Mastering the forced landing
12:13 17 July 2015
How do they do that? A technique that will take the mystery out of flying glide circuits
During training everyone is taught how to make a forced landing following an engine failure and it is surprising to find how little consistency there is in the way it is taught. Of course a method that is suitable for a Tiger Moth is not going to work in Hunter or a Hawk, but the various schemes for similar aircraft seem somehow to be lacking in a basic principle that can be easily understood by a beginner. It is my experience that students soon acquire a standard of competence in the ritual intonation of the checks and Mayday call which sadly deserts them when it comes to touching down on the right side of the boundary hedge − the bit that really matters.
There are, however, some lucky people who seem to be naturally much better at ending up in the right place than others. They can’t tell you exactly what they are doing but they appear to have discovered some way to extract the necessary information directly from the scene. Could it be that there is something going on out there that they notice and others miss?
Stripped down to fundamentals, the flying side of a practice forced landing (PFL) comes down to this:
Where am I?
Where ought I be?
How do I correct any errors?
Sitting in the average cockpit, most of the pilot’s visual field is filled with aircraft but it is what lies out of the window that is going to provide the information needed to answer these three vital questions.
The human visual system has evolved to be acutely sensitive to visual flow, and here comes a point on final when it is suddenly glaringly obvious where the glide is going to end up. By way of contrast, we have all noticed that an airliner at cruise height appears to be suspended in space.
Unfortunately for pilots, evolution did not anticipate the aviation environment, so some instinctive capabilities which work fine at sea level begin to drop out above a thousand feet or so. When we look down at our chosen strip from this kind of altitude, it is not at all easy to see whether we are allowing the right amount of height to get in. Because of this difference in visual capability with height, the forced landing pattern is inevitably going to be divided into two sections. At the start we will be getting virtually no help from our instinctive system, so we have to work out an alternative to get us to the point when it starts to come on line.
When we are flying along at a couple of thousand feet, our main source of information comes from the apparent distance objects sit below the horizon. Anything on the earth’s surface that is up near the horizon is ‘far’ while the bits underneath the aircraft are ‘near’. When we are in a steady glide we will not reach the farthest places up near the horizon and we will over-fly the near ones. Between these two extremes is the place where our glide is going to end up. Ground features nearer than this position move down in the visual field, to pass eventually beneath the aircraft, and those further away move upwards towards the horizon – but the ‘zero-patch’ itself will stay where it is in relation to the horizon, simply getting bigger as the range closes. At altitude, the drift of nearby objects down the side of the aircraft is easily visible but as we move our line of sight up towards the horizon we will find this movement gets harder and harder to detect and there will be quite a big segment where we can’t really be sure whether it is moving or not.
In a steady glide the aircraft’s nose will be fixed at a constant visual angle down from the horizon, so the relationship between it and the zero-patch will be a constant. Finding the zero-patch at height is a bit fiddly, so once we have learned where it is relative to the nose or some other convenient part of the aircraft structure at lower level, we can use this to locate its approximate position. For example, the zero-patch for calm conditions from the back seat in a Tiger Moth lines up with the point where the front windscreen joins the fuselage; in a Cessna 152 it is just above the nose. There will, of course, be equivalent markers for the various glide angles associated with flap setting and strong head winds.
This zero-patch provides some extremely useful information. It shows us the farthest place we can reach in the current conditions. It automatically allows for wind and flap effects, even incorrect gliding speeds. If we have just trimmed into the glide following an engine failure it shows us how far down from the horizon any prospective landing field must be if we are to reach it. Of course, in practice, we need to choose a landing field a good bit nearer than this because we need to arrive there with enough height in hand to manoeuvre onto a good final approach. This is all very well, but being able to see where the aircraft is going to touch down once we straighten up on final is not much help when we are circling the field wondering how to get there. Fear not, the zero-point has more useful tricks up its sleeve.
The Conical Helix
Imagine that we are gliding about halfway along the downwind leg with the runway threshold lying somewhere between the wingtip and the nose. If we keep flying parallel to the runway the touchdown will move towards the wing until it is abeam. Now suppose that instead we start a gentle turn towards the touchdown such that its angle off the nose remains constant. If it starts to drift towards the wing we turn a bit tighter. If it starts drifting towards the nose we slacken the turn. If it was half way down the right-hand edge of the lefthand side-window when we started then we adjust the turn to keep it there.
What we are doing is putting the aircraft on a conical helix which has the useful property that if we keep the horizontal angle to the touchdown point constant the angle of depression will also be constant. If the runway threshold was depressed fifteen degrees down from the horizon at the start it will remain at that value as we spiral down towards it. If we kept this up long enough we would finally arrive on the landing field in a tightening steep turn. To put some numbers on this, we find that, for still air, in an aircraft with a 9:1 glide angle, such as a Cessna 152 gliding at 60kt, a spiral track that keeps the horizontal angle at 45º off the nose will also keep the angle of depression of the zero-point from the horizon at around 10º. If a greater angle is chosen, say 65º off the nose, then the angle of depression increases to 15º. This spiral is illustrated in Figure 1 (p.61). Note that the aircraft’s glide angle doesn’t change. We are always getting nearer to the touchdown but instead of going straight at it we are taking the long way round.
Of course the thing we are really interested in is the depression angle because that is the one that tells us whether we are still within gliding distance−but it is harder to judge than the horizontal angle so, because the two are linked together in this way, we find it easier to use the former. Note that the aircraft’s ground track is not specified. Where it goes will depend entirely on where we first join the spiral. There will be different spirals for different angles of depression and the wind effect will distort the ground track.
Putting the spiral to work
This is pretty useful stuff and can assist us in flying a PFL. It does, however, have to be used intelligently as is not quite the silver bullet it might at first appear. For a start, although the method compensates for wind and flap effects, we need to take care that we select a depression angle direct to the touchdown well in excess of the gliding angle for the current conditions. A rough guide is the glide angle for full flap into the current wind plus a couple of degrees for the wife and kids.
It is obvious that we wouldn’t want to keep on riding an ever-tightening spiral right down to the deck. Somewhere around five hundred feet we want to stop circling the touchdown and settle onto a final straight-in approach. It is important to appreciate that where we end up pointing on final depends entirely where we were when we joined the spiral. If, for example, there was a strong wind blowing from the north-west in the situation shown in Figure 1 this particular spiral would end up with the aircraft landing downwind. So we can see that it is vital that the spiral is started in approximately the right relationship to the landing direction.
Figure 2 shows how the slack early section of the spiral can be used to arrive abeam the touchdown in a position from which a safe base leg can be flown. Let us suppose that we want to intercept the spiral from the overhead. We fly towards the start of a standard downwind leg until the depression angle of the touchdown point reduces to some predetermined value, in this case 15º. We can assess this either by judging the distance down from the horizon or when the touchdown point is in line with some known point on the aircraft structure. For example the point where the outer struts meet the lower wing on a Tiger.
At this point we will have intercepted a spiral that runs roughly along the downwind leg. By turning onto it and adjusting the curve so that the horizontal angle relative to the aircraft stays constant, we guarantee that the angle of depression will also remain constant until the aircraft reaches the check point abeam the touchdown. The angle of 15º has been chosen because, for a standard trainer like the Cessna 152, it puts the aircraft abeam the touchdown point in position that can cope with the full range of wind conditions.
We would like to be on final with the touchdown somewhere between the cleanglide and full-flap angle, say 9º for calm conditions and around 12º for a strong wind. This means that our clean glide will take us well into the field and our full flap glide can be used to bring the touchdown point towards the near hedge once we are sure of getting in. Figure 3 shows how we can use the flat section of the spirals associated with these two depression angles to fly a base leg that guarantees we intercept the centreline at the right height.
The two limit cases are shown: nil wind and 30kt. When we reach the universal starting position abeam the touchdown we abandon the first spiral. If the wind is light we fly straight downwind while the angle of depression reduces. When it reaches the new angle we turn onto the spiral shown in green.
When the wind is strong we do not delay at all but turn onto the base leg immediately. On the first part of the base leg we can use coarser turns towards or away from the field to correct any errors in the angle of the touchdown point. We then hold it constant by flying a gentle curve along the spiral. When we reach the extended centreline we leave the spiral and commence the final descent with an angle to the field that guarantees it is well within gliding distance.
The last two figures show the above scheme related to a standard circuit. Figure 4 shows the nil-wind cases for two different starting heights. In both cases when the aircraft reaches the check line abeam the touchdown it leaves the 15° spiral path, straightens up and carries on for an amount that is usually judged as being about a wing chord’s worth. The green path shows that this delay has taken the aircraft onto the 9º spiral which it follows until it reaches the extended centre line. The yellow track shows the aircraft starting off from a lower height. In order to illustrate how the adjustment is made, the delay here has been deliberately made too short, which has put the base leg initially too close. The depression angle of 11º is too much and the turn away reduces it. By 500ft the pilot is just coming onto the spiral that will hold the desired value of 9º till the turn onto final. The desired window position is the same for both the near and far spirals.
Figure 5 shows the 30kt headwind case. Note that the height spots are much further apart on the downwind spiral because of the tail wind. Despite the turn starting immediately, the aircraft has been blown a considerable distance downwind. Because the turn is taking the aircraft nearer to the touchdown the depression angle increases but even so it is rather too low for the extreme conditions. This is corrected by tracking inside the spiral track which further increases the depression angle.
Halfway along base leg the angle is satisfactory but the track is still a little inside the theoretical curve, giving a temporary increase in height. Because of the into wind lay-off for drift, the touchdown during the crosswind leg will appear very near the nose horizontally making it easier to see whether it is drifting away or not.
The wisdom of carrying some extra height is clear when the aircraft turns into the wind and ends up making only thirty knots over the ground. Despite being high and close it will be another couple of hundred metres before the full-flap glide lies within the near boundary.
We are now in a position to offer an explanation as to how people who are intuitively good at glide circuits might be managing it. The ‘natural’ method for learning new skills is to ‘suck it and see’ and the human unconscious is extremely good at latching onto actions that produce success. People who have a natural flair for PFLs probably noticed that they could keep the touchdown point in a constant position relative to the aircraft by ‘playing’ the rate of turn. After a few attempts they began to see a relationship between that position and the steepness of the final approach path. Where the standard pupil was intellectually trying to work out some ground key points and relate them to the altitude these folk were concentrating exclusively on the way the touchdown point was behaving in relation to the aircraft structure. From the point of view of their intuitive systems it was as though someone had drawn reference circles on the windscreen and side windows to indicate these key points but, because it all takes place below the conscious threshold, it appears to them and to others as some sort of magic.
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