I can imagine a lot of readers being puzzled about the choice of this subject. What possible interest could there be, that you do not know about, regarding something so simple?
Perhaps there is quite a bit and there are some parts of what follows that may well give instructors cause to reflect upon what they actually do - and why! We will start by saying that Stability is the single most important exercise in flying training upon which every other exercise is based in some form or another - and that Lateral Damping (which is the main part of Stability in roll) is a vital factor in understanding low speed loss of control, the single biggest killer in light aviation!
However, Stability does not appear in the mandatory flying training syllabus for RAAus and GA at all - leaving a high probability that all you know about it is what you have read and understood from your text books. So let us take a look at what you may have missed out on and may still be of great importance to you even though you may be now far from basic training and have experience.
If Stability is taught at all it usually indicates that the 'aircraft is stable' by virtue of the fin preventing the aircraft from indiscriminately yawing; the tailplane prevents the aircraft from casual pitching movements and one (or a combination) of sweep-back, dihedral and pendulous Stability design arrangements prevent the aircraft rolling all over the place of it's own accord.
Perhaps Stability is overlooked, or minimized, because most of it is built into the aircraft by the designer and it is not something the pilot appears to have a great deal of control over. Unfortunately students are therefore robbed of a great deal of mental comfort and understanding of what they are actually doing, and instructors unknowingly discard a powerful and vital tool in shaping their student's psychology - as well as leave them vulnerable to a very real danger that does repeatedly kill!
I will take a look as some of the psychological platform, but initially let us look at Stability itself to get us going.
Taildragger pilots in particular are well aware of the power of weathercock stability (the tendency of the aircraft to point into a cross wind and come off the runway) also they soon get a grip of keeping ground contact and centre of gravity couples well aligned or find themselves ground looping all over the place! But very few pilots ever see exactly how powerful Stability actually is!
My first real look came after many years of flying (including teaching Stability as a matter of course) when I had the opportunity to take a float/amphibian endorsement. That was a real eye opener! In only one or two knots of wind (nearly dead calm) as soon as the aircraft was unmoored it immediately pointed straight into wind and there was no means of stopping it doing so. Get it moving and it is impossible to get it to go any further out of wind than around 30û without getting up to all sorts of taxiing antics most of which revolve around juggling about with the Centre of Buoyancy to give stability a different pivot point to act around so you have normal choice of where you go.
For myself, stability becomes a powerful psychological teaching platform that I in fact rest all of my following instruction upon. Whilst I in no way ever wanted to destroy the achievement and satisfaction of a student becoming a pilot, from the outset I did want to create a scene where any of them felt that they easily could master the machine because most of the apparent 'work' was being done for them automatically! I used stability to do this.
The key factor is that the student is not learning to fly at all (that is just a convenient expression) they are learning to control and operate a machine that is quite capable of flying all by itself. That is all an aircraft is - a flying machine. If sufficient airflow is present around the airframe then the aircraft has no choice but to fly and keep flying as long as that airflow is maintained. With that airflow present the aircraft design ensures that Stability works and maintains the aircraft's flight attitude.
It does not even matter if the engine is not going. Stability will automatically align the aircraft correctly with the airflow. Sure it is now descending, but it does not abruptly stop flying and come out of the sky like a sack of cement! The controls work normally etc. - ask any glider pilot - gliders do not have engines at all and yet the world distance record in a sailplane is over 2,000 km and 300 km trips are now merely afternoon jaunts!
Becoming a pilot is not fundamentally different from learning to drive a car, tractor or piece of plant equipment. There is no black magic about the situation and pilots are not superhuman but just ordinary people! You do not have to have huge strength and flap your arms; you do not have to pedal the thing to keep it going like a push bike. All you really have to do is sit in it!
So for the sake of a 30 minute very simple lecture, plus 10 minutes of in-air demonstration, the instructor will be able to begin establishing in the student a firm confidence base whilst demystifying flying itself.
The student clearly sees that their task is not to 'fly', it is to control! Control is the means of making the aircraft go where you want it to while the task of Stability is to make the aircraft continue going in the direction it is pointing. So we use the controls to overcome Stability.
This is no more difficult than establishing that (for example) the fin provides stability in yaw. This is evidenced by everyday examples of a weathercocks on churches or wind vanes on windmills. The student can even walk outside and take a peer at the airfield windsock!
The rudder is the hinged bit on the back of the fin and is used to modify or overcome the Stability of the fin. The relationship between Stability and control is easily and clearly established. We can go a simple step forward and show that control may be used either positively (to make something happen) or negatively (to prevent something from happening). You can use the rudder to make the aircraft yaw, or you can use it to prevent yaw (e.g. counter aileron drag).
That is important enough, but there is now far more suddenly opening up! An entirely new approach to the process of learning to become a pilot becomes available to student and instructor alike.
This is critically important in Recreational Aviation where we may have a very variable education and comprehension ability level in our students from everyday life - they are not would-be career pilots who automatically have to be (normally) university trained and in near perfect health. For the recreational pilot there is a very real need to keep things simple, conversational and put technical subjects into easily understandable everyday terms. Formally teaching stability is a great aid in achieving this.
As a handful of examples out of dozens: Stability is not just a flying training foundation stone, it is also the cement that binds together the 'building blocks' of individual exercises; it thus gives the instructor access to concepts such as 'Exercise Interlock'; the behaviour of the aircraft in the stall and spin is more readily comprehended; what is happening (and why) in crosswind landings and take-offs starts to make more sense; tendencies to over-control by trying too hard can be eased by the student actually using Stability to help.
There is also an application in judgmental training exercises! Growing manipulative skills usually abruptly deteriorate sharply when the student comes under the new mental pressure. At times such as this just a friendly word across the cockpit of "Hey, stop stirring it up. Just let Stability fly it for you while you look out the window at that angle we want!", Does the trick and calms the self-generated work load.
A lot of this does not happen. The instructor was never taught that way or taught to teach that way, so what was good enough for them is good enough for their students. They forget that they are actual survivors of a system that, world-wide, chews up and defeats thousands of students each year who give up flying for good because the learning process is 'too hard', or they are frightened because they just do not understand what is going on and how things are really working!
But there are two most important factors to using Stability as a base exercise than as an active part of teaching throughout the rest of the flying training syllabus. These both reside in Lateral Damping, that is a main part of Stability itself.
Lateral Damping is normally not taught, it may never be mentioned, but by employing it the instructor can build a firm bridge between theory and practical instruction in a manner where the student is appreciating and using angle of attack and airspeed as an actual part of their flying skills, rather than the usual 'go read about that and I will examine your knowledge later, meantime I will get on and teach you to fly!'
At the same time a firm knowledge of what Lateral Damping is provides the most sure known defence against low speed loss of control that leads to low level spinning and spiral dives! I have a deal to say about the subject but let us see what it is first.
WHAT IS LATERAL DAMPING?
Earlier in this article I stated that Stability in roll (if it is covered at all) is via dihedral etc. The situation is clearer for the student if you express stability in roll as being effected by the entire wing, in comparison with the fin and rudder which are rather small components doing their own specific tasks.
If the aircraft is in level flight and is disturbed by a gust, causing it to roll, then you have one wing going down and one wing going up. Both wings are still moving forward at the same airspeed but the down-going wing is meeting it's own relative airflow from below as well. Expressed differently - it has increased angle of attack as far as the wing and the local air are concerned. More angle of attack = more lift so the wing automatically rises. If it goes higher than the other wing then that is now the down-going wing and the same thing happens.
This is Lateral Damping and is the effect of the aircraft automatically maintaining or regaining it's wing attitude (whether level or banked) quite automatically if the wing is disturbed.
That is all well and good but there is a wrinkle! The aircraft, for certification, has to be positively stable in yaw, roll and pitch. Of the three roll is normally the weakest and Lateral Damping will only work up to modest bank angles. Beyond that we begin to enter what may be termed 'spiral instability'.
In this situation a vector is set up between the inclined lift force and the weight force that pulls the aircraft sideways and downwards slightly into a slip. Yaw stability (normally the strongest) gets all offended and re-aligns the fuselage with the airflow and yaws the aircraft towards the down going wing.
The yaw increases the airspeed on the upper wing. More airspeed = more lift so the bank angle increases and the same process repeats. This, my friends is the entry into the 'Dreaded Spiral Dive'.
Personally I feel that the almost psychotic alarm people have to a spiral dive is a load of crud! Spiral instability will not normally begin happening at the angles of bank we use in circuit and general flying work and this is well within the capabilities of Lateral Damping to look after, or the controls themselves if you choose to fly in turbulence. All you have to damn well do is begin reducing bank and lifting the nose and you have stopped the situation in its tracks! Nothing alarming or difficult about that!
There is not normally anything abrupt. The aircraft progresses to the condition and if you ever wind up in a steep nose down, high G and high bank spiral with the airspeed increasing rapidly then you have either lost it in cloud (where you should most definitely not have been), were asleep, or concentrating too much on doing something naughty with your passenger!
But you can in fact be put abruptly into the start of a very nasty spiral situation very abruptly if you allow your situation to progress to a point where you 'shed lateral damping' and lose that Stability influence! At that point things can happen very quickly indeed and if you are near the ground then they are going to kill you and anyone with you!
You are a bit poorly placed to identify this may be about to happen if you do not know what Lateral Damping is to start with! The abruptness of the process is actually that alarming it is then sufficient to totally unnerve you and slow you right down in responding.
What is more amazing is that despite this situation killing several pilots each year, and spinning no longer being taught (other than in the gliding world), there appears to be so little interest in developing more knowledge and defensive measures in training!
SHEDDING LATERAL DAMPING
We need to re-state some basics. If the aircraft is above minimum flying speed then Stability will be working in all three axis and the aircraft may be controlled normally. If the aircraft is below minimum flying speed (stalled or partially stalled) then it's subsequent behaviour will be at least partly controlled by pitch and yaw stability but stability in roll will be considerably less and may be absent altogether. In these circumstances the controls have to be used in a set manner to regain normal control.
To remain under normal control and stability the wing must remain below the critical angle of attack (about 14û-15û). This angle remains constant because it is associated with airflow separation from the wing. The airspeed this happens at however will be variable on the weight of the aircraft at the time and also if the aircraft is turning and therefore subject to a degree of G loading.
Now, if we superimpose Lateral Damping on the above and consider an aircraft that is just above the stall (for it's weight/turn configuration at the time) and a gust or something else disturbs the machine in roll, with the lower wing going down as a result, then the increase in angle of attack could take that wing beyond the critical angle and it will perforce keep going down because there is nothing left to keep it up!
Unfortunately, this deepens the angle of attack and entrenches the situation. Normal instinctive behaviour is to apply opposite stick to the roll and that makes the whole thing worse! The down-going aileron makes the mean chord line of the wing more positive and deepens the angle of attack even more!
Any protection you normally would have had from in-built roll stability via Lateral Damping has gone and you paradoxically have an aircraft that is still very much controlled by other parts of stability but is now unstable in roll!
The consequences of this are that you will have now met all the parameters required for entry into auto rotation (the spin). It does not really matter whether the aircraft will remain in the spin - that is academic if you have just pulled this stunt during a base leg turn!
What happens, and why, with shedding of Lateral Damping is simple and easy enough to read and understand. What is more difficult is how fast it happens, how little warning you are given, and how extreme the aircraft attitude becomes so quickly.
On an aircraft with very benign stalling behaviour and that gives ample pre-stall symptoms plus is known to be difficult or even impossible to spin - people may feel secure that even if they do make a mistake then not a great deal happens and the situation is easily recovered from. That is not so in these circumstances!
Stability quietly slips out the back door with no fanfare and less warning. The aircraft rolls rapidly into the turn, slips and then yaws into the turn, then is abruptly pointing straight at the ground in an attitude that requires a fair bit of height and very prompt, but careful, control use to get out of. The only pre-event symptoms you get is the stick steadily having to come back to hold attitude and the airspeed consequently creeping down. Those can and do go totally unnoticed if you are busy and distracted.
To underline this a little, when Mike Valentine of the GFA did research into Lateral Damping and inclusion of it into the flying training syllabus, a great deal of video footage was taken during the testing. These included extremely benign trainers abruptly departing into spin entry very quickly, from entry attitudes with the noses well below the horizon and both the slip balls and yaw strings pretty much in the middle!
How much height is lost? I will give you an example from personal experience. This was not actually a true shedding of Lateral Damping case but the speed of activity sequence is a good replication.
Part of the gliding training syllabus, for winch or auto tow launching, is a critical decision height cable break recovery. The glider is deliberately detached from the launching cable in a full 45û climb at a point around the middle of the airfield. The height has to be low enough to suggest a straight ahead landing is possible but is actually just too high for this to be done safely so the aircraft has to be turned. The instructor is primarily assessing decision making, reaction speed, attitude control and energy management - because this is the area most of the fatals come from in gliding.
On this session I us at 300' agl and the student (for some reason) overloaded and panicked. The stick hit the back stop and the left rudder hit it's front stop and we were still at 45û climb with the airspeed dropping rapidly.
We immediately went into an enthusiastic spin entry and I stopped it almost instantly. But we were already pointing straight at the ground! Coming around the bottom I estimate we missed the ground by less than 2'!
Now, at that time I was extremely current, specialized in teaching low level emergencies, and was teaching spinning most days in various forms. I also had several thousand flights on the type. Plus I always pump myself up to high state of arousal for these exercises to cover the rare time anything got 'interesting'. It does not get much better than that - but I still needed every foot of our 300' starting height!
You may now see a bit more relevance to the requirement for a minimum height for the base leg turn of 500'! A recreational pilot, just enjoying themselves, who inadvertently gets into a shedding of Lateral Damping situation will need every inch of that extra height if they wish to survive!
PUTTING THINGS TOGETHER
Earlier in this article I made a statement that Stability and Lateral Damping were not taught probably because the pilot has little direct control over them - they are just 'there' so a general awareness is deemed sufficient.
I do not believe that is sufficient at all! Stability by itself creates so many doors. It becomes a foundation stone of confident knowledge that can underpin virtually every part of flying training and significantly aid student and instructor alike.
To understand Stability the basic Principles of Flight have to be first introduced. So angle of attack, airspeed, airflow, centre of pressure, centre of gravity, moment arms, vector diagrams etc. become not just some dry, abstract, theoretical 'so what' thing - they become an active part of flying instruction! Stability, by itself, builds a practical bridge between principles and practical - and this can be used all the time to produce safer and wiser pilots, with the additional advantage that it can be done in conversational terms and without a University degree being required to do so!
With Lateral Damping it can certainly lead you into very scary situations. Now, you may curse and spit about the sun coming up every morning and waking you up when you want to carry on snoozing in bed - but there is nothing you can do about that! But you can actually do something very practical with Lateral Damping! With that knowledge and awareness that it is in fact there then you can barricade it out of your life and never have to worry about it!.
The gliding world uses a very simple process. This is termed 'Safe Speed Near the Ground'. It operates on the basic principle that if you do not stall then you cannot spin (or shed Lateral Damping) so you build in an airspeed safety margin when in circuit to prevent anything like this from happening.
Safe speed is normally a circuit speed of 1.5 times stalling speed - e.g. if your stall speed is 40 knots then your circuit speed is 60 knots. If you fly medium turns and maintain this airspeed below 800' agl then you cannot possibly come to any harm.
But Lateral Damping and your knowledge and understanding of it (because it has been taught to you) lurks in the back of your mind as the Dragon waiting to pounce - so just avoid entering it's territory!
That knowledge and training will cut through the human distraction elements of deteriorating conditions or situations, the maybe slightly crippled aircraft you are having to deal with, or the lunatic in the circuit with you that is giving you a hassle.
When you so desperately feel you should slow down, or crank more bank on, or crank rudder on because you dare not increase bank, or something horrible has happened and you have had to pull up and so much want to turn immediately before you have the energy re-established - you will not! Because that knowledge and training will be a small voice in your head reminding you of what is waiting for you and ANYTHING is better than going there!
So I suppose I will have to do another article on what Safe Speed Near the Ground actually is and why, consequently that will haul me into the Drag Bucket and realistic energy control. Been told about that have you?
Ah well, it will keep us off the streets! Catch you later!
