Originally Posted by
A Squared
Any time you're changing the angle of a stabilizer, the potential exists for loss of control of the aircraft if you lose control of the system adjusting the stabilizer. I doubt anyone disagrees on that point.
Don't disagree per se, but this statement requires clarification. Runaway trim (from whatever cause) can cause loss of control only in a very small corner of the flight envelope (specifically, when slow, at high AOA, and with the engines at high power.) The accident aircraft was not in this corner of the envelope so the pilots had sufficient elevator authority to over ride the trim. Assuming (and yes this remains a BIG assumption) the trim ran away due to the MCAS getting erroneous AOA sensor data, the pilots should have been able to maintain control and overcome the mis trim. The mystery is why they did not.
Further, on the subject of why Boeing did not include the MCAS in the flight manual, could it be because the
existing procedures for runaway trim are applicable whether the cause is a stuck switch, stuck sensor, misbehaving STS, or misbehaving MCAS. Troubleshooting the systems to determine what the root cause is for a runaway trim condition just delays the pilot from executing the runaway trim procedures. Further still, this could also explain why Boeing chose to implement MCAS the way they did, using the trim system vs a stick pusher. It makes operation of the system and any emergency procedures related to malfunction of the system completely transparent to the pilots.