debodine said:
Boeing, during their flight testing, decided that moving the elevator did not provide enough authority to get the nose down in the specific critical situation, so they decided to have MCAS control the trim.
I see how you got to that hypothesis, but it's not quite right. The elevator has equal-or-better authority to the full stabilizer, depending on pitch attitude/speed etc. It is much smaller, yes, but has a MUCH larger range of travel. The full authority of the elevator is capable of counteracting the rear stab, but the control forces required to hold the elevator in that position are extremely high.
MCAS exists for one reason: because the additional lift vector from the larger, relocated engine cowlings causes control force non-linearity under certain conditions.
Regulations state that as the airframe approaches stall, there must be a linear increase in control force. Every plane with a type rating (i.e. every plane) operates this way, Cessna on up. Pilots learn quickly, and it becomes ingrained, that increasing control forces mean increasing angle of attack and that their aircraft is approaching some edge of the flight envelope.
The new engines created a situation where under certain conditions, increased angle of attack would result in reduced rate-of-change of the control column forces. That's a counter-intuitive behavior for every pilot in the world, and also a dangerous control system behavior in general.
MCAS' sole purpose is this: when the flight computer thinks the aircraft is approaching pitch attitude that will result in a stall, it adds nose-down trim so that the pilot's control forces continually increase up to the point where the aircraft enters a stall.
Electric, automatic adjustment of rear stab for the purpose of trim already existed; MCAS is just a little piece of software that activates the automatic trim system in a very specific situation, in order to provide control force linearity.
