Originally Posted by
Dave Therhino
MCAS can be interrupted with the electric trim switches on the yoke each time it activates, and trim can be returned to a low/zero column force state each time if the pilot puts in adequate opposite trim inputs. That is what the Lion Air crew did for approximately 2 dozen cycles of MCAS before the final few cycles, where the pilot flying at that point failed to put in an adequate amount of opposite trim in those final cycles, allowing the out of trim condition to increase to the point where they couldn't recover in the altitude available.
An undocumented system of which the pilots have no clue it even exists can interrupt airplane trim. If the pilot doesn't want to end in a coffin corner, they need to figure out on the spot that:
- this is not STS
- because it's not STS it must be either a bug or an undocumented system
- to save their lives they need to return the trim manually to a low/zero column force each time the undocumented command is triggered.
Despite facing those conditions, the Lion Air crew managed to react appropriately about 2 dozen cycles. Fighting the unknown system raised pilot's adrenaline and fear and induced panic. Thus, incapacitated by stress put on them, they no longer managed to manually correct the erroneous system input that was trying to - and succeeded to - kill them.