter rho, the accepted symbol for air density, and may display r or ? instead.) The dynamic pressure expression, ½rV², is very similar to the kinetic energy expression ½mv², where m = mass. Air density is mass per unit volume; i.e. kg/m³, so the dynamic pressure of the airflow is the kinetic energy per unit volume.

    The values in the expression are:
    r (the Greek letter rho) is the density of the air, item (c), in kg/m³ V² is the airspeed, item (d), in m/s S is the wing area, item (e), in m² CL (C sub L) is a dimensionless quantity — the lift coefficient — which relates mostly to item (a), but also to item (b).  
    In normal operations for very light aircraft, and when there are no high lift devices incorporated in the wing structure, CL usually has a value between 0.1 and 1.5. It can be regarded as the ratio of the conversion of dynamic pressure into lift, by the wing, at varying angles of attack.

    Angle of attack and the lift coefficient
    Item (a) above, the angle at which the wings meet the flight path — more properly termed the geometric angle of attack — is near 16° at minimum controllable airspeed and around 2 to 5° when cruising at low altitudes; less at higher speeds, greater at higher altitudes. We will cover the close relationship between CL, angle of attack (aoa or alpha) and airspeed in the aerofoils and wings module.
    The diagram shows a typical CL vs angle of attack curve for a light aircraft not equipped with flaps or high-lift devices. From it you can read the CL value for each aoa, for example at 10° the ratio for conversion of dynamic pressure to lift is about 1.0.

    Note that CL still has a positive value (about 0.1) even when the aoa is –1°. This is because of the higher camber in the upper half of the wing; some highly cambered wings may still have a positive CL value when the aoa is as low as –4°. A light non-aerobatic aircraft pilot would not normally utilise negative aoa because it involves operating the aircraft in a high-speed descent, but we will discuss this further in the 'Flight at excessive speed' module.

    Also note that the lift coefficient increases in direct relationship to the increase in angle of attack, until near 16° aoa where CL reaches its maximum and then decreases rapidly as aoa passes that critical angle. A rule of thumb for light aircraft with simple wings is that each 1° aoa change — starting from –2° and continuing to about 14° — equates to a 0.1 CL change.

    Also, it is not just the wings that produce lift. Parts of a well-designed fuselage — the aircraft body — can also produce lift and the vertical component of the thrust vector can supplement lift when that vector is angled upwards.

    We can calculate CL for the Jabiru cruising at an altitude of 6500 feet and an airspeed of 97 knots (50 m/s). The wing area is very close to 8 m²:

    • lift = weight = 4000 N
    • r = 1.0 kg/m³ (the approximate density of air at 6500 feet altitude)
    • V² = 50 × 50= 2500 m/s
    • S = 8 m²

    Lift = CL × ½rV² × S

    Dynamic pressure = ½ ×1.0 × 2500 = 1250 N/m²

    So, 4000 = CL × 1250 × 8, — thus — CL = 0.4.

    Changing the angle of attack
    Now what happens if the pilot decides to decrease airspeed to 88 knots (45 m/s) while maintaining the same altitude? First, the pilot decreases power to give 88 knots then adjusts control pressure to maintain the same altitude — but look at the changes in the lift equation.

    • lift still equals weight = 4000 N
    • air density still = 1.0 kg/m³
    • V² changes and is now = 45 × 45 = 2025 m/s
    • S can't change = 8 m²

    Dynamic pressure = ½×1.0 × 2025 = 1012.5 N/m²
    So, 4000 = CL × 1012.5 × 8, — and — CL = 0.5 approximately.

    So, the result of decreasing airspeed, while maintaining straight and level flight, is an increase in the lift coefficient from 0.4 to 0.5. That has two possible contributors — the shape of the aerofoil and the angle of attack; items (a) and (b) above. Because the pilot can't change the aerofoil shape (unless flaps are extended, which we discuss in the 'Aerofoils and wings' module) the angle of attack must have changed. How? By the pilot adjusting control pressure to apply an aerodynamic force to the aircraft's tailplane (or some other control surface), which has the effect of rotating the aircraft just a degree or so about its lateral axis. Once the pilot has achieved the desired aoa, as indicated by the new airspeed (which will be explained in the 'Airspeed' module), the tailplane trim control is adjusted and the aircraft will then maintain that aoa.

    It may be appropriate to slip in another slight complication at this point. Lift, like weight, may be taken as acting through a central point — the centre of pressure [cp]. The position of the cp changes with aoa and this movement has a significant effect — it causes the nose of the aircraft to pitch up or down. So, the lift and weight are usually not in equilibrium and the rotational moment must be counteracted by aerodynamic forces produced by the horizontal stabiliser. Other tailplane surfaces also produce aerodynamic forces for trim and control, so to maintain an aircraft in straight and level flight — apart from the four forces mentioned — there will always be another force, or forces, generated by the fixed tailplane of most aeroplanes or its movable surfaces. We will look at this in the 'Stability' module.

    1.5 Thrust
    Action and reaction
    In the Jabiru the engine supplies torque directly to the propeller shaft and the propeller converts the torque to thrust; we will amplify how this is accomplished in the 'Engine and propeller performance' module. The propeller pushes backwards a tube of air with the same diameter as itself; i.e. it adds momentum to the tube of air where momentum = mass × velocity, and is also a vector quantity. Increasing the speed also imparts kinetic energy to the air. This tube of accelerated, energised air is the slipstream.

    Considering Isaac Newton's third law you expect an equal and opposite reaction to the action of adding momentum. This reaction is the application of forward momentum to the propeller, which pulls the rest of the aircraft along behind if the engine/propeller installation is a 'tractor' type, or pushes it if the engine/propeller installation is a 'pusher' type.

    The line of thrust
    We need to clarify the line of thrust. This line is extended forward through the propeller shaft, which is usually aligned with the longitudinal axis of the aircraft, but not always. For instance, the engine and propeller installation in the carrier-borne Hellcat, of Second World War fame, was vertically offset so that the thrust vector was 3° down; consequently, the aircraft flew with a rather jaunty tail-down attitude. I think the reason was that the thrust line then extended back over the cg, making the aircraft more stable at the very low speeds required for deck landing. (You can read a little about the deck landing techniques of those days — my youth — in this magazine article).

    The relationship of the longitudinal axis with the horizontal flight path — the aircraft's attitude — varies with the speed of the aircraft. At maximum allowable airspeed, or Vne, the longitudinal axis might coincide exactly with the flight path but as speed decreases, the axis starts to angle up and could be inclined 15° to the flight path at minimum controllable airspeed. Because the line of drag is always aligned with the flight path, then the thrust vector does not directly oppose the drag vector.

    The diagram, slightly exaggerated for clarity, shows the relationship between angle of attack and line of thrust to the flight path, for an aircraft maintaining level flight at a very slow speed. The flight path is horizontal so the drag vector will also be horizontal; i.e. aligned with the relative airflow. The line of thrust is aligned with the longitudinal axis, so the angle between the thrust line and the horizontal flight path is the aircraft attitude — in this case, its attitude in pitch. The wing chord line is extended so that the geometric angle of attack can be seen — the angle between the chord line and the flight path. The lift and weight vectors would both be at right angles to the flight path.

    You might notice from the diagram that the thrust vector will have quite a substantial vertical component, so that part of the thrust is supplementing lift. Thus we have just destroyed our previous assertion that if an aircraft is flying straight and level, lift must always equal weight. In this instance, the lift is less than weight and the (very small) shortfall is provided by the vertical component of thrust. So it is more correct to say that, if an aircraft is flying straight and level, lift plus the vertical component of thrust must equal weight.
    We can estimate the thrust delivered by the Jabiru's propeller, cruising at a speed of 97 knots at 6500 feet:
    From the pilot's operating handbook we find that the engine is rated at 80 hp × 746 = 60 kilowatts [kW], and cruise power for 97 knots is 65%, or 39 kW. The fixed pitch propeller is about 70% efficient at cruising speed so the effective power from the propeller is 27 kW.

    One watt is the work accomplished by a force of one newton moving an object one metre in one second. The aircraft is moving at 97 knots, or 50 m/s (knots × 0.514 = m/s), so, 27 000 W divided by 50 [m/s] equals 540 N and that is the thrust being provided, which also means the drag is 540 N. Compare that to the weight and lift of 4000 N, stated in section 1.3, and you see that the lift force — silently and efficiently generated simply by the angle of the wings and the velocity of the airflow — is 7.4 times the thrust force — noisily and inefficiently generated by the engine burning expensive fuel. That is being a bit unfair because the wings are really converting much of the thrust into a lifting force.

    Put another way the lift to drag ratio when cruising at 97 knots is 7.4:1. We will examine lift/drag ratios in the 'Aerofoils and wings' module.

    The slipstream
    The slipstream speed of an aircraft at cruising speed might be 20% greater than the aircraft speed, during a climb it could be 50% faster, and when the aircraft is maintaining height near its minimum controllable speed, slipstream velocity might be 100% greater. Some aircraft are designed so that the slipstream over the centre section of the wings increases V and thus lift, and the combination of the vertical component of thrust plus the slipstream effect means that possibly 25% of the thrust output is contributing lift when flying in a tail-down attitude.

    1.6 Air resistance to motion
    Induced drag and parasite drag
    Drag is the resistance of the air to an aircraft pushing through it. The resistance depends on:
    (a) the streamlining of the aircraft body
    (b) i. the excrescences attached to the airframe
    ii. turbulence at the junctions of structural components
    iii. the cooling airflow around the engine
    (c) the roughness of the surface skin
    (d) the 'wetted' area; i.e. the amount of surface exposed to the airflow
    (e) the density of the air
    (f) the speed of the airflow
    (g) the angle of attack.
    These components of drag are classified in several ways and we will look at them in the 'Aerofoils and wings' module. Part of the air resistance, the induced drag, is a consequence of item (g) the angle of attack. Induced drag is very high, maybe 70% of the total, at the high aoa of the minimum controllable airspeed, but induced drag decreases as speed increases, being possibly less than 10% of the total at full throttle speed. However the balance of the air resistance, known as parasite drag, increases as speed increases until the total air resistance equals the maximum thrust that can be produced.

    You can see from the diagram that parasite drag is directly proportional to dynamic pressure [½rV²] while induced drag is inversely proportional to it.

    Thus in normal straight and level flight, air resistance is high at both minimum and maximum airspeeds and lowest at some mid-range speed where — as resistance is at a minimum — the thrust required to maintain constant height will also be at a minimum; consequently, that is the speed — Vbr — which provides maximum range. If drag is at a minimum, then the lift/drag ratio will be at a maximum; consequently, this is very close to the best engine-off glide speed — Vbg.

    Air density (and thus air resistance) decreases with increasing altitude. So, the parasite drag component for a given airspeed decreases with increasing altitude while the induced drag component increases, because the wing has to fly at a greater aoa to produce the lift required.

    The standard expression for total aircraft drag is very similar to the lift equation:

    (Equation #1.2) Total drag [newtons] = CD × ½rV² × S

    where CD is the total drag coefficient and the ratio of total aircraft drag to dynamic pressure. CD increases as aoa increases.
    Things that are handy to know The generic term aircraft covers a wide range of airborne vehicles: lighter-than-air (aerostats); e.g. airships and hot-air balloons, and heavier-than-air (aerodynes). The latter includes gliders, powered parachutes, weight-shift controlled trikes or microlights, rotorcraft, helicopters, unmanned aerial vehicles and aeroplanes of all types from high performance supersonic fighters, jump jets or jumbo jets, to those like our friendly single-engine Jabiru.
      However, for the purpose of these notes we use the term aircraft to refer only to a class of 'general aviation' and 'sport and recreational aviation' aeroplanes with piston-engines driving propellers; relatively simple wing configurations including parawings; minimum power-off controllable airspeeds possibly as low as 20 knots; and possibly a maximum cruise airspeed, for a really high-performance aeroplane in that class, of 250 knots. These aircraft generally have one or two engines rated at 20 hp to 400 hp. The general aviation aircraft have normal wing loadings (i.e. weight/wing area) between 7 and 24 pounds per square foot (35–120 kg/m²) and maximum lift/drag ratios of 9:1 to 12:1, while ultralights have wing loadings between 4 and 12 pounds per square foot (20–60 kg/m²) and aircraft lift/drag ratios of 5:1 to 12:1.
      A newton [N] is the force required to give a mass of one kilogram an acceleration of one m/s².
      Isaac Newton's third law: If one body exerts a force on another there is an equal and opposite force — a reaction — exerted on the first body by the second.
      A knot is a speed of one nautical mile per hour, 0.5144 m/s or roughly 100 feet per minute. One nautical mile is the length, at the Earth's surface, of one minute of arc of a great circle and currently accepted to equal 1852 metres or 6076.115 feet.
    Stuff you don't need to know
    • Objects do not fall freely in the Earth's atmosphere. The air resistance (drag) increases as both fall velocity and air density increase until a terminal velocity is reached — where the drag force and the weight (the force due to gravity) are balanced — and the object stops accelerating. If the fall continues the object will start to slow slightly because of increasing air density at lower altitude, which increases drag. A streamlined body will have a higher terminal velocity than a non-streamlined body, of the same mass, because of the lower drag.

    12.1 The Bureau of Meteorology's Aviation Weather Service
    The Australian Government's Bureau of Meteorology (BoM) is required to support civil and military aviation by the provision of aviation weather services in the form of weather observations, forecasts and warning or advisory material. The BoM also supplies selected aviation products to Airservices Australia for their online pilot briefing system — the NAIPS Internet Service [NIS].

    The following aviation products can be accessed from the BoM Aviation Weather Services page — select the product category from those listed in the left-hand frame of the page.

    Aviation forecasts Low-level Area Forecasts [ARFOR] are a coded statement of the general weather situation for the lower levels of the atmosphere (up to 18 500 feet) and the expected conditions for a particular forecast area — the latter as detailed on the PCA or as indicated on the clickable map of Australia. The forecast period is not less than 9 hours or greater than 15 hours. The forecast is available at least one hour before commencement of the validity period. Pilots should regard forecasts as the best possible predictions from professional meteorologists supported with extensive computer modelling. However, meteorologists and computer modelling may not predict local micrometeorological events.
      Terminal Aerodrome Forecasts [TAF] are a statement of the most likely meteorological conditions expected, for a specified period, in the airspace within the vicinity of the aerodrome. TAFs are issued for about one third of Australian aerodromes, at not less than six hourly intervals, and are usually valid for 12 hours. Most of the weather reports and forecasts are encoded using the World Meteorological Organization/International Civil Aviation Organization international weather code.
      Area QNH
      (Terminal) Trend Forecasts [TTF] are only issued for the 20 or so major airports and military bases. TTFs are an aerodrome actual weather report combined with a forecast of changes to conditions during the next three hours. The TTF was introduced to overcome the time-span deficiencies of the TAF.  
    Instructions on how to read the ARFORs, TAFs and METARS are available online at the BoM's 'Knowledge Centre', accessible from the right hand side of the Aviation Weather Services page. The older aviation eHelp section still exists on the BoM website. (If a user name is requested use 'bomw0007' and the password 'aviation'.) You may find other useful material via the 'Educational and reference' box.

    Aviation observations
    Aerodrome routine meteorological reports [METARs] are routine observations of weather conditions at an aerodrome issued on the hour or half hour, often through automatic weather stations. SPECI are special reports issued when conditions meet specified criteria.
      Aerological diagrams and low level wind profiles are useful information for glider pilots.  
    Aviation weather packages
    Click the 'Charts only' button from the options provided to display all of the following:
    The latest Australian mean sea level pressure analysis The latest Australian mean sea level pressure forecasts The latest satellite image  
    The aerodrome weather information service [AWIS]
    Automatic weather stations [AWS] are located at about 190 airfields. All the stations are accessible by telephone and about 70 are also accessible by VHF NAV/COMM radio. The access telephone numbers and the VHF frequencies of the AWS can be found by entering the 'Location information' page and downloading the pdf for the relevant state. For an example of the service from an AWS call 08 8091 5549 to hear the current automatic weather information broadcast at Wilcannia, NSW.

    'Plain English' area forecasts, terminal aerodrome forecasts and meteorological observations
    Ian Boag has produced an excellent, freely available, online, well-tested, plain language meteorological translator [PLMT] available here on Recreational Flying (.com) under Resources , providing current ARFOR, METAR and TAF within all Australian ARFOR areas decoded into 'plain English'. However, pilots must still get the NOTAM from the Airservices site.

    Bear in mind that CAR 120 imposes penalties for use of forecasts that were not made with the authority of the Director of Meteorology, or by a person approved for the purpose by CASA, and it may be that plain English conversions are not authorised by the Director, but as the original section of code is presented under the decoded text, it is most likely that there is no problem with Ian Boag's excellent facility; it could be conceived as an learning tool for student pilots. Student pilots should be aware that the ability to decode BoM aviation reports and forecasts will be tested in some of the aviation examinations.

    General weather observations, forecasts and radar images
    Access to the latest general rather than aviation specific weather observations and forecasts plus satellite imagery (visible and infrared) are obtained via the BoM home page. Weather radar images (precipitation location and intensity), from about 50 weather watch radars, are updated at 10 minute intervals. The images from individual radars cover an area of 256 km radius but may be combined into a larger mosaic. The last four snapshots from each radar can be looped to provide a good indication of current storm development, intensity plus the direction and rate of movement. Lightning tracker websites such as Weatherzone provide useful information on current storm location and movement.

    12.2 Airservices Australia's NAIPS Internet Service
    The most convenient way to download the coded ARFOR, TAF and METAR plus the NOTAM is from Airservices Australia's NAIPS Internet Service [NIS], 'a multi-function, computerised, aeronautical information system. It processes and stores meteorological and NOTAM information as well as enabling the provision of briefing products and services to pilots and the Australian Air Traffic Control platform'.

    NIS is accessed through the internet with any web browser or access may be integrated within flight planning software. The Bureau of Meteorology provides all the weather products to the NIS.

    You must register with AsA before you can access the NIS. You are required to create a 'user name' and a password. If you don't have an ARN or Pilot Licence Number leave that field blank, don't use your RA-Aus or other sport and recreational organisation membership/Pilot Certificate number, it may conflict with someone's Aviation Reference Number. Download the NIS user manual (1.6 MB).

    When registered, you can log in; enter user name and password, and then click the required link. If you choose 'Area Briefing' you can select up to five briefing areas by clicking on the map or by entering the required areas in the entry boxes, and then click on the 'Submit Request' button. The ARFOR plus TAFs and METARs and NOTAM for the aerodromes in that area will be presented in the form of a pre-flight briefing. See an actual briefing with explanatory notes added. For further information read the weather check section of the Flight Planning and Navigation Guide.

    12.3 Acquiring weather information in flight
    There are several means of obtaining a limited amount of weather information while airborne:
    AERIS — the Automatic Enroute Information Service network ATIS — the Automatic Terminal Information Service at some aerodromes AWIS — the Aerodrome Weather Information Service at all automatic weather stations can be accessed by telephone and about 70 of them also provide VHF access. FLIGHTWATCH — the on-request service provided by Airservices Australia.  
    For further information read the acquiring weather information section of the VHF Radiocommunications Guide.

    Inflight weather warning broadcasts by Air Traffic Services
    SIGMETs report the occurrence or expectation of significant meteorological events such as widespread duststorms, a severe line squall or heavy hail. SIGMETs are issued by the BoM but broadcast by the Air Traffic Service for the affected area as a hazard alert; see AIP GEN section 5.1.
      AIRMETs report the occurrence or expectation of less severe meteorological events and applies only to aircraft operating below 10 000 feet. AIRMETs are issued by the BoM but broadcast by the Air Traffic Service as a hazard alert for the affected area; see AIP GEN section 5.3.
    12.4 AIP Book and ERSA
    Airservices Australia publishes online versions of the AIP Book and ERSA at www.airservicesaustralia.com/publications/aip.asp. You must click the 'I agree' button to gain entry. For further information about the meteorological service reports and forecasts, read the section AIP GEN 3.5 (about 50 pages). To find a particular section of AIP or ERSA you have to click through a number of index pages. The section/sub-section/paragraph numbering system is designed for an amendable loose leaf print document and you may find it a little confusing as an on-line document.

    11.1 Light scatter
    11.1.1 Rayleigh and Mie scatter
    Some of the visible light radiation from the sun, passing through the atmosphere bounces off atoms, molecules and other particles, and is scattered in all directions without losing energy or altering frequency. Gas molecules, being very much smaller than the wavelength of visible light (0.4 to 0.8 microns, see section 1.8 Electromagnetic wave spectrum), scatter the shorter violet and blue wavelengths much more strongly than the longer yellow and red wavelengths. But as the human eye is not very sensitive to violet light, the skyglow appears blue.

    Atmospheric dust and smoke particles are considerably larger than the gas molecules. But they may still be smaller than the wavelengths of visible light and thus also selectively scatter the blue end of the spectrum, but more strongly than the gas molecules. This phenomenon is termed selective scatter or Rayleigh scatter.

    Cloud droplets and small ice crystals are some 50 times larger than the light wavelengths and scatter all equally. Thus the light scattered from clouds retains the white light spectra, Mie scatter, and even though the droplets are colourless and transparent, the clouds appear white. Thicker clouds have darker bases because most of the light is scattered out the top and sides.

    When the sun is directly overhead, the direct parallel rays that reach the eye from the sun's disc travel only a short distance through the atmosphere, so the sun's disc appears white. As the sun lowers, the distance travelled through the atmosphere increases, as does the scattering of the blue end. The depleted unscattered light that reaches the eye makes the disc appear yellow to orange to red, depending on the number and size of non-gaseous particles in the air. If there is a lot of dust or smoke haze in the path, only the red end of the sun's rays will remain unscattered — even the scattered light becomes reddish.

    The amount of the visible light spectrum scattered is dependent on line-of-sight distance through the atmosphere. The sky near the horizon appears less blue, or whiter, at midday than the sky overhead; thus if the atmosphere were thicker, the sky would be whiter.

    Similarly, when looking horizontally at a series of mountain ranges they appear bluer at a distance, until a point where the far ranges start to appear whiter than those in the middle distance. The trees on the ranges emit terpenes or essential oils — hydrocarbon molecules about 0.2 micron diameter, which combine with ozone infiltrating from the stratosphere. These molecules selectively scatter blue light — hence the blue haze on warm days.

    Air molecules selectively scatter sunlight forward and backward equally, and at about twice the intensity of the light scattered at right angles to the beam. For particles larger than the wavelengths of light, back-scattered light is less intense than that for gas molecules but forward scatter is much more intense. Thus, in an atmosphere containing many large particles, the sky is less bright than blue sky when looking 'down sun' and much brighter when looking in the azimuth of the sun.

    White-out conditions can occur when the surface has a complete snow or ice cover, matched with an extensive cloud cover. The brightness of the cloud cover is increased by light that is successively scattered many times between surface and cloud, with little absorption. The light travels in all directions and at all angles. In such conditions there can be no shadows, the horizon line disappears and the form of the landscape is no longer discernible. This leads to spatial disorientation. Partial white-out or flat light is a less severe condition where a pilot's ability to judge ground references for distance, height and attitude are detrimentally affected.

    11.1.2 Twilight effects
    The characteristic light, during the morning and evening twilight periods, is due to atmospheric scattering. The duration of twilight is geometrically dependent on latitude, season and the observer's elevation. Evening civil twilight is the period from sunset until the centre of the sun's disc is 6° below the normal horizon; i.e. ignoring the topography. If the sky is clear, it is usually practicable to carry out normal outdoor activities without artificial light; thick overcast will reduce available light at the surface considerably during the civil twilight periods, as may elevated topography to the west in the evening and to the east in the morning.

    Last light is the end of evening civil twilight; and the official end of daylight in VFR air navigation regulations.

    First light is the beginning of morning civil twilight and the official start of daylight in the regulations. It is not the time at which a line of light appears on the eastern horizon — if you take-off in those conditions you will be night flying.

    Evening nautical twilight ends when the sun is 12° below the horizon. During this period the western horizon is still clearly defined, weather permitting, and the brighter stars are visible — thus providing good conditions for ocean navigators to take star sights; hence nautical. Noctilucent clouds may be seen in higher latitudes. Evening astronomical twilight ends when the sun is 18° below the horizon, after which all scattered sunlight disappears from the upper atmosphere and the stargazers have good viewing conditions. The morning twilight periods are reversed, of course.

    The twilight wedge, or curve, divides the Earth's shadow from that part of the sky lit by direct sunlight. It appears on clear days as a blue-grey arc next to the eastern horizon as the sun disappears, highest at the antisolar point and curving down to the horizon. Initially there is a fairly sharp boundary bordered by a reddish band, the counterglow, then becoming diffuse as it rises. An airborne observer should see a sharp boundary above the horizon. Similar shadowing occurs at sunrise on the western horizon.

    Usually after sunset the sky above that point is pale yellow with a blue-white arch above, the twilight arch, with yellow above and orange sky to either side. As twilight progresses, the arch above the sunset point becomes pink with yellow and orange below. These areas gradually flatten as the sky above changes from blue-grey through to dark blue. The final glimmers on the horizon are possibly greenish-yellow. Very rarely, and mostly when viewed over water when the air is free from any form of haze, a green flash is seen on the top of the sun's disc just before it disappears.

    Zodiacal light is a faint, luminous glow in the night sky, easily seen in low to mid-latitudes at twilight in moonless conditions. It is caused by sunlight scattered by dust particles in interplanetary space. Zodiacal light extends over the entire sky but is brightest in the zodiacal band, and at about 30° angular distance from the sun, where the intensity is about three times that of the brightest part of the Milky Way. It is best seen when the ecliptic is close to vertical; i.e. autumn evenings and spring mornings. Brightness decreases with angular distance from the sun, being lowest at 120° then gradually increasing to the 180° antisolar point. The enhanced brightness near the solar point, and covering an area 6° by 10°, is the Gegenschein or counter-glow.

    Airglow is visible infrared [IR] and ultraviolet [UV] emissions from the atoms and molecules in the ionisation layers caused by absorption of much of the solar UV radiation and of cosmic radiation. Daytime airglow, dayglow, may be seen from the surface at twilight when the blue skyglow is sufficiently weak. Dayglow is caused mainly by the dissociation of atoms, whereas nightglow emissions are due to recombination. The sum of all visible nightglow emissions, together with zodiacal light and scattered starlight, can be seen as the faint light between stars.

    Crepuscular (twilight) rays are alternate light and dark bands that appear to diverge fan-like from the sun's position when it is hidden behind a cloud bank or the topography, in a humid or hazy atmosphere. The rays pass through gaps, like light beams shining through high windows. The divergence is due to perspective, if the rays pass overhead they then appear to converge on the antisolar point — anticrepuscular rays.

    There are three types of crepuscular rays:
    rays of light passing through gaps in low clouds rays of light diverging from behind a cloud bank pinkish rays radiating from below the horizon.  
    11.2 Atmospheric optical displays
    11.2.1 Electromagnetic wave refraction, reflection and diffraction
    When a light ray passes obliquely from one transparent medium to another, or between layers of different density within the same medium, part of the ray is returned back at the boundary. The remainder, passing through, is deviated from its original course; i.e. its direction changes. The deviation is dependent on angle of incidence; the wave lengths of the light beam, or radio wave; and the refractive index for that medium. The refractive index is the ratio of the speed of electro-magnetic radiation in free space to the speed of radiation in that medium; in air it is effectively 1.0, and in water it is 1.33.

    Refraction has two components — deviation and dispersion. As the components of sunlight have different wavelengths, in the atmosphere the deviated light ray is dispersed into its component colours but the red light deviates less than the blue light when passing from air through ice crystals or water droplets.

    Radio waves in the High Frequency [HF] bands are refracted by the ionisation layers in the atmosphere. The downward bending of the wave is sufficient to redirect the wave back to the Earth's surface but at a distance from the transmission point. If there is sufficient energy, the wave may then be reflected back to the ionosphere. Thus a high-energy HF transmission is able to 'skip', between the surface and the ionosphere, for a considerable distance around the world.

    Reflection is the bounce back of all, or part, of a light ray when it encounters the boundary of the two media, and the angle of reflection equals the angle of incidence. The amount of light reflected depends on the ratio of the refractive indices for the two media.

    Diffraction is the bending of a light beam (or radio wave) into the region of the geometric shadow of an obstacle, or the spreading of light waves around obstacles. This produces a series of light and dark bands or rings or coloured spectra, from the inter-ray interference; constructive interference results in light bands, while destructive interference results in dark bands. The degree of diffraction depends on wavelength — red light is diffracted more than blue — and particle size.

    11.2.2 Ice crystal displays
    Halos are a range of optical phenomena that result when the sun or moon shines through thin cloud — particularly CS — fog or haze composed of ice crystals. The small ice crystals that grow in the troposphere tend to be hexagonal flat plates or hexagonal columns. Light passing through the sides of a hexagonal ice crystal is refracted in exactly the same way as if it were passing through a 60° prism.

    The magnitude of the deviation angle depends on the orientation of the crystal. For a 60° ice prism the minimum deviation angle for all orientations is 22°; and for small rotations of the crystal, at the minimum deviation angle, the variation from 22° is insignificant. Thus in an atmosphere of randomly oriented crystals there will be a concentration of rays deviated by 22°. The deviation of light from its original path, through many hexagonal crystals, brings sunlight or moonlight to the observer's eye from different directions and in varying intensities. However, the concentration of refracted rays around 22° produces a solar or lunar halo whose inner, red edge has an angular radius of 22° from the observer's eye. The red edge merges into a yellow band then all the colours overlap in an outer white band. Halos are minimum deviation effects; each colour has a concentration at its minimum deviation angle, but also has a significant amount of light refracted at greater angles and overlaps other colours. Only the red, with the lowest deviation, cannot be overlapped.

    Light passing through one side and an end of a hexagonal crystal is refracted in the same way as in a 90° prism and, in this case, there will be a concentration of rays at a 46° deviation angle. In suitable conditions a very large solar or lunar halo with an angular radius of 46° may appear, but it will be much less intense than the 22° halo and will rarely be complete. The 22° halo is the most frequently observed of all the ice crystal displays; the 46° halo is rather rare.

    As cloud crystals grow during fall (flat plates perhaps 50 microns thick and several millimetres across, columns perhaps 100 microns across and several millimetres long), the drag creates lee eddies and the crystals tend to orient with their longest dimension near horizontal. They oscillate randomly as they fall in a spiral path, producing complicated optical effects through reflection, refraction and diffraction.

    Sun pillars are vertical columns of light that appear above or below the sun, or both, when the sun is near the horizon. They are caused by reflection of sunlight from the near-horizontal surfaces of ice crystals and are similar to the glitter path of sunlight reflected on water. Light pillars are also associated with the moon. A subsun is a particular form of sun pillar seen from an aircraft when the sun is high — becoming a reflected, elongated image of the sun in nearly horizontal ice crystals in lower clouds. The image appears as far below the horizon as the sun is above. Sun pillars may be associated with AC.

    The parhelic circle is a reflection from the vertical surfaces of horizontally oriented flat plate or columnar crystals when very small ice crystals, diamond dust , fall through the air. The crystals reflect the light in all directions of the azimuth but always downward at the same elevation as the sun. Thus if the sun's elevation is 25° an observer would see the parhelic circle 360° around the horizon by looking up 25°, but usually only part of the faint white circle is seen. The parhelic circle and a sun pillar may form a cross in the sky, centred on the sun.

    If falling plate crystals maintain a horizontal position, with the sun low in the sky, they have the possibility to refract light to the observer from the sides of the 22° halo, but not from other positions in the halo. The result is a spot of increased light intensity and colour separation — red towards the sun — in the 22° halo each side of the sun, where the halo would intersect the parhelic circle; sometimes it appears with a white tail pointing away from the sun. As the sun elevation increases, the spots move further from the sun and outside the halo, disappearing at sun elevations greater than 60°. These intensified light spots are called parhelia, sundogs or mock suns and are the most common ice crystal phenomenon after 22° halos; they are often associated with CI, CS and possibly AC. Similar effects associated with the moon are paraselena or mock moons.

    Refraction through the edges of plate crystals with nearly horizontal bases may produce a circumzenithal arc. This is part of a circle, possibly one third, centred directly above the observer's head and above the sun, just outside the 46° halo position. The halo may also be visible. The circumzenithal arc cannot occur when the sun's elevation exceeds 32°. Colour separation occurs with red on the outer rim, blue on the inner. The arc may be associated with CI and CS.

    An anthelion is a concentration of back reflected light at the anthelic point, 180° from the sun and at the same elevation. The anthelic point may be the centrepoint for various reflection / refraction phenomena — the anthelic arcs.

    Various other light intensifications are associated chiefly with refraction and may appear in ice crystal displays in Antarctic conditions. Among them are:
    Parry arcs circumhorizontal arcs supralateral arcs infralateral arcs contact arcs upper and lower tangents to the 22° halo.  
    11.2.3 Cloud droplet effects
    The moon or sun when viewed through CC, AC, thin AS or SC may be surrounded by a diffraction disc, or aureole of light, of varying size and intensity. The aureole is bluish near the sun or moon,and whiter further out with a red/brown periphery. The aureole may be enclosed by rings with blue inner and red outer edges forming a corona. The size of the rings depends on droplet size, smaller droplets produce larger rings. If there is a wide mixture of droplets of varying size then the diffraction rings will be of widely varying size, overlapping each other and blurring into a uniform illumination, leaving only the aureole visible.

    Cloud irisation or iridescence ( Iris = the Greek rainbow goddess ) appears when a cloud element or streak, usually AC or CC and sometimes lenticularis, is evaporating around its edges so that the droplet size changes quickly over a short angular distance. Also the entire element or small cloud is contained in roughly the same angular distance from the sun. The diffraction pattern traces blue light around the edge of the cloud where the droplets are smallest, and red light where the drops are uniformly larger. The result is iridescent bands — predominantly pinks and blues or greens with pastel shades — appearing along the thinner edges of individual cloud elements. Cloud iridescence is common but the cloud must be within 20° of the sun and thus not readily noticeable. It can occur in thin SC or AS, and also in nacreous clouds.

    The corona is the diffraction pattern seen in cloud droplets when looking towards the sun. The glory is the diffraction pattern seen in cloud or fog droplets when looking toward the antisolar point. (A glory is the circle of light or aureole around the depiction of the head of a saint, etc.) When flying in sunlight over a cloud layer, the coloured rings of glory may be seen around the antisolar point; i.e. around the aircraft shadow if it is not diffused. The antisolar point is that of the observer, so the luminous coloured halos are centred on the position of the observer's head shadow. As in other diffraction rings, the blue halo is on the inside and the red on the outside.

    The 'silver lining' that may be seen around the outer edges of heavier clouds, containing larger droplets, is a diffraction effect.

    11.2.4 Rainbows
    As a light ray from the sun strikes a small spherical raindrop (drops less than 150 microns diameter are held as a sphere by surface tension, while larger raindrops are distorted by drag into a flattened sphere) some light is reflected by the outer surface. Some light passes through and reaches the opposite inner surface, where a fraction of the light is reflected internally and the rest passes out of the drop. A ray may be reflected only once inside a drop, or many times, but each reflection is accompanied by light leaving the drop, so each internal reflection diminishes the reflected ray.

    Each spherical raindrop reflects and refracts, in all directions, the light rays that are striking it. However, due to the spherical surface there is a concentration of first reflection rays reflected back towards the sun, around a maximum angle of about 42° to the axis line joining the raindrop and the sun. The red light is refracted less than the other colours and has a concentration at about 42°. The blue light is concentrated at 40° with the other colours in between.

    The observer will see this concentration of reflected light rays as an intensified coloured light band. This band consists of the first reflection rays from all the raindrops that lie on the surface of a cone, subtended at the observer's eye, with an angular radius of 42° from an axis line drawn from the sun (directly behind the observer) through the observer's head and extended down-sun to the antisolar point; i.e. below the horizon where the shadow of the observer's head might be. This primary rainbow will have the red band on the outer edge. An observer on the Earth's surface sees only an arc of the rainbow circle. When the sun is 40° above the horizon, just the top of the bow can be seen. The rainbow will rise as the sun lowers, until much of the circle can be seen. The lower ends may appear very close to the observer. An airborne observer could possibly see the full circle.

    Light that is reflected twice within the raindrops has a deviation angle of 51° and produces the weaker secondary rainbow — concentric with and outside the primary, but with the red band on the inner edge. Thus the observer is seeing the concentration of twice reflected rays from all the raindrops that lie on the surface of a 51° cone, at the same time they are seeing the first reflections from the raindrops on the 42° cone. Third and fourth reflection rays would also form rainbows with angular radii of 40° and 46° respectively. These are so weak, and would also form up-sun, so that they are most unlikely to be seen except against a dark cloud.

    As the first reflection rays from spherical raindrops have a maximum deviation angle of 42°, it follows that all the low-angle reflections coming back to the observer's eye, from all the raindrops enclosed within the 42° cone, will increase the brightness of the sky within the primary bow. Similarly the sky is also brighter outside the secondary bow.

    The rainbow ends are frequently brighter than the rest of the bow, particularly when the sun is low. This comes from the approximate straight back reflection / refraction in the larger, flatter raindrops added to the reflection / refraction of the smaller, spherical drops.

    Diffraction interference of light rays ( the waves are out of synchronisation ) produces changes in light intensity, which may appear as a series of light / dark bands within, and close to, the primary rainbow.

    When rainbow rays pass through very small water droplets (e.g. cloud or fog droplets) they are spread by diffraction, and each colour band is broadened and overlaps adjoining bands. Where all the colours overlap, the result is a white rainbow, cloud bow or fog bow; this is often seen from an aircraft flying over a smooth, extensive cloud layer. Near sunset, a white rainbow may appear as a red rainbow in a low cloud bank. A full moon can produce a rainbow that appears to be white in the low light conditions but, when photographed, is revealed as a normal rainbow.

    11.2.5 Atmospheric density layer effects
    When a light ray passes through the atmosphere, where the density changes gradually, the light ray changes direction in a curved path rather than abruptly as when passing through an ice crystal. With changes in atmospheric density, the deviation path curves toward the denser air. Thus when a star is low in the sky, the change in atmospheric density with height, particularly with a cold surface layer under an inversion, causes refraction to bend the light rays so that the star's apparent position is higher than actual and the dispersion may produce a multi-colour image — upper part blue, middle white and lower part red. This gives the impression of an aircraft's lights, and is often reported as strange, moving lights in the sky, as the atmospheric effects make the object appear to jiggle. At sunset or sunrise, refraction can cause the sun's image to appear above the horizon when it is actually below.

    Small-scale atmospheric temperature and density variations in the line of sight between the observer and a star, or other light-emitting object, produce the twinkling effect scintillation, and the shimmering of distant landscape. Parcels of cooler or warmer air can act as lenses, reducing or increasing the apparent brightness or size of the object.

    Mirages are optical phenomena produced by refraction of light rays through air layers with large temperature gradients. An inferior mirage (i.e. it appears below its actual position) occurs when the temperature initially decreases rapidly with height. For example, the heat flux from a hot surface, such as tarmac or sand, greatly increases the temperature of the adjacent shallow air layer and consequently the density of that layer decreases (see equation of state). The result is a layer of less dense air underlying denser air, the reverse of the normal lapse rate. Light rays from the sky moving through the layers will be refracted upward in the less dense air (i.e. bent toward the denser air), giving the appearance of a layer of water.

    When seen from the ground or water, a superior mirage (i.e. it appears above its actual position) occurs when there is a pronounced inversion near the surface, and normally over the sea or a large body of water. A distant object within the inversion layer, even something below the horizon, will appear in the sky above its actual position — possibly totally upside down or the upper portion upside down, but certainly distorted and wavering. For more information google the phrase "superior mirage".

    An inversion layer of cooler air, with warmer air above and below, acts as a wave guide for light rays introduced into the layer at a small angle to the horizontal. Unless there is a discontinuity in the layer, the trapped rays cannot escape and may be confined within the wave guide for very long distances, following the curvature of the Earth. In such circumstances, a spectacular superior mirage might be seen from an aircraft flying over land within that wave guide. Whit Landvater is a Nevada balloonist who experienced such a display on November 27, 2003 and said "It was like "living inside a Photoshop document while someone was going crazy with the clone tool and filters!"

    11.3 Moon phases
    The geometry of the sun–Earth–moon orbits gives rise to the eight commonly recognised moon phases and the associated moonrise/moonset periods.. The elapsed time from one full moon to the next is about 29.5 days.
    Moon phases and moonrise / moonset periods Phase Appearance Rises Sets New moon Waxing crescent dawn dusk First quarter Waxing gibbous noon midnight Full moon Waning gibbous dusk dawn Last quarter Waning crescent midnight noon  

    10.1 The global electrical circuit
    The Earth's surface — ocean and solid — and the ionosphere are highly conductive. The atmosphere conducts electricity because of the presence of positive and negative ions plus free electrons. Conductivity is poor near sea level but increases rapidly with height up to the ionosphere; also it is greater at polar latitudes than equatorial. The conductivity near sea level is low because there are fewer ions, and those ions tend to become attached to the larger aerosol particles that are more common near the surface. Refer to section '1.5 Atmospheric moisture'.

    During fair weather there is an electric potential difference of 250 000 to 500 000 volts between the ionosphere and the Earth's surface, the surface being negative relative to the ionosphere. This gives rise to the fair weather current, which is a steady flow of electrons from the surface at about one microwatt per square metre.

    The three main generators in the global electrical circuit are the solar wind entering the magnetosphere, the ionospheric wind and thunderstorms. The average CB generates a current of about one amp during its active period. With an estimated 1000 to 2000 thunderstorms continually active around the globe, emitting possibly 5000 lightning strokes per minute, there is an electrical current of 1000 to 2000 amps continually transferring a negative charge to the surface, and an equal and opposite charge to the upper atmosphere. The electrical charge continually flowing into the stratosphere/ionosphere from the CBs maintains the fair weather current flowing to the surface.

    10.2 Static charge and discharge
    Apart from the CB clouds, the atmosphere carries a net positive charge and the electric potential increases with height, and in cloud and fog. Strong electrical forces also exist in and around rain showers, which can transfer a charge of either polarity to the surface, or to an aircraft. Static electricity is the imbalance of negative and positive charge.

    Aircraft accumulate electrical charges in two ways. The most substantial is from flying through the extremely high voltage electrical fields associated with CB, or potential CB development. The static charge can pervade the whole aircraft, internally and externally, and render navaids useless. The rapid discharge of this charge — a single-channel spark discharge rather than a slow bleed-off from the airframe — may happen in any conditions, but the chances are more probable in temperatures between 10 °C and –10 °C, and where flying in rain mixed with snow.

    The other lesser type is precipitation static. The aircraft charge accumulates from the charge carried by precipitation particles, particularly snow crystals, and separates when the particles break up against the aircraft. Maximum build-up occurs in temperatures a few degrees either side of 0 °C.

    Static charges imparted to antennae will affect communications, particularly navaids where the effect on signal-to-noise ratio may be considerable. The built-up static charge is usually slowly bled off into the atmosphere, or as a quiet, non-luminous point discharge. In extreme build-ups, the consequent corona discharge streamers or brush discharge are manifested as St Elmo's fire, which is usually not visible in daylight but visible at night as a continuous, luminous blue-green discharge from wing tips, propellers and protuberances.

    10.3 Lightning
    The electrostatic structure within CB, or CU CON, is such that pockets of different charge exist throughout the cloud. Generally, the main net positive charge resides on the ice crystals in the upper part of the cloud and the main net negative charge of similar magnitude is centred near the middle or lower part of the cloud at the sub-freezing level. That charge mainly resides on supercooled droplets. A smaller positive charge centre may exist at the bottom of the cloud where temperatures are above freezing. The electrostatic forces of repulsion and attraction induce secondary charge accumulations outside the cloud, a positive region accumulates on the Earth's surface directly below the cloud. Above the cloud, positive ions are transferred away from, and negative ions are transferred toward, the cloud.

    One favoured theory for the charge separation mechanism is the 'precipitation' theory. This suggests that the disintegration of large raindrops, and the interaction between the smaller cloud particles and the larger precipitation particles in the updrafts and downdrafts, causes the separation of electrical charge — with downward motion of negatively charged cloud and precipitation particles, and upward motion of positively charged cloud particles.

    Discharge channels
    Lightning is a flow of current, or discharge, along an ionised channel that equalises the charge difference between two regions of opposite charge; this occurs when the charge potentials exceed the electrical resistance of the intervening air. These discharges can be between the charged regions of the same cloud (intra-cloud), between the cloud and the ground (cloud-to-ground), between separate clouds (cloud-to-cloud) or between the base of a cloud and a charge centre in the atmosphere underneath it (cloud-to-air). The discharge channels, or streamers, propagate themselves through the air by establishing, and maintaining, an avalanche effect of free electrons that ionise atoms in their path. Lightning rates, particularly intra-cloud strokes, increase greatly with increase in the depth of clouds. Cloud-to-cloud and cloud-to-air discharges are rare but tend to be more common in the high-base CB found in the drier areas of Australia. Discharges above the CB anvil into the stratosphere and mesosphere also occur.

    When intra-cloud lightning — the most common discharge — occurs, it is most often between the upper positive and the middle negative centres. The discharge path is established by a 'stepped leader', the initial lightning streamer that grows in stages and splits into more and more branches, as it moves forward seeking an optimal path between the charge centres. The second, and subsequent, lightning strokes in a composite flash are initiated by 'dart leaders', streamers that generally follow the optimum ionised channel established by the stepped leader. The associated electrical current probably peaks at a few thousand amperes. A distant observer cannot see the streamers but sees a portion of the cloud become luminous, for maybe less than 0.5 seconds, hence 'sheet lightning'.

    Cloud-to-ground discharges
    Most cloud-to-ground discharges occur between the main negatively charged region and the surface — initially by a stepped leader from the region, which usually exhibits branching channels as it seeks an optimal path. When the stepped leader makes contact, directly with the surface or with a 'ground streamer' (which is another electrical breakdown initiated from the surface positive charge region and which rises a short distance from the surface), the cloud is short-circuited to ground; to complete each lightning stroke, a 'return streamer', or return stroke, propagates upwards. (The return streamer starts as positive ions that capture the free electrons flowing down the channel and emit photons. The streamer carries more positive ions upward, and their interaction with the free-flowing electrons gives the impression of upwards movement.) The charge on the branches of the stepped leader that have not been grounded flow into the return streamer. Subsequent strokes in the composite flash are initiated by dart leaders, with a return streamer following each contact. The return streamer, lasting 20–40 microseconds, propagates a current-carrying core a few centimetres in diameter with a current density of 1000 amperes per cm² and a total current typically 20 000 amps, but peaks could be much greater. A charged sheath or corona, a few metres in diameter, exists around the core. The stroke sequence of dart leader–return streamer occurs several times in each flash to ground, giving it a flickering appearance. Each stroke draws charge from successively higher regions of the CB and transfers a negative charge to the surface. Return streamers occur only in cloud-to-ground discharges and are so intense because of the Earth's high conductivity. Some rare discharges between cloud and ground are initiated from high surface structures or mountain peaks, by an upward-moving stepped leader and referred to as a ground-to-cloud discharge. Rather rarely an overhanging anvil-to-ground discharge can be triggered by heavy charge accumulation in the anvil, and the high-magnitude strike can move many kilometres from the storm — a 'bolt from the blue', but another reason for recreational pilots to give large storm cells a very wide berth.

    The temperature of the ionised plasma in the return streamer is at least 30 000 °C and the pressure is greater than 10 atmospheres. This causes supersonic expansion of the channel, which absorbs most of the dissipated energy in the flash. The shock wave lasts for 10–20 microseconds and moves out several hundred metres before decaying into the sound wave — thunder — with maximum energy at about 50 hertz. The shock wave can damage objects in its path. The channel length is typically 5 km. Channel length can be roughly determined by timing the thunder rumble after the initial clap; e.g. a rumble lasting for 10 seconds x 335 m/sec = 3.3 km channel length. When a lightning stroke occurs within 150 m or so, the observer hears the shock wave as a single, high-pitched bang.

    Effect on aircraft instruments
    The lightning discharges emit radio waves — atmospherics or 'sferics — at the low end of the AM broadcast band and at TV band 1. These radio waves are the basis for airborne storm mapping instruments such as Stormscope and Strikefinder. The NDB/ADF navigation aids also operate near the low end of the AM band, so that the tremendous radio frequency energy of the storm will divert the radio compass needle. Weather radars map storms from the associated precipitation.

    Strike effect on aircraft
    When most aeroplanes, excluding ultralights, are struck by lightning the streamer attaches initially to an extremity such as the nose or wing tip, then reattaches itself to the fuselage at other locations as the aircraft moves through the channel. The current is conducted through the electrically bonded aluminium skin and structures of the aircraft, and exits from an extremity such as the tail. If an ultralight is struck by lightning, the consequences cannot be determined but are likely to be very unpleasant. Ultralights particularly should give all CBs a wide berth; supercells and line squalls should be cleared by 25–30 nm at least.

    Although a basic level of protection is provided in most light aeroplanes for the airframe, fuel system and engines, there may be damage to wing tips, propellers and navigation lights, and the current has the potential to induce transients into electrical cables or electronic equipment. The other main area of concern is the fuel tanks, lines, vents, filler caps and their supporting structure, where extra design precautions prevent sparking or burn-through. In heavier aircraft, radomes constructed of non-conductive material are at risk.

    10.4 Red sprites and blue jets
    When large cloud-to-ground lightning discharges occur below an extensive CB cluster with a spreading stratiform anvil, other discharges are generated above the anvil. These discharges are in the form of flashes of light lasting just a few milliseconds and probably not observable by the untrained, naked eye but readily recorded on low-light video.

    Red sprites are very large but weak flashes of light emitted by excited nitrogen atoms and equivalent in intensity to a moderate auroral arc. They extend from the anvil to the mesopause at an altitude up to 90 km. The brightest parts exist between 60–75 km, red in colour and with a faint red glow extending above. Blue filaments may appear below the brightest region. Sprites usually occur in clusters that may extend 50 km horizontally. Blue jets are ejected above the CB core and flash upward in narrow cones, which fade out at about 50 km. These optical emissions are not aligned with the local magnetic field. Images and further information are available at the University of Alaska site.

    10.5 Auroral displays
    The Aurora Australis is usually only seen from latitudes higher than 60° south but may sometimes be seen from the Australian mainland. The displays, or aurora storms, take place at altitudes of 100–300 km. The auroral glow is caused by an increase in the number of high-energy, charged particles in the solar wind (separated hydrogen protons and electrons) associated with increased solar flare activity. Some of these particles, captured by the magnetosphere, are accelerated along the Earth's open magnetic field lines (which are only open in the polar regions) and penetrate to the inner Van Allen belt, overloading it and causing a discharge of the charged particles into the ionosphere. The discharges extend in narrow belts 20–25° or so from each magnetic pole. The excitation of oxygen and nitrogen atoms by collision with the particles causes them to emit visible radiation — forming moving patches, bands and columns of limited colours.

    The display colour depends on the gas and the altitude. Oxygen atoms emit a red glow at high levels, orange at medium levels and pale green at low levels. Nitrogen emits blue and violet at high levels and red at low levels.

    The major forms of auroral display, and typical sequence of appearance, are: glow — a faint glow near the horizon, usually the first indication of an aurora arch — a bow-shaped arc running east to west, usually with a well-defined base and small waves or curls rays — vertical rays or streaks, often signifying the start of an aurora substorm and forming into bands band — a broad, folded curtain moving in waves and curves, and indicating maximum activity is near corona — rays appear to converge near the zenith veil — a weak, even light across a large part of the sky often preceding the end of the display patch — an indistinct nebulous cloud-like area which may appear to pulsate.  
    Extensive auroral displays, which are associated with high sunspot activity, are accompanied by disturbances in radio communications. The period of maximum and minimum intensity of the aurora follows the 11-year sunspot cycle.

    9.1 Airframe icing
    High humidity and low winter freezing levels in south-east Australia provide likely conditions for icing at low levels. Hopefully it is unlikely that an ultralight or VFR GA pilot would venture into possible icing conditions, but the pilot of an enclosed cockpit ultralight may be tempted to fly through freezing rain or drizzle. Aircraft cruising in VMC above the freezing level, and then descending through a cloud layer, may pick up ice.

    The prerequisites for airframe icing are: the aircraft must be flying through visible, supercooled liquid; i.e. cloud, rain or drizzle the airframe temperature, at the point where the liquid strikes the surface, must be zero or sub-zero.  
    The severity of icing is dependent on the supercooled water content, the temperature and the size of the cloud droplets or raindrops. The terms used in the Australian Bureau of Meteorology icing forecasts are:
    light: less than 0.5 g/m³ of supercooled water in the cloud — no change of course or altitude is considered necessary for an aircraft equipped to handle icing. No ultralight and very few light aircraft are equipped to handle any form of airframe ice moderate: between 0.5 and 1.0 g/m³ — a diversion is desirable but the ice accretion is insufficient to affect safety if anti-icing/de-icing is used; unless the flight is continued for an extended period severe: more than 1.0 g/m³ — a diversion is essential. The ice accretion is continuous and such that de-icing/anti-icing equipment will not control it and the condition is hazardous.  
    The diagram below shows the ice accretion in millimetres on a small probe, for the air miles flown in clouds with a liquid water content varying from 0.2 g/m³ to 1.5 g/m³.
    The small, supercooled droplets in stratiform cloud tend to instantaneous freezing when disturbed and form rime ice — rough, white ice that appears opaque because of the entrapped air. In the stable conditions usually associated with stratiform cloud, icing will form where the outside air temperature [OAT] is in the range 0 °C to –10 °C. The continuous icing layer is usually 3000 to 4000 feet thick.

    The larger, supercooled droplets in convective cloud tend to freeze more slowly when disturbed by the aircraft; the droplets spread back over the surface and form glossy clear or glaze ice. Moderate to severe icing may form in unstable air where the OAT is in the range –4 °C to –20 °C. Where temperature is between –20 °C and –40 °C the chances of moderate or severe icing are small except in CB CAL; i.e. newly developed cells. Icing is normally most severe between –4 °C and –7 °C where the concentration of free supercooled droplets is usually at maximum; i.e. the minimum number have turned to ice crystals. Refer to section 3.1 Cloud formation. Mixed rime and clear ice can build into a heavy, rough conglomerate.

    Flying through snow crystals or snowflakes will not form ice, but may form a line of heavy frosting on the wing leading edge at the point of stagnation, which could increase stalling speed on landing. Flying through wet mushy snow, which is a mixture of snow crystals and supercooled raindrops, will form pack snow on the aircraft.

    The degree and type of ice formation in cloud genera are: CI, CS and CC; icing is rare but will be light should it occur AC, AS and ST; usually light to moderate rime SC; moderate rime NS; moderate to severe rime, clear ice or mixed ice. As the vertical extent of NS plus AS may be 15 000 or 20 000 feet the tops of the cloud may still contain supercooled droplets at temperatures as low as –25 °C TCU and CB; rime, clear or mixed ice, possibly severe.  
    Freezing rain creates the worst icing conditions, and occurs when the aircraft flies through supercooled rain or drizzle above the freezing level in CU or CB. The rain, striking an airframe at sub-zero temperature, freezes and glaze ice accumulates rapidly — as much as one centimetre per four air miles.

    Freezing rain or drizzle, occurring in clear air below the cloud base, is the most likely airframe icing condition to be encountered by the VFR or ultralight pilot. As it is unlikely to occur much above 5000 feet amsl, choices for descent are possibly limited.

    9.2 Effect of airframe ice
    Ice accretion on the wing leading edge is a major concern for aircraft not equipped with anti-icing or de-icing. Airflow disruption will reduce the maximum lift coefficient attainable by as much as 30–50%, thus raising the stalling speed considerably. Because the aircraft has to fly at a greater angle of attack to maintain lift, the induced drag also increases and the aircraft continues to lose airspeed, making it impossible to sustain altitude if the stall is to be avoided. Fuel consumption will also increase considerably.
    The weight of 25 mm of ice on a small GA aircraft might be about 30 to 40 kg but the increased weight is usually a lesser problem than the change in weight distribution. Also, accretion is often not symmetrical, which adds to increasing uncontrollability. Forward visibility may be lost as ice forms on the windshield. Icing of the propeller blades reduces thrust and may cause dangerous imbalance. Ice may jam or restrict control and trim surface movement; or may unbalance the control surface and possibly lead to the development of flutter. Communication antennae may be rendered ineffective or even snapped off. Extension of flaps may result in rudder ineffectiveness or even increase the stalling speed. Aircraft operating from high-altitude airfields in freezing conditions may be affected by picking up runway snow or slush, which subsequently forms ice and possibly causes problems such as engine induction icing or frozen brakes.  
    Engine air intake icing
    Impact icing may occur at the engine air intake filter. If 'alternate air' (which draws air from within the engine cowling) is not selected or is ineffective, power loss will ensue. When air is near freezing, movement of water molecules over an object such as the air filter may sometimes cause instantaneous freezing. Ice may also form on the cowling intakes and cause engine overheating.

    Pitot or static vent icing
    Pitot or static vent blockage will seriously affect the ASI, VSI and altimeter, as shown in the table below, but be aware that blockage of the static vent tubing from causes other than icing — water for example — will render the ASI, VSI and altimeter useless, unless the aircraft is fitted with an alternative static source.
    If the static vent is totally blocked by ice Flight stage Altimeter reading VSI reading ASI reading During climb constant zero under During descent constant zero over During cruise +constant zero OK On take-off constant zero under   If the pitot tube is totally blocked Flight stage Altimeter reading VSI reading ASI reading During climb no effect no effect over* During descent no effect no effect under* During cruise no effect no effect constant* On take-off no effect no effect zero*   If the pitot tube is partially blocked Flight stage Altimeter reading VSI reading ASI reading During climb constant zero under* During descent constant zero under* During cruise +constant zero under* On take-off constant zero under*
    9.3 Ice jamming control surfaces and cables
    Many aircraft are prone to accumulation of water from dew or rain in areas which, if that water freezes during flight, will inhibit control movement and affect hinge, cable or torque tube movement. This particularly applies to ailerons and elevators if the gap between the control surface and main structure contains some form of flexible seal (to improve aerodynamic efficiency) that allows accumulation of water. Engine controls may also be affected if exposed cables or cable runs are wet and subsequently ice up.

    If water has accumulated within a control surface and frozen before it has the opportunity to drain, then the mass balance of the surface will be degraded and there is a possibility of flutter development.

    Before flight, water should be removed from areas that may affect controls. Care must be taken to avoid flight into freezing conditions after flying through rain.

    9.4 Hoar frost obscuring vision on take-off
    In frosty, still, early morning, winter conditions the air layer adjacent to the ground will be much colder and drier than the air just 10 or 20 feet higher. Pilots planning a post-first light departure in these conditions should be aware that, while on the ground, the airframe will have cooled to freezing point or below. On take-off, the aircraft will quickly rise into the warmer, moister air and it is quite possible, in an unheated cockpit, that atmospheric moisture condensing onto the cold canopy will immediately form an external light, crystalline hoar frost; refer to 'Atmospheric moisture'. The hoar frost will suddenly and completely wipe out vision through the canopy for a short period, and at a most critical time.

    Under slightly warmer conditions it is possible that a dense internal fogging of the canopy and instrument faces will occur during take-off, which will also wipe out forward vision for a short, but critical, period.

    If dewpoint is below freezing, hoar frost may be deposited on parked aircraft in clear humid conditions at night when the skin temperature falls below 0 °C. Rime ice will form on parked aircraft in freezing fog.

    9.5 Carburettor icing
    Ice is formed in venturi-type and slide-type carburettors in ambient air temperatures ranging from about –10 °C to +30 °C if refrigeration and adiabatic cooling within the airways are sufficient to lower the air/fuel mixture temperature — and consequently the metal of the carburettor — below the freezing point. There must also be sufficient moisture in the air, but this need not be visible moisture. Ice may form at the fuel inlet, around the valve or slide, in the venturi and in curved passages, choking off the engine's air supply. If icing continues, this will cause the engine to stop. Carburettor ice may form in flight or when taxying; the latter event will severely degrade take-off performance.

    Temperature reduction within the carburettor
    Adiabatic cooling — in the induction system the constrictions at the throttle valve and choke venturi cause a local increase in air velocity, with consequent increase in dynamic pressure and decrease in static pressure. Density remains constant, so the temperature instantly decreases in line with the decrease in static pressure, refer to section 1.2 Equation of state. This adiabatic cooling is more noticeable when the throttle is closed or partly closed for extended periods, but it is unlikely to be more than a 5 °C drop at the coldest part, and probably much less — say 2 to 3 °C.

    Refrigeration cooling — when fuel is injected into the airstream a certain amount evaporates. The latent heat for fuel evaporation is taken from the surrounding air and metal, which is already being cooled adiabatically. The temperature drop caused by refrigeration may be as much as 15 °C, giving a total drop within the carburettor as high as 20 °C. If the metal of the carburettor is thus reduced to a temperature at or below freezing then cooled or supercooled water droplets will freeze on contact — as in airframe icing.

    Sublimation of water vapour
    Even if there is no visible water in the air, the temperature reduction may cause ice to be deposited on the freezing metal by sublimation of the water vapour in contact with it; refer to sections 1.5 Atmospheric moisture and 1.6 Evaporation and latent heat. The amount forming depends on the absolute humidity of the atmosphere. Normally the higher the temperature, the greater the absolute humidity can be. Thus it is possible that when flying in OAT as high as 20 °C, even 25 °C, carburettor ice can form. Air with a relative humidity of 25% at 20 °C, or 50% at 10 °C, will reach saturation at 0 °C.

    However, an OAT range of 0 °C to 25 °C, peaking at around 10 °C to 15 °C and with relative humidity exceeding 60%, are the most significant conditions for moderate to severe clear air icing — particularly at low throttle openings — as shown in the probability diagram below. Note that the region to the left of the 100% relative humidity line would be visible moisture — mist, fog and cloud.
    Locally high absolute humidity may also occur in the following conditions: poor atmospheric visibility at low levels, especially early morning and late evening after heavy rainfall in light wind conditions in clear air just after morning fog has dispersed just below a stratiform cloud base.  
    When flying through visible moisture, cloud patches or light rain, some of this moisture will evaporate in the carburettor, further reducing the temperature in the airstream. The drop is slight but may be enough to tip the scales. The probability of icing is increased if fuel flow is not leaned — the excess fuel injected into the intake airstream increases the refrigeration.

    Combatting carburettor icing
    The formation of carburettor ice is indicated by a slow decrease in manifold pressure in aircraft equipped with a constant speed propeller, or a decrease in rpm in fixed-pitch aircraft, probably with ensuing rough running as the ice build-up further restricts the airflow and enriches the mixture. Corrective action is usually by FULL application of carburettor heat, which pre-heats the air entering the carburettor. Full carburettor heat should also be applied in conditions conducive to icing, particularly at low throttle settings such as on descent or taxying, but never on take-off. Carburettor heat will increase the fuel vaporisation in a cold engine. Application of partial heat may cause otherwise harmless ice crystals in the airstream to melt then refreeze on contact with freezing metal.

    Rough running may increase temporarily after application of full heat, as the less dense air will further enrich an over-rich mixture; however, full heat must be maintained until the engine eventually settles into smooth running.

    Pre take-off checks: note the rpm and apply full heat — the rpm should drop. Return the heat to the cold position — the rpm should return to the initial reading. If a higher reading is obtained, then icing was — and is — present.

    Non-venturi carburettors, such as the various slide types attached to two-stroke engines — the throttle slide performs as a throttle valve and venturi — are considered, for various reasons, not to be very susceptible to icing. Consequently, they are usually not fitted for carburettor heat, or intake air heating, on the principle that any ice formed will be immediately downstream of the slide, or multi-hole spray bar, or around the main jet, and movement of the throttle slide will dislodge it. This is provided of course, that the rpm drop is noticed before things get out of hand.

    Minor atmospheric turbulence continually disturbs aircraft flight, but a three-axis (rather than weight-shift controlled) aeroplane's stability system normally copes with such events without pilot intervention. However, there are some atmospheric phenomena that produce moderately to severely hazardous wind shear and turbulence events, depending — to some extent — on the height at which they are encountered and how they are encountered. Such events may result in temporary loss of control or even structural damage, particularly in very light aircraft. Paragliders and powered parachutes require fairly smooth air for normal operations.

    For an explanation of the effects of these phenomena on aircraft handling read the 'Wind shear and turbulence' module of the 'Decreasing your exposure to risk' guide.

    8.1 Boundary layer turbulence
    In meteorology, the term boundary layer is used to describe the lowest layer of the atmosphere in which the influence of surface friction and surface temperature on air motion is important. It is also referred to as the friction layer, planetary boundary layer or the mixed layer and is perhaps 1000 to 5000 feet thick by day and thinning by night. (Under high surface temperature conditions the depth of the layer affected by thermals can be much more extensive; see 'Dry thermals in the superadiabatic layer'.) The term 'surface boundary layer' or surface layer is applied to the thin layer (roughly 50 feet deep) immediately adjacent to the surface (and part of the boundary layer) within which the friction effects are more or less constant throughout, and the effects of daytime heating and night-time cooling are at a maximum.

    Air flow becomes turbulent when its natural viscosity cannot dampen out pressure forces arising when air flows past obstacles, through temperature gradients or over/around curved boundaries. In the wake of a topographic or constructed obstacle, the average wind speed is reduced but mechanical turbulence is increased. Some of the velocity energy is converted to turbulence energy; thus intense, intermittent gusts and matching lulls can be experienced on the lee side of sentinel hills, ridge lines and mountain ranges. Turbulence may take any form — eddies, vortices, upflow or downflow — and be aligned in any plane. Turbulence increases with the square of the wind speed. Doubling of wind speed will increase pressure forces, and thus turbulence, by a factor of four. Such mechanical turbulence will affect the aoa of an aircraft flying into it, even exceeding the critical aoa.

    The downward vertical component of eddies and gusts can cause an aircraft to sink rapidly. Such turbulence that occurs when an aircraft is flying near the surface, particularly in take-off and landing, may place the aircraft in a dangerous, possibly irrecoverable, situation.

    Extract from an RA-Aus accident report: "The pilot took off ... towards a saddle in a range of hills which rise 400–600 feet above the airstrip. While attempting to turn 180 degrees in the lee of the saddle he experienced strong turbulence and sink and was unable to complete the turn before the aircraft collided with the ground."

    8.2 Low-level wind shear
    Generally, below 2000 feet agl and over flat terrain, the amount of horizontal and vertical shear, in both direction and speed, is largely dependent on temperature lapse rate conditions:
    Greater lapse rate » greater instability » greater vertical mixing » more uniformity of flow through layer and less shear. An exception is in extremely turbulent conditions below a cumulonimbus. But if the environment lapse rate exceeds about 3º C per 1000 feet then convective thermal turbulence will be severe.

    Convective turbulence is minimised in stable conditions, so vertical shear in the boundary layer is enhanced, with highest values in the lower 300 feet. That will affect aircraft taking off and landing.

    High vertical wind shear values are often attained at the upper boundary of an inversion. An aircraft climbing through the inversion layer, in the same direction as the overlaying wind, would experience a momentary loss of air speed — and lift — through the effect of inertia. Also, the difference in wind velocity between the layers, with shearing instability at the interface, causes the formation of short-lived waves across the interface; much the same way as ocean waves — which grow in amplitude until they curl up and break. The waves produce an extensive but shallow area of moderate to severe clear air turbulence.

    However, severe low-level wind shear can also be associated with other phenomena; for example, lee eddies, lee waves and solitary waves.

    8.3 Convection currents
    When air flows over a surface heated by solar radiation, the surface contact layer is heated by conduction. If the incoming energy is sufficient, the temperature in the lower layer increases and thermals (upward convection currents) rise from the heated contact layer — perhaps initially as bubbles of buoyant air and then developing into downwind slanted, vertical currents of 50–300 metres diameter. The strength of the thermal depends on the heating and thus on the time of day, being weak in the early morning and strongest in mid to late afternoon. But if the wind builds up, turbulent mixing will disorganise the thermals. Areas of sinking air accompany the thermals, surrounding the weaker thermals and, as the day progresses, extending to fill in the inter-thermal gaps.

    The thermal cools at about 3° C/1000 feet and if it reaches dewpoint — the convection condensation level — cumulus will form. The release of the latent heat of condensation of the included water vapour warms the air in the thermal, and the rising cumulus convection current increases its buoyancy. If developed enough, it can draw in surrounding moist air and maintain itself as a single, steady, organised updraft or 'pulse', perhaps even forming a towering cumulus or a cumulonimbus. As the thermals grow higher, the spacing between them generally becomes wider, although adjoining thermals may merge at height.

    Thermals are a principal source of good atmospheric lift for soaring paragliders, hang gliders and sailplanes, and particularly so in the summer.

    Dry thermals in the superadiabatic layer
    In the arid inland areas of Australia, the very dry continental air produces generally cloudless skies with little or none of the sun's energy being absorbed as latent heat. Most of that insolation is available to heat the surface, making it far warmer than the adjacent air; ground temperatures of 80° C plus have been recorded. (Conversely, at night both the surface and the adjacent air cool rapidly, by long-wave radiation into space, dropping surface temperatures to near zero.) The daytime heating of air in contact with that heated ground produces a superadiabatic layer where the temperature lapse rate exceeds 3º C per 1000 feet. The layer is particularly unstable, with vigorous, accelerating dry thermals, and associated downflow, which may extend to 15 000 feet or more, above the terrain. Such dry thermal convection is much more powerful than that experienced in Europe where the operating limits for recreational aircraft designed for those environments is established. Powered aeroplanes flying in likely conditions should expect vertical gust shear, often with velocities greater than 20 feet per second — occasionally very much greater — and reduce cruising speed accordingly.

    A surface eddy flowing into the bottom of a thermal tends to circulate around the central core, which may develop into a vortex stretching up as a spinning column usually for hundreds, but possibly thousands, of feet. A dust devil, dust whirl or willy-willy, 30–50 feet in diameter, is sometimes visible near the surface. Rotation increases as the column elongates. Because of the added vorticity, such thermals are very dangerous to light aircraft taking off, landing or flying at low altitude. The disturbance may not be visible unless it is picking up dust, dry grass or other debris. If you sight dust whirls or disturbed vegetation in the airfield area be prepared for very turbulent conditions. Taxying, parked, even tied-down aircraft, are at risk of considerable damage.

    In coastal areas, cooler maritime air moving over heated, arid ground also provides conditions for propagation of willy-willies. The worst dust or sand whirls — extending to perhaps 3000 feet or more — occur in the dry, sandy interior, and can cause engine and visibility problems.

    Encounters of willy-willies in flight usually involve a major upset in attitude and height loss, which should generally be countered using the upset recovery technique outlined in the 'Wind shear and turbulence' module of the 'Decreasing your exposure to risk' guide.

    8.4 Shear and turbulence near thunderstorms
    Thunderstorms may be classified in four generalised types — single-cell, isolated multicell cluster, multicell squall line and supercell; although supercells may also be multicellular. Their associated surface winds — originating from the downdraughts of cold, dense air — may be both high velocity and extremely turbulent.

    Single-cell storms are usually isolated storms moving with the mid-level wind. They are common in summer and occur in conditions where the wind velocity, relative to the cell motion, does not change markedly with height. (CB development has to be strong to overcome the detrimental effects of vertical wind shear). A single-cell storm may last less than 30 minutes, its life being limited to the growth and collapse of a single, large updraught pulse. The diameter of the storm may be less than one nautical mile and it will not move very far during its lifetime — less than 3 nm in light winds. Such storms do not usually produce violent wind shear near the surface, although microbursts may descend from even a mild-looking CB prior to its collapse. Single-cell storms tend to form in the afternoon when convection is stronger. The strong updraughts are very dangerous for hang-glider pilots.

    Isolated, single-cell storms, embedded in low-level cloud layers, commonly form in cold winter air streams entering the south-west of Western Australia, southern South Australia and Victoria. They are generally frequent but short-lived, with soft hail and shallow wind gusts, and are caused by destabilisation of the cold air mass. They can be accentuated by orographic effects. The passage of vigorous winter-time cold fronts, preceding Antarctic polar maritime air moving into the same areas, are likely to produce the more severe multicell storms. In summer 'cool changes' of unstable maritime air moving into South Australia and Victoria from the west/south-west sometimes produce severe storms.

    Multicell cluster storms (the most common thunderstorm) consist of a series of organised updraft pulses that may be separated by time and/or distance, and be closely or widely spaced. They move as a single unit and perhaps cycle through strong and weak phases. Frontal, pre-frontal, heat-trough and convergence zone systems may produce very vigorous storms several miles wide. By continually propagating new cells, these last an hour or more before the cold downdraft and outflow finally undercuts and chokes off, or smothers, the warm inflow that produces the updraft, and the system then collapses. Each new cell is usually formed in the 'zone of maximum convergence' where the gust front directly opposes the low-level wind.

    Weaker multicell storms advance with, or to the left of, the prevailing mid-level wind at an average rate of 10 knots or so; but the strongest storms may turn almost at right angles to the wind. The storm turns towards the flank where the new updrafts are building — the flanking line, which is a line of CU or TCU stepped up to the most active CB. If the new cells are forming on the upwind side, usually to the west or north-west (a back-building storm), it may appear to move slowly, possibly staying in one place for considerable time.

    Strong updraught/weak downdraught storms often form in conditions where there is moist air at most levels. Such storms produce heavy rain and may produce severe hail but, because of the lack of dry air inflow, severe low-level shear is unlikely.

    In severe storms, with strong updraughts and downdraughts, updraught velocities increase with height, typically 1500 feet per minute at 5000 feet and 3000 feet per minute at 20 000 feet. Updraughts of 5000 feet per minute in the upper part of a storm are not unusual. Downdraught velocities tend to be slightly less at corresponding altitudes. Vertical acceleration loads of 2–3g may be experienced in horizontal flight.

    The areas that most concern light aircraft are the low-level outflow regions, where downburst gusts of 50 knots or more may be reached in the initial line squall; also, lightning and hail may exist. The spreading, cold, dense current of the outflow — the gust front — may last for 10 to 30 minutes and be 1500 to 6000 feet deep. This forces the warm, moist, low-level air up and so continuously regenerates the updraught. Thus, an area up to 15–25 nm from a large storm, and 10–20 nm for a medium storm, should be regarded as a 'no-go' area for very light aircraft. An intense, narrow, initial microburst may sometimes be produced, bringing short-lived but potentially disastrous wind gusts of possibly 80 knots.

    There is an area of extreme, low-level shear at the leading edge of the storm, between the nose of any identifiable shelf cloud and the position the gust front has reached; possibly 1–3 nm ahead of any rain curtain.

    Vertical wind shear is usually detrimental to early development of CB cells. However, if there is:
    strong vertical wind shear, backing and strengthening with height, associated with a deep surface layer of warm moist air, below a mid-level layer of dry air, with an inversion separating the layers, and a rapid decrease in temperature with height above the inversion, then the ideal conditions are created for a severe multicell storm; or a supercell storm if the surface wind is greater than 20 knots and the vertical wind shear exceeds about five knots for each 3000 feet.  
    The capping inversion keeps a lid on development until the lifting force builds up sufficiently to burst through the inversion and great buoyancy develops in the colder, upper layer. Upper-level divergence and a jetstream will also enhance the vertical motion.

    Strong wind shear both tilts the updraught and provides the means to rotate it (storm updraughts usually do not rotate) leading to the development of a supercell storm.

    A supercell is a severe storm with a strong, continuing, organised main updraft and co-existing strong downdrafts, controlling and directing the inflow (which may have a velocity of 30–50 knots) into the cell from the surrounding atmosphere. It will usually diverge to the left of the prevailing mid-level wind. There may be broad, anti-clockwise rotation — as viewed from below — of the cloud base beneath the main updraught. Humid, rain cooled air from the downdraught may also be pulled into the normal inflow (which is often visible as scud beneath the CB). This causes part of the cloud base to lower, forming a circular wall cloud at the updraught base. If vorticity increases within the cloud, a tornadic funnel may form. A gustnado may form on the leading edge of a gust front under a shelf cloud or similar cloud bank, lasting up to several minutes. The gustnado is a brief, intense downburst vortex indicated by rotating scud.

    Broad-scale rotation of a storm cell forms a mesocyclone, 1–10 nm in diameter, with a surface pressure drop of a few hPa at the centre; although a 30 hPa drop has been recorded. Supercells may last for several hours as organised systems and commonly form in warm, moist, north/north-east flow into a surface trough, and along the Great Dividing Range during summer.

    8.5 Convective downbursts
    The CB downdraft can become concentrated into a downburst — a fast-moving plunge of cold, dense air. Peak wind gusts in the squall* usually last less than ten minutes, often 3 to 5 minutes, but extremely hazardous vertical gust and horizontal shear results, with extreme turbulence at the leading edge or 'gust front'. The downburst may be 'dry' or associated with precipitation ranging from virga* showers to heavy rain showers — 'wet'. The cold outflow wedges under warmer, moister air and pushes it up. A curling outflow foot of dust, tree movement or precipitation from the surface touchdown point may be visible on or near the surface. A shelf cloud often forms above the leading edge as the warmer, moister air condenses.

    (*In meteorological terms a squall is a wind that rises suddenly, exceeds a velocity of 22 knots and is sustained for a least a minute then dies quickly. Gusts are shorter lived. Virga is precipitation that evaporates before reaching the surface.)

    Microbursts are a more concentrated downburst form, often associated with warm to hot and relatively dry conditions at low levels, and convectively unstable moist air aloft with high (5000 to 10 000 feet) based CU or TCU. If the cloud is forming when the surface temperature/dewpoint spread is 15 °C to 25 °C then the microburst potential is high. The high spread means the atmosphere can retain much more water vapour. Rain falling in, and from, the cloud is evaporating (virga), thus cooling the entrained air, resulting in downward acceleration of the denser air. Consequently, flight through, under or near precipitation from a large CU involves considerable risk. Significant hail is unlikely. The most dangerous area is the horizontal density current vortex ring close to the touchdown point. The ring moves outward from the contact point at high speed until it disintegrates into several horizontal roll vortices spread around the periphery. The vortices may continue to provide extreme turbulence for several minutes; inflight breakup of aircraft is possible. The maximum horizontal winds occur about 100–200 feet above ground level. Flying directly through the outflow ring would see a 180° reversal in gust direction, and extreme shear.

    In bushfire conditions the firestorms associated with dry microbursts are particularly dangerous to firefighters.

    Microbursts occur under only 5–10% of CB but a less concentrated, longer-lasting gust front macroburst is normally associated with the entire cold air outflow of the larger storm cells. The severe gust fronts from a microburst extend for less than 2 nm, while those from a macroburst extend much further. The vertical gusts within the downburst, perhaps with a velocity twice the mean, may produce a microburst within the macroburst. (Unfortunately as a consequence of some high-profile airliner disasters in the USA, probably due to storm downbursts, the 'microburst' term now seems to be applied to all downburst events.)

    The following is extracted from a report by an RA-Aus pilot who apparently encountered a springtime cluster storm on the southern edges of the Great Dividing Range, north-east of Melbourne, only 13 nm from home, but — fortunately — in a very tough recreational aeroplane.

    "I had encountered a few small rain showers that lasted 15-20 seconds when all of a sudden I noticed the altimeter going nuts ... the next thing to happen was the Cobra Arrow was lifted and it felt like it was just thrown over end first, I pulled the power and then the fun really started; I was now heading to the ground 2000 feet below at over 160 knots ... inverted and going down quick. I can recall just yelling. I pushed down elevator and commenced a bunt — or the upward half of an inverted loop — then a half roll. That's got it up the right way then I was thrown to the right at the same time dislocating my left shoulder, inverted again and rolled back to upright then to the left and bang in went the shoulder; all the time just flying and waiting for something to give! I managed with good luck and a lot of skill to get out of this situation ... I have done a fair amount of aerobatics and I think it more than saved my life this day. I started to ease the power and flew clear of the main front, leaving the mountains two minutes later in blue skies and sunshine and almost nil wind.

    The most worrying thing about the whole ordeal was that I had seen a small front about 3 miles to the west. It had actually run past me. I was looking towards home and feeling pretty good but in the mountains anything can happen. The microburst came back up a valley and changed direction almost 180 degrees. I can remember the trees just getting smashed about. I got a real close-up view of them as the back blast of the burst was shoving me upwards. I was only about 200 feet above them.

    After landing at Coldstream we were able to watch the cell's continuing progress from the ground. It moved around the hills over Healesville then south towards Silvan before coming back around and passing directly over the airfield."

    8.6 Squall lines
    The usual precipitation downdraft associated with an individual CB cell tends to be concentrated towards the leading edge of the storm where the cold, heavy outflow spreads out at ground level, forming a small, high-pressure cell 10–15 nm across. The dense air lifts the warmer, moist air in its path and may initiate an extremely dangerous, self-amplifying, convective complex.Within this, neighbouring storm cells consolidate into a towering squall line of large thunderstorm cells ranged across the prevailing wind direction. At locations in the path of the squall line, the resultant line squall occurs as a sharp backing in wind direction, severe gusts, temperature drop, hail or heavy rain and possibly tornadoes. If the squall line is formed in an environment of strong mid-level winds the surface gusts may exceed 50 knots.
    Squall lines vary in length; some of the longest are those that develop in a pre-frontal trough 50–100 nm ahead of a cold front. These squall lines may be several hundred nautical miles in length and 10–25 nm wide moving at typically 25 knots; their very high altitude anvils extend considerably further. The squall line shown in the adjacent BoM weather radar plot is about 250 nm long. The squall lines form ahead of the front as upper air flow develops waves ahead of the front; downward wave flow inhibits and upward wave flow favours uplift.

    Squall lines are a common northern Australian feature. They develop along active areas of the Inter Tropical Convergence Zone, within the feeder bands of tropical storms, along sea breeze fronts or other convergence zones, and in the summer heat trough. In south-east Australia they may also be associated with fast-moving winter cold fronts, producing severe winds and heavy rainfall.

    During daylight hours the squall line may appear as a wall of advancing cloud with spreading cirrus plumes; the most severe effects will be close to each of the numerous CB cells. The convective complex releases a tremendous amount of latent heat and moisture, which may be sufficient to generate a warm core mesoscale cyclone, and consequent poor flying weather, lasting several days.

    8.7 Storm avoidance
    It can be seen that any downburst encounter — whether the vertical gust or the turbulent horizontal outflow — will be deadly to any light aircraft; any thunderstorm activity or potential activity should be given a very wide berth. Stay well away from any storm sighted — perhaps 10 nm for single cells to 25 nm for the largest storms — and never attempt to fly between storm cells. Be prepared to reverse course if it looks doubtful. Never fly under a CB base, and expect that storm cells may be embedded within an otherwise innocuous cloud layer. It is known for hail to fall from an apparently clear sky; this, in fact, originates from the high anvil of a CB many miles away and, of course, a lightning strike will certainly ruin your day. An encounter with heavy rain may produce total loss of visibility combined with a loss in both airspeed and lift.

    Before any flight, check the online BOM weather watch radar and the area forecasts for storm activity or developing winds. Don't place total faith in the written forecast — check the latest surface chart for the position of pre-frontal zones, convergence zones, developing inland lows, surface troughs, dips in the isobars or other conditions that might indicate possible storm development or increasing winds. Remember that the latter also brings increasing gusts and thus low-level shear and turbulence; 15 minutes spent checking might save 15 weeks repair — for you and/or your aircraft. Check the sky all round at a reasonable height after take-off; if you have any doubts about what you see, scrub the flight!

    Light aircraft should not be operating in the vicinity of thunderstorms. The following is an extract from an RA-Aus fatal accident investigation report.

    "The pilot departed Holbrook airfield in a Sapphire aircraft for his private strip about 30 minutes away ... a line of large thunderstorms were active in the area and a witness reported that one of the nearby cells not only had virga visible below the cloud but also exiting horizontally ... the pilot was aware of the approaching weather and, indeed, was trying to beat it home ... the aircraft impacted the ground in a near vertical attitude ... about 100 metres short of the threshold of his strip ... the owner of the adjacent farm on which the aircraft crashed stated that there were thunderstorms within five kilometres and that a wind squall had passed through the area at the precise time the sound of impact was heard."

    Michael Thompson's storm chasing diary at ozthunder.com/chase/chase.html provides some excellent reports and photographs of storm encounters in eastern Australia.

    8.8 Tornadoes, landspouts and waterspouts
    A tornado is a rapidly rotating, narrow air column extending from the updraught base of a CB to the ground. Intense tornadoes usually develop from areas of rotation inside supercells. One theory is that the horizontal vortices produced by the low-level shear are tilted upward by the updraught inflow initiating the rotation within the cell, which develops into a mesocyclone. The vortex — deriving its energy from the latent heat of condensation released from the warm, moist inflow — spins at perhaps 30 knots, accelerating if the column contracts. Another theory is that the tornado forms when a smaller, more rapidly rotating updraught causes part of the storm base to lower — thus forming a rotating wall cloud from which a condensation funnel cloud appears, which may reach the ground. The funnel is usually located on the edge of the storm?s main updraught, close to the downdraught.

    The tornado diameter at the tip can vary from a few metres to a few hundred metres. Winds at the outer edge may reach 100 knots and there may be a substantial pressure drop within the core, with the magnitude being about 30 hPa per 1000 feet of funnel length.

    Some 15 to 20 tornadoes are reported annually in south-east and south-west Australia. Their intensity and size is predominantly classified as ?weak and short-lived? (1–3 minutes). They usually move from the north-west at 30 knots or so and damage a strip perhaps 50 metres wide by 2 kilometres long. (In April 1960, though, a tornado in jarrah forest near Collie, Western Australia cut a swathe 240 metres wide and 30 kilometres long, destroying tens of thousands of trees.) Although tornadic storms can occur in any season, day or night, they are often associated with dewpoint temperatures exceeding 10 °C and an inversion at 6000 feet or so. Bushfires may trigger their development. Areas of high incidence are west of the Dividing Range from southern NSW to central Queensland, western Victoria and the south-west corner of Western Australia. A tornado that struck Brisbane in November 1973 produced winds estimated at 135 knots. Also a wind velocity of 90 knots was reported in the fatal tornado at Sandon, Victoria in 1976.

    Fujita damage scale number for tornadic winds:
    F0 35–62 knots: light damage (covers Beaufort scale 8 to 11) F1 63–95 knots: moderate damage — caravans overturned, cars pushed off roads. (Beaufort scale 12 starts at 63 knots) F2 96–135 knots: considerable damage — roofs off, large trees uprooted, light missiles F3 136–180 knots: severe damage — house walls off, heavy cars lifted and thrown F4 181–225 knots: devastating damage — well constructed houses levelled, structures blown some distance, large missiles generated F5 226–275 knots: incredible damage — strong timber houses lifted and carried considerable distance to disintegrate, car sized missiles fly in excess of 100 m.  
    Landspouts and fair weather waterspouts develop, from the surface up, in a superadiabatic or similar layer within an environment with little vertical shear. The landspouts and waterspouts tend to develop from low-lying eddies along wind shifts which, in the unstable atmosphere, roll up into vertical vortices about 0.5 nm in diameter. If a vortex happens to get caught in the updraught under a TCU or developing CB then the updraught stretches (and contracts) the vortex, and the tornado-like landspout or waterspout may form. The funnel is usually indicated by dust in a landspout, but the moist sea air will provide a visible condensation funnel, plus a sheath of spray, around a 'fair weather' waterspout. In Australia most waterspouts occur in northern waters. But the world record height of a waterspout, off the New South Wales south coast in 1898, was measured from land by theodolite at 5014 feet, but this was most likely a tornadic waterspout; i.e. a tornado moving out over coastal waters. Multiple or cluster spouts may form in the one location.
    Photographs and descriptions of tornadoes, gustnadoes and waterspouts observed in south-east Queensland can be viewed in the Brisbane Storm Chasers Web site.
    8.9 Other pre-frontal turbulence
    Cold fronts generally travel south of 25° S latitude and west to east. Their passage produces pre-frontal/frontal wind shear, the severity of which increases with the speed of frontal movement and the temperature differential across the front. For example, a front moving at 10 knots with 5° C differential would probably produce only light/moderate shear, while one moving at 30 knots with 10° C differential is likely to produce very severe shear.

    New South Wales Southerly Buster
    The NSW Southerly Buster is an intense, pre-frontal squall leading a cold front moving up from the Southern Ocean. It occurs maybe 30 times per year, with about 10 major events usually in spring and summer. The phenomenon is a shallow density current, 20–50 nm wide, centred on the coast and surging northward at 15 knots with 30–60 knot gusts. The temperature may fall 10–15 °C over a few minutes and there may be extreme low-level turbulence. A spectacular roll cloud may form above the nose of any frontal cloud, but usually there is little cloud and consequently little warning.

    A prime cause of the Southerly Buster is the interaction of a shallow cold front with the blocking mountain range that parallels the coast; frictional differences over land and sea uncouple the flow. Other phenomena lead to intensification of the temperature gradient between the warm air mass and the cold density current; for example, a hot north-westerly or a warm dry foehn wind preceding the squall. Severe thunderstorm activity may result from the forced lifting of warm, humid air.

    Sea breeze fronts
    In coastal areas, differential diurnal heating promotes development of on-shore breezes which, during the day, grow in strength to 'moderate breeze' and, due to Coriolis effect, begin to back. The surface wind is a resultant of the sea breeze vector and the gradient wind vector. In hot land conditions, the sea breeze front (a density current) can travel 100–200 nm inland by midnight, if not blocked or diverted by terrain.

    The cool air lifts the warmer inland air (providing a lift source for gliders) and, if conditions are suitable for deep convection, a squall line may develop and propagate along the convergence line of the surface flow. Opposing sea breeze fronts, such as occur in Cape York, may cause strong convergence disturbances when they meet. Along the eastern Queensland coast, typically between September and March, storm lines of CB up to 100 nm in length form inland in mid- to late-afternoon then move towards the coast, and are out to sea by mid-evening. Such squall lines may be difficult to avoid if encountered unexpectedly.

    8.10 Low-level jets
    Low-level jets may form by interaction between anticyclones and mountain barriers — particularly in the area west of the Dividing Range in northern NSW and southern Queensland. This produces a zone in the friction layer, which may extend 50 nm plus, where wind velocity is highly geostrophic and concentrated both vertically and horizontally, so that large, low-level shears are produced.

    Core speeds of 25–30 knots,and up to 50 knots, occur in an otherwise light surface wind area, particularly early to mid-morning in winter, with the anticyclone centred over the interior. The overnight cooling of the western slopes produces a horizontal temperature gradient. A low-level jet in a circuit area is very dangerous to light aircraft.

    8.11 Lee wind downflow, eddies, rotors and vortices
    Pilots of aircraft flying on the lee side of higher topographic features — particularly if taking off or landing, or flying parallel to a ridge — should be aware that the downflow (sinking air) encountered can exceed a powered aircraft's climb capability; there is usually no indication of the downflow other than that sinking feeling!

    (Of course glider pilots will find atmospheric upflow on the windward side of the ridge providing the opportunity for 'ridge soaring'.)

    Strong sink conditions may occur on the lee side of mountains, ridges, valley walls, hills and islands, and even extend above the height of the barrier. The severe sink associated with this lee side downflow is a function of wind speed and slope angle. For example, if the horizontal wind speed is 29 knots and the slope angle is 15 degrees then the ambient downslope velocity is about 30 knots [29 / cosine 15° = 29 / 0.97 = 30]. The sink vector is equivalent to sine 15 degrees [15 / 60] = 0.25 x 30 = 7.5 knots or about 750 feet per minute — greater than the maximum climb rate of many ultralights. This downflow airstream may be non-turbulent, particularly when associated with standing wave conditions, so a pilot may not have an early indication of the danger. Turbulent eddies/curl-overs within the downflow may add to the ambient sink rate.

    The following is an extract from an RA-Aus fatal accident investigation. Note: the Capella aircraft was last sighted in flight over a lightly forested area not far above tree-top height and thought to be intending to land in the grounds of a winery familiar to the pilot. The aircraft impacted the ground almost on the apex of a small rise and about halfway down the slope in a lawn area. Weather was fine with good visibility, and wind was 10 to 15 knot northerly with strong gusts.

    "Indentations in the ground and damage to the aircraft indicate that the aircraft had initially contacted the ground travelling in a north-westerly direction at a relatively low forward speed but with high downward force. The wind direction and strength combined with the topography at the accident site (a long east-west ridge to the north) would have combined to produce a small standing wave with significant downflow. An aircraft approaching at minimum speed and tree top height could expect significant sink in that area. This could translate to loss of airspeed if the pilot was concentrating solely on touching down on a given spot."

    Injuries suffered when an aircraft sinks with high vertical decelerations are usually very much more severe than those suffered in horizontal decelerations of similar magnitude.

    Some pilots have expressed the opinion that a light aircraft cannot get into real trouble in a lee sink situation because the airstream must level out before reaching the surface and so will take the aircraft with it. This is not so; inertia is related to mass and the mass of a molecule of metal is far greater than that of an air molecule.

    Eddies with large sink rates, possibly greater than 1000 feet per minute — lee wind eddies — may occur, in only moderate wind conditions, on the lee side of mountains, ridges, hills and islands. Sink will be particularly dangerous when accompanied by high temperature (i.e. high density altitude) and high aircraft loading. Airfields along the eastern Australian coastal strip will be influenced by lee downflow and eddies when the westerlies are blowing during August to October.

    Vortex-like turbulence tends to develop when slope gradients exceed one in three [18°] and it appears at a lower level than the long horizontal vortices associated with lee waves. As the vortices stream downwind, severe turbulence may be encountered at and below the hilltop level and for some distance downstream. Pre-conditions for these streaming or trailing rotors are a stable layer, a wind vector component across the barrier exceeding 20 knots, and this component should decrease considerably not far above the barrier.

    Horizontal lee eddies can also develop from friction with the mountain side; this normally requires an inversion at or below the mountain top with a strong, sustained wind exceeding 20 knots. The eddies may be visible if cloud forms under the inversion.

    Wake vortices, similar to those produced from aircraft wingtips, can develop in the lee of lone hills and peaks in strong, sustained wind conditions. The strong — often twin — spiral turbulence can be felt at a distance ten times hill height and at altitudes considerably above and below hill height. In 1966 a BOAC Boeing 707 suffered in-flight breakup in such conditions, while giving passengers a view of Mt Fuji on a cloudless day. A search and rescue aircraft recorded airframe loads of +9g /−4g when flying through the same vortices. Ravine winds can also develop wake vortices.

    Ravine or gap winds occur in narrow gaps which that part a mountain range. The pressure difference between the two sides of the barrier when moderate to strong wind flows across the range creates a pressure gradient — with consequent strong, turbulent winds in the ravine and flowing from the exit. This also applies to gullies, to some extent.

    Effect of windbreak eddies
    Turbulent windbreak eddies will form in the lee of obstacles such as trees adjoining an airstrip. The distance they spread from the windbreak is dependent on the density and height of the trees. Generally, the windbreak affects airflow for a horizontal distance equal to ten times the height of the tree line, if the flow is perpendicular to the windbreak; the more turbulent flow is closest to the trees. There will also be a significant lee-side downflow extending over the windbreak shadow, its vertical component being dependent on the ambient windspeed. Such downflow conditions require that take-off and approach speeds are higher than normal, and that ample clearance is provided — not a place to be low and slow! In addition, in conditions of high solar radiation, the differential heating of airstrip surfaces caused by partial shading can promote turbulent vertical eddies over the take-off area.

    The following is an extract from an ATSB fatal accident investigation.
    "The pilot and his passenger were conducting a private flight in the pilot's Jabiru aircraft in the Southport area. Several other pilots heard the pilot advise over the radio that he was conducting a simulated engine failure and glide approach. The aircraft subsequently impacted a steep embankment short of runway 19 at Southport aerodrome and on the extended runway centreline. The embankment was approximately 2 m high, about 210 m from the displaced approach threshold and 30 m short of the sealed runway surface.

    An examination of the wreckage indicated that the aircraft had impacted the embankment in a moderately nose-high, left wing-low attitude. Damage to the propeller indicated that the engine was delivering significant power at the time of impact.

    Local procedures required that pilots conduct right circuits when operating on runway 19. Tall trees adjacent to the aerodrome induced localised mechanical turbulence, windshear and downdrafts when the wind was from the southeast. At the time of the accident, the wind was recorded on the Gold Coast Seaway as 150 degrees at 15 knots, gusting to 18 knots.

    It is likely that the aircraft entered an area of turbulence and high sink rate generated by the prevailing wind over the adjacent trees. Given the evidence of significant power at the time of impact, it is possible that the pilot had initiated a go-around at a stage in the approach from which it was not possible to establish a positive rate of climb."

    8.12 Mountain waves
    Mountain waves or lee waves are a manifestation of an internal gravity wave. Such waves occur fairly frequently over, and in the lee of, the mountainous areas of south-eastern Australia, and in the lee of the mountains along the east coast in strong westerly wind flow conditions.

    Conditions favourable for the formation of strong mountain waves, and which would be provided in the outer fringes of a high pressure system, are:
    an isothermal layer or inversion at about ridge height, sandwiched between a low-level unstable layer and instability, or low stability, aloft a wind, in excess of 20 knots, crossing a ridge at a high angle and increasing in velocity with height. A sharp change in wind direction within the stable layer and a large amplitude wave may induce stationary vortex or rotor flow. These vortices differ from the streaming rotors formed in lee wind eddies. They are closed with a long horizontal axis; form in the lee of, and parallel to, a well-defined escarpment, and remain fixed in position. Curl-overs may also be produced by friction slowing the near-surface downflow. Usually cloud will not form in the vortex but should it do so, it may range from scraps of scud to a long, solid roll cloud.

    Turbulence in and under the rotor area, i.e. from the mountain height down, will be severe to very severe. Some evidence of the rotor may be seen on the surface — rising dust, sudden and erratic wind changes, etc. Readers interested in the techniques recommended for flight in such conditions should check www.mountainflying.com

    If conditions are suitable, lenticular cloud that appears along the crests may reveal the waves; the stationary clouds continuously form and dissipate in the vertical air motion.

    Vertical movement of 2000 feet per minute is common in lee waves and could be much greater; the vertical component being dependent on wave length and amplitude. Lee wave downflow can easily exceed the climb capability of any powered light aircraft. In suspected lee wave, or potential vortex, conditions it is advisable to clear the lee side of a ridge or escarpment at an altitude well above it and to cross the ridge lines at an oblique angle; never attempt to cross a ridge at 90° when flying into wind in potential lee wave conditions.

    Wave length tends to increase with stronger wind aloft, and is also affected by temperature and stability conditions. The shorter the wave length, the steeper the ascents and descents.

    Amplitude depends on airstream plus the shape and size of the ridge. It will be at a maximum within the stable layer, particularly if the layer is shallow with great stability. The larger the amplitude, the further the air moves up and down. Over a plain, the wave effect can continue for 100 nm. The disturbance may extend to the stratosphere. Depending on length and amplitude, mountain waves may produce considerable areas of smooth, laminar uplift and sink — much sought-after by experienced sailplane pilots. Mountain waves are unlikely to break unless the amplitude is high, but if they do break then moderate to severe clear air turbulence will result.

    A resonating mountain or orographic wave will produce strong, adiabatically warming downslope winds — called foehn in Europe, chinook in the Rocky Mountains area and Canterbury north-wester in New Zealand. In January 1943, a temperature rise of 27 °C (− 20 °C to 7 °C ) was recorded in the space of two minutes in Spearfish, South Dakota. The resonating waves may reach extreme heights and may produce downslope windstorms exceeding 100 knots in the lee of high, extensive mountain barriers. Updrafts and downdrafts in excess of 3000 feet per minute are common; 7000 feet per minute has been reported in the USA.

    Weak foehn winds occur regularly in the south-east Australian coastal strip under the influence of westerly or north-westerly flows; they can bring unseasonal warming to areas around Lakes Entrance, Victoria, for example.

    8.13 Valley winds
    Valleys and gullies tend to develop their own rather turbulent air circulation, somewhat independently of the ambient wind overflow. They have a tendency to flow up or down the valley/gully regardless of the general wind direction. However, if the overflowing wind exceeds 20 knots or so then significant downflow and turbulent eddies may form over the windward slopes of larger valleys, whilst rising air may be experienced over the leeward slopes. Thus aircraft contemplating a 180-degree turn within such a valley should first move over to the leeward side before commencing the turn; if available an appropriate flap setting should be used to allow a slower speed, smaller radius turn. This minimises the risk of encountering turbulent downflow on the windward side.

    Circulation within valleys may also be modified by solar heating of the valley slopes. Anabatic winds form during the day when hillside slopes are heated more than the valley floor. The differential heating of contact air causes air to flow upslope. Wind speeds of 10 knots or more may be achieved.

    8.14 Solitary waves
    Solitary waves — external gravity waves or undular bores — are common in the dry interior of northern Australia, particularly in spring prior to the wet season. They occur as severe, low-level clear air disturbances (a horizontal vortex) accompanied by a transient surface wind squall. When sufficient moisture is present a long, continuously forming roll cloud may appear with base at 500–1000 feet agl and top at 3000–5000 feet agl. Long distance soaring capability is provided by the uplift at the front of solitary waves.

    The roll cloud (and thus the vortex) may extend for several hundred nautical miles. Because it forms along the wave leading edge updraft and evaporates in the trailing edge downdraft, it appears to roll backwards. The wave may manifest itself as one large amplitude wave closely followed by several smaller diminishing waves.

    Solitary wave disturbances seem to be generated on an inversion by a disturbance such as late afternoon thunderstorm activity, the collision of opposing sea breeze fronts or the interaction of the northern end of a cold front with a developing nocturnal inversion.

    The waves, usually a 'family', propagate at a speed of 15–30 knots, relative to the ambient air flow, in a low-level stable layer under an inversion at 1500–2000 feet or so with a deep stable layer above. The neutral layer enables the wave to propagate without being damped and to travel long distances; i.e. the layer acts as a wave guide.

    The Gulf of Carpentaria Morning Glory is a product of the late-afternoon interaction of the sea breeze fronts on Cape York. The north-easterly sea breeze, aided by the prevailing easterly/south-easterly winds, is more dominant than the westerly sea breeze. The westerly breeze increases the depth of the cooled surface layer and produces a sharp gradient in the low-level wind profile. The surging higher-density air from the north-east collides with the westerly flow. This builds a long ridge of the cooler, denser air protruding into the inversion. The resulting disturbance in the inversion layer, when the convergence collapses at night, produces solitary waves in the boundary layer that propagate to the south-west on the nocturnal inversion. The waves reach the southern Gulf coastline about dawn and provide an amazing soaring ride for sailplane and hang-glider enthusiasts. Similar phenomena occur in other parts of the world but are not as extensive, or as regular, as the Gulf of Carpentaria phenomenon.
    Photographs of magnificent Gulf of Carpentaria roll clouds can be viewed on the Morning Glory web site. Be sure to view the Gulf of Carpentaria satellite image for 8 October 1992 (8:00 am local time) to see the Morning Glory threaded diagonally right across the Gulf.
    The occurrence of several roll clouds arriving in the Burketown, Queensland area from the north-east, south-east and south, during the same morning has been recorded. When opposing solitary waves meet, they pass through each other and reform their shape and velocity.

    If unaccompanied by a roll cloud, solitary waves arrive unannounced, presenting a very severe low-level wind shear and turbulence hazard to aircraft. With suitable surface conditions, aircraft flying at low levels may be warned by a line of raised dust. With the passage of a wave, the closely spaced updrafts and downdrafts may each exceed 2000 feet per minute and the transient wind gusts may vary surface wind by 30 knots or more; not something to fly into head-on, but providing an outstanding ride for the capable pilot.

    8.15 Aircraft wake vortices
    Aircraft themselves induce another form of mechanical turbulence. All aeroplanes (and helicopters) develop wake vortices in flight, their size and energy being dependent on both the aircraft's mass and the dimension of the lift coefficient. The latter, of course, is dependent on aoa and wing configuration (i.e. flap and high-lift device settings) so, for any particular aircraft, its wake vortices are greatest at the slowest flight speeds — at rotation for take-off followed by the climb out, plus the approach followed by the flare for landing.

    The relatively large surface area and the shape of weight-shift trike wings, at high aoa during take-off and landing, generate significant vortices that may trap any following aircraft with a low wing loading. In light winds, the vortices generated by aircraft the size of twin turboprops tend to persist for at least a couple of minutes as they slowly sink a couple of hundred feet below the flight path and, of course, drift with the wind. Gusty wind conditions or contact with the ground will dissipate vortices more rapidly but will spread additional turbulence while doing so.

    It is often thought that an aircraft encountering the wake vortices from an aircraft of similar size would not be unduly upset; however, this is not so and particularly if the vortex is of higher energy such as that generated by a high lift coefficient STOL aeroplane. Such encounters with relatively small vortices can be very dangerous if there is insufficient height to recover from any consequent uncommanded roll and yaw; and, of course, the upset will increase in severity as the relative mass of the vortex-generating aircraft increases. The most likely points of wake encounter are when turning base to final behind an aircraft landing from a straight-in approach and before touchdown or after lift-off if too close to any aeroplane.

    Certainly it is wise for light aircraft to anticipate and avoid encounters with the vortices from significantly larger aircraft. The general concept is to follow at least two minutes behind them in take-off or landing, and try to maintain a flight path somewhat above (which may not be possible) and upwind of the preceding aircraft. (In 1994 a Mooney 201 aircraft failed to do that when taking off behind an RAAF Hercules at Wagga Wagga, New South Wales, and ended up as wreckage alongside the runway.)

    8.16 Clear air turbulence
    Turbulence above the boundary layer and not directly associated with convective cloud is clear air turbulence [CAT]. CAT is usually associated with regions of strong vertical wind shear and temperature inversions; with jet streams, particularly in convergence/divergence areas; or with internal gravity waves, generated in the lee of mountain regions. The waves may break at various altitudes and distances from origin, generating many patches of CAT. Thus CAT is not just a concern for high-altitude aircraft; it can also adversely affect aircraft flying at comparatively low altitudes.

    Gravity waves, with consequent turbulence near thunderstorm tops [TNTT], also propagate from the intrusion of strong convective clouds into a stable upper layer. Upper-level turbulent patches vary in length from one to thirty nautical miles and are usually less than 2000 feet deep. Aircraft loads of minus 1g to plus 3g may occur.

    Upper-level frontal zones form independently of surface fronts in conjunction with jet stream intensification and with strong temperature gradients. The frontal zones are characterised by subsiding dry air and a downfold in the tropopause. Strong wind shear at the zone produces severe CAT.

    8.17 Effect of heavy rain
    Flight through rain causes a water film to form over the wings and fuselage; if the rain falls at a rate exceeding perhaps 20 mm per hour, the film over the wings is roughened by the cratering of drop impacts and the formation of waves. The effect, which increases with rainfall rate, is a lowering of the lift coefficient value at all angles of attack, with laminar flow wings being most affected and fabric wings least affected. The stall will occur at a smaller angle of attack; i.e. the stalling speed increases, which is further compounded by the increased weight of the aircraft.

    The water film will increase drag, and the encounter with falling rain will apply a downward/backward momentum, which may be significant to a light aircraft. Propeller performance is degraded and water ingestion may affect engine output.

    Thus the rain effect can be hazardous when operating in conditions of low excess aircraft energy — typically when taking off, landing or conducting a go-around. Visibility through a windscreen may be zero in such conditions, so a non IFR-equipped aircraft will be in difficulties.

    Further reading
    The online version of CASA's magazine Flight Safety Australia contains some articles relating to microscale meteorological events, which are recommended reading. A categorised index of articles of interest to recreational pilots contained in Flight Safety Australia since 1998 is available on this site. Particularly check the articles in the 'Micrometeorology and weather emergencies' category; there are also relevant articles within the other categories.

    7.1 Thermal systems
    Density or gravity currents
    A density or gravity current is formed whenever denser air intrudes into and displaces less dense air, and (usually) flows across the surface; for example, katabatic winds, convective cloud downbursts and the New South Wales Southerly Buster. Density current motion is dependent on dynamic pressure, hydrostatic pressure and surface friction. These, in turn, are dependent on the height of the intrusion and the relative densities. The flow speed is also a function of the ambient wind flow.

    Two circulations evolve within the head of a density current, and provide the mass for the mixing billows and eddies. One is below the nose, or point of stagnation (as with an aerofoil), due to surface friction. The other is above the nose where the internal speed is greater than the current propagation speed. The nose tends to repeatedly collapse and reform as the current advances, thereby adding to the turbulence of the squall. A strong, opposing, ambient wind would tend to flatten the nose into a wedge shape. The advancing head of density currents, such as the NSW Southerly Buster, often have no warning cloud associated with them. On the other hand they may produce a spectacular shelf cloud, or arcus, by forcing the warmer inflow air to rise. The leading edge of the shelf may become detached to produce a horizontal cloud tube — a roll cloud.

    The passage of the leading edge of a density current is marked by a temperature fall, pressure jump and a strong gust-line with large, rotational shear.

    Other thermal systems include:
    thunderstorms squall lines tornadoes and sea breeze fronts.  
    7.2 Wave systems
    Gravity or buoyancy waves
    Wave motion is the basic mechanism by which local disturbances are transferred from one part of the atmosphere to another without net mass transport. Gravity waves, or buoyancy waves, are pressure waves generated by disturbances within the atmosphere, where the restoring forces (potential energy) for the wave motion are provided by buoyancy and gravity, rather than compression and expansion as in higher-frequency acoustic waves. The kinetic energy is provided by mass; i.e. an air parcel, vertically displaced by a disturbance, will be acted on by gravity because its density differs from its environment. The potential energy of displacement is converted to kinetic energy when buoyancy returns the parcel to its original level. However, kinetic energy reaches a maximum at its original position, so the parcel overshoots that position and again is returned by the restoring force of buoyancy. The air parcel tends to oscillate around its undisturbed position, at a typical frequency of 5–10 minutes. If successive parcels of air are subject to displacement then a gravity wave is generated in the direction of propagation.

    The source of the disturbance could be orographic effects, frontal lines, density currents, jet streams, convection penetrating a stable layer, squall lines or low level turbulence.

    Gravity waves can be external waves or internal waves. External waves are those propagating on a discontinuity surface such as an inversion or — in regions where the gradient is strong enough to guide the propagation in a direction perpendicular to the gradient — a solitary wave. Ocean waves are external gravity waves. Internal waves propagate horizontally or obliquely to the density strata. If propagating obliquely they transport energy to the upper atmosphere and produce clear air turbulence.

    If the layer in which internal waves are produced is bounded above and below by discontinuity surfaces — for example the ground, or density or wind discontinuities — then the upward oblique waves may then be deflected downward, so the waves are then effectively contained within a wave guide. Mountain waves are an example where, depending on the thickness of the layers and the intrusion of the mountain into the airstream, the deflected energy may return in phase with the following primary waves. In this case, the amplitude of the deflected waves adds to the primary wave and the wave grows by resonance.

    Strong convective cloud punching into a stable layer aloft may generate internal gravity waves and consequent clear air turbulence within the upper layer; e.g. turbulence near thunderstorm tops.

    Passage of a gravity wave is marked by a pressure jump and a wind change but no change in temperature or humidity, as there is no air mass change. The vertical lifting may initiate cloud and precipitation. Solitary waves are well-known wave systems.

    7.3 Orographic systems
    The orographic systems of interest are:
    Slope and valley winds Low-level jets Lee wind downflow, eddies, rotors and vortices STRICT COPYRIGHT JOHN BRANDON AND RECREATIONAL FLYING (.com)

    6.1 Geostrophic and cyclostrophic winds
    Winds exist because of horizontal and vertical pressure gradients, so atmospheric motion can be deduced from isobaric surface charts. In the absence of surface friction, if the horizontal pressure gradient force is exactly balanced in magnitude by Coriolis effect then accelerations of the air will be relatively small and a geostrophic wind (from the Greek 'geo' = earth, strophe = turning ) will flow horizontally at a constant speed that is proportional to the isobaric spacing gradient. The flow will be perpendicular to the two opposing forces and parallel to straight isobars. Air will be accelerated to the extent that these forces are unbalanced. Transitory disturbances and vertical movement will create imbalance. When vertical motion is present the horizontal wind cannot be exactly geostrophic.
    Geostrophic flow is predominant above the friction layer in very large-scale weather systems, where the pressure gradient force and the Coriolis force are nearly equal and opposite; e.g. the Southern Ocean west wind belt.

    Between 15°S and 15°N latitudes there is little geostrophic flow due to weak Coriolis effect (it being zero at the equator), and winds tend to flow across the isobars. (In which case it is more useful to show wind flow on upper air charts as streamlines. A streamline arrow shows the direction of flow, whereas an isotach is a line along which the speed of flow is constant.)

    At the other end of the scale in short-lived mesoscale systems, Coriolis has insufficient time to take effect or is relatively weak compared to other forces, so geostrophic balance is not present and air accelerations can be quite large.

    If atmospheric circulation was always in perfect balance between geostrophic forces and pressure gradient forces, geostrophic winds would flow and there would be no change in pressure systems. In reality the pressure distribution takes the form of curved isobars resulting in a third force — the centripetal acceleration — which pushes the flow inward of the curve.

    The gradient wind is the equilibrium wind for the three forces — centripetal acceleration, pressure gradient force and Coriolis (or geostrophic). It is roughly aligned with the isobars on the meteorological surface chart. The vector difference between the geostrophic and the gradient winds is the ageostrophic wind. Thus, ageostrophic movement is large for small-scale systems and small for large-scale systems.

    When the centripetal acceleration becomes the major control of the gradient wind, there is an extremely strong curvature of the airflow and the winds are called cyclostrophic (Greek = circle – turning); for example, tropical cyclones and tornadoes. When a body is moving in a curved path, centripetal force is the radial inward force that constrains the body to move in that curved path and, even at constant speed, there is an inward acceleration resulting from the body's continually changing velocity. (The same applies to an aircraft in a constant-speed level turn.) The equal and opposite centrifugal force that appears to act outward on a body moving in a curved path is a fictitious force, but convenient to show the equilibrium forces for air moving in a cyclonically curved path; e.g. around a surface low pressure system, thus:
    For the gradient wind to follow cyclonically curved isobars, the pressure gradient force must be slightly stronger than Coriolis to provide the centripetal force. As the magnitude of the Coriolis is directly dependent on wind speed, it follows that the wind speed around a low is less than would be expected from the pressure gradient force and the gradient wind is sub-geostrophic.

    For air moving in an anticyclonically curved path (e.g. around a high), the opposite occurs, and the Coriolis provides the centripetal force.
    For the three forces to be in equilibrium, the Coriolis must exceed the pressure gradient force. Consequently, the gradient wind speed must be greater than would be expected from the pressure gradient force — and thus is super-geostrophic.

    Air moving within a pressure pattern possesses momentum. If the air moves into a different pressure pattern and gradient it will tend to maintain its speed and Coriolis for some time, even though the pressure gradient force has changed. The resultant imbalance will temporarily deflect the airflow across the isobars in the direction of the stronger force — Coriolis or the pressure gradient force.

    6.2 Effect of surface friction
    The Earth's surface has a frictional interaction with atmospheric motion that reduces the wind speed and thus the Coriolis effect. The pressure gradient force remains the same, so the wind is deflected towards the region of lower pressure. The friction effect is greatest at the Earth's surface and reduces with height until, at the top of the friction layer or boundary layer, the wind velocity is the gradient wind. This will usually occur somewhere between 1500 feet and 5000 feet above the terrain — much lower over the sea. In this 'spiral layer' the cross-isobar flow is greatest at the surface and decreases with height, while the speed of the flow is least at the surface and increases with height.

    So, the gradient wind flow tops the boundary layer and, as height within the layer decreases, the wind speed decreases and the wind direction veers* (in the southern hemisphere, backs* in the northern) until the wind velocity at the surface has the maximum cross-isobar component and a much lower speed. Thus, in the presence of surface friction — a force that always acts opposite to wind direction — the veering boundary layer air spirals in towards a low (clockwise rotation) and out from a high (anticlockwise rotation) in the southern hemisphere.

    *The terms veering and backing originally referred to the shift of surface wind direction with time, but meteorologists now also use the terms when referring to the shift in wind direction with height. Winds shifting anti-clockwise around the compass (e.g. from west to south) are 'backing', while those shifting clockwise (e.g. from south to west) are 'veering'.

    Velocity change between surface wind and gradient wind

    Over land, the surface wind speed may be only 30–50% of the gradient wind speed. In the boundary layer, wind slants across the isobars in the direction of the gradient force; i.e. towards the lower pressure. The stability of the boundary layer affects the strength of the friction force; a very stable layer suppresses turbulence and friction is weak, except near the surface. In a superadiabatic layer, convective turbulence is strong and the friction force will also be strong (refer to sections 3.3.2 and 9.1). The following table is for a typical neutrally stable layer, and shows the daytime average angular change in wind direction for an average wind profile over various terrains and beneath a moderately strong gradient wind of 30 knots or so.
      Typical vertical wind profile   Height (feet) Flat country Rolling country Hilly country Wind speed (knots) below 500 +30° +36° +43° 12 500 – 1000 +22° +30° +36° 20 1000 – 2000 +10° +17° +25° 25 2000 – 3000 +2° +5° +10° 28
    Within the friction layer the wind is backing as height increases; the change in direction in the first 300 feet is negligible in strong winds but greatest in light winds (below 10 knots) and may be as much as 15–20° if the surface wind is less than 5 knots. The greatest change in wind speed occurs at night and early morning.

    Also read the 'Wind shear and turbulence' module of the 'Decreasing your exposure to risk' guide.

    6.3 Calculating low-level geostrophic wind speed
    The geostrophic wind can be estimated from the isobar spacing on a surface (mean sea level) synoptic chart. The estimation is usually a reasonable approximation of the wind speed around 3000 feet agl over much of Australia. The equation applied is:
      Geostrophic wind speed (knots) = 3832 G × T / P sine L
    where G = horizontal pressure gradient in hPa/km
    T = air temperature in Kelvin units
    P = msl pressure in hPa
    L = the latitude in degrees
    Because the proportion T/P normally doesn't vary greatly at msl, the equation can be simplified to:
      Geostrophic wind speed (knots) = 2175/D sine L
    where D = the distance in kilometres between the 2 hPa isobars on the chart.
    The sine of an angle less than 60° can be estimated easily without reference to tables by using the 1-in-60 rule of thumb; i.e. the sine of an angle is roughly degrees × 0.0167 [or 1/60]; e.g. sine 36°S = 36 × 0.0167= 0.601; or 36/60 = 0.6

    The following table is derived from the preceding simplification and shows the estimated wind speed in knots for spacings between the 2 hPa isobars, from 40 to 600 km. If the surface chart shows 4 hPa spacing, then just halve the estimated distance between the isobars and still use the table below.
    Estimated wind speed from 2 hPa isobar spacings of 40 to 600 km Latitude 40 km 60 km 80 km 100 km 120 km 160 km 200 km 400 km 600 km 10°S 300 210 160 130 110 80 60 30 20 20°S 160 110 80 65 55 40 30 16 10 30°S 110 75 60 45 35 30 25 12 8 40°S 90 60 45 35 30 25 18 10 6
    6.4 Slope and valley winds
    Valleys tend to develop their own air circulation, somewhat independent of the ambient wind overflow. They have a tendency to flow up or down the valley regardless of the prevailing wind direction. This circulation is modified by solar heating of the valley slopes.

    Anabatic winds form during the day when hillside slopes are heated more than the valley floor. The differential heating of contact air causes air to flow upslope. Wind speeds of 10 knots plus may be achieved.

    Katabatic winds normally form in the evening. They are the result of re-radiative cooling of upper slopes, which lowers the temperature of air in contact with the slope and causes colder, denser air to sink rapidly downslope. In some circumstances, katabatic winds can grow to strong breeze force during the night but cease with morning warming. Anabatic and katabatic winds are usually confined to a layer less than 500 feet deep. However, the turbulence — and the sink — associated with a katabatic wind will adversely affect aircraft. Aircraft flying in a southern Australian valley late in a warm evening should expect the onset of katabatic winds.

    Katabatic winds are density or gravity currents. They can also occur in the tropics; for example, the Atherton tablelands in northern Queensland form a plateau adjacent to the tropical coast. Winter nocturnal temperatures on the plateau can reduce to near freezing and the cold, dense surface layer air flows downslope onto the warm coastal strip. In some cases katabatic winds can persist for days; an extreme example is the large-scale diurnal katabatic winds flowing from the dome of intensely cold, dense air over the Antarctic ice plateau — average elevation 6500 feet. These winds can achieve sustained speeds exceeding 80 knots, though speeds of 160 knots have been recorded at Commonwealth Bay — the windiest place on earth.

    6.5 Squalls and gusts
    Squalls or 'squally winds' are a sudden onset of strong wind lasting at least a minute then dying quickly. Wind speeds exceed 22 knots, and possibly reach 70–90 knots. They may be associated with a thunderstorm (rain squall, snow squall), with a squall line, with a dry outflow from a thunderstorm in the interior (dust squall) or with an intense cyclone where the squall reinforces the strong environment wind. Gusts or 'gusty winds' are onsets of increased wind speed that exceeds the mean wind speed by at least 30% but are shorter-lived than squalls, and often complemented by matching lulls.

    6.6 Tropical cyclones
    Tropical cyclones form only in very moist air in ocean regions where surface water temperatures exceed 26 °C. They generally occur between November and April, and in latitudes 5° – 20° South and are prominent features on the synoptic charts. Coriolis effect within 5° of the equator is too weak to develop the initial vorticity and sea surface temperatures are too low at latitudes higher than 20°. To be named as a tropical cyclone (typhoon in South-east Asia) the storm must have sustained wind speeds exceeding 33 knots; if wind speed is less, it is a tropical depression. In the eastern Pacific and the Atlantic the tropical cyclone would be named as a tropical storm for wind speed between 34 and 62 knots, then upgraded to hurricane status when the sustained wind speed exceeds 62 knots; the hurricane is then downgraded back to tropical storm when it weakens.

    Small tropical depressions (warm-core lows) form on a trough line. Warm-core lows usually become less intense with increasing height but — powered by the latent heat of condensation and if the vertical wind shear is low (below 20 knots) — some become more intense with height. They develop into a tropical storm or a monsoon low, with a very rapid updraught. This may create a cyclostrophic vortex and possibly grow, over two or three days or even less, into a full-scale tropical cyclone, with wind speeds often much greater than 62 knots. A gust of 139 knots was recorded at Mardia, near the Pilbara coast of Western Australia, in February 1975.

    The pressure drop within the tropical cyclone may be 50 to 100 hPa. (TC Orson produced a msl pressure of 905 hPa at the North Rankin gas platform in April 1989.) The diameter of the vortex may be 400 km, with a central eye 20–40 km in diameter surrounded by spiral feeder bands of CB cloud reaching the tropopause. The dry air in the eye usually descends slowly and warms adiabatically; near the surface it may be 5–8 °C warmer than the surrounding cooled air. The enormous energy within a large tropical cyclone can result in a local lifting of the tropopause; the Atlantic hurricane Bonnie of August 1998 produced chimney clouds reaching 59 000 feet.

    The tropical cyclones affecting Australia mainly form in the Coral Sea, Arafura Sea, Timor Sea and the Gulf of Carpentaria. They are usually more compact, but no less severe, than their counterparts elsewhere. While developing, the cyclone usually drifts to the west or south-west at about 10 knots. Sometimes it recurves and accelerates to the south-east and, unless it crosses a coastline, loses its impact by 30° S. They last about six to 10 days (although TC Justin persisted for three weeks off the Queensland coast in 1997. When a cyclone crosses a coast it loses the source of latent heat from the warm, moist ocean air, and weakens into a rain depression, which has high potential for major flooding. About nine tropical cyclones occur around Australia each year.

    Wind speeds felt at the surface in the south-west quadrant, before recurving, will be much greater than those in the north-east quadrant, due to the addition or subtraction of the forward movement to the rotational movement. Wind speeds of 148 knots, with a core pressure of 877 hPa, have been recorded in Pacific Ocean tropical cyclones.

    Monsoon lows are a feature of the active period of the northern Australian wet season. They develop over land from tropical depressions but don't grow into a tropical cyclone unless they move offshore. Monsoon lows bring turbulence, low cloud and heavy rain with reduced visibility over an extensive area for a considerable time; as does a tropical cyclone when it weakens into a rain depression.
      Further information concerning tropical cyclones can be found at the Australian Bureau of Meteorology's tropical cyclone page. Tropical cyclone categories
    The Australian Bureau of Meteorology defines cyclone intensity in its area of responsibility, 90°E to 160°E, from category 1 to category 5, according to the expected strongest gust, as below:
    1 below 69 knots Negligible damage to houses. Damage to crops, trees and caravans 2 69 to 92 Minor house damage, significant damage to trees and caravans. Heavy damage to crops 3 93 to 120 Roof and structural damage. Power failure likely 4 121 to 150 Caravans blown away. Dangerous airborne debris 5 above 150 knots Extremely dangerous with widespread destruction
    Cyclone Tracy, which wrecked Darwin (24/12/1974) was category 4, but the highest recorded gust in the city was 117 knots. Cyclone Vance (22/3/99) was category 5.

    The Saffir-Simpson scale, however, is used in the Atlantic and Eastern Pacific for categorising hurricane intensity:
      Saffir-Simpson scale   Class Central pressure Max. 1 minute sustained speed Damage potential Tropical depression   below 33 knots nil Tropical storm   33 – 62 minimal Hurricane cat.1 above 980 mb 63 – 83 minimal Hurricane cat.2 965 – 980 84 – 96 moderate Hurricane cat.3 945 – 965 97 – 113 extensive Hurricane cat.4 921 – 945 114 – 135 extreme Hurricane cat.5 below 921 over 135 knots catastrophic
    6.7 Determining wind velocity
    During pre-flight planning, pilots determine the forecast wind velocities, at various cruising levels and at aerodromes along their route, by reference to forecast information provided by an authority such as the Australian Bureau of Meteorology or Airservices Australia.

    Meteorological forecast information for an area [an ARFOR] can be obtained from Airservices Australia's NAIPS Internet Service. See Obtaining weather forecasts, NOTAM, first light and last light.

    The real-time weather observations, at about 190 airfields, can be obtained by telephoning the Australian Bureau of Meteorology automatic weather station at the location and listening to the audio data. See AWIS in the VHF radiocommunications guide.

    As the flight progresses, the navigation techniques employed enable calculation of the actual wind velocity at cruising level. While airborne, a radio-equipped aircraft can usually obtain a report of actual weather conditions at the larger aerodromes; see 'Acquiring weather and other information in-flight' in the VHF radiocommunications guide. If a mobile phone is carried, the AWIS (if available) can be used to obtain surface wind and some other weather data. However, surface wind velocity at smaller airfields can be estimated from the probable wind profile knowing the upper level velocity — see 'Effect of surface friction' above — or determined by observation.

    Determining surface wind direction visually while airborne
    Apart from an airfield windsock, the most obvious indicators of surface wind direction are dust from agricultural operations or moving vehicles and smoke from chimneys or smaller fires.

    Wind ripples in grassland, crops or tree tops provide a reasonable indication in light to moderate winds, as does wave movement on small to larger lakes.

    In lighter winds the wind shadow of still water, at the upwind edge of a small lake or dam, is usually apparent. And, of course, when the aircraft is flying at a lower level the aircraft's drift is a strong indicator of the near-surface winds.
    The Beaufort wind speed scale (land)
    No. Wind speed Gust speed Meteorological classification Terms used in general forecast Wind effect on land 0 <1 knot   calm calm Smoke rises vertically 1 1 – 3   light air light winds Smoke drifts 2 4 – 6   light breeze light winds Leaves rustle, water ripples; '15 knot' dry windsock tail drooping 45° or so 3 7 – 10   gentle breeze light winds Wind felt, leaves in constant motion, smooth wavelets form on farm dams and small lakes, smoke rises at an angle above 30°; '15 knot' dry windsock tail 15° or so below horizontal 4 11 – 16   moderate breeze moderate wind Small branches move, dust blown into air, crested wavelets form 5 17 – 21   fresh breeze fresh wind Small trees sway, smoke from small fires blown horizontally; '15 knot' dry windsock horizontal 6 22 – 27   strong breeze strong wind Large branches sway, whistling in wires 7 28 – 33   near gale strong wind Whole trees in motion 8 34 – 40 43 - 51 fresh gale gale wind Twigs break off, difficulty in walking 9 41 – 47 52 - 60 strong gale severe gale Some building damage 10 48 – 56 61 - 68 whole gale storm Trees down 11 57 – 62 69 - 77 storm violent storm Widespread damage 12 63 + 78 + tropical cyclone tropical cyclone Severe extensive damage
    The Beaufort wind speed scale (sea — and perhaps large lakes)
    0 – Sea is mirror-like
    1 – Ripples present but without foam crests
    2 – Small wavelets, glassy appearance and do not break
    3 – Large wavelets, crests begin to break, with scattered white horses
    4 – Small waves becoming longer, fairly frequent white horses
    5 – Moderate waves, many white horses with chance of spray
    6 – Large waves are forming with extensive white foam crests, spray probable
    7 – The sea heaps up, white foam from breaking waves is blown in streaks
    8 – The edges of crests break into spindrift with well marked, foam streak lines
    9 – High waves with tumbling crests and spray, dense foam streaks
    10 – Very high waves with overhanging crests, surface appearance white, visibility affected
    11 – Chaotic sea, large parts of waves blown into spume with foam everywhere
    12 – Air filled with foam and spray, visibility severely impaired

    State of seas classification
    The following table is the state of seas classification, with likely maximum wave height in metres, used in general meteorology reports and warnings for Australian coastal waters: Calm zero No waves Rippled 0.1 m No waves breaking on beach Smooth 0.5 m Small breaking waves on beach Slight 1.3 m Waves rock buoys and small boats Moderate 2.5 m Sea becoming furrowed Rough 4 m Sea deeply furrowed Very rough 6 m Disturbed sea with steep-faced rollers High 9 m Very disturbed sea with steep-faced rollers Very high 14 m Towering seas Phenomenal >14 m Hurricane seas
    State of swell classification
    The following table is the state of swell classifications used for reporting the wave-train height and length:
    Swell height Swell length Low swell 0 - 2 m Short 0 – 100 m Moderate 2 - 4 m Average 100 – 200 m Heavy >4 m Long >200 m
    The length and speed of the wave-train can be calculated readily if the period (in seconds) is measured; i.e. the length in metres is 1.56 × the period squared and the speed in knots is 3.1 × the period.

    e.g. if period = 10 seconds, then train lengths = 156 metres and propagation speed = 31 knots

    6.8 The compass rose and the wind rose
    In nautical terms there are 32 compass 'points' each division being 11.25° of azimuth. Winds shifting anticlockwise around the compass rose are 'backing', those shifting clockwise are 'veering'. The names of the compass points and the associated compass direction in degrees are shown in the following table. The term 'by' indicates plus or minus one point (11.25°) in the stated direction; e.g. 'nor'east by north' indicates north-east (45°) minus 11.25° = 33.75°.

    Compass rose points
      11.25 North by (one point) east 191.25 South by (one point) west 22.50 Nor'nor east 202.50 Sou'sou'west 33.75 Nor'east by north 213.75 Sou'west by south 45.00 North east 225.00 South west 56.25 Nor'east by east 236.25 Sou'west by west 7.50 East nor'east 247.50 West sou'west 78.75 East by north 258.75 West by south 90.00 East 270.00 West 101.25 East by south 281.25 West by north 112.50 East sou'east 292.50 West nor'west 123.75 Sou'east by east 303.75 Nor'west by west 135.00 South east 315.00 North west 146.25 Sou'east by south 326.25 Nor'west by north 157.50 Sou'sou'east 337.50 Nor'nor'west 168.75 South by east 348.75 North by west 180.00 South 360.00 North
    The wind rose
    The term 'wind rose' nowadays applies to the diagram meteorologists use to represent the wind velocity statistical data collected for a particular location. To view the wind rose for a specific location in Australia, go to the Bureau of Meteorology's wind rose page.

    5.1 Air masses
    An air mass is a relatively homogeneous body of air usually covering millions of square kilometres of the Earth's surface and perhaps around 20 000 feet thick; even extending to the tropopause. To be homogeneous, the air mass source region must be exclusively continental (dry air) or exclusively maritime (moist air). All air mass source regions lie in tropical (warm air) or polar (cold air) latitudes. The air masses originating there are modified by passage into — and interaction within — the mid-latitudes, so producing 'mid-latitude air'.

    The modification of the air mass, by heating or cooling from the surface it is passing over, will change stability. Additional heating will make moist air more unstable, while additional cooling makes moist air more stable. Low-level convergence produces upper-level instability and low-level divergence produces upper-level stability.

    The air masses, and their source regions, affecting the Australian climate are:
    Equatorial maritime: Hot, humid air with dewpoint around 25 °C, bringing monsoon conditions to northern Australia. Tropical maritime: Warm, humid air with dewpoint around 20 °C, bringing showers, rain and tropical cyclones. Tropical continental: The source region is northern Australia. Hot, dry air in summer, dewpoint around 0 °C, bringing heat waves to southern Australia. Warm, dry air in winter, dewpoint around 4 °C. Southern Ocean maritime: Cool, moist air with dewpoint around 10 °C, bringing clouds, rain and drizzle to southern Australia. Antarctic polar continental/maritime: Cold, moist air with dewpoint around 5 °C, bringing cold outbreaks to southern Australia with snow and sleet. (South of Australia, the coastline of Antarctica lies north of the Antarctic Circle with the ice pack extending to about 60°S, only 1200 km from Tasmania).  
    Frontal zones, or fronts, separate air masses of different characteristics. They usually extend from the surface to the middle troposphere, and occasionally to the upper troposphere. Within the frontal zone, changes of temperature, pressure, density and wind velocity are large compared to changes outside the frontal zone.

    In section 4 we established that the Antarctic front is the boundary region between the intensely cold Antarctic polar continental air and the warmer, moister polar maritime air. Also that the polar fronts are the major frontal regions of the southern hemisphere — mixing between polar air, mid-latitude air and returning tropical air. The Antarctic and polar fronts are quasi-stationary frontal regions, and may extend for several thousand nautical miles. They are distinct from the mobile cold fronts that directly affect southern Australia's daily weather patterns.

    The diagrams below indicate typical positions of the air masses, and the polar and Antarctic fronts, in the summer and winter seasons.
    5.2 Extra-tropical cyclones
    The effect of the potential energy stored in the zone of strong surface temperature gradient in the polar frontal regions, with the cold air masses pushing north-west and wedging under the warmer air pushing south-east, is that the polar fronts spawn a series of migratory depressions south and west of Australia — typically in latitudes 35°S – 45°S. The depressions tend to be intense — surface pressures below 940 hPa with gradients of 50 hPa over 1500 km have been recorded.

    These transient depressions forming in the westerly wind belt (also known as cold-core cyclones, lows, storm depressions or, more correctly, extra-tropical cyclones) — often with embedded, smaller-scale storms — are the principal cause of day-to-day weather changes in southern Australia.

    A common theory for the development of these extra-tropical cyclones is that the interaction of the air masses cause a disturbance to develop on the line of the polar front. This initiates the process of converting the potential energy of the strong temperature gradient into the kinetic energy of a developing extra-tropical cyclone, so distorting the polar front into a wave-like configuration. The extent of each wave/trough is dependent on which air mass is stronger at that point. A wave crest may develop into an extra-tropical cyclone after several days (see following diagrams A to G) forcing southward movement of the warm air and northward movement of the cold air as mobile fronts. The intense, mobile cold front moves at 15–30 knots, faster than the warm front which it may eventually overtake to form an occluded front. That may then lead to an intensified storm.

    The development of the low also requires that the mass of the vertical column of air over the area is reduced by mass divergence, thus reducing the surface pressure. Consequently, the upper-air Rossby waves — and the jet streams — support and direct (and may enhance) the development of surface cyclones and other features.

    The maturing storm depression usually moves south-east to about 60°S – 65°S, into the Ross Sea and the sub-polar low belt. Here, cut off from the warmer moister air, it decays. Depressions may have a life cycle of one week or so. Some primary depressions may head north-east into the high-pressure belt. As they are then isolated from the westerly wind belt, they are consequently termed cut-off lows.

    Depressions tend to travel in groups of three or four, creating large eddies in the westerly wind belt. Secondary depressions occur on the trailing arm of the primary low cold front, and may curve north-east before decaying or swinging to the south-east. These secondary lows are often fast-developing, intense, short-lived storms.

    The spring-time msl analysis (below) from the World Meteorological Centre, Melbourne, shows the synoptic features in a polar projection of half the southern hemisphere, from the prime to the 180° meridians. It covers the area of southern Africa at the left, the Indian and Southern oceans, Antarctica at the bottom, and Australia/New Zealand at the right. The planetary-scale synoptic features displayed are the Antarctic polar high and the two anticyclones of the sub-tropical high belt extending a ridge right across the chart and centred at 35°S — also with a spur extending south to link into the polar high.

    There are also three or four centres of low pressure in the sub-polar low belt just off the Antarctic coastline at 65°S, each associated with an extensive front — some extending for maybe 3000 nautical miles. These are the polar fronts.

    There are about four migratory lows in the westerly wind belt at 55°S, one at 150°E and a group around 30°E — each associated with mobile cold and warm fronts. The unusual element is the long trough (the dashed line) extending from north-west Australia into the Tasman Sea and the Southern Ocean. The front passing over the south-east corner of Australia brought with it a cold outbreak of polar maritime air.

    The diagrams below are a four-day msl pressure forecast issued by the Australian Bureau of Meteorology [BoM]. Note the position of the fragmentary warm fronts well south of the mainland, and the frontal trough systems between the highs. A wide selection of the Bureau's daily msl analysis and prognosis charts can be viewed at BOM charts.
    In winter, intense primary depressions can develop at rates of one hPa per hour with the pressure gradient steepening towards the centre. Lows also develop in regions where no significant surface temperature gradient exists. They develop from the interaction of airstream flow and consequential frontal development. Weak lows may also form on the lee side of the Great Dividing Range.

    Occasionally a cold-core high — which unlike a warm-core high, decreases in intensity with height — will form in the southern polar maritime air mass behind a cold front. They are usually short-lived, as the upper levels are warmed by subsidence, and the system moves north-east and merges with the high pressure belt. However, such highs behind an intense low can direct a major cold outbreak of sub-Antarctic air into south-eastern Australia.

    If the cold-core anticyclone stays in the Southern Ocean and persists, it may form a blocking high, which interrupts and diverts the normal movement of the mobile cyclones. The same result is achieved if a warm-core high extends further south than normal.

    5.3 Mobile cold fronts
    The mobile cold fronts, which develop with the extra-tropical cyclones, are typically 5000 feet deep at the nose and expand with depth. They may be 150 to 800 nm long and advance eastward at speeds of 15 to 40 knots — as indicated on the surface chart below. Mesoscale fronts may be much smaller. Small but sharp fronts also develop in the middle and upper troposphere.

    Warm fronts occur in the region where warm, less dense air is moving in the general direction of the south pole and sliding up over the semi-stationary colder, denser air. The resultant slope is in the region of 1:100 to 1:300. Cold fronts — where colder, denser air is pushing under semi-stationary, warmer air — have a typical slope of 1:60, but the warmer air is tending to ascend slantwise across the slope of the cold front.

    As the extra-tropical cyclones generally develop south of Australia — and the consequent warm fronts move south — the passage of a warm front over the mainland is rare. Part of a weak warm front may pass over Tasmania from a low developing in the south-east mainland corner or in Bass Strait. Such warm front occurrences over land are fragmentary, weak and transient. The BoM surface chart below shows a weak warm front forming at the south-east mainland corner, it subsequently disappeared within 24 hours.

    Similarly occluded fronts are rare occurrences in Australia; so, the remainder of this section deals solely with the structure and effects of cold fronts. The presence of a front does not of itself imply cloud formation and rain. Convergence is necessary to produce rain, and when the front is remote from a depression, then convergence may be absent. Cold fronts moving northward into south-west Queensland are usually shallow and diffused but may trigger a surge in the prevailing easterlies.

    The two diagrams below show the cross-section of typical summer cold fronts. The upper diagram is that of an active summer cold front. When the low pressure system weakens, or the cold front trails towards the high pressure region, the air aloft subsides and warms, the upper cloud disappears and the front weakens — as shown in the lower diagram. Note that the diagrams greatly exaggerate the frontal slope.
    In winter, if the normal pattern of eastward movement is halted then cold fronts will cross south-east Australia every few days. They are usually relatively weak but with widespread cloud bands, low cloud bases and showery precipitation. Some winter cold fronts may be vigorous and fast moving, with embedded thunderstorms and a narrow band of cloud and precipitation. Such winter fronts are usually associated with a very deep depression forming further north than usual.

    Cross-section of an unstable cold front
    When an active cold front moves north-east — particularly in spring and summer — a subsidence may occur in the cold air behind the frontal zone, which causes the frontal zone to bulge ahead of its surface position. Thus, the lifting of the warm air occurs ahead of the frontal surface position and is accompanied by increased instability — the nose of the cold front pushes up a bow wave that creates lift similar to orographic lift. Depending on the moisture content of the lifted air, thunderstorms — or even a squall line — may form ahead of the front.

    The sequence of events associated with the passage of such a front moving at 25 knots (but without a squall line) might be as follows:
    In the transition zone ahead of the front, warm to hot north to north-westerly winds freshen, pressure is falling and cirrus clouds are moving from the west, three to six hours prior to passage of the front; this is followed by lowering cloud (Ac, As and Ns). Some rain occurs just ahead of the front, then thunderstorms and violent gusts, and the temperature drops suddenly as the frontal zone passes. In the cold air behind the front, the clouds and showers clear quickly, the wind backs to south/south-west and the pressure rises.

    There may be a number of pressure changes in the transition zone ahead of any cold front, usually including wind squalls. The airflow in the zone is very unstable, producing large changes in wind velocity — both horizontal and vertical — and distinct lines of convection cells, which may form a squall line particularly in spring and summer.

    5.4 Synoptic isobaric features
    East coast lows and cut-off lows
    Depressions forming off south-west and south-east Australia tend to be large, deep and slow moving. They may dominate the local weather system, bringing heavy rain for several days, particularly in the cooler season. These depressions may be cut off from the westerly wind belt by a high pressure cell or ridge to their south.
    Deep cut-off low off Western Australia coast
      Slow moving, cut-off low — eastern coast
    Blocking pairs
    About ten times per year a semi-stationary system of high and low pressure cells, located in the Tasman Sea, can block the normal easterly procession of the highs and lows. The blocking pairs occur most frequently in winter with the low pressure cell or trough closer to the equator and the high pressure cell on the polar side, both out of their normal zone. (The high could be a warm-core high that has drifted south-east or a persistent cold-core high). A strong north/south wind is set up between them and the upper, westerly wind flow is split — with one part passing on the northern side of the blocking pair and the other part passing on the southern side.

    Blocking pairs can cause abnormal weather patterns in south-east Australia. Persistent and recurring pairs lead to low rainfall and drought conditions.
    5.5 The north-west cloud band
    The north-west cloud band is a frequent feature in satellite weather images, typically extending over 2500 nm and existing for two to four days. Most occurrences disintegrate after six days. It originates in a convective system in the Indian Ocean south and west of Indonesia, where tropical maritime air flowing poleward on the western flank of a high pressure ridge — extending through eastern and northern Australia — conflicts with a pre-frontal trough of colder, drier air extending from southern Australia into north-western Australia. 
    The maritime air is forced to rise, producing heavy stratiform cloud that eventually extends from the convective source (which continues to feed moisture into the system) to south-eastern Australia. The phenomenon occurs once or twice a month during the colder months. The vertical extent of the cloud band increases toward the south-east with a lowering base and an increasing height of the tops. Two or three times a year a fully active band will present cloud cover right across Australia, extending — unbroken — from very low levels to above 20 000 feet and joining with a low pressure system in the south-east corner. Heavy rain is often associated with the bands and conditions less than standard visual meteorological conditions [VMC] can exist for days.

    4.1 General global circulation
    As the Earth rotates at a constant rate and the winds continue, the transfer of momentum between Earth–atmosphere–Earth must be in balance and the angular velocity of the system maintained. (The atmosphere is rotating in the same direction as the Earth but westerly winds move faster and easterly winds move slower than the Earth's surface. Remember, winds are identified by the direction they are coming from not heading to!)

    The broad and very deep band of fast-moving westerlies in the westerly wind belt, centred around 45°S (but interrupted at intervals by small, migrating lows moving east — not shown in the diagram above) lose momentum to the ocean through surface friction, resulting in the Southern Ocean's west wind drift surface current. The equatorial easterlies or trade winds and, to a lesser extent the polar easterlies, gain momentum from the Earth's surface. That gain in momentum is transferred, to maintain the westerlies, via large atmospheric eddies and waves — the sub-tropical high and the sub-polar low belts.

    These eddies and waves are also part of the mechanism by which excess insolation heat energy is transferred from the low to higher latitudes.

    Globally, the equatorial low pressure trough is situated at about 5°S during January and about 10°N during July. Over the Pacific Ocean the trough does not shift very far from that average position — but due to differential heating it moves considerably further north and south over continental land masses. In Australia the trough will sometimes approach Alice Springs — latitude 23°S in the hot centre of the continent. The average summer msl pressure chart shows the position of the three most intense low pressure areas of the trough over South America, Africa and Australia/Papua-New Guinea.

    The low-level air moving towards the trough from the sub-tropical high belts at about 30°S and 30°N is deflected by Coriolis, and forms the south-east and north-east trade winds. Coriolis effect deflects air moving towards the equator to the west and air moving away from the equator to the east. Thus, when the north-east trade winds cross the equator in the southern summer, they turn to become the north-west monsoon which brings the 'Wet' to northern Australia.

    4.2 Cross-section of tropospheric circulation

    4.3 The intertropical convergence zone and the Hadley cell
    The trade winds converging at a high angle at the equatorial trough, the 'doldrums', form the intertropical convergence zone [ITCZ]. The air in the trade wind belts is forced to rise in the ITCZ and large quantities of latent heat are released as the warm, moist, maritime air cools to its condensation temperature. About half the sensible heat transported within the atmosphere originates in the 0–10°N belt, and most of this sensible heat is released by condensation in the towering cumulus rising within the ITCZ.

    A secondary convergence zone of trade wind easterlies — the South Pacific convergence zone — branches off the ITCZ near Papua-New Guinea, extends south-easterly, and shows little seasonal change in location or occurrence.

    Over land masses the trade winds bring convective cloud, which develops into heavy layer cloud with embedded thunderstorms when the air mass is lifted at the ITCZ.

    The ITCZ is the 'boiler room' of the Hadley tropical cells, which provide the circulation that forms the weather patterns and climate of the southern hemisphere north of 40°S. The lower-level air rises in the ITCZ then moves poleward at upper levels — because of the temperature gradient effect — and is deflected to the east by Coriolis, at heights of 40 000 – 50 000 feet, while losing heat to space by radiative cooling.

    The cooling air subsides in the sub-tropical region, warming by compression and forming the sub-tropical high pressure belt. Part of the subsiding air returns to the ITCZ as the south-east trade winds thus completing the Hadley cellular cycle. (The system is named after George Hadley [1685-1768], a British meteorologist who formulated the trade wind theory.)

    At latitudes greater than about 30°S the further southerly movement of Hadley cell air is limited by instability, due to conservation of momentum effects, and collapses into the Rossby wave system. The Hadley cell and the Rossby wave system — combined with the cold, dry polar high pressure area over the elevated Antarctic continent — dominate the southern hemisphere atmosphere. Fifty per cent of the Earth's surface is contained between 30°N and 30°S, so the southern and northern Hadley cells directly affect half the globe.

    4.4 The sub-tropical anticyclones
    The subsiding high-level air of the Hadley cells forms the persistent sub-tropical high pressure belt, or ridge, that encircles the globe and which is usually located between 30°S and 50°S. Within the belt there are three semi-permanent year-round high-pressure centres in the South Indian, South Pacific and South Atlantic oceans. In summer, anticyclonicity also peaks in the Great Australian Bight.

    In winter the high-pressure belt moves northward, the high in the Bight extends and migrates into a large, semi-permanent winter anticyclone over southern Australia.

    The Indian Ocean centre produces about 40 anticyclones annually which, as they develop, slowly pass from west to east, with their centres at about 38°S in February and about 30°S in September. The anticyclones, or warm-core highs, are generally large, covering 10° of latitude or more, roughly elliptical, vertically extensive and persistent, and with the pressure gradient weakening towards the centre. The anticyclones are separated by lower-pressure troughs.

    Winds move anticlockwise around the high, with easterlies on the northern edge and westerlies on the southern edge. Air moving equatorward on the eastern side is colder than air moving poleward on the western side. The high-level subsiding air spreads out, chiefly to the north and south of the ridge due to the higher surface pressures in the east and west. Thus the position of the sub-tropical high belt dominates Australian weather. In summer, when it is centred just south of the continent, sub-tropical easterlies cover much of Australia, with monsoonal movement in the north. In winter the belt, being further north, allows the strong, cold fronts that are embedded in the westerlies to affect southern Australia (refer to section 5.2).

    4.5 The Antarctic polar high and the sub-polar low belt
    The lowest surface temperatures on Earth occur at the Antarctic continent, at minus 80 °C or less. The very dry air allows any long-wave radiation to escape without any appreciable atmospheric warming. The cold-core Antarctic polar high is quite shallow — 5000 to 10 000 feet deep — which decreases in intensity with height, and has a very steep inversion and an extensive upper-level low aloft; the combination of high pressure and low temperatures producing very dense air.

    The air moving in an anti-clockwise direction around the anticyclone produces the surface outflow belt of polar easterlies. But, over the high-altitude icecap, tropospheric circulation consists of mid and upper-level inflow and katabatic outflow in a shallow surface layer. (A monthly mean katabatic wind of 58 knots has been recorded at Commonwealth Bay.) Very cold air masses and minor highs can split off the main Antarctic air mass — following passage of a major cyclone — and move northwards in winter, bringing the very cold Antarctic continental/maritime air towards Australia. By contrast, due to the Antarctic ice cap elevation of 6000 to 13 000 feet, Southern Ocean storms usually do not penetrate the Antarctic region south of Australia and surface pressure mainly depends on elevation.

    A series of deep lows — usually centred between 50°S and 60°S and tending further south during the equinoctial periods (the Antarctic sub-polar low belt) — surround the Antarctic polar high, the boundary between the two systems is formed by the polar easterlies. This boundary between the intensely cold continental air and the warmer, moister polar maritime air is termed the Antarctic front.

    4.6 Rossby waves and the westerly wind belt
    Upper westerlies blow over most of the troposphere between the ITCZ and the upper polar front. They are concentrated in the westerly wind belt where they undulate north and south in smooth, broad waves. These waves comprise one, two or three semi-stationary, long wave, peaks and troughs. They occur during each global circumnavigation and have a number of distinct mobile short waves; each about half the length of the long waves.

    The amplitude of these mobile Rossby waves, as shown on upper atmosphere pressure charts, varies considerably and can be as much as 30° of latitude. Then the airflow, rather than being predominantly east/west, will be away from or towards the pole. The gradient wind speed in the equatorward swing will be super-geostrophic and the speed in the poleward swing will be sub-geostrophic.The poleward swing of each wave is associated with decreasing vorticity and an upper-level high pressure ridge and the equatorward swing is associated with increasing vorticity and an upper trough.

    Downstream of the ridge, upper-level convergence occurs, with upper-level divergence downstream of the trough. This pattern of the Rossby waves in the upper westerlies results in compensating divergence and convergence at the lower level. This is accompanied by vorticity and the subsequent development of migratory surface depressions — lows or cyclones (cyclogenesis) — and the development of surface highs or anticyclones (anticyclogenesis).

    The long waves do not usually correspond with lower-level features, as they are stable and slow moving, stationary or even retrograding. However, they tend to steer the more mobile movement of the short waves which, in turn, steer the direction of propagation of the low-level systems and weather.

    The swings of the Rossby waves carry heat and momentum towards the poles, and cold air away from the poles. The crests of the short waves can break off, leaving pools of cold or warm air, which assist in the process of heat transfer from the tropics. Wave disturbances at the polar fronts perform a similar function at lower levels.

    An upper-level pool of cold air — an upper low or cut-off low or upper air disturbance — will lead to instability in the underlying air. The term cut-off low is also applied to an enclosed region of low surface pressure that has drifted into the high pressure belt, i.e. cut off from the westerly stream, or is cradled by anticyclones and high pressure ridges. Similarly the term cut-off high is also applied to an enclosed region of high surface pressure cut off from the main high pressure belt (refer to 'blocking pairs') and to an upper-level pool of warm air that is further south than normal — also termed upper high.

    Air thickness charts show the vertical distance between two isobaric surfaces. Usually, 1000 hPa is the lower, and the upper may be 700 hPa, 500 hPa or 300 hPa. The atmosphere in regions of less thickness — upper lows — will be unstable and colder, whereas regions of greater thickness — upper highs — tend to more stability. On these charts, winds blow almost parallel to the geopotential height lines.

    4.7 Southern polar fronts
    The polar fronts, a series of separate fronts globally distributed in the Southern Ocean, are the major frontal zones of the southern hemisphere. They mix between polar air, mid-latitude air and returning tropical air (refer to diagram 4.2). The very cold, dense air moving from the Antarctic high pressure cell and which is deflected by Coriolis into easterlies, contacts the warmer, moister Southern Ocean air moving away from the sub-tropical high pressure belt and which is deflected by Coriolis into westerlies. The returning tropical air is the upper-level air flowing from the Hadley cell, which subsides behind the front and returns to the sub-tropical region at lower levels. Polar fronts are quasi-stationary and generally located about 45°S, but move with the seasons.

    4.8 Upper-level jet streams
    Upper air flow in the Hadley cell moves to about 30°S latitude while cooling and eventually subsiding, forming the sub-tropical high pressure belt or ridge. Applying the principle of conservation of momentum: the rotation at the equator is 464 metres/second while at 30°S the surface rotation is 402 m/sec. Thus at 30°S a molecule of upper air transported from the equator has a surplus momentum of 62 m/sec or 122 knots. This surplus momentum forms the westerly sub-tropical jet stream, with an average velocity of 120 knots — the upper stream represented in the following diagram from The Weather Company www.weatherzone.com.au.
    The polar front jet streams are embedded in the upper-level westerlies, snaking north and south daily and seasonally with the movement of the polar front depressions. They exist because of the strong thermal gradient in that area and they are regions of maximum upper-level air mass transport. As they meander polewards and equatorwards with the general upper air waves, they tend (by their sheer mass) to steer the movement of major low-level air masses. This encourages development of surface pressure features, and intensification of pre-existing features, by the concentrated convergence/divergence within the jet stream. The jet streams are stronger in the winter when the polar front is closest to the equator. The image indicates the position of the sub-tropical and polar front jet streams on 29 August 2009.
    Jet streams are not continuous but can be as much as 3000 – 5000 km long, 100 – 300 km wide and 7000 – 10 000 feet deep. About 60% of the width tends to be on the equatorial side of the core, which is located near the tropopause. Over Australia, core wind speeds normally range from 60 – 150 knots, but occasionally reach 200 knots. The wind speeds usually decrease by 3 – 6 knots per 1000 feet above and below the core, but the rate may reach 20 knots per 1000 feet. Horizontally, the wind speeds are diminished by about 10 knots per 100 km distance from the core. Jet stream cirrus may form on the equatorial side of the core.

    3.1 Cloud formation
    Generally, upward motion of moist air is a prerequisite for cloud formation, downward motion dissipates it. Ascending air expands, cools adiabatically and, if sufficiently moist, some of the water vapour condenses to form cloud droplets. Fog is likely when moist air is cooled not by expansion but by contact with a colder surface.

    Water vapour generally needs something to condense onto to form liquid. Common airborne condensation nuclei are dust, smoke and salt particles; their diameter is typically 0.02 microns (micrometres) but a relatively small number may have a diameter up to 10 microns. Maritime air contains about one billion nuclei per cubic metre (typically salt), while polluted city air contains many more. The diameter of a cloud droplet is typically 10 to 25 microns and the spacing between them is about 50 times the diameter — perhaps 1 mm — with maybe 100 million droplets per cubic metre of cloud. The mass of liquid in an average density cloud is approximately 0.5 gram per cubic metre.

    Above the freezing level in the cloud, some of the droplets will freeze if disturbed by contact with suitable freezing nuclei or with an aircraft. Freezing nuclei are mainly natural clay mineral particles, bacteria and volcanic dust, perhaps 0.1 microns in diameter but up to 50 microns. There are rarely more than one million freezing particles per cubic metre; thus there are only enough to act as a freezing catalyst for a small fraction of the cloud droplets. Most freezing occurs at temperatures between –10 °C and –15 °C.

    The balance of the unfrozen droplets remains in a supercooled liquid state, possibly down to temperatures colder than –20 °C. Eventually, at some temperature warmer than –40 °C, all droplets will freeze by self-nucleation into ice crystals, forming the high-level cirrus clouds. In some cases, fractured or splintered ice crystals will act as freezing nuclei. The ice crystals are usually shaped as columnar hexagons or flat plate hexagons. Refer to sections 3.5.2 and 12.2.2.

    Condensation of atmospheric moisture occurs when: the volume of air remains constant but temperature is reduced to dewpoint; e.g. contact cooling and mixing of different layers the volume of an air parcel is increased through adiabatic expansion evaporation increases the vapour partial pressure beyond the saturation point a change of both temperature and volume reduces the saturation vapour partial pressure.
    3.2 Cloud classification
    3.2.1 Cloud genera
    Cloud forms are based on ten main genera, conventionally grouped into three altitude bands — high, medium and low — plus a vertically developed group. About 90% of atmospheric moisture exists below 20 000 feet with 50% or more in the band below 6500 feet. The altitudes included in each band are dependent on the thickness of the troposphere at nominal locations — tropical, temperate or polar. These are:
      Cloud altitude bands   Tropical Australia Temperate Australia Antarctic High 20 000–60 000 16 000–43 000 10 000–26 000 Medium 6500–26 000 6500–23 000 6500 –13 000 Low 0–6500 0–6500 0–6500
    High clouds
    A two-letter code is used to identify cloud genera in meteorological reports, observations and aviation area forecasts. Cirrus [CI] (Latin for 'curl'): white patches, banners, threads or delicate filaments of ice crystals. They often appear in patches of individual 'generating heads' with streaks of crystals falling from them thus forming comma-shaped or hooked 'mares' tails'. Cirrus clouds may merge into CS or CB. They are formed by widespread ascent, but sometimes by upper level turbulence in a smaller area. Cirrostratus [CS]: a thin, transparent, amorphous, whitish veil of smooth or sometimes finely fibrous appearance, appearing over much of the sky at very high altitudes. They create the appearance of halos about the sun or moon. Cirrostratus may merge into CC or possibly AS, and are formed by widespread ascent and may thicken when preceding a cold front. Cirrocumulus [CC]: thin, white patches, sheets or rows with small, regularly arranged elements or cloudlets in the form of grains or ripples, which may be merged or separate; sometimes with an appearance like fish scales — a 'mackerel sky'. The apparent width of elements is less than one degree. Cirrocumulus elements may merge together to form CS or separate into CI mares' tails. CC are produced by turbulence aloft — often associated with a front or upper-level disturbance.  
    Medium-level clouds
    Altostratus [AS]: grey/bluish sheet, with coverage of possibly 8 oktas, of uniform appearance. They are often striated or fibrous, having parts thin enough to reveal a vague sun without any halo but possibly a corona. Altostratus often merges into NS. They are caused by widespread ascent and are usually associated with a front or upper-level disturbance. Altocumulus [AC]: white/grey patches, bands or sheets of regularly arranged globular elements (sometimes called mackerel sky) — waves or rows with light shading, closely packed or merged. The element width is 2 to 5 degrees. (A finger width at arm's length is approximately 2 degrees; the spread between the tips of the little finger and thumb when a hand is splayed is about 22 degrees.) Altocumulus often shows coloured patches (irisation) around elements when illuminated by the sun or moon; a corona may be visible. They are usually caused by turbulence and are not associated with a change in the weather. Nimbostratus [NS] (from the Latin 'nimbus' = cloud, aureole): thick, dense, dark grey layer, often with a ragged or diffused base, with continuous precipitation. Coverage is often 8 oktas. Scud (pannus) may form beneath it. Invariably they occur at medium level, but usually extend to high level and merge with AS; they may also extend to low levels and envelop hills. Nimbostratus are produced by widespread ascent.  
    Low-level clouds
    Stratocumulus [SC]: grey/whitish patches, sheets or layers of separate or partly merged globular masses or rolls with dark shading and generally irregular appearance. If regularly arranged, the separate elements have apparent width exceeding 5 degrees. Coverage is often 8 oktas and may be penetrated by large CU or CB. Stratocumulus are probably the most frequently seen cloud in south-eastern Australia and are most frequent in winter anticyclones — 'anticyclonic gloom' — when moist air is trapped under an inversion. They are particularly noticeable around Melbourne. Stratus [ST] (Latin = spread, laid down): grey, uniform layer with fairly even base from which drizzle may descend. The sun outline may be visible. Stratus envelops low hills. They sometimes appear in ragged patches, which are produced by frictional turbulence or possibly orographic ascent. Cumulus [CU] (Latin = heap): white, heaped tops with generally grey, horizontal bases. Form is usually sharply outlined but may be ragged if evaporating. Vertical development varies greatly with atmospheric buoyancy, and bases can be at low or medium levels. Cumulus are formed by convection or possibly orographic ascent.  
    Vertically developed clouds
    Cumulonimbus [CB]: heavy, dense cloud with massive vertical development, bases at low or medium levels, with tops possibly reaching (even overshooting) the tropopause. They may have a 'boiling' appearance during their vertical development stage. The base is usually very dark with lighter inflow areas. They are associated with heavy showers or virga — precipitation that evaporates before reaching the surface. Frequently low, ragged, turbulence cloud is mixed beneath it. Cumulonimbus are produced by vigorous convection. Refer to section 3.6.
    For more information on the types and dangers of thunderstorms read sections 9.4 through 9.7. Towering cumulus [TCU]: CU with cauliflower appearance, often of great vertical extent. Properly known as cumulus congestus [CU CON].   Cloud structure and composition Cloud type Height of base Vertical extent Composition Associated precipitation CI 20 000 + usually thin* ice crystals fall streaks CS 20 000 + usually thin ice crystals nil CC 20 000 + usually thin crystals/droplets nil AS 6000 – 20 000+ up to 15 000 usually crystals, occasionally mixed rain/snow AC 6000 – 20 000 usually thin usually droplets to –10 °C, some crystals to –30 °C occasionally mixed rain, drizzle NS 0 – 8000+ merges into AS water droplets steady rain, snow, ice pellets SC 1500 – 4000 500 – 3000 mainly droplets down to –15 °C rain, drizzle, virga ST 0 – 2000 200 – 1000 usually water droplets drizzle CU, TCU 1500 – 15 000 up to 15 000 water droplets rain showers CB 1500 – 5000 15 000 – 35 000+ mainly droplets to –15 °C, mixed at lower temperatures rain/snow showers/virga, hail, ice pellets
    *With fall streaks, the vertical extent of CI may exceed 5000 feet
    Photographs and more information on cloud classes and identification techniques can be found at the Australian Severe Weather website. 3.2.2 Cloud species
    Each of the cloud genera are subdivided into species by the addition of a common species descriptor (with a three-letter code), according to cloud shape and structure. Fibratus [FIB]: CI and CS in the form of long, irregularly curved or nearly straight parallel filaments, but without tufts or hooks. CI FIB, CS FIB Spissatus [SPI]: dense or thickened CI plumes or CS, often originating from, or the remnants of, a CB anvil. Generally has a stormy appearance, looking greyish when viewed towards the sun. CI SPI, CS SPI Uncinus [UNC] (Latin = hook): CI with filaments that are hooked or comma-shaped. 'Mares tail cirrus'. Ice crystals are forming at the high point of the fall streak where a small tuft of cloud may appear — the generating head. The crystals forming the tail are falling through atmospheric layers of varying wind velocity and persist for quite a while before evaporating. CI UNC Nebulosus [NEB]: CS and AS as an indistinct veil lacking any detail. Also applied to low amorphous ST — lifted fog. CS NEB, AS NEB, ST NEB Stratiformus [STR]: AC and SC, occasionally CC, spread out into an extensive sheet or layer. CC STR, AC STR, SC STR Lenticularis [LEN]: (from the Latin 'lentil shaped') AC of orographic standing wave origin, sometimes CC or SC; occurs as a biconvex shape with a sharp margin, and often elongated if produced by a long ridge. They sometimes display iridescence. May form in long bands parallel to the Great Dividing Range and extend 50 to 100 nm downstream, towards the east; see mountain waves. When there are alternating layers of drier and moister air a tall, well-developed lenticularis formation may resemble an inverted stack of dinner plates, occasionally seen in the mountain areas of south-eastern Australia. CC LEN, AC LEN, SC LEN Castellanus [CAS]: having a turreted or crenellated appearance and connected to a common cloud base line. They are generally AS (but forming AC), or sometimes SC, CI or CC, signifying increasing instability. AC CAS may precede the development of CB. Floccus [FLO] (Latin = tuft of wool): CI, CC or particularly AC occurring in chaotic form, like a flock of sheep, each unit having a ragged base and a small cumuliform tuft above; 'thundery skies'. Often accompanied by virga. If developing CU reach this humid and unstable layer then energetic CB may develop. Fractus [FRA]: ST or CU shreds with broken, ragged or wispy appearance, associated with formation or dispersion of low cloud. CU FRA often appears early in the morning, rising only slightly above the condensation level; they are also found in precipitation under CB. ST FRA is much darker than CU FRA when found under CB. ST FRA normally forms below NS or AS, and derives moisture from evaporating raindrops or surface water. Uplift from near-surface turbulence may produce ST FRA, particularly in areas of rising ground or low hills. If forming without overlying cloud, ST FRA forewarns of worsening low-level visibility and ST formation. Pannus or scud is a mix of CU FRA and ST FRA. Humilis [HUM] (Latin = lowly): CU with small development and usually flattened at an inversion that is not far above the condensation level — 'fair weather CU'. Lifetime is 5 to 45 minutes. CU HUM Mediocris [MED] (Latin = of middle degree): CU of intermediate vertical growth, occurring at no more than 3000 feet. They have tops showing small protuberances that are not actively growing. CU MED Congestus [CON] (Latin = piled up): CU with cauliflower appearance, often of great vertical extent, perhaps 10 000 feet; generally known as towering CU [TCU]. Freezing does not occur. CU CON may produce heavy showers or microbursts, the latter particularly so in northern Australia. Calvus [CAL] (Latin = bald): developing CB prior to anvil stage, but at least some of its upper part is losing its CU outline due to freezing. CB CAL Capillatus [CAP] (Latin = hair): CB with distinct icy, upper fibrous or striated cirriform appearance. Frequently anvil-shaped, or untidy plumes, or disordered cirrus mass. CB CAP
    3.2.3 Cloud varieties
    Each of the cloud genera and species can be further classified into varieties by use of a common descriptor for element arrangement, transparency, etc. Intortus [IN]: irregularly curved or tangled CI. Vertebratus [VE]: CI looking like fish bone, ribs or vertebrae. Lacunosus [LA]: thin CC or AC with regularly spaced, net-like holes or a honeycomb appearance. Undulatus [UN]: parallel undulations in patches, sheets or layers of CC, CS, AC, AS, SC or ST caused by waves in the airstream. Radiatus [RA]: broad, parallel bands of CI, AC, AS, CU or SC appearing to converge towards a radiation point on the horizon, or both horizons. Duplicatus [DU]: more than one layer of CI, CS, AC, AS or SC at slightly different levels. The winds at each layer are usually blowing in slightly different directions. Translucidus [TR]: AC, AS, SC or ST in large sheets thin enough to show position of the sun or moon. Perlucidus [PE]: AC or SC in broad layers or patches with small lanes that allow the sky to be seen. Opacus [OP]: AS, AC, SC or ST that completely masks the sun or moon.
    3.2.4 Accessory clouds
    There are three cloud types that only exist in association with one of the main cloud genera: Pileus (Latin = cap, hood, like mushroom cap): a short-lived, smooth lenticular cloud appearing in a humid stable layer above a CB or TCU when the rising thermal deflects the moving air in the layer up and over into the condensation level. Further CB or TCU development will push through the cap cloud, which may hang on as a temporary collar. There is a good photograph of such an event in the Sydney Storm Chasers website. In strong shear conditions, the cap cloud may form downwind. Velum (Latin = veil): a thin, wide and persistent sheet of cloud accompanying a CB or TCU and forming in a humid, stable layer. Velum is dark in contrast to the convective cloud that generally rises through it. Pannus (Latin = piece of cloth): a mix of CU FRA and ST FRA, or just a lump of ST. Scud rapidly forms or reforms generally at lower levels under precipitating CU, AS, CB or NS bases in turbulent lifting conditions, particularly when air rises rapidly at the edge of cool moist outflow, or a downburst or in upflow caused by the topography — and exacerbated by evaporation of moisture from forest canopies. Scud changes size and shape constantly, and may be drawn into the cloud base. Flight in a locality where pannus is forming — scud running — is a very dangerous activity for aviators.
    3.2.5 Cloud features
    Some notable cloud features are: Incus (Latin = anvil): the anvil of a large CB, particularly a multicell or supercell storm, which has spread out, usually when upper-level winds are light. A severe storm attains maximum vertical development when the updraught reaches a stable layer which it is unable to break through — often the tropopause — and the cloud top spreads horizontally in all directions to form an overhanging anvil.

    The photograph and text below appeared in the "NSW Lightning Bolt" of August 1997 — produced by the Severe Weather Section of the Bureau of Meteorology, NSW. That anvil had a spread of about 30 km. The rollover around the underside of the anvil indicates rapid expansion.

    "Rose's magnificent photo (below) of a storm cloud near Millthorpe in NSW is familiar to many Bureau staff from the 1996 Weather Calendar, a 1995 Bureau Christmas card, and the new thunderstorm poster. The story of how the photo came to be taken may attract the writers at the Disney Studios. Rose relates the tale:

    '... my son Ian phoned to tell me about the clouds and to ask if I had a spare film, as his camera was empty. I tied a film to our kelpie's collar and sent him down the hill to Ian. Meanwhile, Ian's daughter Melanie was cycling up to get the film ... by the time they both met Ian the cloud had started to break up. Fortunately by then I had climbed two fences and taken the two shots ...' " Arcus (Latin = arch, bow or curve): a shelf-like cloud indicating the inflow region at the leading edge of a thunderstorm or a squall line. If conditions are very humid the shelf cloud will be a low, thick, curved and well-formed cloud bank. If there is a sharp, severe gust front there may be a vortex indicated by twisting scud under, and leading up to, the shelf. A roll cloud, like a horizontal tube, may develop if the leading edge of the shelf speeds up and detaches. SC, AC roll clouds are also associated with mountain waves and solitary waves.
      Granitus: a localised cloud (always forming below the lowest safe altitude [LSALT] marked on aeronautical charts) enclosing and obscuring a large chunk of land, usually in the form of a hill or peak. Granitus is sometimes known as 'stuffed CU', which refers to both the solid content and the consequences of entering such a cloud.
      Wall cloud: a localised, possibly rotating, lowering from a CB cloud base. Situated at the main updraught with a diameter ranging from 0.5 km to 5 km. Refer to section 9.5.   The Sydney Storm Chasers website has many images of thunderstorm features.
    There are good photos of wall clouds, arcus, pannus and mammatus.   Mammatus: hard, downward protuberances, pouches or bulges from the underside of a CB anvil (frequently) or CI, CC, AS, AC or SC, indicating descending pockets of small droplets or ice crystals. The sinking, saturated air is cooler than the air around it. As it sinks it warms, but warming is retarded because some of the heat is used in evaporating cloud droplets in the saturated air. If more energy is required for evaporation than is generated by adiabatic warming, then the air and the cloud pouches will continue to sink and will elongate the protuberances. The mamma associated with CI and CC are very shallow, forming undulations in the cloud trails. Mamma associated with CB are an indication of a dissipating storm rather than severe turbulence.
      Fall streaks: virga-like showers of ice crystals or snowflakes from CI generating heads, which sink at rates up to 0.5 m/sec but slowing as they sublimate. As they sink through several thousand feet they become deflected by falling into winds of lower velocity, or slow through sublimation, and thus appear to trail back from the parent head as hooks, mares' tails, etc. Dense streaks combined with a strong drop in wind speed produce jet-stream banners — CI features that stream with the wind. AS and most stable cloud features lie across the wind.
      Billow clouds: AC and AS found in a series of regular bands with clear areas between of similar width, occurring most frequently at 15 000 to 25 000 feet. At other times the upper surface (usually but could be the lower surface) of the cloud may have regular wave-like troughs and crests – undulatus.

    When a higher-level inversion occurs, the upper and lower air layers are generally stable. If there is a significant difference in wind velocity between the layers then there is vertical wind shear at the interface, and a phenomenon known as 'Kelvin-Helmholtz shearing instability' causes the formation of long but short-lived waves across the interface — in much the same way as ocean waves — which grow in amplitude until they curl up and break. The waves produce an extensive but shallow area of clear air turbulence. If sufficient moisture exists, the waves become visible as Kelvin-Helmholtz billows. Billows always move with the wind so that in wave clouds they appear to move from the front to the rear of the formation, evaporating in the troughs and re-condensing in the crests. Kelvin-Helmholtz instability produces the ripples seen when a light wind blows across a pond of water.  
    Pyrocumulus: CU initiated by bushfire thermal activity. Ray Kennedy's photograph below shows a CU CON building above the brown smoke during the Gippsland bushfires on New Year's Day 1998.  

    3.2.6 Stratospheric clouds Nacreous (mother-of-pearl) clouds are rare, high-latitude, stratospheric clouds resembling CC LEN or AC LEN. Small patches are occasionally formed in winter, usually in stationary standing waves, and often in the lee of mountain ranges, which provide abrupt uplift. They usually occur in the ozone layer at about 25 km with temperatures down to –80°C or –90°C. Nacreous clouds are probably composed of spherical ice crystals about one to two microns diameter. Brilliant iridescence is shown at angular distances up to 40 degrees from the sun, and green and pink colours predominate. These clouds are brightest at sunset but are rarely seen in daylight.
      Noctilucent clouds [NLC] are rare, tenuous, mesospheric cloud formations only seen from higher-latitude locations, normally around 40° to 60° south, against a twilit (nautical and astronomical) sky in summer. Sufficient contrast for observation occurs when the sun is between 6° and 16° below the horizon with maximum contrast at 10° when solar illumination and light scatter is at the maximum. They are seen close to the sunward horizon and extend maybe 20° above, along the twilight arch, although the clouds can be seen at a much higher elevation. The clouds appear to be near the mesopause at about 80 km and are moving with the zonal easterlies. They resemble high CI with pronounced band or wave structures, commonly herring-bone, bluish-white to pure white with yellow beneath. They are probably composed of cosmic dust with thin ice deposition, saturation of traces of water vapour being reached through orographic waves resonated from the earth's surface, or possibly oxidation of atmospheric methane.  
    The Australian Severe Weather website has many excellent images grouped into cloud classifications, cloud features and atmospheric phenomena. Also the Cloud Appreciation Society website is well worth a visit.

    3.2.7 ICAO / WMO Cloud continuity scale
    SKC — sky clear, no cloud. FEW — few clouds, one to two oktas cover. SCT — scattered, 3 – 4 oktas cover. Clear intervals between clouds predominate. BKN — broken, 5 – 7 oktas. Cloud masses predominate. OVC — overcast, 8 oktas. Continuous, no clear intervals.  
    3.3 Lifting sources
    There are four main processes that provide the lifting source for moist air to form cloud: convection frictional turbulence orographic ascent convergence or widespread ascent.  
    3.3.1 Convection
    When air flows over a surface heated by solar radiation, the surface contact layer is heated by conduction, and some of the heat is transported upward by molecular motion and small turbulent eddies. If the incoming energy is sufficient, the temperature in the lower layer increases and thermals rise from the heated contact layer — initially as bubbles of buoyant air, and then develop as columns with 100 – 300 metre diameters. The strength of the thermal depends on the heating:

    Thermal vertical velocity Thermal strength Knots Feet/min Metres/sec Weak 1 – 2 100 – 200 0.5 – 1 Moderate 2 – 6 200 – 600 1 – 3 Strong 6+ 600+ 3+
    Circling within a thermal (thermalling) is a prime source of uplift for soaring paragliders, hang gliders and sailplanes, and particularly so in the summer. In hot, dry areas of Australia, thermals exceeding 1000 feet/min are common.
    The rising thermal cools at about the DALR of 3 °C/1000 feet and if it reaches dewpoint — the convection or lifting condensation level — cumulus will form. They are initially maintained by a series of random rising eddies, but if developed enough can draw in surrounding moist air and maintain itself, in a steady organised upward flow, from the release of the latent heat of condensation. If the cloud has enough energy to pass the freezing level it may develop into a rain and wind storm, and possibly a CB. Refer to section 3.6.

    In most instances the air providing the water vapour for convective cloud growth comes from within the friction layer. When thermal turbulence of sufficient intensity to penetrate above the friction layer is present, and the condensation level lies above the friction layer, then isolated convective cloud — fair weather cumulus CU HUM — is formed with clear-cut bases and tops to the limit of penetration. A subsidence inversion above the condensation level may limit the vertical extent, with the cloud spreading out in broken SC. Night cooling also has the effect of spreading the cloud into broken SC. Air warmed by advection over a warm surface, particularly the sea, in a summer anticyclone provides ideal conditions for development of fair weather cumulus.

    3.3.2 Frictional turbulence
    An airstream flowing over ground or water produces a turbulent layer, up to 500 feet deep in light winds or 3000 feet plus in strong winds. The vertical eddies within this friction layer or boundary layer transport air from the upper level to the surface, adiabatically warmed to a temperature above that of the surface air. Similarly surface air is transported to the upper level, cooling adiabatically to temperatures below that of the upper level. Thus, as the turbulent mixing continues, the lower level is warmed and the upper level is cooled until the temperature lapse rate through the layer equals the DALR and the layer is in neutral stability — providing the air remains unsaturated. An inversion is formed at the top of the friction layer. A pre-existing inversion, e.g. a subsidence inversion, will strengthen the process. Thermal turbulence will also be present.
    The deep, turbulent mixing also has the effect of evening-out the moisture content throughout the layer and if the humidity mixing ratio is high enough a mixing condensation level will be reached within the friction layer. If the lapse rate of the layer above the friction layer is stable, then layer cloud will form with its base at the mixing condensation level and its top at the inversion. Thus the thickness of the cloud layer will vary from very thin to possibly 3000 feet.

    If the upper air layer is unstable then cloud development would not be halted at the inversion and convective cloud would probably develop. If the wind is light the layer cloud would tend to ST, otherwise SC with undulations in the lower surface continually forming, with breaks where cloud is being evaporated in the down currents. ST FRA may also form with local variations in humidity, temperature and turbulence. Cloud produced by frictional turbulence is not usually associated with precipitation except perhaps for drizzle from dense layers.

    3.3.3 Orographic ascent
    Orography is the branch of physical geography concerned with mountains. An airstream encountering a topographic barrier (i.e. hill, ridge, valley spur, mountain range) is forced to rise, in a broad cross-section from at or near the surface to the upper levels, and cools adiabatically. If the lift and the moisture content are adequate, condensation occurs at the lifting condensation level and cloud is formed on or above the barrier. Stratus is formed if the air is stable, whilst cumulus forms if the air is slightly unstable. If there is instability in depth, coupled with high moisture, CB may develop (refer to section 3.6). Solar heating of ridges may cause the adjacent air to be warmer than air at the same level over the valleys; thus the ridge acts as a higher-level heat source, increasing buoyancy and accentuating the mechanical lifting.

    The orographic lifting of an airstream provides gliders with the opportunity for ridge or hill soaring. Sea breezes crossing relatively small topographic barriers at the coastline (e.g. cliffs) may provide quite smooth uplift.

    Orographic cloud — cap cloud — in stable conditions tends to form continuously on the windward side of mountain ridges, but clears on the lee side. Lenticular cloud may also form a high cap above a hill when there is a layer of near saturated air aloft; orographic lifting causes condensation, and descent causes evaporation. A mountain wave may form — particularly in a sandwiched, stable layer — resulting in the formation of a series of lenticular clouds.

    3.3.4 Convergence and widespread ascent
    The air in the centre of a low pressure centre, trough or heat trough is lifted by convergence, as is the air in the inter-tropic convergence zone.

    The air in the broad area ahead of a cold front is lifted by the frontal action. Generally the air rises very slowly, possibly one to five feet/minute, and cools. If moist enough, the air condenses at the lifting condensation level producing extensive layers of stratus-type cloud: NS, AS, CS and CI. However active or fast-moving fronts may nose the air up much more rapidly, leading to CB development.

    3.4 Fog
    Fog [FG] is defined as an obscurity in the surface layers of the atmosphere that is caused by a suspension of water droplets, with or without smoke particles, and which is defined by international agreement as being associated with visibility less than 1000 metres. If the visibility is between 1000 and 5000 metres then the obscurity is mist — meteorological code BR, from the French brouillard = mist.

    Radiation fogs are the prevalent fogs in Australia, with occurrence peaking in winter. They are caused by lowering of the ground temperature through re-radiation into space of absorbed solar radiation. Radiation fogs mainly occur in moist air on cloudless nights within a high-pressure system, particularly after rainfall. The moist air closest to the colder surface will quickly cool to dewpoint with condensation occurring. As air is a poor conductor, a light wind of 2–6 knots will facilitate the mixing of the cold air throughout the surface layer, creating fog. The fog itself becomes the radiating surface in turn, encouraging further cooling and deepening of the fog. An increase in atmospheric pollution products supplies extra condensation nuclei to enhance the formation of fog; i.e. smog.

    A low-level inversion forms and the contained fog may vary from scattered pools in surface depressions to a general layer 1000 feet in depth. Calm conditions will result in a very shallow fog layer, or just dew or frost. The fog droplets sink at about 1 cm/sec. Surface winds greater than 10 knots may prevent formation of the inversion; the cooled air is mixed with the warmer air above, and so does not cool to dewpoint. If the forecast wind at 3000 feet is 25 knots or more, the low-level inversion may not form and fog is unlikely (refer to 'spread' in section 1.5). In winter, radiation fog may start to form in the evening and persist to midday — or later if the sun is cut off by higher-level cloud and/or the wind does not pick up sufficiently to break up the low-level inversion.

    Advection fog may occur when warm, moist air is carried over a surface that is cooler than the dewpoint of the air. Cooling and some turbulence in the lower layer lowers temperature to dewpoint and fog forms. Sea fogs drifting into New South Wales coastal areas are advection fogs that are formed when the sea surface temperature is lower than the dewpoint, but with a steady breeze to promote air mixing. Dewpoint can be reached by both temperature reduction and by increased water vapour content through evaporation. Advection fogs will form in valleys open to the sea when temperature falls in the evening, and when combined with a sea breeze of 5 – 15 knots to force the air upslope. Thick advection fogs may be persistent in winter, particularly under a mid-level cloud layer.

    Shallow evaporation fogs or steaming fogs result from the immediate condensation of water vapour that has just evaporated from the surface into near-saturated air. Steaming from a sun-warmed road surface after a rain shower demonstrates the process. Sea smoke or frost smoke is an evaporation fog occurring in frigid Antarctic air moving over relatively warm waters, thereby prompting evaporation into the cold air which, in turn, quickly produces saturation.

    Freezing fog is a fog composed of supercooled water droplets that freeze on contact with solid objects; e.g. parked aircraft. When near-saturated air is very cold, below –24 °C at sea level to –45 °C at 50 000 feet, the addition of only a little moisture will produce saturation. Normally, little evaporation takes place in very cold conditions but release of water vapour from engine exhausts, for instance, can quickly saturate calm air (even though the engine exhaust heat tends to lower the relative humidity) and will produce ice fog at the surface or condensation trails [contrails] at altitude. If the temperature is below –40 °C, ice crystals form directly on saturation. Contrails persist if relative humidity is high but evaporate quickly if low. Distrails occur when the engine exhaust heat of an aircraft flying through a thin cloud layer dissipates a clear trail.

    Frontal fog or rain-induced fog occurs when warm rain evaporates at surface level in light wind conditions and then condenses to form fog.

    3.5 Precipitation
    3.5.1 Rain [RA] and drizzle [DZ]
    Cloud droplets tend to fall but their terminal velocity is so low, about 0.01 metres/sec, that they are kept aloft by the vertical currents associated with the cloud construction process; but droplets will evaporate when coming into contact with the drier air outside the cloud. Some of the droplets are larger than others and consequently their falling speed is greater. Larger droplets catch up with smaller ones and merge or coalesce with them, eventually forming raindrops. Raindrops grow with the coalescence process and reach maximum diameters — in tropical conditions — of 4–7 mm before air resistance disintegrates them into smaller raindrops; this starts a self-perpetuating process. It takes about one million cloud droplets to form one raindrop.

    The terminal velocity of a 4 mm raindrop is about 9 metres/sec. Only clouds with extensive depth, 3000 feet plus, will produce rain (rather than drizzle). But very small, high clouds — generating heads — may produce trails of snow crystals, which evaporate at lower levels — fall streaks or virga.

    Drizzle forms by coalescence in stratiform clouds with depths possibly less than 1000 feet and with only weak vertical motion — otherwise the small (0.2 – 0.5 mm) drops would be unable to fall. It also requires only a short distance or a high relative humidity between the cloud base and the surface — otherwise the drops will evaporate before reaching the surface. Terminal velocity approximates 1–2 metres/sec.   Light drizzle [–DZ]: visibility greater than 1000 metres Moderate drizzle [DZ]: visibility 500–1000 metres Heavy drizzle [+DZ]: visibility less than 500 metres   Light rain showers [–SHRA]: precipitation rate under 2 mm/hour Moderate rain showers [SHRA]: 2–10 mm/hour Heavy rain showers[+SHRA]: more than 10 mm/hour   Light rain [–RA]: under 0.5 mm/hour, individual drops easily seen Moderate rain [RA]: 0.5–4 mm/hour, drops not easily seen Heavy rain [+RA]: more than 4 mm/hour, rain falls in sheets
    Weather radar reports precipitation according to the reflectivity level: 1 – light precipitation 2 – light to moderate rain 3 – moderate to heavy rain 4 – heavy rain 5 – very heavy rain, hail possible 6 – very heavy rain and hail, large hail possible
    Scotch mist is a mixture of thick cloud and heavy drizzle on rising ground, formed in conditions of weak uplift of almost saturated stable air.

    3.5.2 Snow [SN]
    At cloud temperatures colder than –10 °C where both ice and supercooled liquid water exist, the saturation vapour pressure over water is greater than that over ice. Air that is just saturated with respect to the supercooled water droplets will be supersaturated with respect to the ice crystals, resulting in vapour being deposited onto the crystal (refer to section 1.5). The reduction in the amount of water vapour means that the air is no longer saturated with respect to the water droplets. To achieve saturation equilibrium again, the water droplets begin to evaporate. Thus ice crystals grow by sublimation and water droplets lessen, i.e. in mixed cloud the ice crystals grow more rapidly than the water droplets. Snow is frozen precipitation resulting from ice crystal growth, and falls in any form between small crystals and large flakes. This is known as the Bergeron-Findeison theory and probably accounts for most precipitation outside the tropics. Snow may fall to the surface or, more often, melt below the freezing level and fall as rain.

    Snowflakes are built by snow crystals colliding and sticking together in clusters of several hundred — known as aggregation. Most aggregation occurs at temperatures just below freezing, as the snow crystals tend to remain separate at colder temperatures.

    3.5.3 Hail and other ice forms
    The growing snow crystals acquire a fall velocity relative to the supercooled droplets. Growth also continues by collision and coalescence with supercooled droplets forming ice pellets [PE]. The process is termed accretion, or opaque riming if the freezing is instantaneous incorporating trapped air, or glazing if the supercooled water freezes more slowly as a clear layer. A similar process occurs with airframe icing. The ice pellets in turn grow by coalescing with other pellets and further accretion — these are termed hail [GR] when the diameter exceeds 5 mm. The size reached by hailstones before falling out of the cloud depends on the velocity and frequency of updraughts within the cloud. Hail is of course a hazard to aviation, particularly when it is unexpected; for example hail falling from a CB anvil can appear to fall from a clear sky. Snow grains [SG] are very small, flattened, opaque ice grains, less than 1 mm and equivalent to drizzle. Snowflakes that, due to accretion, become opaque, rounded and brittle pellets, 2 – 5 mm diameter, are called snow pellets or graupel [GS]. Sleet is transparent ice pellets less than 5 mm diameter that bounce on impact with the ground. Sleet starts as snow, partially melting into rain on descent through a warm layer, then refreezing in a cold near-surface layer. The term is sometimes applied to a snow/rain mixture or just wet snow. Diamond dust [IC] is minute airborne ice crystals that only occur under very cold (Antarctic) conditions.

    When raindrops form in cloud-top temperatures warmer than –10 °C the rain falls as supercooled drops. Such freezing rain or drizzle striking a frozen surface, or an aircraft flying in an outside air temperature [OAT] at or below zero, will rapidly freeze into glaze ice. Freezing rain is responsible for the ice storms of North America and northern Europe, but the formative conditions differ from the preceding.

    3.5.4 The seeder – feeder mechanism
    Any large-scale air flow over mountain areas produces, by orographic effect, ice crystals in cold cloud tops. By themselves, the falling crystals would cause only light drizzle at the ground. However, as the crystals fall through the low-level mountain top clouds they act as seed particles for raindrops that are formed by coalescing cloud droplets with the falling crystals, producing substantial orographic rainfall in mountain areas.

    Aerial cloud seeding involves introducing freezing nuclei (silver-oxide crystals with a similar structure to ice crystals) into parts of the cloud where few naturally exist, in order to initiate the Bergeron-Findeison process.

    3.6 Thunderstorm development
    Like CU, surface heating may provide the initial trigger to create isolated CB within an air mass but the initial lift is more likely to be provided by orographic ascent or convergence effects.

    In the formative stages of a CB, the cloud may have an updraught pulse of 1000–2000 feet/min. The rising parcel of air reaches altitudes where it is much warmer than the surrounding air, by as much as 10 °C, and buoyancy forces accelerate the parcel aloft possibly reaching speeds of 3000–7000 feet/min. Precipitation particles grow with the cloud growth. The upper levels of the cloud gain additional energy from the latent heat released from the freezing of droplets, and the growth of snow crystals and hailstones. When the growth of the particles is such that they can no longer be suspended in the updraught, then precipitation — and its associated drag downdraught — occurs.

    If the updraught path is tilted by wind shear or veer, rather than vertical, then the precipitation and its downdraught will fall away from the updraught, rather than back down through it (consequently weakening or stopping the updraught) and a co-existing updraught/downdraught may become established. An organised cell system controlling its environment and lasting several hours may evolve.

    Middle-level dry air from outside the cloud is entrained into the downdraught of an organised cell. The downdraught is further cooled by the dry inflow air evaporating some of its water and ice crystals, and tends to accelerate downwards in vertical gusts. At the same time, the downdraught maintains the higher horizontal momentum it gained at upper levels from the higher forward speed of the storm at that height. When the cold, plunging air nears the surface, the downburst spreads out in all directions as a cold gust front or squall. This is strongest at the leading edge of the storm and weakest towards the trailing edge.

    Each organised cell system contains an updraught / downdraught core. Beneath this is the outflow region containing the rain shield. The core is bounded by the downdraught gust front, a flanking line with a dark, flat base. Underneath this is the inflow region of warm, moist air. The CU and TCU generated by the inflow within the flanking line are the genesis of new cells. Within the core, the condensation of moisture from the inflow region produces rain, hail and snow and the associated downdraught to the outflow region. When the cool air outflow exceeds and finally smothers(or undercuts and chokes off) the inflow, then the storm dissipates.

    High moisture content in the low-level air with dry, mid-level air and atmospheric instability are required to maintain CB development. The amount of precipitation from a large storm is typically 200 000 tonnes but severe storms have produced 2 million tonnes.

    Anvils may take several forms: Cumuliform: forms when a very strong updraught spreads rapidly and without restriction. Back-sheared: the cloud top spreads upwind, against the high-level flow, this indicates a very strong updraught. Mushroom: a rollover or lip around the underside of an overhanging anvil, which indicates rapid expansion. Overshooting top: a dome-like protusion through the top of an anvil, which indicates a very strong updraught pulse. The overshooting top in large tropical storms has been known to develop into a 'chimney' form, towering maybe 10 000 feet into the stratosphere, with an extensive plume cloud extending downwind from its top. Such clouds transfer moisture to the stratosphere.  
    The Australian Bureau of Meteorology Web site has a storm spotters' guide.
    Parts 1 and 2 briefly describe the structure and types of thunderstorms likely to be encountered in Australia.

    For further information on clouds, fog and precipitation consult the University of Manchester's Intute, an online catalogue of internet resources in Earth sciences.

    3.7 Flight in cloud or without external visual references
    The human vestibular system
    When walking, a person's prime sense of orientation is provided by visual references. When vision is severely degraded, the vestibular system in the inner ears, which senses motion and gravity (thus roll, pitch and yaw), generally allows us to keep our balance when walking without using visual references. However, the vestibular system is not designed for high speed or angular motion, and cannot be used as an in-flight back-up system; i.e. you cannot close your eyes and continue to fly straight and level. Motion of the fluid within the ears' semicircular canals is affected by inertia and will feed quite erroneous prompts to the brain, resulting in various types and levels of vertigo.

    For example, without the external visual references of clear sky, terrain or a horizon, forward deceleration tends to give a pitching-down sensation whilst forward acceleration gives a pitching-up sensation. Once settled into a constant rate turn, the sensation is of not turning at all; but when the turn is halted, the sensation is then of turning in the opposite direction. In addition, the vestibular system will not detect slow rates of bank, so that if the aircraft is banking at the rate of one or two degrees per second the vestibular system will not send any prompts to the brain — it will consider the aircraft is still flying straight and level, while any associated speed changes may provide contrary sensations. For example, if the aircraft is slowly banking and accelerating in a descending turn, the sensation may well be one of pitching-up.

    Spatial disorientation
    Aircraft accidents caused by spatial disorientation are usually fatal and occur when VFR flight is continued in adverse visibility conditions — cloud, fog, smoke, haze, showers, oncoming darkness and combinations thereof. Pilots who have not been trained to fly solely by visual reference to the flight instruments in a 'blind flying' panel will soon find themselves experiencing spatial disorientation should they, inadvertently or deliberately, enter cloud where the external visual references — by which they normally orient themselves in visual meteorological conditions — are lost. The same applies to any atmospheric condition or in adverse weather where the visual references (horizon [principally], terrain and clear sky) are lost or just significantly reduced — smoke from bushfires or extensive burning of sugar cane, for example.

    Even a pilot who is well experienced in flying in instrument meteorological conditions may occasionally experience a phenomenon called 'the leans'. This usually occurs when the aircraft has been inadvertently allowed to slowly bank a few degrees and the pilot then makes a quick correction to level the wings. The vestibular system doesn't register the initial bank but does register the wing levelling as an opposite direction bank (away from a wings-level attitude) — and the pilot's brain produces a leaning sensation while also perceiving from the instrument readings that the aircraft is flying straight and level. The reaction — which can persist for quite a while — may be for the pilot to lean sideways in her/his seat so that everything feels right!

    Read the section titled 'Pressing on in deteriorating conditions' in the Flight Planning and Navigation Guide.

    For more information on the vestibular functions and effects, google the terms 'vestibular spatial disorientation' in a web search.

    This list of Aviation Terms contains those that are most frequently used. To have one added to the list please let me know...thanks
    AGL - Above Ground Level, as a measurement of altitude above a specific land mass, and differentiated from MSL. ADF - Automatic Direction Finding via automated radio. ADI - Attitude direction indicator. Shows the roll and pitch of the aircraft. AFCS - Automatic flight control system that provides inputs to the fight controls to assist the pilot in maneuvering and handling the aircraft. AFT - Referring to the rear of the aircraft. AI - Altitude indicator. Displays the aircraft's altitude above sea level. Aileron - The movable areas of a wingform that control or affect the roll of an aircraft by working opposite one another-up-aileron on the right wing and down-aileron on the left wing. AIM - Airman's Information Manual - A primary FAA publication whose purpose is to instruct airmen about operating in the US airspace system. ADC - Air Data Computer - A primary sensor-based navigation data source. AGR - Air-Ground Ranging - Straight-line distance from the aircraft to a point on the ground. ATC - Air Traffic Control - A service operated by the appropriate authority to promote the safe, orderly, and expeditious flow of air traffic. Airfoil - The shape of the wing when looking at its profile. Usually a teardrop shape. Airframe - The fuselage, booms, nacelles, cowlings, fairings, and airfoil surfaces of an aircraft. Airspeed - The speed of an aircraft relative to its surrounding air mass. See: calibrated airspeed; indicated airspeed; true airspeed. Airspeed Indicator - An onboard instrument which registers velocity through the air, usually in knots. Different from ground speed. AIS - Aeronautical Information Service. ALS - Approach light system. A lighting system installed on the approach end of an airport runway and consists of a series of lightbars, strobe lights, or a combination of the two that extends outward from the runway end. ALT - Short term for Altitude. Altimeter - An onboard instrument which senses air pressure in order to gauge altitude. Altimeter Setting - The barometric pressure reading used to adjust a pressure altimeter for variations in existing atmospheric pressure. Altitude - Height of an aircraft, usually with respect to the terrain below. Angle of Attack - The angle between the chord line of the wing of an aircraft and the relative wind. Annual - Mandatory inspection of airframe and power plant that occurs every 12 months. AO - Aircraft Operator. AOPA - Aircraft Owner and Pilot's Association. APP - Approach (Control). Approach Speed - The recommended speed contained in aircraft manuals used by pilots when making an approach to landing. ARCID - Aircraft Identification. ATA - Actual Time of Arrival. As opposed to ETA (Estimated Time of Arrival) used in filing a flight plan. ATD - Actual Time of Departure. As opposed to ETD (Estimated Time of Departure) used in filing a flight plan. ATIS - Automated Terminal Information Service usually containing vital information on wind direction, velocity, pressure readings, and active runway assignment for that particular airport. Attitude - The primary aircraft angles in the state vector; pitch, roll, and yaw. Attitude Indicator - A vacuum powered instrument which displays pitch and roll movement about the lateral and longitudinal axes. ADF - Automatic Direction Finding - A basic guidance mode, providing lateral guidance to a radio station. Equipment that determines bearing to a radio station. Autopilot - A method of an automatic flight control system which controls primary flight controls to meet specific mission requirements. Autorotation - A rotorcraft flight condition in which the lifting rotor is driven entirely by action of the air when the rotorcraft is in motion. AVGAS - Aviation Gasoline (piston aircraft fuel). Bernoulli Effect - Airflow over the upper surface of an airfoil causes suction (lift) because the airstream has been speeded up in relation to positive pressure of the airflow on the lower surface. CAS - Calibrated Airspeed - The indicated airspeed of an aircraft, corrected for position and instrument error. CAS is equal to true airspeed in standard atmosphere at sea level. Camber - The convex or concave curvature of an airfoil. CAT - Clear Air Turbulance. CAVU - Ceiling and Visibility Unlimited; ideal flying weather. Ceiling - The heights above the earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as "broken," "overcast," or "obscured". CG - Center of Gravity - The longitudinal and lateral point in an aircraft where it is stable; the static balance point. Chord - The measurable distance between the leading and trailing edges of a wingform. CTAF - Common Traffic Advisory Frequency - A frequency designed for the purpose of carrying out airport advisory practices while operating to or from an airport without an operating control tower. The CTAF may be a UNICOM, Multicom, FSS, or tower frequency and is identified in appropriate aeronautical publications. Controlled Airspace - An airspace of defined dimensions within which air traffic control service is provided to IFR flights and to VFR flights in accordance with the airspace classification. Controlled airspace is a generic term that covers Class A, B, C, D, and E airspace. Crabbing - A rudder-controlled yawing motion to compensate for a crosswind in maintaining a desired flight path, as in a landing approach. Dead Reckoning - The process of estimating one's current position based upon a previously determined position, or fix, and advancing that position based upon known speed, elapsed time, and course. Deadstick - Descending flight with engine and propeller stopped. Departure Stall - A stall in the takeoff configuration with power. Deviation (Magnetic) - The error of a Magnetic Compass due to inherent magnetic influences in the structure and equipment of an aircraft. Directional Gyro - A panel instrument providing a gyroscopic reading of an aircraft's compass heading. DME - Distance Measuring Equipment, a radio navigation device that determines an aircraft's distance from a given ground station, as well as its groundspeed and time to/from the station. Drag - The resisting force exerted on an aircraft in its line of flight opposite in direction to its motion. Dry Weight - The weight of an engine exclusive of any fuel, oil, and coolant. Elevator - The movable part of a horizontal airfoil which controls the pitch of an aircraft, the fixed part being the Stabilzer. ETA - Estimated time of arrival. ETD - Estimated time of departure. FBO - Fixed-Base Operator. A commercial operator supplying fuel, maintenance, flight training, and other services at an airport. FAR - Federal Air Regulations. Flap - A movable, usually hinged airfoil set in the trailing edge of an aircraft wing, designed to increase lift or drag by changing the camber of the wing or used to slow an aircraft during landing by increasing lift. Flare - A control wheel maneuver performed moments before landing in which the nose of an aircraft is pitched up to minimize the touchdown rate of speed. Flight Envelope - An aircraft's performance limits, specifically the curves of speed plotted against other variables to indicate the limits of speed, altitude, and acceleration that a particular aircraft cannot safely exceed. Flight Plan - Specified information relating to the intended flight of an aircraft, filed orally or in writing with an FSS or an ATC facility. FSS - Flight Service Station - Air traffic facilities which provide pilot briefing, enroute communications and VFR search and rescue services, and assist lost aircraft. Fuselage - An aircraft's main body structure housing the flight crew, passengers, and cargo and to which the wings, tail and, in most single-engined airplanes, engine are attached. GA - General Aviation - That portion of civil aviation which encompasses all facets of aviation except air carriers holding a certificate of public convenience and necessity from the Civil Aeronautics Board and large aircraft commercial operators. Glass Cockpit - Said of an aircraft's control cabin which has all-electronic, digital and computer-based, instrumentation. Glider - An unpowered aircraft capable of maintaining altitude only briefly after release from tow, then gliding to earth. Glide Scope - (1) The angle between horizontal and the glide path of an aircraft. (2) A tightly-focused radio beam transmitted from the approach end of a runway indicating the minimum approach angle that will clear all obstacles; one component of an instrument landing system (ILS). GPS - Global Positioning System; satellite-based navigation, rapidly replacing dead reckoning methods. Gross Weight - The total weight of an aircraft when fully loaded, including fuel, cargo, and passengers; aka Takeoff Weight. Ground Control - Tower control, by radioed instructions from air traffic control, of aircraft ground movements at an airport. Ground Effect - Increased lift generated by the interaction between a lift system and the ground when an aircraft is within a wingspan distance above the ground. It affects a low-winged aircraft more than a mid- or high-winged aircraft because its wings are closer to the ground. Ground Speed - The actual speed that an aircraft travels over the ground its "shadow speed"; it combines the aircraft's airspeed and the wind's speed relative to the aircraft's direction of flight. IFR - Instrument Flight Rules, governing flight under instrument meteorological conditions. ILS - Instrument Landing System. A radar-based system allowing ILS-equipped aircraft to find a runway and land when clouds may be as low as 200' (or lower for special circumstances). IAS - Indicated Air Speed - A direct instrument reading obtained from an air speed indicator uncorrected for altitude, temperature, atmospheric density, or instrument error. Compare calibrated airspeed and true airspeed. IMC - Instrument Meterological Conditions - Meteorological conditions expressed in terms of visibility, distance from clouds, and ceiling less than minimal specified for visual meteorological conditions (VMC). Knot - One nautical mile, about 1.15 statute miles (6,080'); eg: 125kts = 143.9mph. Lift - The force exerted on the top of a moving airfoil as a low-pressure area [vacuum] that causes a wingform to rise. airfoils do not "float" on air, as is often assumed - like a boat hull floats on water - but are "pulled up" (lifted) by low air pressures trying to equalize. Lift-Drag Ratio - The lift coefficient of a wing divided by the drag coefficient, as the primary measure of the efficiency of an aircraft; aka L/D ratio. Liquid Compass - A non-electronic, calibratable compass floating in a liquid as a panel instrument; aka wet compass. Load Factor - The proportion between lift and weight commonly seen as g (sometimes capitalized) - a unit of force equal to the force of gravity times one. LORAN - Long Range Navigation System - Utilizes timing differences between multiple low-frequency transmissions to provide accurate latitude/longitude position information to within 50'. LTA - Lighter-than-air craft, generally referring to powered blimps and dirigibles, but often also includes free balloons. Magnetic Compass - The most common liquid-type compass, capable of calibration to compensate for magnetic influences within the aircraft. Magnetic Course - Compass course + or - deviation. Magnetic North - The magnetic North pole, located near 71° North latitude and 96° West longitude, that attracts a magnetic compass which is not influenced by local magnetic attraction. MAG - Magneto - An accessory that produces and distributes a high-voltage electric current for ignition of a fuel charge in an internal combustion engine. MSL - Mean Sea Level. The average height off the surface of the sea for all stages of tide; used as a reference for elevations, and differentiated from AGL. METAR - Acronym in FAA pilot briefings and weather reports simply means an "aviation routine weather report". NDB - Non Directional Beacon - An LF, MF, or UHF radio beacon transmitting non-directional signals whereby the pilot of an aircraft equipped with direction finding equipment can determine his bearing to or from the radio beacon and "home" on or track to or from the station. PAR - Precision Approach Radar, a ground-radar-based instrument approach providing both horizontal and vertical guidance. Pattern - The path of aircraft traffic around an airfield, at an established height and direction. At tower-controlled fields the pattern is supervised by radio (or, in non-radio or emergency conditions by red and green light signals) by air traffic controllers. Flying an entire pattern is called a 'Circuit'. PIC - Pilot in Command - The pilot responsible for the operation and safety of an aircraft during flight time. Pitch - Of the three axes in flight, this specifies the vertical action, the up-and-down movement. Pitot Tube - More accurately but less popularly used, Pitot-Static Tube, a small tube most often mounted on the outward leading edge of an airplane wing (out of the propeller stream) that measures the impact pressure of the air it meets in flight, working in conjuction with a closed, perforated, coaxial tube that measures the static pressure. Roll - Of the three axes in flight, this specifies the action around a central point. Rotorcraft - A heavier-than-air aircraft that depends principally for its support in flight on the lift generated by one or more rotors. Includes helicopters and gyroplanes. Rudder - The movable part of a vertical airfoil which controls the YAW of an aircraft; the fixed part being the fin. Scud - A low, foglike cloud layer. Service Ceiling - The height above sea level at which an aircraft with normal rated load is unable to climb faster than 100' per minute under Standard Air conditions. Sideslip - A movement of an aircraft in which a relative flow of air moves along the lateral axis, resulting in a sideways movement from a projected flight path, especially a downward slip toward the inside of a banked turn. Sink, Sinking Speed - The speed at which an aircraft loses altitude, especially in a glide in still air under given conditions of equilibrium. Skid - Too shallow a bank in a turn, causing an aircraft to slide outward from its ideal turning path. Slip - Too steep a bank in a turn, causing an aircraft to slide inward from its ideal turning path. Slipstream - The flow of air driven backward by a propeller or downward by a rotor. Squawk Code - A four-digit number dialed into his transponder by a pilot to identify his aircraft to air traffic controllers. Stabilizer - The fixed part of a horizontal airfoil that controls the pitch of an aircraft; the movable part being the elevator. Stall - (1) Sudden loss of lift when the angle of attack increases to a point where the flow of air breaks away from a wing or airfoil, causing it to drop. (2) A maneuver initiated by the steep raising of an aircraft's nose, resulting in a loss of velocity and an abrupt drop. TAS - True Air Speed - True Air Speed. Because an air speed indicator indicates true air speed only under standard sea-level conditions, true air speed is usually calculated by adjusting an Indicated Air speed according to temperature, density, and pressure. Thrust - The driving force of a propeller in the line of its shaft or the forward force produced in reaction to the gases expelled rearward from a jet or rocket engine. Opposite of drag. Torque - A twisting, gyroscopic force acting in opposition to an axis of rotation, such as with a turning propeller; aka Torsion. Touch-and-Go - Landing practice in which an aircraft does not make a full stop after a landing, but proceeds immediately to another take-off. Transponder - An airborne transmitter that responds to ground-based interrogation signals to provide air traffic controllers with more accurate and reliable position information than would be possible with "passive" radar; may also provide air traffic control with an aircraft's altitude. Trim Tab - A small, auxiliary control surface in the trailing edge of a wingform, adjustable mechanically or by hand, to counteract ("trim") aerodynamic forces on the main control surfaces. Turn & Bank Indicator - Primary air-driven gyro instrument, a combined turn indicator and lateral inclinometer to show forces on an aircraft in banking turns. Also referred to as "needle & ball" indicator, the needle as the gyro's pointer and a ball encased in a liquid-filled, curved tube. Uncontrolled Airspace - Class G Airspace; airspace not designated as Class A, B, C, D or E. UNICOM - Universal Communication - A common radio frequency (usually 121.0 mHz) used at uncontrolled (non-tower) airports for local pilot communication. Useful Load - The weight of crew, passengers, fuel, baggage, and ballast, generally excluding emergency or portable equipment and ordnance. V - Velocity - Used in defining air speeds, listed below: VA = Maneuvering Speed (max structural speed for full control deflection) VD = Max Dive Speed (for certification only) VFE = Max Flaps Extended Speed VLE = Max Landing Gear Extended Speed VLO = Max Landing Gear Operation Speed VNE = Never Exceed Speed VNO = Max Structural Cruising Speed VS0 = Stalling Speed Landing Configuration VS1 = Stalling Speed in a specified Configuration VX = Best Angle of Climb Speed VXSE = Best Angle of Climb Speed, one engine out VY = Best Rate of Climb Speed VYSE = Best Rate of Climb Speed, one engine out VASI - Visual Approach Slope Indicator - A system of lights on the side of an airport runway that provides visual descent guidance information during the approach to a runway. Venturi Tube - A small, hourglass-shaped metal tube, usually set laterally on a fuselage in the slipstream to create suction for gyroscopic panel instruments. Now outdated by more sophisticated means. VFR - Visual Flight Rules that govern the procedures for conducting flight under visual conditions. The term is also used in the US to indicate weather conditions that are equal to or greater than minimum VFR requirements. Also used by pilots and controllers to indicate a specific type of flight plan. VMC - Visual Meteorological Conditions - Expressed in terms of visibility, distance from clouds, and ceiling equal to or better than specified minima. VOR - VHF OmniRange - A ground-based navigation aid transmitting very high-frequency (VHF) navigation signals 360° in azimuth, on radials oriented from magnetic nort. The VOR periodically identifies itself by Morse Code and may have an additional voice identification feature. Voice features can be used by ATC or FSS for transmitting information to pilots. VSI - Vertical Speed Indicator. A panel instrument that gauges rate of climb or descent in feet-per-minute (fpm). Also called the Rate Of Climb Indicator. Yaw - Of the three axes in flight, this specifies the side-to-side movement of an aircraft on its vertical axis, as in skewing. Yoke - The control wheel of an aircraft, akin to a automobile steering wheel.


    ATC Phrases

    By Admin, in Reference Items,

    The following are the most common ATC phrases
    "Cleared to taxi"
    When told by ground control or tower that you are cleared to taxi, the controller has given you instruction to taxi along taxiway centerlines according to taxiway markings. It is important to repeat all controller instructions and runway crossing instructions, as you may be told to "hold short" of a specific runway and wait for further instructions.
    "Position and hold" or "Line up and Wait" (AUS)
    The tower expects you to taxi onto runway centerline and maintain a stopped position while the aircraft in front of you gains separation or clears the runway. It is important that, prior to crossing the hold-short lines, you verify your instructions, verify runway of use, and scan extended final for traffic.
    "Cleared for takeoff"
    The tower controller is the only authority to clear you for takeoff at a controlled airfield. Repeat back your takeoff clearance and call sign, as well as scan final for traffic. The tower may request other specific instructions, so listen closely to your takeoff clearance.
    "Enter closed traffic"
    The tower has acknowledged the pilot's intention to perform successive operations involving takeoffs and landings or low approaches where the aircraft does not exit the traffic pattern.
    "Cleared for the option"
    When you are cleared for the option you have been given permission to either do a touch-and-go, make a low approach, missed approach, stop and go, or full-stop landing. If requesting this clearance, the pilot should do so upon establishing downwind on a VFR traffic pattern.
    "Cleared touch-and-go"
    When authorized by the tower, the touch-and-go procedure allows the pilot to land on the runway, reconfigure the airplane and perform a takeoff to re-enter the traffic pattern. If requesting this approach the pilot should do so upon establishing downwind on a VFR traffic pattern.
    "Cleared low approach"
    A low approach clearance allows the pilot to perform a simulated emergency landing or normal landing down to the runway environment (100' AGL) and then perform a go-around to re-enter or depart the pattern. If requesting this approach you should do so upon establishing downwind on a VFR traffic pattern.
    "Cleared stop-and-go"
    A stop-and-go clearance allows the pilot to land on the runway, come to a full stop, and then takeoff on the remaining length of runway. The pilot must be aware of runway lengths and takeoff distance requirements. This procedure can be beneficial in keeping costs lower when performing night currency. If requesting this clearance the pilot should do so upon establishing downwind on a VFR traffic pattern.
    "Cleared to land"
    When given clearance to land the tower has authorized you to land on the runway in use. The phrase "cleared to land" gives you immediate use of that runway, unless the tower advises that you are in sequence for landing ("number two to land, number three, etc..."). After advising approach or tower that you are inbound for landing at your destination you do not have to make any further request for clearance to land.
    The land-and-hold-short procedure requires the pilot to perform an accurate landing on the runway so that the pilot can stop the aircraft before reaching an intersecting runway, intersecting taxiway, or construction area. If you are unable to comply with landand-hold-short operations, you may request clearance for a different runway.
    "Make Short Approach"
    Used by ATC to have a pilot to alter their traffic pattern so as to make a short final approach. If unable to execute a short approach, simply tell the ATC so.
    "Parking with me"
    Under normal conditions you would exit the runway at the first available taxiway, stop the aircraft after clearing the runway, and call ground control for instructions if you have not already received them. If the controller says "parking with me", he or she has given you clearance to taxi to your destination.
    "Caution: wake turbulence"
    This call from ATC advises the pilot of the potential for encountering wake turbulence from departing or arriving aircraft.
    "Frequency change approved"
    You've reached the edge of the controller's airspace and may change your radio to your next frequency.
    "Proceed direct"
    You may turn to the direct heading of your destination (often followed by this heading). Usually used by ATC once you've been vectored clear of other traffic in the area.
    "Report position"
    The controller wants to pinpoint your position relative to the airport. You should report altitude, distance, and direction. For example: "8081G is five miles southwest of the airport at one thousand two hundred feet"
    ATC would like you to hurry up whatever it is that you're doing; taking off, landing, climbing, descending, or taxiing to your destination.
    ATC request for a pilot to use his aircraft transponder identification feature (usually an IDENT button). This helps the controller to confirm an aircraft identity and position.
    Followed by a squawk code or function button on the transponder. ATC issues individual squawk codes to all aircraft within radar service in order to differentiate traffic.
    "Go around"
    Pilots receiving this transmission should abandon their approach to landing. Additional instructions from ATC may then follow. Unless otherwise instructed, VFR aircraft executing a go around should overfly the runway while climbing to pattern altitude, then enter the traffic pattern by way of the crosswind leg.
    "Watch for Traffic..."
    Usually followed by the direction and distance of the traffic, you should immediately scan for it with "Looking for traffic" and report back to the controller whether you have the aircraft in sight or not.
    "Extend Downwind"
    While this may seem obvious, the controller wants you to continue straight on your downwind until he or she tells you to turn base (often followed by "I'll call your base"). In all likelyhood you're going to have a long final. Keep course and scan for other traffic.

    Birth of a NOTAM
    NOTAM start life as messages on the Aeronautical Fixed System (AFS). They are received centrally at the UK NOTAM office at London Heathrow from originators within the UK and from foreign NOTAM offices. AIS staff check and edit the NOTAM if necessary and they are then placed in the transmit queue for transmission to all UK NOTAM recipients. These include ATC offices, some airlines, briefing services etc.

    There is no central world-wide NOTAM database, databases are built up individually by users from the incoming message stream.

    ICAO NOTAM format
    The format of NOTAM is defined in Annex 15 to the International Convention on Civil Aviation. An explanation of the format can be found here. Here is a typical NOTAM and its decode.

    A1484/02 NOTAMN
    Q) EGTT/QMRXX/IV/NBO/A/000/999/5129N00028W005
    A) EGLL
    B) 0208231540
    C) 0210310500 EST

    Notam Decoder
    A1484/02 - one letter to indicate the Series, a 4-digit NOTAM number followed by a stroke and two digits to indicate the year.

    NOTAMN - Suffix N Indicates this is a new NOTAM. Other options are R for NOTAM replacing another or C for one cancelling another.

    Q) EGTT/QMRXX/IV/NBO/A/000/999/5129N00028W005

    This is the "Q" or qualifier line, it always starts Q) and contains the following fields, each separated by a stroke.

    FIR (here EGTT, London FIR)

    NOTAM Code, a 5 letter code starting with Q, defined in Annexe 15. Here QMR indicates that it concerns a Runway. XX indicates that remaining detail is in Plain Language. In this particular case the text shows that certain runway lighting is unavailable. Strictly speaking under ICAO rules this should have appeared as separate NOTAM for each type of lighting. QLCAS is the code for centreline lighting u/s QLZAS is the code for Touch Down Zone lighting u/s and QLAAS is the code for Approach Lighting u/s (note in all cases AS indicates unserviceable). The use of QMRXX here is a sensible compromise that reduces the number of NOTAM from three to one. A full list of codes is included in ICAO document 8126 (Aeronautical Information Services Manual).

    IV - Indicates that this is significant for IFR and VFR traffic

    NBO - indicates for immediate attention of aircraft operators, for inclusion in PIB's and Operationally significant for IFR flights

    A - Indicates scope, here Aerodrome, others are E (en-route) or W (nav warning)

    000/999 - lower and upper limits expressed as a flight level. In this case it has been left as the default as it is not applicable.

    5129N00028W005 - Indicates the geographical centre and radius of influence, always this number of digits. In this case the radius is 5 n.m.

    A) EGLL - ICAO indicator of the aerodrome or FIR (London Heathrow) can include more than one FIR

    B) 0208231540 - Date/time group (UTC) when this NOTAM becomes effective

    C) 0210310500 EST - Date/time group (UTC) when the NOTAM ceases to be effective. Note "EST" means "estimated" (NOT Eastern Standard Time!). All NOTAM with EST remain in force until cancelled or replaced.

    E) RWY 09R/27L DUE WIP NO CENTRELINE, TDZ OR SALS LIGHTING AVBL - text of the notam using ICAO abbreviations.

    Decode of this is "Runway 09/27 due to work in progress no centreline, touchdown zone or simple approach lighting system available"

    Here's the whole thing again
    A1484/02 NOTAMN
    Q) EGTT/QMRXX/IV/NBO/A/000/999/5129N00028W005
    A) EGLL
    B) 0208231540
    C) 0210310500 EST

    and here's the same thing as it appears in the PIB produced by ANAIS
    AGA : FROM 02/08/23 15:40 TO 02/10/31 05:00 EST A1484/02

    You can see that the Q line is omitted entirely, A) has been stripped out because it appears as the header to the section and B) and C) have been reformatted and placed in the first line. AGA has been derived from the Q Code "QMR" (see Annex 15)

    This appendix is to be used to interpret the contents of coded international NOTAM's.
    A NOTAM code group contains five letters. The first letter is always the letter "Q'' to indicate a code abbreviation for use in the composition of NOTAM's. The second and third letters identify the subject being reported. (See Second and Third Letter Decode Tables). The fourth and fifth letters identify the status of operation of the subject being reported. (See Fourth and Fifth Letter Decode Tables).  
    AGA Lighting Facilities (L)
    Uniform Abbreviated Phraseology
    Approach lighting system (specify runway and type)
    apch lgt
    Aerodrome beacon
    Runway center line lights (specify runway)
    rwy centreline lgt
    Landing direction indicator lights
    ldi lgt
    Runway edge lights (specify runway)
    rwy edge lgt
    Sequenced flashing lights (specify runway)
    sequenced flg lgt
    High intensity runway lights (specify runway)
    high intst rwy lgt
    Runway end identifier lights (specify runway)
    rwy end id lgt
    Runway alignment indicator lights (specify runway)
    rwy alignment indicator lgt
    Category II components of approach lighting system (specify runway)
    category II components apch lgt
    Low intensity runway lights (specify runway)
    low intst rwy lgt
    Medium intensity runway lights (specify runway)
    medium intst rwy lgt
    Precision approach path indicator (PAPI) (specify runway)
    All landing area lighting facilities
    ldg area lgt fac
    Stopway lights (specify runway)
    swy lgt
    Threshold lights (specify runway)
    thr lgt
    Visual approach slope indicator system (specify type and runway)
    Heliport lighting
    heliport lgt
    Taxiway centre line lights (specify taxiway)
    twy centreline lgt
    Taxiway edge lights (specify taxiway)
    twy edge lgt
    Runway touchdown zone lights (specify runway)
    rwy tdz lgt
    AGA Movement and Landing Area (M)
    Uniform Abbreviated Phraseology
    Movement area
    mov area
    Bearing strength (specify part of landing area or movement area)
    bearing strength
    Clearway (specify runway)
    Declared distances (specify runway)
    declared dist
    Taxiing guidance system
    tax guidance system
    Runway arresting gear (specify runway)
    rwy arst gear
    Parking area
    prkg area
    Daylight markings (specify threshold, centre line, etc.)
    day markings
    Aircraft stands (specify)
    acft stand
    Runway (specify runway)
    Stopway (specify runway)
    Threshold (specify runway)
    Runway turning bay (specify runway)
    rwy turning bay
    Strip (specify runway)
    Taxiway(s) (specify)
    AGA Facilities and Services (F)
    Uniform Abbreviated Phraseology
    Braking action measurement equipment (specify type)
    ba measurement eqpt
    Ceiling measurement equipment
    ceiling measurement eqpt
    Docking system (specify AGNIS, BOLDS, etc.)
    dckg system
    Fire fighting and rescue
    fire and rescue
    Ground movement control
    gnd mov ctl
    Helicopter alighting area/platform
    hel alighting area
    Landing direction indicator
    Meteorological service (specify type)
    Fog dispersal system
    fog dispersal
    Snow removal equipment
    snow removal eqpt
    Transmissometer (specify runway and, where applicable, designator(s) of transmissometer(s))
    Fuel availability
    fuel avbl
    Wind direction indicator
    COM Communications and Radar Facilities (C)
    Uniform Abbreviated Phraseology
    Air/ground (specify service and frequency)
    a/g fac
    En route surveillance radar
    Ground controlled approach system (GCA)
    Selective calling system (SELCAL)
    Surface movement radar
    Precision approach radar (PAR) (specify runway)
    Surveillance radar element of precision approach radar system (specify wavelength)
    Secondary surveillance radar (SSR)
    Terminal area surveillance radar (TAR)
    COM Instrument and Microwave Landing System (I)
    Uniform Abbreviated Phraseology
    DME associated with ILS
    ils dme
    Glide path (ILS) (specify runway)
    ils gp
    Inner marker (ILS) (specify runway)
    ils im
    Localizer (ILS) (specify runway)
    ils liz
    Middle marker (ILS) (specify runway)
    ils mm
    Outer marker (ILS) (specify runway)
    ils om
    ILS Category I (specify runway)
    ils I
    ILS Category II (specify runway)
    ils II
    ILS Category III (specify runway)
    ils III
    Microwave landing system (MLS) (specify runway)
    Locator, outer (ILS) (specify runway)
    ils lo
    Locator, middle (ILS) (specify runway)
    ils lm
    COM Terminal and En Route Navigation Facilities (N)
    Uniform Abbreviated Phraseology
    All radio navigation facilities (except...)
    all rdo nav fac
    Nondirectional radio beacon
    Distance measuring equipment (DME)
    Fan marker
    fan mkr
    Locator (specify identification)
    Direction finding station (specify type and frequency)
    RAC Airspace Organization (A)
    Uniform Abbreviated Phraseology
    Minimum altitude (specify en route/crossing/safe)
    mnm alt
    Class B, C, D, or E Surface Area
    Air defense identification zone (ADIZ)
    Control area (CTA)
    Flight information region (FIR)
    Upper control area (UTA)
    Minimum usable flight level
    mnm usable fl
    Area navigation route
    rnav route
    Oceanic control area (OCA)
    Reporting point (specify name or Coded designator)
    ATS route (specify)
    ats route
    Class B Airspace
    Upper flight information region (UIR)
    Upper advisory area (UDA)
    Intersection (INT)
    Aerodrome traffic zone (ATZ)
    RAC Air Traffic and VOLMET Services (S)
    Uniform Abbreviated Phraseology
    Automatic terminal information service (ATIS)
    ATS reporting office
    Area control centre (ACC)
    Flight information service (FIS)
    Aerodrome flight information service (AFIS)
    Flow control centre
    flow ctl centre
    Oceanic area control centre (OAC)
    Approach control service (APP)
    Flight service station (FSS)
    Aerodrome control tower (TWR)
    Upper area control centre (UAC)
    VOLMET broadcast
    Upper advisory service (specify)
    advisory ser
    RAC Air Traffic Procedures (P)
    Uniform Abbreviated Phraseology
    Standard instrument arrival (STAR) (specify route designator)
    Standard VFR arrival std vfr arr PC
    Contingency procedures contingency proc PD
    Standard instrument departure (SID) (specify route designator)
    Standard VFR departure
    std vfr dep PF
    Flow control procedure
    flow ctl proc
    Holding procedure
    hldg proc
    Instrument approach procedure (specify type and runway)
    inst apch proc
    Obstacle clearance limit (specify procedure)
    VFR approach procedure vfr apch proc PM
    Aerodrome operating minima (specify procedure and amended minimum)
    opr minima
    Noise operating restrictions noise opr restrictions PO
    Obstacle clearance altitude
    Obstacle clearance height
    Radio failure procedure
    radio failure proc
    Transition altitude
    transition alt
    Missed approach procedure (specify runway)
    missed apch proc
    Minimum holding altitude (specify fix)
    mnm hldg alt
    ADIZ procedure
    adiz proc
    Navigation Warnings: Airspace Restrictions (R)
    Uniform Abbreviated Phraseology
    Airspace reservation (specify)
    airspace reservation
    Danger area (specify national prefix and number)
    Overflying of ... (specify)
    Prohibited area (specify national prefix and number)
    Restricted area (specify national prefix and number)
    Temporary restricted area
    tempo restricted
    Navigation Warnings: Warnings (W)
    Uniform Abbreviated Phraseology
    Air display
    air display
    Captive balloon or kite
    captive balloon or kite
    Demolition of explosives
    demolition of explosives
    Exercises (specify)
    Air refueling
    air refueling
    Glider flying
    glider flying
    Banner/target towing
    banner/target towing
    Ascent of free balloon
    ascent of free balloon
    Missile, gun or rocket firing
    Parachute jumping exercise (PJE)
    Burning or blowing gas
    burning or blowing gas
    Mass movement of aircraft
    mass mov of acft
    Formation flight
    formation flt
    model flying
    model flying
    Other Information (O)
    Uniform Abbreviated Phraseology
    Aeronautical information service
    Obstacle (specify details)
    Aircraft entry requirements
    acft entry rqmnts
    Obstacle lights on ... (specify)
    obst lgt
    Rescue coordination centre
    Availability (A)
    Uniform Abbreviated Phraseology
    Withdrawn for maintenance
    withdrawn maint
    Available for daylight operation
    avbl day ops
    Flight checked and found reliable
    fltck okay
    Operating but ground checked only, awaiting flight check
    opr awaiting fltck
    Hours of service are now
    hr ser
    Resumed normal operations
    Military operations only
    mil ops only
    Available for night operation
    avbl night ops
    Available, prior permission required
    avbl ppr
    Available on request
    avbl o/r
    Not available (specify reason if appropriate)
    not avbl
    Completely withdrawn
    Previously promulgated shutdown has been cancelled
    promulgated shutdown cnl
    Changes (C)
    Uniform Abbreviated Phraseology
    Operating frequency(ies) changed to
    freq change
    Downgraded to
    downgraded to
    Identification or radio call sign changed to
    ident change
    Operating on reduced power
    opr reduced pwr
    Temporarily replaced by
    tempo rplcd by
    On test, do not use
    on test, do not use
    Hazard Conditions (H)
    Uniform Abbreviated Phraseology
    Braking action is ...
    ba is




    Braking coefficient is ... (specify measurement device used)
    brkg coefficient is
    Covered by compacted snow to depth of
    cov compacted snow depth
    Covered by dry snow to a depth of
    cov dry snow depth
    Covered by water to a depth of
    cov water depth
    Totally free of snow and ice
    free of snow and ice
    Grass cutting in progress
    grass cutting
    Hazard due to (specify)
    hazard due
    Covered by ice
    cov ice
    Launch planned ... (specify balloon flight identification or project Code name, launch site, planned period of launch(es)_date/time, expected climb direction, estimate time to pass 18,000 m (60,000 ft), together with estimated location)
    launch plan
    Migration in progress
    migration inpr
    Snow clearance completed
    snow clr cmpl
    Marked by
    marked by
    Covered by wet snow or slush to a depth of
    cov wet snow depth
    Obscured by snow
    obscured by snow
    Snow clearance in progress
    snow clr inpr
    Operation cancelled ... (specify balloon flight identification or project Code name)
    opr cnl
    Standing water
    standing water
    Sanding in progress
    Approach according to signal area only
    apch according signal area only
    Launch in progress ... (specify balloon flight identification or project Code name, launch site, date/time of launch(es), estimated time passing 18,000 m (60,000 ft), or reaching cruising level if at or below 18,000 m (60,000 ft), together with estimated location, estimated date/time of termination of the flight, and planned location of ground contact when applicable)
    launch inpr
    Work completed
    work cmpl
    Work in progress
    Concentration of birds
    bird concentration
    Snow banks exist (specify height)
    snow banks hgt
    Covered by frozen ruts and ridges
    cov frozen ruts and ridges
    Limitations (L)
    Uniform Abbreviated Phraseology
    Operating on auxiliary power supply
    opr aux pwr
    Reserved for aircraft based therein
    reserved for acft based therein
    Operating without auxiliary power supply
    opr without aux pwr
    Interference from
    interference from
    Operating without identification
    opr without ident
    Unserviceable for aircraft heavier than
    u/s acft heavier than
    Closed to IFR operations
    clsd ifr ops
    Operating as a fixed light
    opr as f lgt
    Usable for length of...and width of...
    usable length/width
    Closed to all night operations
    clsd night ops
    Prohibited to
    prohibited to
    Aircraft restricted to runways and taxiways
    acft restricted to rwy and twy
    Subject to interruption
    subj intrp
    Limited to
    limited to
    Closed to VFR operations
    clsd vfr ops
    Will take place
    will take place
    Operating but caution advised due to
    opr but caution due
    Other (XX)
    Uniform Abbreviated Phraseology
    Where 4th and 5th letter Code does not cover the situation, use XX and supplement by plain language
    (plain language following the
    NOTAM Code)

    Area Forecasts For Operations At or Below 10,000 Feet
    The Area Forecast system is designed primarily to meet the needs of pilots of general aviation. There is an emphasis on plain language and brevity in a simple, easy to read format. The system provides for the routine issue of forecasts for designated areas (see map below) and the prompt issue of amendments when prescribed criteria are satisfied.
    More detail of the area forecast boundaries with place locations is contained in Airservices Australia's Planning Chart Australia (PCA).
    There may be variations in commencement of validity between different regions, and between those times when daylight saving is or is not operating. However the following principles apply: the standard validity period is twelve hours but this may vary from state to state. an area forecast covering daylight hours will be available as soon as practicable in the morning. area forecasts are not prepared for those times when air traffic volume is so low as not to justify routine issues. In these cases a route forecast will service any individual flights. area forecasts will generally be available a minimum of one hour before commencement of validity.  
    Message Structure

    Message Identifier
    The forecast is identified as AREA FORECAST unless the forecast is an amendment in which case it will be denoted AMEND AREA FORECAST. In the case of amended area forecasts, all individual sections that are amended will be annotated with AMD preceding the section heading.

    Validity Period
    The validity period is written DDHHMM TO DDHHMM, where DD is the day of the month and HHMM is the time in hours and minutes UTC.

    Area Number
    The relevant forecast area is specified by an area forecast district number. These are given in more detail on the current Airservices Australia's Planning Chart Australia. Note that Areas 24, 87 and 88 are only designated for the purpose of Area QNH. Any flights in these areas can be provided with a route forecast.

    The overview will highlight any conditions which may inhibit safe operations for pilots flying under visual flight rules, and will make reference, where necessary, to any spatial and temporal variations. It will assist the pilot in making the following types of decisions: Are the meteorological conditions Visual Meteorological Conditions (VMC), marginal, Instrument Flight Rules (IFR) or too poor for flying? Is it better to plan for a coastal or inland track? If bad weather is encountered, what is the contingency plan? Return? Change altitude? Change heading? Land immediately?  
    Area forecasts may be divided into spatial, temporal or weather-related subdivisions. Spation subdivisions are given using PCA (Planning Chart Australia) or lat/lon coordinates

    Winds and Temperatures
    Upper level winds are given for 2000 (or 3000 in elevated regions), 5000, 7000, 10 000, 14 000 and 18 500 feet. The expected mean wind direction is given in three figures to the nearest ten degrees True, followed by a solidus (/), followed by the mean wind speed in two figures to the nearest five knots, 290/40. CALM and VRB05 (wind direction variable at 5 knots) are used when appropriate.

    A REMARKS section may be included below the WIND section to provide further information on winds.

    Upper level temperatures are given for 10 000, 14 000 and 18 500 feet. These are given in whole degrees Celsius, following the forecast of the upper wind for the level concerned. e.g. 290/40 PS08, 300/50 ZERO, 360/10 MS10. The abbreviation PS is used for positive temperatures, and MS (minus) is used for negative temperatures.

    The inclusion of cloud is restricted to:
    any CB or TCU. any cloud with a base at or below 5000 feet above the highest terrain in the area covered by the forecast. any cloud layer of more than 4/8 (broken or overcast) amount with base at or below 20 000 feet above MSL. any cloud associated with any forecast precipitation, moderate or severe icing and moderate or severe turbulence.  
    Cloud amount is given using the following abbreviations:
    FEW - Few (1 to 2 oktas) SCT - Scattered (3 to 4 oktas) BKN - Broken (5 to 7 oktas) OVC - Overcast (8 oktas)
    ...except for cumulonimbus and towering cumulus, for which amount is described as:
    ISOL - Isolated OCNL - Occasional (well separated) FRQ - Frequent (little or no separation) EMBD - Embedded (in layers of other cloud)
    Cloud type is given using the following abbreviations:
    CU - Cumulus SC - Stratocumulus CB - Cumulonimbus TCU - Towering cumulus ST - Stratus AS - Altostratus AC - Altocumulus NS - Nimbostratus
    If subdivisions are used and one or more subdivisions have no cloud associated with it, the format used is NIL CLOUD.

    When CU and SC, or AC and AS, occur together at similar heights, they are combined, i.e. CU/SC or AC/AS.

    Cloud base and tops are given in feet above MSL (mean sea level).

    Weather information relating to the layer below 21 000 feet above MSL is given following the word 'WEATHER'. If subdivisions are used and one or more subdivisions have no weather associated with it the format is, WEATHER A: NIL.

    Horizontal visibility is given in metres to the nearest 100 metres up to and including 5000 metres, and in whole kilometres above that value. Forecast visibilities of 50 metres or less are given as 'ZERO'. The forecast value is followed by the units used e.g. '8KM' or '1000M'. Significant variations of visibility are included. If the visibility is forecast to be above 10 kilometres throughout the area, the words 'UNRESTRICTED' or 'GOOD' are used. Vertical variations of horizontal visibility, which might prevent flight under VMC conditions, are significant. For example, information is supplied on the depth of layers affected by drizzle, haze and dust storms, and the levels of haze layers under inversions. Visibility variations with these phenomena is given.

    Freezing Level
    Freezing level is the height, in feet, above MSL of zero degrees Celsius. Reference is made to any variations in height greater than 1000 feet, and to the occurrence of more than one freezing level.

    The icing section gives information on the expected occurrence of moderate or severe icing in cloud (including convective cloud), or precipitation, in the layer below 20 000 feet above MSL.

    The height above MSL of the bottom and top of the layer is given as, for example, MOD IN RA 5000/8000.

    When the layer of icing is expected to extend above 20 000 feet, descriptions such as MOD ABOVE 14000 are used.

    This section provides information on moderate or severe turbulence including turbulence associated with convective cloud.

    The height above MSL of the bottom and top of any layer(s) is given as, for example, MOD IN CLOUD 12000/16000

    When the turbulence is expected to extend to ground level, descriptions such as BELOW 8000 are used.

    When the turbulence is expected to be confined to clouds, descriptions such as MOD IN CLOUD BELOW 8000 are used.

    When the turbulence is expected to extend above 20 000 feet, descriptions such as SEV ABOVE 15000 are used.

    Critical Locations
    These are locations such as gaps in mountain ranges which are frequently used by general aviation aircraft.

    Currently, critical location forecasts are appended to Area Forecasts for Bowral and Mt Victoria (NSW) on AREA 21; Mt Victoria and Murrurundi (NSW) on AREA 20; and Kilmore Gap (Vic) on AREA 30.

    Critical location forecasts are written in a mixture of plain language and TAF format making reference as necessary to cloud, visibility and weather.

    CAVOK is used to indicate visibility greater than 10 KM, cloud ceiling above 5000 FT above ground level and nil significant weather.

    This section will include any relevant information not included elsewhere in the forecast.

    Abbreviations and Codes Used in Area Forecasts
    AC - Altocumulus AC/AS - Altocumulus and Altostratus with bases at the same level AS - Altostratus AMD - Amendment BKN - Broken CAVOK - Cloud and visibility and weather ok. CB - Cumulonimbus CU - Cumulus CU/SC - Cumulus and Stratocumulus with bases at the same level DZ - Drizzle EMBD - Embedded FEW - Few FG - Fog FM - From (only used in Critical Locations section) FRQ - Frequent GR - Hail GS - Small Hail INTER - Intermittent variations (only used in Critical section Locations) ISOL - Isolated MOD - Moderate NS - Nimbostratus OCNL - Occasional OVC - Overcast RA - Rain SC - Stratocumulus SCT - Scattered SEV - Severe SH - Shower SN - Snow ST - Stratus TCU - Towering Cumulus TEMPO - Temporary variations (only used in Critical Locations section) TS - Thunderstorm Z - Code for UTC (universal time)

    A METAR is a routine report of meteorological conditions at an aerodrome.

    A SPECI is a special report of meteorological conditions, issued when one or more elements meet specified criteria significant to aviation. SPECI is also used to identify reports of observations recorded ten minutes following an improvement (in visibility, weather or cloud) to above SPECI conditions.

    The location is indicated by either the ICAO (International Civil Aviation Organization) location indicator or another approved abbreviation.

    The day of month and the time of the report is given in UTC (Coordinated Universal Time) using six figures followed by the letter Z. The first two digits are the day of the month; the following 4 digits are the time in hours and minutes, e.g. 291741Z (time of report is 1741 on the 29th of the month UTC).

    The abbreviation AUTO will be included when the report contains only automated observations.

    Surface Wind
    The wind direction is given in degrees true, rounded to the nearest 10 degrees. A variable wind direction is given as VRB.

    The wind speed, given in knots (KT), is the mean value over the sampling period which is normally ten minutes. The maximum wind speed during the sampling period is reported when it exceeds the mean speed by 10 knots or more. It is indicated by the letter G which is followed by the gust value, e.g. a wind direction of 280°, with a mean speed of 20 knots and a maximum gust of 31 knots, is given as 28020G31KT.

    The horizontal visibility is given in metres up to 9000 metres; with 9999 being used to indicate a visibility of 10 kilometres or greater.

    When the visibility is estimated manually (i.e. by an observer), two groups may be reported when the visibility is not the same in different directions. In these cases, the higher visibility will be given first, followed by the minimum visibility and its direction (using one of the eight points of the compass) from the observing station e.g. 9000 2000N.

    When visibility is given by an automated sensor (in fully AUTOmated reports), only one group is reported. The value is followed by the letters NDV (Nil Directional Variation) to indicate that, as there is only one visibility sensor in place, any directional variation in visibility that may exist cannot be detected.

    Weather phenomena are reported using the codes listed in the tables:

    Code Weather Descriptor
    MI - Shallow BC - patches PR - partial DR - drifting BL - blowing SH - showers FZ - freezing TS - thunderstorm
    Code Weather Phenomena
    DZ - drizzle RA - rain GR - hail SN - snow SG - snow grains DU - dust SA - sand SS - sandstorm DS - dust storm GS - small hail/snow pellets PL - ice pellets FG - fog BR - mist FU - smoke HZ - haze PO - dust devil SQ - squall FC - funnel cloud VA - volcanic ash IC - ice crystals PL - ice pellets
    Intensity is indicated for precipitation, blowing dust/sand/snow, dust storm and sandstorm by appending:
    the prefix - for light, e.g. -DZ the prefix + for heavy, e.g. +RA no prefix for moderate, e.g. SHRA  
    One or more codes may be grouped, e.g. +TSGR, -TSRASN

    When precipitation is reported with TS, the intensity indicator refers to the precipitation, e.g. -TSRA = thunderstorm with light rain. Well-developed dust/sand whirls (dust devils) and funnel clouds are reported using the indicator +  
    An observation may provide an indication of weather in the vicinity of the aerodrome, i.e. between 8 and 16KM of the aerodrome reference point. In these cases, the weather code is prefixed with the abbreviation VC (vicinity), e.g. VCTS.

    Cloud information is reported from the lowest to the highest layers in accordance with the following rules:
    1st group: the lowest layer regardless of amount. 2nd group: the next layer covering more than 2 oktas of the sky. 3rd group: the next higher layer covering more than 4 oktas of the sky. Extra groups: for cumulonimbus and/or towering cumulus clouds, whenever observed and not reported in any of the above.  
    Cloud amount is described using the codes in the table:

    Code - Cloud Amount
    SKC - sky clear FEW - few (1 to 2 oktas) SCT - scattered (3 to 4 oktas) BKN - broken (5 to 7 oktas) OVC - overcast (8 oktas) NSC - nil significant cloud NCD* - nil cloud detected * NCD is reported (in fully automated reports only) when a cloud sensor detects nil cloud (a human observer will report SKC when the sky is clear.

    Cloud height is given as a three-figure group in hundreds of feet above the aerodrome elevation, e.g. cloud at 700 feet is shown as 007.

    Cloud type is identified only for cumulonimbus and towering cumulus, e.g. FEW030CB, SCT045TCU.

    When an individual layer is composed of cumulonimbus and towering cumulus with a common base, the cloud is reported as CB only.

    If the sky is obscured, due to, for example, bushfire smoke, human observers will report the vertical visibility (when it can be estimated) in lieu of cloud. It is reported with the prefix VV followed by the vertical visibility in hundreds of feet, e.g. the group VV003 reports an estimated vertical visibility of between 300 and 399 feet (values are rounded down to the next hundred foot increment).

    The abbreviation CAVOK (Cloud and Visibility OK) is used when the following conditions are observed simultaneously:
    Visibility is 10 kilometres or more; No cloud below 5000 feet or below the highest 25NM minimum sector altitude, whichever is the higher, and no cumulonimbus and no towering cumulus; and No weather of significance to aviation, i.e. none of the weather phenomena listed in the weather tables above.  
    Air temperature and dew point values are rounded to the nearest whole degree. Negative values are indicated by M (minus) before the numeral, e.g. 34/M04

    Pressure (QNH)
    The QNH value is rounded down to the next whole hectopascal and is given using four figures prefixed by Q, e.g. 999.9 is given as Q0999

    Supplementary Information
    Supplementary information is used to report:
    Recent Weather - significant weather observed since the last report but not at the time of observation is given after the prefix RE, e.g. RERA. Wind Shear - reports of wind shear experienced on take-off or landing are given after the indicator WS, e.g. WS RWY16.  
    The Remarks section (indicated by RMK) may contain the following:
    Quantitative information on past rainfall is given in millimetres in the form RFRR.R/RRR.R or RFRR.R/RRR.R/RRR.R. The former, e.g. RF00.2/004.2, gives the rainfall recorded in the ten minutes prior to the observation time, followed by the rainfall recorded in the period since 0900 local time. The second format, e.g. RF00.2/003.0/004.2, gives the rainfall recorded in the ten minutes prior to the observation, followed by the rainfall in the sixty minutes prior to the observation, followed by the rainfall recorded in the period since 0900 local time. Information of operational significance not reported in the body of the message, for example: information about significant conditions (such as bushfires and distant thunderstorms) beyond the immediate vicinity of the aerodrome, any BKN or OVC low or middle cloud present at or above 5000 feet when CAVOK has been included in the body of the message, CLD:SKY MAY BE OBSC may be reported in fully automated reports when the ceilometer (cloud sensor) detects nil cloud and the visibility sensor estimates horizontal visibility as being less than 1000 metres  
    Elements of report not available
    Where an element of a report is not available, solidi will be reported in lieu of the missing element, e.g. //// for visibility, // for weather and ////// for cloud.

    SPECI Criteria
    SPECI is used to identify reports of observations when conditions are below specified levels of visibility and cloud base; when certain weather phenomena are present; and when temperature, pressure or wind change by defined amounts (outlined in the table on the right).

    SPECI is also used to identify reports of observations recorded 10 minutes following an improvement in visibility, weather or cloud to METAR conditions.

    Element And Criterion
    Wind Direction - Changes of 30° or more, the mean speed before or after the change being 20KT or more
    Wind Speed - Changes of 10KT or more, the mean speed before or after the change being 30KT or more
    Wind Gust
    Gusts of 10KT or more above a mean speed of 15KT or more Gust exceeds the last reported gust by 10KT or more Visibility - When the horizontal visibility is below the aerodrome’s highest alternate minimum visibility*
    Weather - When any of the following begins, ends or changes in intensity:
    thunderstorm hailstorm mixed snow and rain freezing precipitation drifting snow fog (including shallow fog, fog patches and fog at a distance) dust storm sand storm squall funnel cloud moderate or heavy precipitation Cloud - When there is BKN or OVC cloud below the aerodrome's highest alternate minimum cloud base*
    Temperature - When the temperature changes by 5°C or more since last report
    Pressure - When the QNH changes by 2hPa or more since last report
    Upon receipt of advice of the existence of wind shear The incidence of any other phenomenon likely to be significant *Where no descent procedure is established for an aerodrome, the aerodrome’s alternate ceiling and visibility are 1500 feet and 8 kilometres respectively.

    METAR/SPECI Examples
    METAR YPPH 221130Z 28012G23KT 9000 -SHRA FEW005 BKN050 27/22
    Q0999 RETS RMK RF00.6/003.4 DISTANT TS
    METAR Routine meteorological observation
    YPPH ICAO location indicator for Perth Airport
    221130Z Time of observation is 1130 on the 22nd of the month UTC
    28012G23KT Wind from the west (280 degrees True) at 12 knots; gusting to 23 knots
    9000 Visibility is 9 kilometres.
    -SHRA Present weather is light rain shower
    FEW005 There are 1 to 2 oktas of cloud with base at 500 feet
    BKN050 There are also 5 to 7 oktas of cloud with base at 5000 feet
    27/22 The air temperature is 27°C; the dewpoint temperature is 22°C
    Q0999 The QNH is between 999 and 999.9 hectopascals
    RETS Recent weather was a thunderstorm
    RMK Remarks section follows
    RF00.6/003.4 0.6 mm of rain has fallen in the last 10 minutes; 3.4 mm has fallen since 0900 local time
    DISTANT TS Distant thunderstorm (greater than 16 kilometres from the aerodrome reference point)

    SPECI YSCB 171515Z AUTO 22015G25KT 9000NDV // NCD 13/09 Q1003 RMK
    SPECI Special meteorological observation (for wind gust)
    YSCB ICAO location indicator for Canberra Airport
    171515Z Time of observation is 1515 on the 17th of the month UTC
    AUTO This report is fully automated
    22015G25KT Wind from the southwest (220 degrees True) at 15 knots, gusting to 25 knots
    9000NDV Visibility is 9000 metres; from a single visibility sensor, therefore no directional variation (NDV) in visibility can be detected
    // Present weather is unavailable
    NCD Nil cloud has been detected (by ceilometer)
    13/09 The air temperature is 13°C; the dewpoint temperature is 09°C
    OVC110 There are also 8 oktas of cloud with base at 11 000 feet
    Q1003 The QNH is between 1003 and 1003.9 hectopascals
    RMK Remarks section follows
    RF00.8/003.0 0.8 mm of rain has fallen in the last 10 minutes; 3.0 mm has fallen since 0900 local time

    SIGMETs are issued to provide urgent advice to aircraft of actual or expected weather developments or trends that are potentially hazardous.

    SIGMETs are issued to advise of the occurrence or expected occurrence of the following phenomena:

    Code and Description
    OBSC TS - Obscured thunderstorm(s) EMBD TS - Frequent thunderstorm(s) SQL TS - Squall line thunderstorms OBSC TSGR - Obscured thunderstorm(s) with hail EMBD TSGR - Embedded thunderstorm(s) with hail FRQ TSGR - Frequent thunderstorm(s) with hail SQL TSGR - Squall line thunderstorms with hail TC - Tropical cyclone SEV TURB - Severe Turbulence SEV ICE - Severe icing SEV ICE FZRA - Severe icing due to freezing rain SEV MTW - Severe mountain wave HVY DS - Heavy duststorm HVY SS - Heavy sandstorm VA - Volcanic ash RDOACT CLD - Radioactive Cloud
    Pilots in command of aircraft encountering any of the above phenomena not notified by SIGMET advices must report details of the phenomena in an AIREP SPECIAL.

    SIGMET for thunderstorms are only issued when they are:
    obscured (OBSC) by haze or smoke embedded (EMBD) within cloud layers frequent (FRQ), i.e. with little or no separation between clouds and covering more than 75% of the area affected squall line (SQL) thunderstorms along a line of about 100 nautical miles or more in length, with little or no separation between clouds  
    SIGMET for thunderstorms and tropical cyclones do not include reference to icing and turbulence as these are implied as occurring with thunderstorms and tropical cyclones.

    Responsibility for the issuance of SIGMET within Australian FIRs
    SIGMETs for volcanic ash are the responsibility of the Volcanic Ash Advisory Centre, Darwin.

    SIGMETs for tropical cyclones are the responsibility of the Tropical Cyclone Advisory Centres in Perth, Darwin and Brisbane.

    SIGMETs for turbulence and icing above FL185 are the responsibility of the Aviation Weather Centre, Melbourne.

    SIGMET for all other phenomenon are the responsibility of the Meteorological Watch Offices located in Perth, Darwin, Adelaide, Hobart, Melbourne, Sydney, Brisbane and Townsville.

    SIGMET Structure

    Bulletin Identification
    WCAU01 for SIGMET on tropical cyclones WVAU01 for SIGMET on volcanic ash cloud WSAU21 for SIGMET for other phenomena  
    Originating Office (WMO Indicator)
    The World Meteorological Organisation (WMO) location indicators for Australian Meteorological Watch Offices are:

    APRM - Adelaide Regional Forecasting Centre
    APRF - Perth Regional Forecasting Centre
    ABRF - Brisbane Regional Forecasting Centre
    ASRF - Sydney Regional Forecasting Centre
    ADRM - Darwin Regional Forecasting Centre
    AMRF - Melbourne Regional Forecasting Centre
    AMHF - Hobart Regional Forecasting Centre
    ABTL - Townsville Meteorological Office
    AMMC - Melbourne Aviation Weather Centre

    Note: These differ from the ICAO indicators (beginning with Y) used elsewhere in the message.

    Issue Date/Time
    Issue date/time is given in UTC in the form DDHHMM, where DD is day of month, and HHMM is time in hours and minutes.

    Flight Information Region
    Gives the abbreviation for the FIR (YMMM or YBBB) in which the phenomenon is located.

    The message identifier is SIGMET.

    Daily Sequence Number
    The four-character sequence number consists of:
    a two-letter designator to indicate the general location of the event (as given in the two maps below), and a two-digit number, giving the sequence number of SIGMETs issued by the relevant office within the FIR (Brisbane or Melbourne) since 0000 UTC.  
    The tropical cyclone and low-level SIGMET two-letter designators and their associated geographical extent are:

    The high-level (above FL185) icing and turbulence SIGMET two-letter designators and their general associated geographical extent are:

    Validity Period
    The validity period is given in the format DDHHMM/DDHHMM, where DD is the day of the month and HHMM is the time in hours and minutes UTC.

    Tropical cyclone and volcanic ash SIGMETs can have a validity of up to six hours. SIGMETs for other phenomena can be valid for up to four hours.

    Originating Office (ICAO Indicator)
    The International Civil Aviation Organization (ICAO) location indicators for Australian Meteorological Watch Offices are:
    YPRM - Adelaide Regional Forecasting Centre YPRF - Perth Regional Forecasting Centre YBRF - Brisbane Regional Forecasting Centre YSRF - Sydney Regional Forecasting Centre YPDM - Darwin Regional Forecasting Centre YMRF - Melbourne Regional Forecasting Centre YMHF - Hobart Regional Forecasting Centre YBTL - Townsville Meteorological Office YMMC - Melbourne Aviation Weather Centre
    Flight Information Region
    This gives the abbreviation and full name for the FIR in which the phenomenon is located.

    Meteorological Information
    This section includes:
    type of phenomenon observed or forecast location, both horizontal and vertical extents movement or expected movement expected change in intensity SIGMET for tropical cyclone and volcanic ash cloud include a forecast position for the end of the validity period message status  
    Cancel SIGMET
    If during the validity period of a SIGMET, the phenomenon for which the SIGMET is no longer occurring or is no longer expected, the SIGMET is cancelled by issuing a SIGMET with the abbreviation CNL in lieu of meteorological information.

    SIGMET Status
    The status line indicates whether the SIGMET is:
    NEW - the SIGMET is for a new phenomenon. REV - the SIGMET reviews an earlier SIGMET for the phenomenon. CNL - cancels a current SIGMET.  
    The following abbreviations are used in SIGMET:
    Code and Description
    A - Altitude
    ABV - Above
    APRX - Approximately
    BLW - Below
    CNL - Cancel
    DS - Dust storm
    EMBD - Embedded
    FIR - Flight Information Region
    FCST - Forecast
    FL - Flight level
    FRQ - Frequent
    FZRA - Freezing rain
    GR - Hail
    HVY - Heavy
    ICE - Icing
    INTSF - Intensifying
    LOC - Location
    MOV - Moving
    NC - No Change (intensity)
    OBS - Observed
    OBSC - Obscured
    RDOACT CLD - Radioactive cloud
    REV - Review
    SEV - Severe
    SQL - Squall line
    SS - Sand storm
    STNR - Stationary
    STS - Status
    TC - Tropical cyclone
    TS - Thunderstorm
    TURB - Turbulence
    VA - Volcanic ash
    WI - Within (area)
    WKN - Weakening (intensity)
    Z - Universal Time

    SIGMET Examples
    WSAU21 AMMC 180357
    YMMM SIGMET MM01 VALID 180439/180839 YMMC-
    YMMM MELBOURNE FIR SEV TURB FCST WI S3200 E12800 - S3200 E13000 - S4700 E13600 - S4700 E13400 FL260/400 MOV E 25KT NC

    WSAU21 AMMC 180720
    YMMM SIGMET MM02 VALID 180720/180839 YMMC-
    STS:CNL SIGMET MM01 180439/180839

    WCAU01 APRF 180217
    YMMM SIGMET PH01 VALID 180215/180815 YPRF-

    WVAU01 ADRM 200100
    YBBB SIGMET BT04 VALID 200100/200700 YPDM-
    YBBB BRISBANE FIR VA ERUPTION LOC S0416 E15212 VA CLD OBS AT 200100Z A100/180 APRX 120NM BY 40NM S1130 E14530 - S1330 E14900 - S1030 E15030 - S0830 E14700 - S1130 E14430 MOV SW 20KT FCST 0700Z VA CLD APRX S110 E144530 - S1230 E14930 - S1050 E15130 - S0800 E14700 - S1130 E14400 STS:REV SIGMET BT03 191900/200100

  • Create New...