Failure to promptly reduce the wing AOA at stall warning can result in an increasing AOA and a stall. A high AOA can also cause engine surge and compressor stalls because of inlet flow interruptions. And the asymmetric lift associated with wing airflow separation or loss of thrust can result in a lateral-directional upset — that is a rapid wing drop-off.
Manufacturers' flight test crews and non-routine flight operations pilots who do actual stalls in their aircraft during post-maintenance flight checks report that stalls can be wicked. Colleagues who have done post-maintenance stall checks on Hawker 800s after replacement of the TKS panels have described the event as a sudden, rapid roll, and this in an aircraft otherwise known for fine flying qualities.
While stall training is conducted in full flight simulators (FFS), as noted in “Upset Recovery in Sims” (BCA, April 2012, page 34), even the most advanced of those systems have some inherent limitations involving the fidelity of their aerodynamic model near the edges of the flight envelope, the absence of g-loads, which could affect stall recovery and incomplete motion cues at the first indication of stall. Instructors need to be aware of a simulator's limitations to mitigate negative training.
According to the FAA's Advisory Circular, the first step in recovering from a stall is to ensure the autopilot and autothrottle are disconnected. Specifically, it states, “While maintaining the attitude of the airplane, disconnect the autopilot and autothrottle. Ensure the pitch attitude does not increase when disconnecting the autopilot. This may be important in out-of-trim situations. Manual control is essential to recovery in all situations. Leaving the autopilot or autothrottle connected may result in inadvertent changes or adjustments that may not be easily recognized or appropriate, especially during high workload situations.”
The second step in recovery is to pitch the nose down to immediately decrease the AOA, and implement nose-down pitch trim.
Step number three is wings level, and step number four is to add thrust as needed. Transport Canada points out that turbojet engines normally require 8 sec. to achieve go-around thrust from idle thrust at low altitudes, and even more time at higher altitudes.
Step five is to ensure the speed brakes/spoilers are retracted, and step six is return to the desired flight path.
For some time there has been concern among training, regulatory and investigatory officials as to whether flight crews have been adequately trained for the proper response to a stick pusher. Transport Canada's Advisory Circular 0247 states, “From observations, most instructors state that, regardless of previous academic training, pilots [on their first encounter with a stick pusher] usually resist the stick pusher and immediately pull back on the control wheel rather than releasing pressure as they have been taught. Therefore, pilots should receive practical stick-pusher training in an FFS in order to develop the proper response [allowing the pusher to reduce AOA] when confronted with a stick-pusher activation. Stick-pusher training should be completed as a demonstration practice exercise, including repetitions, until the pilot's reaction is to permit the reduction in AOA even at low altitudes.”
Meanwhile, the FAA's Advisory Circular encourages incorporation of stick-pusher training into flight training scenarios. Similarly, the Joint Aviation Authorities Joint Operational Evaluation Board Report regarding Bombardier Challenger 300 training recommends that flight crews be exposed to operation of the Stall Identification System (stick pusher) since unfamiliar pilots could be misled into taking inappropriate action.