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Coping with emergencies

Knowing the aircraft


Rev. 13 — page content was last changed 13 December 2009

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The first three modules in this "Coping with emergencies" guide deal with the circumstance where:
  • an immediate landing is forced upon the pilot in command because of engine/propeller failure or fuel starvation/exhaustion or carburettor icing
  • the aircraft remains under control, at least up to the initial impact with the terrain, trees or a water surface
  • all efforts are primarily directed to avoid/minimise injury to persons rather than trying to minimise damage to the aircraft or other property.

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'.


      Content



1.1 Know the best lift/drag ratio

L/D and the angle of attack
L/D ratio plotThe maximum L/D ratio (pronounced "L over D") for light aircraft — a measure of the aerodynamic efficiency — is usually between 6 and 12. However there is a very wide range; that for a powered 'chute is probably about 3 while some of the recreational aircraft designed with wide span, high aspect ratio wings — to provide soaring capability — have much higher maximum L/D. For example, the Alpin TST-3 motor glider achieves an L/D of 33 when the engine is stowed within the fuselage and can achieve a minimum sink rate of only 150 feet per minute. However, when the elastic breaks most powered recreational aircraft exhibit the flight characteristics of a very low-performance glider — or worse. (Surprisingly perhaps, most Boeing and Airbus jet transports have maximum L/D around 17–18; better than their piston-engined predecessors.)

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.
Forces in the glide
glide forces In a gliding descent, the forces are as shown in the diagram on the left. In the case of a constant-rate descent 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 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.

1.2 Know the best glide and minimum descent airspeeds

The aoa associated with maximum L/D decides the best engine-off glide speed (Vbg) according to the operating weight of the aircraft. There are two glide speeds that the pilot must know and, more importantly, to also be familiar with the aircraft attitude — in relation to the horizon — associated with those airspeeds, so that when the engine fails you can immediately assume (and continue to hold) the glide attitude without more than occasional reference to the ASI.

     •  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.
 
speed polar
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.

 speed polarTo 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.

1.3 Know the effect of a windmilling propeller

 Fig. 5.4.4   diagram: windmilling propellerThe 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.

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.

 Fig. 5.4.4   diagram: windmilling 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.


1.4 Know the practical glide ratio and terrain footprint

For accuracy you should measure (preferably by stop-watch and altimeter) the actual rate of sink achieved at Vbg with the throttle closed (engine idling), and from that you can calculate the practical glide ratio for your aircraft. The practical glide ratio is Vbg (in knots multiplied by 100 to convert to feet per minute) divided by the rate of sink (measured in fpm). For example, the glide ratio when Vbg is 60 knots and actual rate of sink is 750 fpm = 60 × 100/750 = 8; thus in still air that aircraft might glide for a straight line distance of 8000 feet for each 1000 feet of height.

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:
  • the practical glide ratio
  • the topography (e.g. limited directional choice within a valley)
  • the height above suitable landing areas
  • turbulence, eddies and downflow conditions
  • manoeuvring requirements
  • the average wind velocity between current height and the ground.
The footprint is shifted downwind; i.e. the into-wind radius of action will be reduced while the downwind radius will be increased. The wind velocity is going to have a greater effect on an aircraft whose Vbg is 45 knots than on another whose Vbg is 65 knots. Atmospheric turbulence, eddies and downflows will all contribute to loss of height. Rising air might reduce the rate of descent.

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.

120° footprint



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.

1.5 Know the height lost during manoeuvres

Any manoeuvring involved in changing direction(s) will lead to an increased loss of height and thus reduce the footprint. This reduction will be insignificant when high but may be highly significant when low. The increase in height loss during a gliding turn is, of course, dependent on the angle of bank used and the duration of the turn. Properly executed, gently banked turns that only change the heading 15° or so produce a small increase in rate of descent and a slight reduction in the margin between Vbg and stalling speed. Steeply banked turns through 210° will produce a significant increase in rate of descent, and a major reduction in the margin between Vbg and stalling speed. It is height loss per degree turned, rather than sink rate, which is important. So, you should be very aware of the height loss in 30°, 45° and 60° changes of heading because they are representative of the most likely turns executed at low levels.

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.
Steepening the final descent path
If it is necessary to steepen the descent path to make it into a clearing, it is recommended using full flaps and/or a full 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.


1.6 Know the height loss in a turn-back following engine failure

If the engine fails soon after take-off the conventional and long-proven wisdom is to, more or less, land straight ahead — provided that course of action is not going to affect others on the ground — for example, put you into a building. If the engine fails well into the climb-out one of the possible options is to turn back and land on the departure field. If the take-off and climb was into wind and a height of perhaps 1500 feet agl had been attained (and the rate of sink is significantly less than the rate of climb) then there would be every reason to turn back and land on that perfectly good airfield. There might be sufficient height to manoeuvre for a crosswind landing rather than a downwind landing.

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.

210° turn-backTurning 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.
Radius of turn and height loss
In a turn-back to land on the departure runway it is important to minimise both the distance the aircraft moves away from the extended line of the runway and the time spent in the turn. The slowest possible speed and the steepest possible bank angle will provide both the smallest radius and the fastest rate of turn. However, these advantages will be more than offset by the following:
  • When the steepest bank angle and slowest speed is applied, the necessary centripetal force for the turn is provided by the extra lift 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.

  • When turning, it is not L/D that determines glide performance but rather the ratio to the drag of the vertical component of lift [Lvc] that offsets the normal 1g weight, or Lvc /D. Thus, due to the increase in induced drag, Lvc /D will be less than normal L/D, resulting in an increase in the rate of sink and a steeper glide path. Lvc /D degrades as bank angle in the turn increases. See the diagram 'turn forces and bank angle' and read the text that follows it.

  • The stall speed increases with bank angle, or more correctly with wing loading; see wing loading in a turn. Thus the lowest possible flight speed increases as bank in a gliding turn increases.

  • Any mishandling or turbulence during turns at high bank angles and low speeds may result in a violent wing and nose drop, with substantial loss of height; see 'Safety brief: loss of control in low-level turns'.
Choosing the bank angle
In some faster aircraft it might be found that the turn-back requires a steep turn, entered at a safe airspeed (e.g. 1.2 × Vsturn), where the wings are slightly unloaded by allowing the nose to lower a little further throughout the turn. Then, having levelled the wings, convert any airspeed gained into saving altitude by holding back pressure until the airspeed again nears the target glide speed. The bank angle usually recommended is 45°, because at that angle the lift force generated by the wing is equally distributed between weight and centripetal force, although the Vsturn will be increased to about 1.2 × Vs1. Thus the safe airspeed would be 1.2 × 1.2 × Vs1 = 1.44 Vs1. (The speed 1.5 Vs1 is usually accepted as a 'safe speed near the ground' for gentle manoeuvres.) If the aircraft has a high wing loading, the sink rate in a steep turn may be excessive. Refer to 'turn forces and bank angle'.

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 |



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