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Decreasing your exposure to aerodynamic risk
7. Wind shear and turbulence
Rev. 6 — page content was last changed 22 June 2012
The general air flow in the atmospheric boundary layer — which exists between the surface and perhaps 2000–5000 feet agl — is normally subject to irregular small-scale horizontal and vertical fluctuations and eddies; more so in an unstable atmosphere. Light turbulence usually doesn't significantly alter the aircraft's flight path — bearing in mind that what is a minor variation in flight path at a reasonable altitude may be hazardous when operating at slower speeds very close to the ground in take-off, landing, go-around or perhaps stock mustering and other low-level work. However, there are some — often apparently insignificant — atmospheric phenomena which, if occurring at lower levels, induce moderate to severe turbulence or wind shear, and can result in very dangerous loss of height and/or loss of control. Such small-scale phenomena may last only seconds or minutes and their likely existence at low levels may not be mentioned in aviation weather forecasts.
In addition, aircraft operating at normal recreational cruising altitudes can encounter very rough and possibly dangerous air motion in the form of thermals, downdrafts and shear-induced turbulence. Such encounters add gust accelerations (i.e. loads) to the aircraft structure additional to any existing manoeuvring accelerations.
TurbulenceIt is usual to classify all the changes in atmospheric motion that significantly disturb aircraft flight as turbulence, but in some wind shear events there may be no air turbulence involved. It is difficult to define the degree of turbulence or the load effects of shear in a way that is meaningful to a recreational and sport aviation pilot. Measuring by the airflow velocity change or the gust velocity measured in feet per second doesn't really enable the pilot to judge how turbulent the conditions are in their circumstances, particularly so if the instrument panel is not equipped with an accelerometer or variometer. The following is based on an old ICAO turbulence scale which, though classifying by the induced positive or negative accelerations (only as measured near the aircraft cg), does provide a descriptive definition of sorts that is appropriate for three-axis aircraft, but perhaps not so meaningful for flexible-wing weight-shift aircraft (powered or unpowered) and certainly not meaningful for powered-parachutes or paragliders.
The wake vortices from aeroplanes and helicopters add another form of turbulence that is extremely hazardous to all recreational aircraft, particularly because of the strong rotational effects that lead to sudden height loss. Such vortices must be anticipated and avoided.
Note: inertia is the property of resisting any change in motion, or continuing in the same state of rest or state of motion relative to the Earth's cg. The mass of a body is a measure of its inertia; i.e. its resistance to being accelerated or decelerated by an applied force (such as a change in aerodynamic lift) increases with mass. An aircraft in flight is 'airborne' and its true airspeed is relative to the surrounding air, not the Earth's surface. However, when the aircraft encounters a sudden change in the ambient air energy/velocity — even just a transient gust, horizontal or vertical — inertia comes into play and momentarily maintains the aircraft motion relative to the Earth or — more correctly — relative to space. This changes aoa and airspeed, and imparts other forces (e.g. drag and pitching moments) to the aircraft. A heavier aircraft has more inertia than a lighter one, so is more resistant to irregular, random displacement forces — atmospheric turbulence. The fact that inertia momentarily overrides the physics of aerodynamics is sometimes a cause of confusion. As long as an aircraft's mass remains unchanged so will its inertia whether it is at rest or moving; i.e. motion or pulling g has absolutely no effect on an aircraft's inertia but speed does affect momentum, which is mass (or inertia) × velocity.
Wind shear can be induced by the terrain, constructed obstructions, passage of cold fronts, convective downbursts, thermals, temperature inversions, low-level jets and other sources; all of which will be described later. The closer to the surface that the shear occurs the more hazardous for aircraft — and particularly so for very light aircraft. For an aircraft taking off, landing or going around, the shear may be large enough and rapid enough to exceed the airspeed safety margin and the aircraft's capability to accelerate or climb; or the pilot just may not be able to recover an uncommanded roll due to a crosswind gust, before a ground strike occurs.
The shear is the rate of change of wind speed and/or direction experienced by the aircraft, Such events tend to be classified as 'vertical' or 'horizontal' shear, though many, perhaps most, shear encounters are a combination of both. There is a third classification — 'vertical gust' shear — which has the greatest potential to produce extreme structural loads and which we will examine first.
Gust categorisationVertical gust shear, chiefly associated with updrafts and downdrafts, is the change in the predominantly vertical air motion with horizontal distance flown. Thermals can produce very severe updraft shear when flying in the unstable, high-temperature superadiabatic boundary layer conditions endemic to inland Australia, though their vertical speed near the surface may be relatively low but accelerating with height.
There is a noticeable transition gradient between the surrounding air and a well-ordered updraft/downdraft core; also the ascending/descending column tends to entrain some surrounding air, creating a turbulent interface around it. It is possible that a cruising aircraft suddenly encounters an area of substantial vertical motion. The sudden entry (an aircraft cruising at just 60 knots is moving at 100 feet per second) into such a strong vertical gust is a hazardous form of shear. Apart from fast-rising thermals, such events are also associated with downdrafts from large, vertically developed convective clouds.
Note: the categorisation of vertical gusts is not the same as the usual atmospheric turbulence. The meteorological categories for wind gusts in general (as measured with an anemometer) are:
It is probable that 60% of vertical gusts associated with thunderstorms have velocities of 10 fps or less, while 35% are in the 10 to 25 fps range. An encounter with a gust over 50 fps would be rare — but of course it does happen and always when you don't expect it; see this recreational pilot's report.
Table 7.1 shows the approximate addition to, or subtraction from, the original aoa experienced by four imaginary aircraft each in level flight at a cruise speed where the aoa is 4° and encountering vertical updrafts or downdrafts of the speed shown. Such angles are readily calculated using the 1-in-60 rule; i.e. angular change = gust speed/aircraft speed × 60. The values in red indicate where the stalling aoa, presumed to be 16°, would be exceeded and thus any gust-induced loading is alleviated (with a momentary delay due the aircraft inertia), but the stall indication is applicable only to updrafts and not downdrafts.
I have used these reasonably close approximations: (a) to convert knots to metres per second, divide by 2; (b) to convert knots to feet per minute, multiply by 100; (c) to convert feet per minute to metres per second, divide by 200.
Encounter with an updraftFor example let's look at a two-seat aeroplane (we'll call it 'Model A') with a wing area of 10 m², cruising at 50 m/s (100 knots TAS) at its MTOW of 540 kg and at an altitude where the air density is 1 kg/m³ — about 6000 feet. The lift force being produced must equal the gross aircraft weight (mass multiplied by the acceleration of gravity, which is near enough to 10 m/s²), thus the weight is 540 × 10 = 5400 newtons [N] and the lift from the wing must be the same — ignoring the tailplane/canard balance needs.
The lift equation in level flight is: lift = CL × ½rV² × S = weight
where CL is the non-dimensional lift coefficient, r is the air density in kg/m³, V is the true airspeed in m/s and S is the wing area in m². Substituting the Model A values in the equation, then CL × ½ × 1 × 50 × 50 × 10 = 5400 so the value of CL in the cruise with zero flap must be 0.43 and aoa would be around 4°.
The 'lift coefficient — aoa curve' diagram is a generalisation of the relationship between CL and aoa for a normally cambered wing, which reaches the zero lift aoa at around 2° negative, and the critical aoa at 16° where CLmax is 1.3. The slope of the 'lift curve' is such that each 1° aoa change, within the 2° to 12° range, increases/decreases CL by around 0.1.
Now suppose our Model A aeroplane cruising at 50 m/s encounters a sharp-edged thermal that has a velocity of 7.5 m/s (1500 fpm or 25 fps), then the aircraft aoa will increase by 9° to about 13°. (Using the 1-in-60 rule: 7.5/50 × 60 = 9.) In addition, the speed of the airflow relative to the wing will increase very slightly to 50.5 m/s as shown in the diagram 'Effect of updraft encounter on aoa' below. (The diagram is much the same as the wind triangle plot you might use in navigation — the 'effective change in aoa' is comparable to the drift angle, and the 'effective airspeed' is comparable to the ground speed.)
The lift coefficient increases by around 0.1 per 1° aoa change so the value of CL will now be around 1.3, the 'CL — aoa curve' indicates 1.2. Ignoring the very slight airspeed change we can calculate the lift force produced under the changed conditions:
i.e. lift force = 1.2 × ½ × 1 × 50 × 50 × 10 = 15 000 N.
Thus entry to the gust has increased CL from 0.43 to 1.2 (i.e. 2.8 times) and induced a momentary increase in total wing loading from 5400 to 15 000 N. This applies a rapid bending moment to the wings, flexing them up but well within the design limit load for normal category aircraft of +3.8g, or 20 520 N (5400 × 3.8) total wing loading for our aircraft. (Design limit loads were discussed in the module 'Don't fly real fast', but be aware that the extension of flaps reduces the limit load factors by as much as 50%.)
A very short time after that initial entry into the gust, the inertial effects are overcome, the +2.8g load (15 000/5400) accelerates the aircraft upwards — felt by the occupants as a very severe jolt pushing the seat up under them but also 'felt' as a sudden 2.8g load by all other parts of the aircraft's structure — and the wings' elastic reaction also adds some impetus to the fuselage. The acceleration alleviates the gust loads on the wing while the aircraft restores itself to its trimmed angle of attack and flight continues normally; except that the new flight path will incorporate a rate of ascent relative to the Earth, equivalent to the updraft speed.
When the aircraft flies out of the updraft it will again momentarily maintain its flight path relative to the Earth. During that time the effective airflow around the wings will no longer be directly aligned with the flight path but will have acquired a vertical component opposite to that at entry. The aoa and consequently CL will decrease, producing a momentary decrease in wing loading. The airframe will experience a negative g load, and perhaps the occupants will feel the shoulder harness stopping them being thrown out of the seat, before the aircraft is finally restored to level, unaccelerated flight.
Encounter with a strong updraftNow let's consider the Model A aircraft cruising at 120 knots (60 m/s) with CL of 0.3 encountering a 2000 fpm (10 m/s or 33 fps) gust. The encounter would increase aoa by 10° and CL to about 1.15, so:
lift produced = 1.15 × ½ × 1 × 60 × 60 × 10 = 20 700 N.
Thus entry to the gust has produced a momentary increase in total wing loading from 5400 to 20 700 N imparting a +3.8g load (20 700/5400), which is around the 20 520 N wing load limit as well as the 3.8g airframe load limit. The change in load is from +1g to +3.8g, which will impart a 2.8g acceleration. Note that if our aircraft had been cruising at less than 50 m/s, when the 10 m/s gust was encountered the aoa change would exceed 12° and consequently the critical aoa. The airflow over the wing would separate instantly and alleviate the gust load; this is relevant to Va, the design manoeuvre speed.
It is also assumed above that the aircraft is in unaccelerated flight when the gust is encountered. If the aircraft were in a 40° banked turn then the manoeuvring load factor would be 1.3g rather than 1g, and the gust-induced load would be added to the basic manoeuvring load. If it were in a 60° banked turn then the basic load would be 2g, and the manoeuvre plus gust acceleration would be 4.8g; this is getting very close to the ultimate load factor and in the zone where component fatigue could cause premature structural failure.
Effect of lower aircraft weightIf an aircraft is well below MTOW there is a significant effect on structural loads developed in a vertical gust. Let's take our Model A with only one person on board and less fuel so that weight is reduced to 450 kg or 4500 N, again cruising at 60 m/s and still at an altitude where the air density is 1 kg/m³. So substituting those values in the equation then CL × ½ × 1 × 60 × 60 × 10 = 4500 and the value of CL in the cruise must now be reduced to 0.25 and the aoa reduced to about 1°.
Now suppose that aircraft encounters the same 2000 fpm (10 m/s) updraft. Then the aircraft aoa will again increase by 10° but to about 11° and CL of 1.05. We can calculate the lift force produced under the changed conditions:
lift produced = 1.05 × ½ × 1 × 60 × 60 × 10 = 18 900 N.
Thus entry to the gust at the lower weight has increased CL from 0.25 to 1.05 (i.e. 4.2 times) and induced a momentary increase in total wing loading from 4500 to 18 900 N (4.2g). This is well within the 20 520 N (5400 × 3.8) total wing loading limit for this aircraft but outside the 3.8g design load limit for the structural parts — the mounting structures for the engine, battery and occupant seats, for example.
So when operating at significantly lower weight (and thus lower wing loading) an encounter with a vertical gust at a particular flight speed will induce greater accelerations than when operating near MTOW, which obviously affects choice of speed in turbulent conditions.
Effect of speedFrom the foregoing we could deduce that the faster a particular aircraft's speed is when encountering vertical gust shear, the lesser the structural loads developed. This is because the change in effective angle of attack will lessen as forward speed increases. However, angles of attack at higher speeds are much lower, so the effective CL change from a gust encounter is then proportionately greater. (But it depends to some extent on the slope of the lift curve and the wing loading.)
For example, take our fully laden Model A flying at both 80 knots and 120 knots at 6000 feet; CL at the slower speed would be 0.7 and 0.3 at the faster speed. If, in both cases, the aircraft encountered a 500 fpm thermal the aoa changes would be about 4° and 2.5°, increasing CL to 1.0 and 0.5 respectively. The acceleration would be about 1.4g (1.0/0.7) at the slower speed but 1.7g (0.5/0.3) at the faster, so acceleration loads increase as airspeed increases and that increase is amplified by increasing gust velocity.
This doesn't alter the fact that a high wing-loading aircraft will provide a better ride in turbulence than a low wing-loading [W/S] aircraft, at the same high speed. Imagine two different aircraft types having the same weight but different wing area; if they are flying at the same speed and encounter the same vertical gust, the change in aoa and thus CL will be roughly the same for both. However, the low W/S aircraft will experience a higher acceleration because its wing area is greater and thus the total induced load is greater. We will discuss speed to fly in turbulence later in this module.
Encounter with a downdraftNow suppose the Model A aircraft cruising at 50 m/s (4° aoa) encounters a sharp-edged downdraft that has a velocity of 7.5 metres/second (1500 fpm). Then the aircraft aoa will decrease by 9° to about 5° negative where, from the CL – aoa curve, CL will perhaps be around 0.2 negative. The airspeed relative to the wing will change slightly but can be ignored.
We can calculate the lift force produced under the changed conditions:
lift force produced = –0.2 × ½ × 1 × 50 × 50 × 10 = −2500 N
A negative value means the lift force is acting opposite to the norm. Thus the entry to the downdraft has produced a momentary change in total wing loading from 5400 N positive to 2500 N negative, producing a 0.5 negative g load (–2500/5400) and resulting in a 1.5g negative acceleration from +1g to –0.5g. The occupants will be restrained by their harnesses while the seat drops away from them. Following initial entry into the downdraft the inertial effects are overcome and the aircraft will restore itself to its trimmed angle of attack and flight will continue normally — except that the new flight path will incorporate a rate of sink relative to the Earth and equivalent to the atmospheric downflow.
Note the difference in the acceleration between the 1500 fpm updraft and 1500 downdraft encounter at the same cruise speed — the updraft produced a 2.8g acceleration, the downdraft only a 1.5g acceleration.
When the aircraft flies out of the downflow it will again momentarily maintain its flight path relative to the Earth. During that time the effective airflow around the wings will no longer be directly aligned with the flight path but will have acquired a vertical component opposite to that at entry. The aoa and consequently CL will increase producing a momentary increase in wing loading, and the airframe will experience a positive g load before the aircraft is finally re-established in level flight.
Thus encountering changes in vertical flow induces momentary changes in aoa and wing loading. The gust accelerations and the variation in the vertical profile of the flight path will be considerable if extensive and higher-speed vertical gusts are encountered.
But it's a bit more complex!The foregoing assessments of aircraft reaction to updraft/downdraft shear is simplified, but aircraft reactions are much more complex. For example:
The accelerations calculated in the foregoing are those measured at the aircraft's cg. Accelerations at the aircraft's extremities may be much greater due to added yawing, rolling and/or pitching motions and they will also affect the control surfaces.
Gust ratiosIn normal flying weather the velocity of any near-surface wind is changing constantly. Due to the eddies that usually exist within the flow, fluctuations in direction of 20° or so and in speed perhaps 25% either side of the mean, occur every minute. In other than very light wind conditions these variations are evident in the form of wind gusts. In stronger wind conditions, gust ratios (maximum gust to mean wind speed) are typically 1.6:1 over open country and 2:1 or greater over rough terrain, adding more turbulence to the flow. In an unstable atmospheric boundary layer the rising air in thermals is complemented by colder air sinking from the top of the layer, where the wind velocity approximates the gradient flow; i.e. the direction may be backed by 20–30° from the wind at the surface, and the speed is greater. The descending air retains most of these characteristics when it arrives at the surface as a strong gust, thus backing (i.e. shifting anticlockwise around the compass) and increasing in speed.
Very light aircraft are of course more susceptible than others to low-level gusts. Such gusts figure in light aircraft accidents perhaps ten times more often than all other forms of windshear or turbulence combined. However, such upsets don't often result in serious injury to the occupants, and coping with such conditions in take-off and landing is an everyday part of pilot development. This module is concerned with more unusual events.
Horizontal shear effectsHorizontal shear is the change in horizontal wind velocity (speed and/or direction — gusts and lulls) with horizontal distance flown; i.e. a substantial change in the ambient energy state of the air mass in which the aircraft is borne. Horizontal shear is particularly dangerous when landing, taking off or going around as headwinds can suddenly disappear or change to strong crosswind gusts. These can last anywhere from a few seconds to several minutes. Crosswinds can change to tailwinds, resulting in loss of control and ground collision. There are two general classifications for horizontal shear.
Increasing-performance shear. If a low-flying aircraft suddenly encounters an increase in the headwind component of wind speed (or a decrease in a tailwind) then due to its inertia the aircraft will momentarily maintain its speed (and flight path) relative to the Earth. Thus there will be a brief increase in speed of flow over the wings with consequent increase in lift. The aircraft will rise, gaining potential energy, until the inertial effects are overcome and the aircraft restores itself to the previous flight state at a higher altitude than previously; but at a changed ground speed and track — if the changed wind velocity is maintained.
Decreasing-performance shear. Similarly should the aircraft encounter a decrease in the headwind component (a lull, or a gust from the rear) then airspeed and lift will decrease, and the aircraft will sink until the inertial effects are overcome. In recreational light aircraft flight conditions (and in accordance with the lift equation) the percentage increase or decrease in lift will be about double the percentage increase or decrease in airspeed; i.e. if airspeed dropped by 10% then lift will drop by 20% and the aircraft will sink very quickly. The worst situations to encounter such shear are where loss of airspeed and/or a sudden loss of height, take-off or climb performance could be critical — on the final approach to landing, on take-off or during a go-around . The time taken for the aircraft to restore itself to the original airspeed will be much the same as that taken to gain — in normal conditions — the same increase in airspeed by increasing power.
Usually increasing-performance shear should not present any problem to an aircraft on approach or take-off, as long as the pilot continues to maintain the appropriate attitude in pitch, ignoring the speed increase(s), and is prepared for a possible decreasing-performance shear encounter to follow. On the other hand decreasing-performance shear will be very dangerous if the aircraft has insufficient height to clear obstacles while the pilot takes action to accelerate the aircraft through the shear and minimise height loss. If the aircraft's initial airspeed was less than the safe speed near the ground, including the normal 50% gust estimate allowance, then the shear effect is exacerbated and the rate of sink could be extremely high. Wind shear occurring just after wheels-off can cause the aircraft to stop accelerating.
Wind shear events are usually a combination of wind speed variations and variations in three-dimensional direction, which will affect aircraft speed, angle of attack and attitude in the three axes. Thus encountering changes in horizontal flow within the airmass causes momentary changes in lift. There are consequent variations in the vertical profile of the flight path plus — with significant wind direction changes — diversions from the planned ground path and uncommanded motions in roll, yaw and pitch. Increasing-performance shear could even stall the aircraft: imagine an aircraft in slower level flight encountering a 20-knot gust vector at 45° to the horizontal. The effect would be like encountering a 14-knot head-on gust combined with a 25 fps updraft — the airspeed would be increased but the vertical gust component could take aoa past critical, so we have an accelerated stall.
Something you can be sure of is that no matter what scientists and aviators may do to place wind shear and turbulence events into tidy boxes, the atmosphere has never been a party to such classification and will, on occasion and literally out of the blue, produce a demonstration of staggering power that just confounds our experience and expectations.
Various scenarios were outlined in the 'Don't stall and spin in from a turn' module where the aircraft could be flying with little margin between effective and critical aoa; it is on occasions like these that Murphy's Law springs into action. What can and will go wrong at those worse possible times is an encounter with wind shear or turbulence that suddenly increases the effective aoa of the wing and instantly switches on a stall/spin event or a high sink rate at the worst possible time.
Vertical shear (unrelated to vertical gust shear) is the term used for the change in the roughly horizontal wind velocity with change in height; i.e. as the aircraft is climbing or descending. As the vertical component of a light aircraft's velocity during climb or descent is probably no more than 12% of its horizontal velocity, the outcome of vertical shear is much the same as that for horizontal shear so we'll ignore the term and talk about the wind gradient.
The wind gradientThe Earth's surface has a frictional interaction with the atmosphere. Its effect decreases with height, until between 1500 and 3000 feet agl the gradient wind (i.e. the wind more or less is aligned with the isobars on the meteorological surface chart) dominates. The stability of that 'friction' or 'boundary' layer between the surface and the gradient wind level 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 very strong and the friction force will be strong. In a typically neutral layer, with a moderately strong gradient wind of about 30 knots at 2500 feet, the wind speed might be 20 knots at 750 feet but only 10 knots at the surface.
There is also a change in wind direction within the layer, perhaps as much as 40°. The rate of change in the gradient wind speed is generally more pronounced within the lower 300 feet, while the change in direction in that first 300 feet is negligible in strong winds but greatest in light winds, perhaps as much as 15–20° if the surface wind is less than 5 knots. The profile of the wind velocity change between the gradient wind and the surface wind is called the wind gradient. The greatest change in wind gradient velocities occurs at night and early morning.
If the gradient wind speed is 30 knots at 2500 feet agl and reduces uniformly to 10 knots at the surface then, although the 20-knot change is relatively high, the time taken for a light aircraft to descend for landing through that wind profile is measured in minutes; thus there is no shear because the rate of change is slow. Remember in aviation terms wind shear is a sudden but sustained 'variation in wind along the flight path of a pattern, intensity and duration, that displaces the aircraft abruptly from its intended path and sufficiently that substantial and timely control action is needed.'
So the average recreational light aircraft is not adversely affected by the normal (see Note 3) wind gradient provided that the minimum safe speed is maintained during take-off and landing, and the pilot remains aware of the gradient effects on the flight path/speed profile, adjusting the usual piloting techniques accordingly.
(Note: 'normal' for the average recreational light aircraft implies a surface wind no more than 'moderate'; i.e. less than 16 knots or the point at which a dry '15 knot' windsock becomes horizontal. Greater surface wind speed could indicate a more pronounced gradient and thus possible shear conditions within the gradient.)
Vno — maximum structural cruise speedIn light aircraft the green arc on the airspeed indicator should indicate the 'normal operating' range between Vs1 at the lower limit, and Vno, or perhaps Vc, at the upper limit. Vno is the maximum structural cruise speed and, when cruising at and below Vno, the airframe would not be put at risk of overstressing in an encounter with turbulence in the upper end of the moderate range. Flight in the yellow arc speed range between Vno/Vc and Vne should only be conducted using controls cautiously and in reasonably smooth atmospheric conditions.
Vc — design cruising speedVc is normally not a limiting speed — it is a value chosen by the designer as a basis for stress calculations. In the FAR Part 23 regulation normal category, Vc in knots may not be less than 33 times the square root of the aircraft wing loading in pounds/square feet. For example 'minimum' Vc for an aircraft with 9 lb/ft² weight/wing area ratio (about 45 kg/m²) would be 33 × 3 = 99 knots. Alternatively the designer is allowed to obtain a lower speed value by setting Vc at 90% of Vh — the maximum level flight speed attainable at sea level whilst utilising maximum continuous engine power. Vno must not be less than the minimum Vc and for light aircraft Vc and Vno can be considered synonymous.
The representative gust envelope, below, for a normal category aircraft superimposes positive and negative vertical gust load lines over a manoeuvring V-n diagram similar to that shown in the 'Airspeed and properties of air' tutorial. The horizontal light blue line indicates airspeed increasing from zero. The calculated gust load lines originate at the normal flight load condition of +1g, the strong 50 fps (30 knot) gust line extending to Vc and the moderate 25 fps (15 knot) line extending to Vd (the design diving speed). You can see that the moderate line intersects with Va at about 2g (i.e. a 1g acceleration) and with Vc at about 2.8g (a 1.8g acceleration). The strong gust line intersects with Va at about 3g and with Vc at something in excess of the 3.8g limit load.
The brown arrow shows the 2.5g load — added to the normal 1g load — when the aircraft, flying at a speed about 30% higher than Va, encounters a 50 fps updraught. The total load factor is then about 3.5g; within the limit load factor, even though the gust is at the low end of the strong gust category.
The pink arrow shows the resultant 2g load when the aircraft, flying at a speed about 30% higher than Vc, encounters a 25 fps updraught. Adding the normal 1g load the total load factor is then 3g. So even when flying at a speed somewhat greater than Vc/Vno in a certified light aircraft, an encounter with a vertical gust at the low end of the moderate gust range would be no problem.
The calculations designers use to establish the gust load lines is similar to those used in the preceding section 'Vertical gust effects' but more refined; e.g. using a 'gust alleviation factor' rather simplifying the calculation with the concept of a 'sharp-edged gust'.
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 of interest to us 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 is probably set at 90% of Vh, as mentioned above.
At higher speeds the wing lift coefficient (i.e. the aoa plus flap/slat/spoiler configuration) is relatively low, with much of the lift being provided by the dynamic pressure due to the airspeed — so the lifting force potential of the wing is very high. In these circumstances the wing loading might be readily tripled or quadrupled through abrupt and excessive elevator movement. For example if the aircraft was flying with CL = 0.25 and the pilot suddenly pulled back hard on the stick then CL might increase to 0.75 applying a momentary 3g load made up of the pre-existing 1g normal load plus the 2g acceleration. So it is unwise to make full or abrupt applications of any one primary flight control if flying at a speed greater than Va. This is because at the higher speeds it is easy to apply forces that could exceed the airframe structural limitations, and particularly so if you apply non-symmetrical loads; e.g. apply lots of elevator and rudder together. Misuse of controls in light aircraft at high speed can generate greater structural loads than those likely to be encountered in turbulence, so Va is also useful as a 'turbulent air operating speed'. At this compromise speed the aircraft will produce an accelerated stall, and thus alleviate the aerodynamic load on the wings, if it encounters a vertical gust imparting an acceleration sufficient to exceed the load limit factor.
Aircraft design rules generally state that the minimum acceptable manoeuvring speed is a fixed calculation relative to Vs1 for all aircraft within the same category; for a normal category light aircraft (whose certificated vertical load limit factor is +3.8g) minimum Va = Ö3.8 Vs1 or 1.95 × Vs1. Of course the aircraft designer may specify a Va speed that is greater than the minimum requirement. The sample gust envelope diagram indicates that particular aircraft at Va could handle a vertical gust speed greater than 50 fps without reaching the load limit.
All the preceding assumes the airspeed indicator has been properly calibrated and Va is stated as a calibrated airspeed. If it has not been accurately calibrated each one knot error in IAS around Va speeds will make a 0.1g difference in wing loading; i.e. if the ASI is understating airspeed by 10 knots then the load is 1g greater than thought.
For more Va information see 'Critical limiting speeds'.
What speed then?Follow the recommendations in the manufacturer's approved documentation. However, if that lacks substance then the following is relevant.
If in cruising flight at speeds at or below Vno/Vc and 'very low' to 'low' turbulence is encountered, speed could be maintained without detriment, but be prepared to slow down if you suspect it may become rougher or if conditions suit the development of fast-rising thermals. If turbulence is in the moderate to severe category, reduce speed to the weight-adjusted Va. If flight at this speed is still worrying then speed could be reduced further. But when flying in moderate to severe turbulence at speeds below the weight-adjusted Va, then although the potential for exceeding load limits is no longer a problem, the potential for loss of control is much increased.
As speed is reduced below Va updraft encounters produce increasing changes in aoa. This increases the potential for stall and in rough conditions even transient stalls may lead to longer-term loss of control with possible spin entry — and spins in severe turbulence may not be quickly recoverable. Loss of control may also lead to airspeed exceeding Va and thus restoring the potential to exceed load limits. Normal minimum safe speed in fairly smooth air is 1.5 × Vs1, but in very rough conditions the lower limit should be perhaps 1.7 × Vs1. You can see from the V-n diagram that the intersection of the 50 fps gust line and the Cna curve corresponds with a load factor of about 2.8g. The stall speed is escalated by the square root of the load factor and the square root of 2.8 = 1.7.
If there is no manufacturer's documentation or instrument panel placard indicating manoeuvring speed, or a speed to fly in turbulence, then assume maximum weight Va is twice Vs1 CAS. If weight is below MTOW reduce Va by one half of the percentage weight reduction; e.g. if weight is 16% below MTOW, reduce Va by 8%.
In addition it should be recognised that some sport and recreational light aircraft are ageing, the strength of their airframe components is not 'as new' and the designed ultimate load limit factor is no longer achievable.
Prudent actionsThe action to take in encounters with turbulence or shear very much depends on your interpretation of events. For example if, in cruising flight, you believe you have entered just a limited layer of turbulence then you would initiate a shallow descent or climb to find smoother air. At the other extreme is a cruising flight encounter with a severe vertical gust or other severe turbulence, where your actions might be:
In excessive uplift, rather than going with the flow it may be prudent to reduce power and lower the nose somewhat to maintain target speed. It depends on what is above you — a towering cumulus or Class C airspace for example. If you are likely to violate controlled airspace notify Flight Service on the area frequency and ensure the transponder, if available, is operating. If you have the ability, then inform Flight Service of the turbulence encounter and location — the pilot report may help others to avoid it.
Generally an aircraft will not run into a severe downdraft at low levels, more likely it will meet the turbulent low-level horizontal outflow from the downdraft. However, if you encounter a severe downdraft at lower levels, the only option is to immediately apply full power and either adopt the attitude for best rate of climb or allow airspeed to increase even beyond Va and trust you will fly out of the downdraft quickly. Whichever way the pilot is in a hazardous situation, and the aim should be to recognise and avoid extreme shear conditions.
Upset recoveryMany windshear or turbulence encounters will result in an uncommanded roll perhaps combined with strong yaw, pitch-up or pitch-down. The result may well be an aircraft in a most abnormal attitude and losing height fast. Such events are very dangerous at low level.
Most light aircraft don't have much roll capability, perhaps 15–30° per second is the norm. If a condition such as a curl-over, a lee wave rotor or the wing tip vortices from a preceding larger aircraft (for example when turning base to final following a larger aircraft landing from a straight-in approach), is encountered, the resulting induced roll may well exceed the countering capability of the ailerons. As mentioned in the 'Engine failure after take-off' module under the sub-heading 'Unloading the wings is a good practice to practise' a light aircraft (but not a trike) can be better controlled for at least a few seconds, even at sub-Vs speeds, by pushing forward to unload the wings so that the aircraft is operating in the reduced-g zone (between perhaps +0.25g and +0.75g) but not in the negative-g zone'.
In the 'Don't stall and spin in from a turn' module under the sub-heading 'When I recognise a stall with wing drop what's the best way to recover?' a stall recovery technique was presented. That same technique is applicable to recovery from an abnormal attitude (where the primary aim is to get all lift force directed away from the ground), except that the initial forward stick movement should unload the wings rather than just reducing aoa below critical.
This concludes the series for the present, but I suggest you read a discussion, in the Aviation Meteorology Guide, of the atmospheric phenomena that are major sources of wind shear and turbulence.
'Decreasing your exposure to risk' articles
| Introduction and contents | Recent RA-Aus accident history | Don't fly real fast | Don't stall and spin in from a turn |
| Don't land too fast in an emergency | Engine failure after take-off | The turn back: possible or impossible — or just unwise? |
| Wind shear and turbulence |
Copyright © 2007–2012 John Brandon (contact information)