Originally Posted by
ernst_mulder
A (possibly stupid) question from SLF,
What I am wondering about is why, after a struggle with MCAS, the plane ends up in a nosedown dive. Wouldn't the pilots in principle be able to keep fighting the unwanted trim commands indefinitely? Also earlier in this thread I read that it is possible to fly an airplane even with a full downward trimmed HS. Could another mechanical problem be the cause, i.e. elevator(s) breaking off after too much stress trying to compensate for the full downward trimmed HS?
Yes, the pilot could keep counteracting the MCAS input with equal and opposite trim, and get into a trim cycle where, while the ride is wild, control is never lost until the electric trim is shut off or by flap selection for landing. Remember the MCAS function uses the high trim rate and can put in up to 2.5 degrees each cycle, so the pilot's trim inputs counteracting each cycle need to be fairly large. That is what the crew apparently successfully did in the flight prior to the Lion Air accident flight. Remember, by putting in opposite trim, the pilot interrupts the MCAS function and causes it to re-arm for another cycle, which will be triggered five seconds after trim switch release if the MCAS logic is still satisfied (manual flight, flaps up, same side AOA sensed beyond some threshold). However, if on repeated successive cycles of MCAS input, the pilot fails to fully reverse the amount of trim MCAS has put in, the nose down trim will continue to build until the column input can no longer override its effect. This can happen in just a few MCAS cycles.
Think of this in terms of the total scenario that results from an erroneous high AOA input to the flight control computer that is in control as opposed to imagining flying along just fine and MCAS starts acting up. The stick shaker activates on rotation, and the airspeed difference between the left and right side increases as speed builds due to the AOA correction in the air data computation logic. The pilot recognizes he has unreliable airspeed, and is getting stick shaker and possibly other aural warnings about speed (not sure about that last part), so he flies manually and never engages the autopilot He thinks he's on the verge of stalling, and despite keeping the nose down and accelerating, the stick shaker remains on and he has unreliable airspeed. After a minute or two he gets to the speed where he's getting cues to pull the flaps up, so he does, and now MCAS starts putting in trim inputs on top of his already high workload and stress. Apparently an assumption was made that the population of crews flying the 737 Max could consistently handle this situation correctly given the additional information put out by Boeing after the Lion Air accident.