|| Tutorials home | Decreasing risk exposure | Safety tour | Meteorology | Flight Theory | Navigation | Communications | Builders guide ||
Coping with emergencies
Knowing the aircraft
Rev. 13 — page content was last changed 13 December 2009
The first three modules in this "Coping with emergencies" guide deal with the circumstance where:|
Skill in forced landing approaches is a vital asset that can only be developed, and maintained, by regular practice and self-assessment. There is no economic way for a pilot to practise vehicle control following first impact on rough terrain. However, competence in accurate handling of the aircraft in adverse conditions, at least up to the final stages of the approach, can be achieved by regular simulations of engine failure from all flight states.
Low flying training for the final stages of the forced landing approach — where to survive the pilot may have to manoeuvre an aircraft without power at slow speed around trees or under powerlines — is best undertaken with an experienced bush pilot. See the Safety brief: loss of control in low-level turns.
There is some element of chance in every emergency landing (Murphy's Law proposes that what can go wrong will go wrong, and at the worst possible time) but being well prepared is by far the most important factor in deciding the outcome. The main constituent of that preparation is for the pilot to know the aircraft and – faced with the situation where there is no option but to put it down immediately — keep cool, maintain command of the aircraft, decide the landing site (if this is an option) and fly the approach by maintaining a suitable flight speed, and touch down at the lowest controllable vertical and horizontal flight speeds with the wings level and the aircraft in a nose-up attitude — even if landing in tree-tops. That is, the pilot must maintain complete control of the flight path, airspeed, sink rate and attitude right up to the point of first impact.
A bit of fear is normal — even desirable — but excessive stress may cause the pilot to concentrate on very few features of the situation to the detriment of other equally important features. Panic or acceptance that there is nothing much she or he can do about the situation will not improve the outcome, but applied knowledge will ensure the best possible result.
Before continuing with this page I suggest you review the document 'Airmanship, flight discipline and human factors training'.
Maximum L/D usually occurs at an angle of attack between 4° and 5° or where the CL is around 0.6. — L/Dmax is sometimes termed the glide ratio because for light aircraft it is just about the same ratio as distance covered/height lost in an engine-off glide at the optimum still-air gliding speed. For example, if L/Dmax = 8 then the glide ratio is 8:1 meaning the aircraft might glide a horizontal distance of 8000 feet for each 1000 feet of height lost, in still air with the wings held level.
We can use the '1-in-60' rule to calculate the angle of the glide path relative to the horizon, for example L/Dmax = 10 then 60/10 = 6° glide path angle. If the aircraft is maintained in a glide at an airspeed higher or lower than L/Dmax then L/D will be degraded and the glide path will be steeper; for example if L/D is degraded to 8 then 60/8 = 7.5° glide path angle.
Because of the slight flattening of the curve around L/Dmax, the aoa — and thus the airspeed that will provide maximum air distance travelled from the potential energy of height — is more akin to a limited range rather than one particular best glide speed. An aoa either side of that top arc of the curve results in higher drag and thus a decrease in L/D and less air distance travelled without power.
However, we may also need to glide at a speed that results in the lowest rate of sink (the vertical component of the velocity vector) so providing the longest time in the air from the potential energy of height. The lowest rate of sink occurs at the minimum value of drag × velocity and the corresponding minimum descent airspeed may be around 80% of the L/Dmax speed. So, the aircraft is moving rather slowly and will not cover as much distance as when moving at the best glide speed, but will take a little longer to lose height. See the speed polar diagram in section 1.2.
For example, assuming a glide angle of 10°, from the abridged trigonometrical table the tangent of 10° is 0.176, so the ratio of drag/lift in this case is then 1 : 5.7. (This is a little little more accurate than using the '1-in-60' rule but inconsequential anyway.)
Conversely we can say that the angle of glide is dependent on the ratio of lift/drag at the airspeed being flown. The lower that ratio is, then the greater the glide angle — and consequently the greater the rate of sink and the lesser the distance the aircraft will glide from a given height. The rate of sink is the resultant of the gliding angle and the airspeed.
Be aware that the aircraft manufacturer's quoted L/Dmax may be overstated and generally will not take into account the considerable drag generated by a windmilling propeller so, for glide ratio purposes, it might be advisable to discount the quoted L/Dmax by maybe 20%. But the best option is to check it yourself.
• Vmp — minimum power — the speed that results in the lowest rate of sink in a power-off glide, providing the longest time in the air from the potential energy of height. The lowest rate of sink occurs at the minimum value of drag × velocity and may be around 80% of Vbg. Vmp is the airspeed used by gliders when utilising the atmospheric uplift from thermals or waves. This is the airspeed to select if you are very close to a favourable landing site with ample height and a little more time to plan the approach would be welcome. It is also the airspeed you should reduce to in the last stage of a forced landing in order to minimise both vertical and horizontal velocities, and thus impact forces.
Vmp decreases as the aircraft weight decreases from MTOW, the percentage reduction in Vmp is half the percentage reduction in weight. So, if weight is 10% below MTOW then Vmp is reduced by 5%. Vbg is also reduced in the same way if weight is less than MTOW.
• Vbg — the best power-off glide — the CAS that provides minimum drag thus maximum L/D, or glide ratio; consequently this provides greatest straight-line flight (i.e. air) distance available from the potential energy of height. The ratio of airspeed to rate of sink is about the same as the L/D ratio, so if Vbg is 50 knots (5 000 feet per minute) and L/Dmax is 7 then the rate of sink is about 700 fpm.
This 'speed polar' diagram is a representative plot of the relationship between rate of sink and airspeed when gliding. Vmp is at the highest point of the curve. Vbg is ascertained by drawing the red line from the zero coordinate intersection tangential to the curve: Vbg is directly above the point of contact. Stall point is shown at Vs1.
Much is said about the importance of maintaining the 'best gliding speed' but what is important is to maintain an optimum glide speed; a penetration speed that takes atmospheric conditions into account; for example, sinking air or a headwind. The gliding community refers to this as the speed to fly. The normal recommendation for countering a headwind is to add one third to one half of the estimated wind speed to Vbg, which increases the rate of sink but also increases the ground speed. For a tailwind, deduct one third to one half the estimated wind speed from Vbg, which will reduce both the rate of sink and the groundspeed. Bear in mind that, for safety, it is better to err towards higher rather than lower airspeeds.
To illustrate the speed to fly, the polar curve on the left indicates the optimum glide speed when adjusted for headwind, tailwind or sinking air. For a tailwind the starting point on the horizontal scale has been moved a distance to the left corresponding to the tailwind velocity. Consequently the green tangential line contacts the curve at an optimal glide speed that is lower than Vbg with a slightly lower rate of sink. This is the opposite for a headwind — shown by the purple line. For sinking air the starting point on the vertical scale has been moved up a distance corresponding to the vertical velocity of the air. Consequently the pink tangential line contacts the curve at a glide speed higher than Vbg.
If you want further explanation of speed polar curves (with excellent diagrams) read this article on glider performance airspeeds.
The foregoing does not apply to a powered parachute as the glide speed is normally fixed at the aircraft's designed speed.
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.
Thus both Vbg distance and Vmp time are adversely affected by the extra drag of a windmilling propeller, which creates much more drag than a stopped propeller following engine shut-down.
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.
However, do not attempt to halt a windmilling propeller unless: (1) you have more than ample height to recover from a possible stall; and (2) stopping it will make a significant difference to the distance covered in the glide. Sometimes it may not be possible to stop the windmilling. Never be distracted from the job in hand by trying to stop a two-blade propeller in the horizontal position in order to minimise propeller damage during the landing.
Should the PSRU fail in flight, the propeller is thereby disconnected from the engine and may 'freewheel' rather than 'windmill'.
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.
These measurements should be taken at MTOW and then, if a two-seater, at the one person-on-board [POB] weight with the reduced Vbg.
The airspeed used should really be the TAS but, if the ASI is known to be reasonably accurate, using IAS will err on the side of caution. Also with the engine idling, a fixed-pitch propeller will probably be producing drag rather than thrust, so that too will be closer to the effect of a windmilling propeller. You should also confirm the rate(s) of sink at Vmp.
Having established the rates of sink you then know the maximum airborne time available. For example, if the rate of sink at Vbg with one POB is 500 fpm and the engine fails at 1500 feet agl then the absolute maximum airborne time available is three minutes. If failure occurs at 250 feet whilst climbing then time to impact is 30 seconds — but 3 or 4 seconds might elapse before reaction occurs plus 4 or 5 seconds might be needed to establish the safe glide speed. Read the section on conserving energy in the Flight Theory Guide.
Following engine failure it is important to be able to judge the available radius of action; i.e. the maximum glide distance in any direction. This distance is dependent on the following factors, each of which involves a considerable degree of uncertainty:
Considering the uncertainties involved (not least being the pilot's ability to judge distance) and particularly should the engine fail at lower heights where time is in short supply, it may be valid to just consider the radius of the footprint as twice the current height — which would encompass all the terrain within a 120° cone and include some allowance for manoeuvring. The cone encompasses all the area contained within a sight-line 30° below the horizon. If you extend your arm and fully spread the fingers and thumb the angular distance between the tips of thumb and little finger is about 20°. There is a drawback, in that total area available from which to select a landing site is considerably reduced; the area encompassed within a radius of 60% of the theoretical glide distance is only about one third of the total area.
For powered 'chutes the radius of the footprint might be equivalent to the current height, providing a 90° cone from a sight-line 45° below the horizon.
Just because an aircraft has a good glide ratio does not mean it will perform equally well in a turn; it may lose more height in a turn than an aircraft that has a poorer glide ratio. For example, a nice slippery aircraft with a glide ratio of 15 may lose 1000 feet in a 210° turn, whereas a draggy aircraft with a glide ratio of only 8 might lose only 600 feet in a 210° turn. Of course, the radius of turn is greater in the faster, slippery aircraft. sideslip, and a sideslipping turn from base. A series of 'S' turns will reduce the forward travel. These techniques are certainly not something tried out for the first time in an actual emergency; they should only be used after adequate instruction and adequate competency has been reached — and maintained. The use of full flaps plus full sideslip may be frowned upon by the aircraft manufacturer, but in an emergency situation use everything available.
Except for 'S' turns, these techniques are not available with weight-shift aircraft. For powered 'chutes braking both wings simultaneously will slow the aircraft and increase rate of sink but excessive braking may stall the wing.
Please read the 'Safety brief: loss of control in low-level turns ' section of the Flight Theory Guide before continuing.
On the other hand, there will be a minimum safe height below which a turn-back for a landing in any direction could clearly not be accomplished. To judge whether a safe turn-back is feasible the pilot must know the air radius of turn and how much height will be lost during the turn-back in that particular aircraft in similar conditions, then double it for the minimum safe height. Such knowledge can only be gained by practising turn-backs at a safe height and measuring the height loss.
Turning back to land on, or parallel to, the departure runway requires a turn through maybe 210° onto an intercept path for the extended runway line. At interception a small opposite direction turn may be needed to align with the selected landing path. If the take-off has a crosswind component, the initial turn should be conducted into the crosswind so that it will drift the aircraft back toward the extended runway line and reduce the ground radius of the turn. If the take-off has been downwind then the minimum height for a turn-back would be greatly increased. Any doubt whatsoever — do not turn back.
Of course, if you have departed from a large aerodrome rather than a small airstrip then there is ample cleared area available for a landing; there is no need to opt just for a runway.
For aircraft at the lower end of the performance spectrum it may be found that a 20° to 25° bank angle provides a good compromise, with an appreciable direction change and a reasonable sink rate. There may be other techniques for an aircraft fitted with high lift devices. All of this indicates that performance will vary widely, and you must know your aircraft and establish its safe turn-back performance under varying conditions — otherwise don't turn back!
More turn-back discussion can be read in 'The turn back: possible or impossible — or just unwise?'
[ The next section in the airmanship and safety sequence is section 11.7 'Precautions when taking off towards rising terrain']
The next module in this 'Coping with emergencies' guide deals with deceleration forces.
Groundschool – Coping with emergencies
| Guide contents | Knowing the aircraft | Deceleration forces | Forced landing procedures |
| Overcoming aircraft control failures | Procedure when lost | Safety and emergency communication procedures |
| Aviation distress beacons | Understanding SAR services | ERSA emergency and survival procedures |
Copyright © 2004–2012 John Brandon [contact information]