I can see it now. 737 in cruise everyone relaxed down the back. Two pilots "monitoring" up front - one reading the newspaper the other filling in the tech log. Then a loud bang like a compressor stall which it probably was. And if you haven't seen or heard a compressor stall then believe me the bang is real loud and scary.
Aircraft yaws because the autopilot does not control the rudders. "Wot the f..k"? sez the captain. Both look at the engine instruments. The captain decides to shut down the engine (which by now has probably recovered itself from the compressor stall) and closes the offending engine thrust lever while still on autopilot. Big yaw now occurs because of the closed throttle and someone up front is a bit slow to prevent further yaw.
Someone maybe disconnects the autopilot and instinctively applies rapid aileron and spoilers to get the aircraft on a even keel. Spoilers at high altitude in cruise can cause rapid wing waggling caused by overcontrolling.
By now a couple of hundred few feet of altitude loss is evident, the altitude alerter sounds causing someone up front to exclaim "s..t!" and further overcontrols both ailerons and elevators to get back to cleared level. Finally everything is under control again and the autopilot is carefully re-engaged and a PA is made to the people down the back who were the recipients of the momentary harsh over-controlling made in haste rather than deliberate.
To the startled passengers it felt like the 737 was porpoising and rolling rapidly for maybe 5-10 seconds as the crew got over their own momentary fright. Any dead heading pilot down the back I'm sure would be equally startled as the passengers at the sudden yaw and rolls that occured.
It is not like a planned simulator session where the pilot knows roughly when the instructor will fail an engine and is ready on the rudder. From a smooth cruise to a sudden loud unexpected bang or multiple bangs and yaw would likely cause even the most switched on crew up front to be victim of the startle factor and lose a few seconds before they had things calmly under control up front within the regulatory allowance of +/- 20 degrees in initial yaw to +/- 5 degrees in subsequent heading.
Last edited by A37575; 9th May 2011 at 10:16.