The turn-back following engine failure
Mike Valentine
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The turn-back following engine failureMike Valentine |
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The following article, written by Mike Valentine, appeared in the June and July 2004 issues of Australian Ultralights.
One of the most persistent accidents in the operation of single-engined aircraft is the so-called turn-back manoeuvre. It is pretty topical at the moment and has received a bit of an airing in this magazine. Maybe its time to take a look at it in a bit more detail. We will do it in two parts, firstly the manoeuvre itself and then the pilot strategies to deal with it.
The turn-back is defined as a partial or full failure of the engine, followed by an attempt to turn back to the runway just used for the take-off instead of landing on whatever is available straight ahead, or within about 30 degrees each side of straight ahead.
Historically attempts to turn all the way back have resulted in a very low success rate; in fact the turn-back has acquired the reputation of being one of the “killer”manoeuvres, something that should never be attempted, no matter what.
What are the facts? Is the turn-back as dangerous as it is painted? Must it be ruled out at all costs or is there room for maybe attempting it under certain circumstances? Let us examine all the pertinent factors and then you, as pilots, can make up your own minds.
The starting point for this examination is the simple fact that an engine failure should not, of itself, cause a serious or fatal accident. In essence, the problem consists of maintaining the aircraft’s energy level following the partial or full loss of power, maintaining a safe speed no matter what other factors are present and keeping the aircraft under full control to enable a safe landing to be carried out. On the face of it, this should not be too difficult a task, but the facts seem to indicate otherwise.
The need to maintain a safe speed is paramount. So is avoidance of “loading up” (applying G loadings, in other words pulling back on the stick) the aircraft. These factors are not hard to understand, but the frustrating thing is that they often escape a pilot who, in spite of being only a matter of 300 or 400 feet above the ground and losing all power in a machine that may lose speed very rapidly, will attempt a 180+ degree turn back to the runway he has just left and unfortunately crash to his death without completing the manoeuvre.
There appears to be a missing link in our training. There is an old saying that there are four useless things in aviation; the altitude above you, the runway behind you , the fuel in the bowser and the last five seconds. It is the last five seconds we will examine closely here.
Actually, it’s probably a bit longer than five seconds, but the exact numbers don’t matter. The principle, however, matters very much indeed. Taking no particular aircraft as an example, but assuming a climb speed or about 1.3 to 1.5 times the stall speed, if complete failure of the engine occurs, the following sequence of events ensues:
As the Cadbury professor said “Why is this so”?
Let’s go back to the very instant the engine failed. The pilot realizes that something has happened, but needs time to work out what it is. This is one of the most surprising factors in occurrences of this kind. A simulator study carried out some years ago (Kentley, 1975) found that the average time from engine failure to brake application in an aborted take-off situation was 4.45 seconds. This is for a pilot who was under check and therefore fully expecting something to be thrown at him, although he didn’t know exactly what. Add an element of surprise and the delay will inevitably increase.
Then we have the aircraft’s inertia to consider. We often hear pilots talk about “delays” in airspeed indicating systems. If you have ever calibrated an airspeed indicator you will know that the time between changing the pressure in the air pump of the manometer and the change registering on the ASI is negligible – in the order of fractions of a second. In real life, if there is a delay in getting an airspeed indication following an engine failure in the climb, the delay is caused by the aircraft needing time to accelerate to its new (nose-down) attitude, not to any delay in the ASI. In other words, if the ASI says you are slow, YOU ARE SLOW. Period.
Following an engine failure, the aircraft describes a curved flight path as the pilot pushes over into the approach attitude. During this curved manoeuvre, everything is changing, pitch attitude, airspeed, G loading. When the approach attitude is eventually attained, the pushover is terminated and aircraft is apparently stable at its new attitude.
Apparently? Yes, apparently is the correct word because, although the aircraft is at the new attitude, it has just been though an energy-changing manoeuvre without the benefit of the engine to sustain it and its own inertia, aided and abetted by its drag, prevents it from attaining the new value of airspeed without allowing some time to pass for the aircraft to accelerate to its new speed. You just can’t accelerate an aircraft of several hundred kilograms mass, and robbed of its power, over a 10 to 15 degree pitch change and a similar speed increase without taking time. Any attempt to manoeuvre the aircraft without allowing the full measure of this time to elapse will risk loss of control.
This is where we get the five seconds referred to earlier. From the time the engine fails to the time the energy level (=speed) of the aircraft is safe enough to carry out a manoeuvre, at least five seconds have elapsed. See diagram below.
This pattern has been tried in flight by the author and was measured with one pilot flying the aircraft and the other pilot observing and starting/stopping a stopwatch. The exercise was tried in a Grob 103 Twin 2 glider (590 kg, 35 knot stall, 35:1 glide ratio) in a winch-launch failure situation and a Skyfox Gazelle ultralight trainer (520 kg, 43 knot stall, about 9:1 glide ratio) in a simulated engine failure after take-off. Despite the difference in characteristics between the two aircraft, the results were remarkably similar and deliver a clear message. Inertia and drag, especially inertia due to the aircraft’s mass, are powerful forces and must be recognised and respected. Keep in mind too that the diagram doesn’t take into account the 4.45 seconds we discussed earlier to realize that some kind of failure had occurred!
We can now see the nature of the beast we are trying to tame when the engine quits. We can sum it up so far as follows:
We have covered the human reaction time to any abnormal event and went on to discuss the recovery problems from an engine failure due to aircraft inertia. We will now look at energy management, pilot actions and a subtle trap which might be described as a sharp little sting in the tail.
The total energy stored in an engineless aircraft is the sum of two component parts:
These two sources of energy can be interchanged (up to a point). Indeed they must be interchanged when an engine failure occurs after take-off. Height must be traded for speed – in other words the nose must be lowered – if the total energy level of the aircraft is to be maintained. If the nose is not lowered sufficiently, and if sufficient time is not allowed for the speed to increase and stabilize, it may not be possible to control the aircraft to a safe landing.
Let’s take an example of a high-drag aircraft of relatively low mass, such as a Thruster or a Drifter. Both these aircraft have thick high-lift wings which give a good climb rate at about 50 knots. The trouble is that, should the engine fail at under about 150 feet AGL, taking into account the factors covered earlier, it is unlikely that the pilot will be able to get enough speed in the available height to enable the aircraft to be “flared” for a normal landing. A heavy landing, possibly a very heavy landing, is almost inevitable. The theorists would tell us that 150 feet gives insufficient potential energy to allow a pilot to attain the required level of kinetic energy (speed) to level the aircraft off near the ground and achieve a smooth landing. From 150 feet, there just isn’t enough room to get the required speed.
What should we do? Well, not much can be done about the height — you can’t make height if you haven’t got it! What you can do is choose not to climb at 50 knots and instead climb the aircraft a bit faster, say 55 knots. The extra 5 knots may make all the difference if the engine spits the dummy at a critical height and the loss of climb performance caused by increasing the climb speed by 5 knots is relatively unimportant.
The glider people learned the hard way that careful energy management at the start of a winch-launch is the only way to protect a pilot against a low-level launch failure such as a cable-break. That is why winch-launch climbs are graded, from quite shallow in the first 150 to 200 feet, smoothly steepening to the full climb of about 45 degrees at 1.3 to 1.5 times the stall speed above that height. Exactly the same principle applies to managing the total energy in an ultralight take-off.
Try this little jingle. “Am I high, am I low, am I fast, am I slow?” It can be applied equally well to a climb after take-off or an approach to a landing. In the climb situation, the combinations look like this:
Refreshing our memories about the speed trend diagram above, the pilot’s first action following loss of power is to apply forward stick to start the pitch-down manoeuvre. The next thing to do is wait for the speed to catch up with the attitude, whereupon the aircraft will be fully controllable and you can do what you want.
That deals with the mental and physical actions needed to cope with take-off engine failures at the actual time they occur. However there is also a preparation element. What pilots need to do before they ever open the throttle for take-off is to run a little failure plan through their minds. Mental rehearsal is a powerful teaching tool and it works just as well for an ultralight pilot preparing for take-off as it does for an aerobatic ace mentally rehearsing a complex sequence on the ground before performing it in the air. Something along the lines of “On this runway, in this aircraft, in these weather conditions, where do I expect the aircraft to be airborne? What do I do if it isn’t? If I get into the air OK, what are my prospects for engine failure? What is likely to be the last point in the take-off path that I can suffer an engine failure and still land ahead on the available runway? Beyond that point, what are my options?” And so on, depending on individual circumstances on each and every flight. It only takes a few seconds and it is time well spent.
When an engine fails after take-off, there is a strong psychological urge to try to get back to a landing area that is not only familiar to you, but also of known quality. The reasoning seems to be “Why put down in a paddock which might contain a variety of unpleasant surprises when I can return to my familiar airfield which I know I can land on because I have done it hundreds of times”? Attempting to return to the airfield is therefore a very human and entirely understandable reaction to an engine failure after take-off. Unfortunately it is almost always the wrong decision at low-level and it simply has to be trained out of a pilot. This is what all those tedious practice engine-failures are all about – instructors are keen to develop a “conditioned response” which will take over under the stress of an engine failure and overcome the “return to the womb” tendency lurking just beneath the surface of a pilot’s thinking and which is quite likely to kill him/her if allowed to dominate.
Nobody is suggesting that it is easy to persuade a pilot to forsake a known safe landing area and opt for an unknown quantity, but the effort must be made.
As mentioned earlier, turnback accidents occur from time to time in circumstances where there was so much runway available after the engine failure that the pilots never needed to turn back at all. Either that, or the runway itself ended, but there was a quite satisfactory over-run area beyond the end of the strip, which could have easily accommodated an aircraft gliding to a landing after an engine failure. What persuaded these pilots to turn back in these circumstances is a mystery. Perhaps they didn't have a contingency plan for an engine failure.
This is another trap for the unwary, born of an urge to subconsciously squeeze the stick back in an imagined attempt to keep the aircraft in the air a little bit longer and thereby stretch the glide to attain a landing area which is just out of range. It is dangerous enough on its own, in straight and level flight. In a turn, it is deadly. Get an instructor to demonstrate why, at a safe altitude.
Going back to when we first realised the engine had failed, we smartly moved the stick forward to start the recovery manoeuvre. When we did so, we probably felt ourselves getting a bit light on the seat, as a small increment of negative g* was applied to the aircraft. This is quite normal in this kind of manoeuvre, as anyone who has experienced a winch-launch cable break in a glider and watched the biros and half-eaten Mars bars floating around the cockpit will attest.
If we feel lighter, so does the entire aircraft. If the effective weight of the aircraft is less than it normally is, even by only a small amount (and we really are talking only small amounts here), then its stalling speed is reduced. Taken to its logical conclusion, if the pilot pushes hard enough on the stick to reduce the aircraft to a weightless condition, the wing has no weight to support and it therefore cannot stall.
Such a flight condition is highly transient, lasting only a second or two (the top portion of the arc in the diagram above). This is where we come to the reason for bringing up this point. As we reach the apex of the curved flight path, if we decide to manoeuvre the aircraft to turn back to the airfield, the aircraft responds perfectly well to our control inputs, because the controls are functioning quite normally and the aircraft behaves itself because it is protected by the artificially reduced stalling speed during the manoeuvre.
It does not take long for payback time to arrive. When the pushover manoeuvre ceases and the aircraft settles back to its normal 1g state, the wing will promptly stall and control of the aircraft will be lost.
In case anyone thinks this is a pretty exotic argument and unlikely to occur in practice, I would suggest otherwise. I was the reluctant witness of a glider fatal accident some years ago, where the pilot did exactly what is described here. The winch cable broke at about 400 ft and the glider started its turn, apparently under full control, then rolled over into a spin and impacted vertically. The pilot had 1000 metres of strip remaining ahead of him and never needed to turn at all.
Turn back or no turn back, energy management is the key to survival in low-level engine failures. If you are low and slow, you are playing with fire. Oh, and don't forget to plan ahead. Engine failures are not a matter of "if", but "when".
Read the Coping with Emergencies Guide.
Mike Valentine,
Operations Manager
* "g" is the load factor on the aircraft, 1g is the normal state of an aircraft in straight and level flight. Any increase of g loading, for example pulling back on the stick at high speed, makes the pilot (and aircraft) feel heavier. At 2g the pilot and aircraft are effectively twice their normal weights, and so on for 3g, 4g etc. Negative g is the opposite, the stick being pushed forward and the pilot and aircraft becoming effectively lighter. Zero g is weightlessness and pushing the stick further forward will have the pilot floating off the seat, hopefully restrained from departing the aircraft by the harness!
Copyright © 2004 Mike Valentine
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