Gremline Flight Safety Report: Forced Landings in Light Aircraft - The Constant Aspect Approach

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the gremline digest — the constant aspect approach

Forced Landings in Light Aircraft — The ‘Constant Aspect’ Approach

If you have arrived here from the preceding article on forced landings in light aircraft, carry on reading. If not, we suggest that that you study our previous piece before proceeding. Click Here.


Introduction
The old method of performing a forced landing after engine failure was to attempt to fly a square circuit around your chosen field, with “key” heights to be achieved at the beginning of the downwind leg and another “key” height at the start of the base leg. How do you accurately judge these heights above ground of uncertain height? If you have a radar altimeter then you can get some idea, otherwise you are guessing and “key” heights are pretty meaningless. Why fly a square circuit at all? It demands precise decisions and a modicum of luck to get the turning points exactly right and your luck today isn’t too good because you’ve already had an engine failure.
      There is a much easier way to fly a successful forced landing pattern without having to judge any “key” heights or points that will be affected by wind speed and direction. This simple method is THE CONSTANT ASPECT APPROACH.

 


The Initial Aiming Point (IAP)
It may be worth reminding pilots that the maximum gliding range for your aircraft at its present weight in the present conditions is FIXED. Believe me, if you are flying at your best gliding speed it’s no good raising the nose on the approach in the hope of stretching the glide. The wreckage may bounce over the near hedge, but you certainly will not fly over it. First we’ll define the INITIAL AIMING POINT (IAP). This is NOT your planned touchdown point in your selected field. It is a datum within the landing area that you select to allow you to adjust your circuit and approach. It is a point that you fix your eyes and brain on while you are getting closer to your chosen field. The IAP is the pivot on which you fly your whole forced landing pattern, certainly from the downwind position to the wings-level final approach. A guide to the selection of your IAP is to choose a point that is about one third of the available landing run from the planned touchdown point. Your planned touchdown point should be about 100 yards from the near field boundary, depending on the size of field selected.
      The actual dimensions of the forced landing pattern will vary from aircraft type to aircraft type. You don’t have to worry about this as the performance of the aircraft will automatically determine the size of the pattern flown. The first diagram, which I happen to have available, illustrates the pattern followed by a RAF “Bulldog” flying a constant aspect approach style forced landing pattern. The diagram is copied from the RAF “Bulldog” Instructors’ Manual, courtesy of the Central Flying School, Royal Air Force.

The diagram is self-explanatory if you study it step by step but perhaps it needs to be emphasised that the IAP is held closer to the aircraft fuselage than to the wingtip throughout the pattern. It is also worth noting that there is no mention of aircraft altitude after leaving the “high key” point at the beginning of the exercise. The “high key” position is purely a training aid for use during PRACTICE forced landings and is not an essential part of the pattern for a real forced landing. The pattern is best visualised as flying the aircraft around the surface of an irregular cone which has its apex at the IAP and its angle determined by the aircraft’s gliding angle. One may join this cone at any position and height relative to the IAP and, if necessary, fly several orbits while losing height – or less than one orbit if short of altitude. There is no rigid pattern over the ground. This varies automatically to take account of wind velocity (Velocity = speed and direction) and rate of descent, and may be a continuous gentle curve. The angle of the cone is determined by the Sight Line Angle, as defined below.

 


The Sight Line Angle (SLA)
This is the nub of the whole technique. If you practice this technique, and refresh yourself regularly, you will amaze yourself just how easy it is to fly an accurate, controlled and safe forced landing pattern in any wind speed. This technique corrects automatically for variations in surface wind so you will arrive at the planned touchdown point without having to juggle with any numbers or to make last second corrections. Magic? Read on.

      The concept of Sight Line Angle (SLA) needs a diagram to make it absolutely clear, so here is my second little picture.

The Sight Line Angle is the angle below the horizon of the INITIAL AIMING POINT. It doesn’t matter what size this angle is in degrees or radians or anything else. You select your IAP while gliding abeam your chosen field, or sooner if possible, and note the angle subtended below the horizon. If this SLA begins to INCREASE then you are OVERSHOOTING your IAP. If the SLA begins to DECREASE then you are UNDERSHOOTING your IAP. Think about the concept for a while and get the idea fixed in your mind. In fact, you already knew this, but may not have consciously recognised the fact. If the SLA is increasing then you are closing in on your IAP. If the SLA is decreasing then you are drifting away from your IAP. That’s all there is to think about.
      There is no fixed, ideal SLA for one particular aircraft. It will vary from day to day depending on variables such as the wind vector. If it sounds a bit complex don’t worry about it. Just fly the pattern accurately at the best gliding speed, holding a constant SLA and you’ll arrive at the chosen spot at the correct speed, still the right way up.
      All you have to do it to look at the SLA and hold it constant throughout the downwind leg and the turn onto final. FLY THE AIRCRAFT. If the SLA seems to be decreasing then turn gently towards the IAP. If the SLA seems to be increasing then slacken the turn until it corrects itself. Do not change the IAP throughout the pattern. Don’t be tempted to change your concentration from the IAP to your still hoped for touchdown point. Continue to control the SLA all the way downwind, around the continuous base curve and onto the final approach. Continue to fly the aircraft at the best glide speed for the configuration. If your aircraft calls for 68 knots then stick to that speed. If it needs 72 knots then that’s what it needs. Near enough is not good enough in this case. You must FLY THE AIRCRAFT. Do you know how weight affects your best gliding speed? Do you KNOW what YOUR aircraft weighs on each and every flight?

 


The Final Approach
So there you are, lined up on your chosen field, concentrating on the IAP and keeping the SLA as steady as a rock while flying the aircraft as accurately as you can manage. If you have flown the IAP and the SLA accurately you will find your aircraft rolling out neatly on the final approach at about 300 feet agl. Having concentrated so hard on the IAP you can now forget it. FLY THE AIRCRAFT. Adjust the Touchdown Point towards you from the IAP and concentrate on the touchdown point. Use drag flaps and/or sideslip to steepen your approach if this is necessary to achieve your touchdown point. It should be almost impossible to be undershooting your planned touchdown point but it may appear that, initially, you are overshooting the touchdown point. Ease the nose down as the flaps take effect, control the speed and retrim in pitch. If necessary INCREASE you airspeed by a few knots by lowering the nose slightly. Now all you need to concentrate on is your AIRSPEED and the touchdown point. Do not let the speed decay on the final approach.


The Touchdown

Your touchdown point will have been chosen to give you at least a few yards inside the near hedge so it doesn’t matter if you undershoot or overshoot by a couple of yards. Do not grope for the ground. Rotate the aircraft into the landing attitude and then land. You may notice a slightly different feel on the rudder and elevator because of the lack of prop wash over the controls. Your airspeed should be right for landing so put it on the ground in the correct attitude for your type. No nosewheel type takes kindly to landing nosewheel first and the only way you can do that is to be either too fast or fly into the ground without any flare. “Wind shear” seems to be the currently favoured excuse for bad landings. Bad landings result from bad approaches flown by careless pilots. Once you are on the ground apply maximum safe braking. The technique varies from type to type, but probably involves holding the elevator control well back as the brakes are applied. Locked wheels offer less retardation than braked rotating wheels, so try not to lock the wheels. Beware of harsh braking on wet grass. Don’t forget to steer the aircraft until it comes to a full stop  - or the far hedge! If you are going to hit the far hedge, have a go for a gateway or the gap between two trees. It’ll make a great picture to hang in the Flying Club.
      Make the aircraft safe and do not relax until everyone is safely out of the aircraft. Finally tell somebody that you are on the ground and try to tell them where you are.
      There, that was easy, wasn’t it?

 


Conclusion
No, it wasn’t easy. It needs regular practice, but the more you practice the easier it gets. You may not grasp the beauty of this technique until you have tried it several times. Don’t be put off by your first attempt. No forced landing is ever going to be easy, but most should be survivable. To stall and spin because you forgot to control the aircraft is a silly way to kill yourself and your passengers.
      There are many things I have left out of this article because I have tried to make it as non-specific to type as possible. Persuade your instructors to try this system if they do not already teach it and I believe everyone will soon be more confident and proficient at forced landings. It may make practice forced landings even easier if you can persuade air traffic that you really need to paint a big white spot on their airfield to mark the IAP for early PFL training.
      I have used the constant aspect approach technique after engine failure for real in very different aircraft. One required an approach speed of 50 knots and another required a minimum altitude of 6000 feet abeam the IAP and a minimum of 180 knots on final with the roundout beginning at 400 feet agl. The constant aspect approach technique worked for both aircraft types. That’s all one can ask of any technique. I have also used the same technique on over 700 landings in gliders, but all glider pilots use a constant aspect approach for every landing without even thinking about it. The Space Shuttle re-enters the atmosphere at Mach 24 and the pilots then fly a dead-stick constant aspect approach to touchdown.


My thanks to the Commandant, Central Flying School, Royal Air Force for allowing me to fly one of their aircraft to refresh this technique, and to members of CFS Examining Wing for valuable discussion on the flight safety aspects of flying training standards and operations. However, I could have done without the comment from one member of Examining Wing who said, “Do you know, you graduated from here before I was born.” The “Trappers” still know how to make you feel good!

 

 

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