Originally Posted by
WideScreen
However, with a B737, we should not forget, this is not a Pitts, Extra, Van, or even a C172. A B737 does have a large (roll-) inertia, implying, once it rolls, it'll take significant forces and time to "undo" that. And, for this case, they had "wings level" (but significant nose down) within some 6-8 seconds after the AP alarm kicked in. Not bad, I'd say, for a large aircraft. So, yeah, they reacted pretty fast (though initially forgetting the throttles).
Please reconsider your statement on roll inertia. The B737, depending on the version has adequate aileron authority, other than below crossover AOA (it is not speed, please stop calling it speed, the aircraft doesn't care about speed, it only cares about AOA... well, yes, speed for... anyway, crossover is not a speed, it is an AOA).
Once a rapid roll has been initiated, neutralising the roll control input will cease the roll, quite promptly. An opposite roll control input will reduce the time to go to zero roll rate, however the B737 will damp a roll rate in about a second, so at the most optimistic and enthusiastic roll rates, say 40 deg/sec, you get an additional 20 degrees of roll approximately. If you used any opposite input, then the 40 deg/sec rate will be removed almost instantly, coffee will spill in both cases, more in the second.
The B737 ailerons are effective through to stall, and actually below, but with caution, as in most aircraft, large aileron inputs in the stall or above stall AOA can lead to a roll reversal. Spoiler roll control remains effective. Crossover AOA relates to the relative authority of the rudder vs aileron in roll control. At normal speeds, the ailerons are adequate. At high speed, the ailerons respond normally. Above VNE/MMO the ailerons generally act normally, however, at extremely high speed excursions, large aileron inputs can result in roll reversal, that is in the 450KCAS and above range, and at that point it is time to stop playing about. There is a single set of data that shows a roll reversal occurring, and it did not end well, and the whole trainwreck was caused by the Pilot-In-The-Left-Seat-Being-Paid-To-Nominally-Be-In-Command.
Roll rate is determined by control deflection, (aileron/roll spoiler/rudder, AOA, CAS, and speed brake effects on lift distribution. Thrust asymmetry gives a secondary roll... which is what the AP was managing in this case, until it couldn't. Roll rate can also be affected by any mis scheduling of a FBW gain that has erroneous sensor data such as actual flap deflection, Roll damping has additionally density effects. (not the whole answer, it is the bits of interest)
At 07:40:05 UTC:
- the control wheel trim switch activated and A/P disengaged,
- the aircraft rolled to the left with the roll angle of about 49°,
- the highest aircraft altitude recorded was about 10,700 feet, thereafter the aircraft continued to descend until the end of FDR recording,
- the left thrust lever decreased to a position of about 8° and the N1 speed of the left engine was at about 34%,
- the right thrust lever position and the N1 speed of the right engine remain unchanged, and
- after the A/P was disengaged, the control wheel was deflected to the left for four seconds and recorded deflection value up to 18°.
1:25 after the anomaly, the crew didn't comprehend what had happened. The AP disconnected because the trim switch was actuated. Once that happened, a condition that the AP had about 1/3 RWD input of the ailerons, with the aircraft already rolled to the left "inexplicably", the pilot puts in up to 2/3 LWD aileron for a short period.
"BANK ANGLE" GPWS Mode 6 triggered 2 seconds before this....