Low-Speed Loss of Control
Understanding angle of attack is the key.
By Richard L. Collins
July 2008
From an NTSB preliminary report on an accident involving a Piper PA-46-500TP (Meridian turboprop single): "An eyewitness, a retired Navy instructor pilot, located about one half-mile northwest of the accident site, reported observing the accident airplane descending through the overcast about 1227. The airplane leveled off about 300 feet above ground level (agl) and turned south on a compass heading of approximately 195 degrees. A couple of minutes later the witness observed the airplane heading north on an approximate heading of 15 degrees but at a 'much slower' airspeed. The eyewitness then observed the airplane roll right to an approximately 60-degree angle before the nose of the airplane fell through to an 'extreme nose-low attitude.' After the airplane disappeared behind trees, the witness heard a crash and observed black smoke. The witness further stated, 'The [airplane] exhibited a classic approach turn stall maneuver I had taught many times, but this time with no altitude to recover.' "
The pilot was trying to fly an ILS approach and the accident occurred after the pilot reported he was having trouble performing a "coupled" approach and that he was trying to "get control" of the airplane.
A lot of dominoes tumbled there, but in the end it apparently became what is referred to as a stall/spin accident. We often think of such accidents as typically involving lower-powered airplanes maneuvering at low altitude, and a lot do, but a surprising number of them, about half, occur in airplanes used for transportation. There is usually a distraction, as there was in this example, and in the end the pilot simply doesn't allow the airplane to fly.
There were a couple of other accidents in the recent past that relate to this. One was a turboprop, one a turbojet. One had an owner-pilot and an experienced keeper-pilot; the other had two professional pilots. In both cases the airplane was apparently leveled off after a descent but no power was added, and a low-speed loss of control was the result.
In the all-time classic book on flying technique, Stick and Rudder, Wolfgang Langewiesche devoted the first 24 pages to a discussion of the importance of understanding angle of attack. He didn't think most pilots understood much about the subject. Do they now?
Years ago, in the early '50s, the FAA got caught up in this and launched an educational program on angle of attack for flight instructors. This is from a distant memory, but as I recall they equipped one of the first PA-18s with angle of attack instrumentation that measured five different angles. Lights in the cockpit would show when each angle was reached and the drill was to honk the airplane around and watch the angle of attack change. Get four lights on and it was close to the stall; five lights and it let go.
Also in the early '50s, Leonard Greene of Safe Flight Instrument Company developed what he initially called the "Landing Speed Indicator." It was basically an angle of attack indicator that showed, with a centered vertical needle, when the wing was at the angle of attack for maximum lift. This was later marketed as the Safe Flight Speed Control System, $345 for a basic model, and I had one in my Piper Pacer. This did not catch on as a product, though Safe Flight is still very much in the business of developing and manufacturing sophisticated systems for turbine airplanes.
Pilots must feel that they can do just fine using the airspeed indicator, and they can, but an awareness of angle of attack becomes critical when there are distractions or when the chips are down. A pilot who levels off but doesn't advance the power has to realize that the angle of attack will increase to the point of a stall if the airplane is flown level without power. That is terribly basic but it is sometimes neglected with bad results.
I don't know how widely understood angle of attack is today, so bear with some basics for just a second. I used to describe it to students as the difference between where the nose is pointed and where the airplane is going. I would demonstrate it in slow flight, with the nose up, say, 10 degrees with the altimeter steady and/or the vertical speed on zero. The nose is up 10 degrees, the airplane is not going up, so the angle of attack is approximately 10 degrees.
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