Originally Posted by
EDLB
Seems to me that the final hole in the cheese which dictated the outcome of both flights is, that at higher speeds the trim is jammed (in the direction of nose up) if trimmed nose down. On the Ethopian flight a single nose down command by the MCAS system was enough to seal their fate.
I do not believe that in the time where they clung on the column for nose up, that they did not try to use the trim switches to unload the back pressure. Elevator trimming you learn from day one in your SEP trainer.
On the Lion air you can see that the PF did constant fight the MCAS AND trim with nose up. As soon as he transferred command there are only few blips of nose up trim to see on the FDR. Same here on the Ethopian flight.
I can’t believe that you only try with two short blips if the landscape becomes larger fast.
Not a pilot, my field is electronic engineering. The FDR traces on both Lion Air & Ethiopia show short electrical trim ANU blips when longer trim activation would be expected.
I am curious as to where the FDR data for the stabilizer trim is read from - yoke trim switches or the motor drive electronics? Most high power motor drives have feedback that detects if the power demand is exceeded. If this happens, power to the motor is disabled, in order to protect it. Therefore a situation could arise where even though the pilot is activating ANU trim, it results in just a short ANU motor movement, before the motor power limit is exceeded. So if the FDR records from the drive electronics, the trim ANU 'request' by the pilot, will be seen as just a 'blip' although the trim switch is still activated,
This would account for AND trim running the full period as motor power demand is not exceeded (low aerodynamic load), but ANU trim is fighting against high aerodynamic load, which activates the motor protection. Thoughts?