Aviation Safety Letter 4-2003

Stall-spin Accidents, Follow-up from ASL 1/2003

by Alain Gauthier, Engineer-Physicist and pilot

I read your publication very conscientiously and always find ample food for thought. The article Stall-Spin Accidents May Be Hazardous to Your Health caught my attention, and I would like to stress some aspects of it. All pilots, of course, have their own store of knowledge and experience, and they have to form their opinions from their own backgrounds and broaden their performance envelopes. but, if we do not study ourselves, nature will teach us by example.

Identical aircraft? An ideal. Certified aircraft are issued a type certificate. In theory, all such aircraft should be identical, but pilots know that every aircraft has its own personality at any given moment. In a critical flight situation, the smallest of these differences counts.

Wing condition: All industrial production tolerates a margin of error. Wings of the same type are therefore all slightly different. What is more, the average angle of incidence and dihedral are not necessarily identical on both the left and the right. Their weights also vary. And even if two aircraft were identical to begin with, their operating lives will change them in different ways (fatigue, overloads, turbulence, dirtiness of their lifting surfaces and so on).

And lift beyond — V2/2: Stalling occurs on the upper wing, reducing the lift component due to the Venturi effect, but increasing the angle of attack tends to increase the pressure on the lower wing. Total lift is therefore the algebraic sum of the forces on the bottom AND the top of the wing. The top may contribute as much as three quarters of the total lift, and, without its contribution, the aircraft cannot maintain steady level flight, unless it has the necessary power (F-18), and controlled flight becomes harder and harder to sustain.

Stall and spin: The rudder can help to maintain control in slow-flight or stall situations because it can speed up one wing and slow down the other. At the stall limit, therefore, kicking in rudder can restore lift on one wing while increasing stall even more on the other. The upshot is that the pilot has given the aircraft a very effective way to turn round on itself.

Speed and spin: Stall and spin certainly go hand in hand, but bear in mind that it is not the aircraft that stalls, but the wings. Here are two cases that are often linked.

First case: Like every Canadian pilot, I was initiated into spin. Demonstrating spin from VS is often not clear, because the controls are ineffective in this situation. To counter this, I begin in slow flight — about 1.2 VS — but I do not have to wait to stall. In the classic manoeuvre, yanking back the column and kicking in rudder produces a very clear stall on one wing while the other gains lift. The moral is that one stalled wing is enough to start to spin! So, I feel that spin is always lying in wait when turning on short final, virtually in slow flight, even if the aircraft has not stalled.

Second case: We were told again and again that you have to make shallow turns in the circuit, but.you gradually forget what you were taught about the load factor "g". I think it is important to explore our own performance envelope with our aircraft. During basic training, I learned to make 45° turns at 1.2 VS. The first time was somewhat daunting: it required a load factor of 1.41 g, and, at 1.2 VS, the limit is 1.44 g (not good at low altitude). Yet, some pilots attempt this manoeuvre at 100 m while turning on final.

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