Tuesday, July 9, 2013
The Reality Of Deep Stalls
Stalls are simply stalls, except when they’re not
One possible solution some pilots have used to facilitate recovery from a deep stall is to employ the ailerons to roll the airplane to knife edge and force the nose to pitch down sideways. In this manner, a pilot may be able to escape with a semi-normal stall recovery. Some pilots have also managed to recover by rocking the nose with what little elevator control remains until the angle of attack becomes so high that the nose finally falls through.
On multi-engine aircraft, asymmetric thrust may also succeed in breaking the airplane out of the deep stall.
Manufacturers have installed "stick shakers" and "stick pushers" to provide a semi-automatic means of recovery from such situations.
Deep stalls can be insidious, however. Until the age of voice and flight recorders that preserved the last few hours of every commercial flight, accident investigators were sometimes puzzled by crashes that seemed almost inexplicable. Airplanes would sometimes mush into the ground at high descent rates but with wings level and nose 20 to 30 degrees above the horizon. Most were in military or airline jets, usually fatal to all aboard, so there were no survivors left to describe what led up to the crash.
Four years ago, an Air France Airbus A330 crossing the South Atlantic in the middle of the night crashed into the ocean off the coast of Brazil in what many interpreted as a deep stall. The crash killed all 216 passengers and 12 crew, and the Airbus sank to the bottom of the Atlantic. For two years, investigators searched for the wreckage with a variety of submersibles, and by a stroke of what many searchers regarded as blind, dumb luck, finally found the black boxes in 13,000 feet of water.
Trouble was/is, the Air France accident was not a deep stall, despite the probable cause finding by the French investigation BEA team that the crew "executed inappropriate control inputs that destabilized the flight path, leading to an unrecoverable aerodynamic stall."
The key word here is "unrecoverable." Although the Airbus A330 was definitely in a fully stalled attitude when it hit the water, wings level and nose roughly 20 degrees above the horizon, the crew was receiving mixed signals from the airplane regarding airspeed and altitude, apparently the result of flying through a super-cold event that iced up all the airplane's pitot tubes.
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