Aviation Safety Letter 1/2003

Stall-Spin Accidents May Be Hazardous to Your Health

There is one thing common to all aircraft; whether you're flying a large or a small aircraft, an ultralight, a trike or a glider, you must not - at any cost - exceed the critical angle of attack (AOA) in order to sustain controllable flight. The AOA, as you all know, is the angle between the mean aerodynamic chord (MAC) and the relative wind. If this occurs, you will lose all lift, stall and fall out of control to the ground. Note that the relative wind is that which is created by the motion of the aircraft through the air.

As a pilot, you must remember the AOA concept and repeat it as often as it takes in order not to forget this physical fact that can be life-threatening at low altitude. I have read too many times of friends, colleagues, pilots, or associates losing their lives or seriously injuring themselves following a stall-spin accident at low level.

What is the cause? What is the problem? The answer to these questions is not easy; otherwise there wouldn't be as many stall-spin accidents as there are. As you can see, this type of failure is often difficult to foresee. Can we say that it is insidious or dare we say, inevitable? Well, yes it can be insidious and no it isn't inevitable! It's pure physics.

There are of course, two elements required for such accidents. First, the aircraft must be in a stalled condition, and second, there must be a yaw moment introduced. Students must be able to recognize the conditions leading to an imminent stall and prevent it from happening.

No stall, no spin. It is as simple as that. Even more fundamental is the necessity to instill in the student a complete understanding of the AOA concept. This seems to be a particularly difficult concept for many students to grasp, and quite a few pilots out there are unaware of the relationship between power and AOA; how increasing or reducing power changes the direction of the relative wind, and thus the AOA.

You have a set of wings, and as long as the air is flowing over them at the same velocity, and the AOA remains the same, all is well. If, however, you change the AOA and the relative wind factor - such as in a tight left turn from base to final to try and line up with the center line of the runway - and you pull back the control column as you force the aircraft to make a tighter turn, you can have one wing slowing down because you are using the ailerons to try and lower the upper wing that is rising, often too quickly. The left aileron will cause the lower wing to drag and slow down even more. In a left turn, the engine torque factor and gravity may pull the wing and aircraft down too early for your comfort, and if you kick in rudder to keep your nose up where you want it to be, the lower wing may keep coming down no matter what you do. Releasing back pressure, lowering the nose, adding power, changing the AOA, and bringing the wings level may save the day right there; especially if you have some altitude available, otherwise there is a possibility that you may become another statistic.

If you don't have enough altitude for recovery, you may see your life flash before your eyes! Your last thoughts would probably be: What happened? How did this come about? This can't be happening to me!

It's sudden and often deadly at low pattern altitude! What is the solution? Well certainly any regular, recurrent and well structured training at a flying school, with a qualified instructor, will go a long way to instill in you the qualities, the knowledge and most of all the confidence to recognize the imminent stall and spin parameters and environment.

Your instructor trains pilots day in and day out, and can perceive the little subtleties that make a difference in safe flying abilities, and will give you the knowledge to recognize hazardous situations well before they occur. Recurrent training will do more than save your life; it will ensure that you will be a safer, happier and more confident pilot.

Happy flying!

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