Stall and High Angle of Attack

Aerodynamic stall and flight at high angle of attack are discussed with criticism of certain stall recovery training procedures, suggestions of why these procedures were adopted and research proving them to be faulty.
In an aerodynamic stall, aside from the most important action of immediate reduction of the angle of attack (AOA), some other important considerations are:

  1. Because aircraft designers make the empennage as small as possible to reduce drag, elevator authority may be insufficient to recover if the elevator trim is opposite to the elevator, i.e., elevator trim nose up. If such is the case, attention must be paid to the elevator trim position. In a very high AOA stall (significantly greater than initial stall AOA), it may be possible to roll the aircraft into a high bank angle and cause the nose to drop, but envelope protection in some fly-by-wire aircraft might limit this possibility.
  2. S.S. Hoerner, in his book on Fluid Dynamics, identifies a relatively unknown problem of a dynamic stall where the AOA at which the airflow re-attaches to the upper surface of the wing is considerably less than the AOA at initial stall.
  3. One of the results of the mistaken belief that aircraft performance is enhanced at high AOA resulted in the stick shaker technique for windshear encounters. To support this belief, training departments began, in the 1980s, to teach a stall recovery technique of beginning with a power off stall and recovering with thrust while attempting to hold the pitch attitude at the value when the stall occurred. Pilots were told this was the procedure for low altitude stall recovery and the purpose was to minimize altitude loss, despite the fact that in the worst case windshear, a stall would be power on and it would be impossible to recover with the suggested procedure. The fact that a 1g stall speed is typically 5 to 7 percent greater than the FAA certified stall speed,  making the stick shaker warning dangerously close to a true stall, further complicates this suggested maneuver (see “Vmin and Stall Speed”). I had thought this stall recovery procedure was not presently being taught, but a March 7, 2011 AW&ST issue contains an article pointing out several accidents where pilots (allegedly) kept an aircraft in a stall without recovering. Further inquiry through an aviation safety blog confirmed that this was still being taught as late as mid 2011.

The last work (1989) on the optimal trajectory studies for windshear (conducted by Professor Angelo Miele and the Aero-Astronautics Group at Rice University) answered the question of attempting to keep an aircraft at a high AOA.

Two reports are “Optimal Recovery Techniques From High-Angle-of-Attack Windshear Encounters: Take-Off Problem.” and the same title for the “Landing Problem”. These reports analyzed aircraft performance in strong windshears at low altitude with the aircraft put at the stick shaker AOA and say; “Maintaining the aircraft at the stick shaker is a poor strategy in terms of altitude loss and survival capability.”; and “If the pilot accidentally or deliberately increases the angle of attack to the stick shaker value, following the windshear onset, the optimal recovery trajectory (ORT) requires that the angle of attack be reduced quickly to a lower value and then increased gradually in such a way that the stick shaker value is reached again near the end of the shear.”; with substantiating proof that the ORT can recover from a windshear 50 percent stronger than when a high AOA is maintained, and if a high AOA is avoided in the beginning, the aircraft can survive a windshear twice as strong.

Being aware of the training problem, I discussed it in a 1990 SAE paper (901995; Windshear—Optimum Trajectory, Human Factors and Miscellaneous Information), referencing one of the above reports; and also noted the hysteresis effect in a dynamic stall (Hoerner) and the FAA Vmin stick shaker   issue.

May, 2011

Please also see Pilot Error in the Air France 447 Accident