The Alpha Measurement

December 28, 2010 by Bruce Landsberg

Sometimes an accident hits so close to home it causes us to question the fundamentals. A friend lost some business acquaintances landing a Bonanza at his private strip when the 5,000 hour pilot inexplicably stalled and they spun in. He began to wonder if an angle of attack (AOA or Alpha) indicator would have helped and why they weren’t installed on many more aircraft. I’ve wondered myself.

Most of us were taught to use airspeed as a proxy for AOA. But it’s a derivative that measures indirectly and not always accurately. Instructors the world over have reminded pilots to watch airspeed but perhaps the warning should be to “Mind the Alpha.”  The FAA, through knowledge and practical tests, has attempted to ensure that we know that airplanes can be stalled at any airspeed or attitude. Yet every year about two dozen pilots, rookie and veteran alike, fatally misjudge that critical angle. Why?

My guess: distraction. The old bromide is to “Aviate, navigate, communicate.” But we get distracted by any number of shiny things or operational procedures that clamor for attention at the worst possible time during approach and landing. Typically, the aircraft is bleeding energy as the pilot reduces power, sets flaps, negotiates with ATC, follows traffic, etc. Alpha increases.  Turning and/or poor coordination often close out the scenario.

So why hasn’t the industry adopted Alpha and routinely given us a gauge to measure it? Good question. Are the current stall warning devices adequate on light aircraft? In today’s glass cockpits it would be relatively simple to add Alpha and appropriate warnings. The nice lady whose voice advises of 500 feet and IFR minimums might also let us know when Alpha was about to become critical:  “Alpha”, “Don’t stall”, “Lower the nose Dummy” could be spoken with increasing volume and stridency.  Some new light aircraft are developing a measure of automatic envelope protection through the autopilot system.  Is this what’s needed? Perhaps still more training is the ticket –  Air Safety Institute produced an award-winning online course, Essential Aerodynamics: Stalls, Spins, and Safety.

What about retrofit to the existing hardware? We did it to Air Safety Institute’s Piper Archer for about $4,000 installed. It allows for impressive short field performance but not many of the pilots have embraced it despite the fact that one can fly with precision and make impressive short field landings. I’m perplexed.

What do you think?

Bruce Landsberg
Senior Safety Advisor, Air Safety Institute

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