Spins represent a real hazard to all pilots, and glider pilots in particular since they often fly with less margins above the stall. Unfortunately the spins themselves are quite disorienting and as a result there is rarely time in normal training to gain mastery of this manoeuvre. We are left with the next best strategy which is to recognize the spin, and gain sufficient practise to recover safely.
Since the spin is a condition of stalled flight the first preventative measure is to recognize the symptoms of the stall and promote the proper recovery at this point. This has been the task of training at the basic and intermediate stages. The delineation between the stall and the spin is the rotational motion as evidenced by the nose of the aircraft swinging across the horizon. There has been considerable acceptance of using standard stall recovery techniques when aircraft are stalled in turns, or experiences only a wing dropping. At the end of the intermediate stage, full spin recovery technique was introduced for what we term the incipient spin. The incipient spin by my definition is a stall where spin rotation has started. Usually once the spin has progressed somewhere between a quarter to a half a turn it demonstrates all of the more classical signs of a fully developed spin. These include:
- Nose down attitude.
- Constant rotation rate.
- Constant airspeed, although airspeed indicator may fluctuate.
- High sink rate.
With students we want to keep recovery procedures simple. If they can discern no rotation then do stall recovery, otherwise use the full-spin recovery technique. However if there is any doubt do the full-spin recovery technique. It may be a crude way of recovering from a stall but will do no harm. The standard spin recovery technique is:
- Apply full-opposite rudder to the direction of the spin.
- Centralize the controls.
- Pause briefly.
- Lower the nose to unstall the wings, until the rotation stops.
- When the dive starts level the wings and centralize the rudder.
- Start recovery from the dive. Do not pull-up such that visual contact is lost with the horizon.
- When the airspeed approaches the normal gliding attitude airspeed lower the nose to the normal gliding attitude.
The use of the rudder first is important as stopping the rotation is often necessary to make the elevator effective so that the aircraft can be unstalled. When the aircraft rotates the spinning causes a certain amount of inertia about the spinning axes. This means that there will be resistance to the tilting of the aircraft by the elevator. Additionally in a spin there are often pronounced slipping motions which will cause the elevator to either be blanketed by the vertical fin or be moving sideways through the air. Both of these events may cause the elevator to be ineffective at pitching the aircraft to unstall the wings. Having said this there are a number of factors affecting spin recovery:
- Weight distribution - any addition of weight any distance away from the centre of gravity will increase the inertia about the spinning axes. Independent of the Centre of Gravity location or weight, this inertia will promote a less steep spin attitude and more sluggish recoveries.
- An aft centre of gravity promotes a flatter spin with increased speed. The aircraft may not have enough control authority to recover from this attitude.
- High altitude promotes slower recovery as the controls respond slower when the air is less dense.
- Aileron position may increase the time for recovery.
- Positive flap settings may make the spin flatter.
- Negative flap settings allow the aircraft to stall at lower angles of attack and may promote more rapid wing drop situations.
Lastly a word of caution. Contrary to popular wisdom there is no such thing as a ‘stable’ spin. Even a quick examination of the more serious works on this subject will indicate that the spin motions are very complex and hard to model past about three turns. Even minor alterations to spin entry can have marked effects on the nature of the spin as can improper control inputs during the recovery phases. Minor changes to the surface of the wing due to bug accumulation, dirt, or moisture can also dramatically influence the stall/spin characteristics for both laminar and non-laminar airfoils.
The use of multiple turn spins has been promoted in the past by instructors wishing the student to notice all of the symptoms of the spin and to give them time to do recovery when they have not been prompt in their actions. Again the rationale for this approach has been based upon the notion of a ‘stable’ spin.
My arguments against this approach are twofold. First the spin is quite a disorienting manoeuvre and unless the training curriculum is going to include a large number of them, the student is not likely to really notice what the instructor is talking about. Secondly those that still believe in the ‘stable’ spin likely have not reached the stage themselves when they can detect the changes that go on during the spin. I did spin training for a number of years before I started to relax and really begin to pay attention to the motion of the aircraft. It was only then that I began to see that they really weren’t as stable as I had originally surmised.
What I think that students can learn in a short spin-training program is recognition that the aircraft is in a spin and the appropriate recovery. This is important and can still be done safely and effectively by performing single turn spins in aircraft loaded at least in the middle of their safe balance range. Also this is an area where the instructor should take over and do the recovery if the student does not react promptly before a second turn is past. Doing more entries and recoveries is more effective training than one spin with more turns.