Originally Posted by
yoko1
In the accident scenario, when MCAS was commanding the stab nose down the forces were increasing on the jackscrew mechanism. When the pilots were commanding nose up, the forces were decreasing. If the stab motor was going to stall under an aerodynamic load, it would have happened in the direction of increasing (MCAS), and not decreasing force (pilot trimming). On the contrary, MCAS had absolutely no problem using that one trim motor to drive against an ever increasing force until it hit the physical stops. There is no evidence that the motor was ever overtasked.
While I don't dispute your conclusion, I don't think that your description of forces on the horizontal stab are correct. I would agree that under normal circumstances the load would increase as you trim away from the previously trimmed airspeed, but that would assume no deflection of the elevator.
In this circumstance the pilots were applying substantial nose up elevator. This would tend to increase the force upwards on the front of the horizontal stab, and require the jackscrew to apply an opposite downward force to the front of the horizontal stab. If there were no screw and you applied a nose up elevator and the horizontal stab could slide around position at will, and you would likely see the stab go front up and by default AND which is the same direction the MCAS is driving the trim. The Alaskan Airllines MD80 accident even highlights this. Once the jackscrew let go the horizontal stab went full nose up with full after elevator (i recognize the MD80 uses tabs) and went beyond limits up creating a substantial nose down tendency of the aircraft which wasn't recoverable.
At the end of the day maybe it stalled out maybe it didn't, but from the FDR traces it certainly seems it was moving in the commanded direction.
From Alaskan Airlines Report
As the jam was overcome, the acme screw was being pulled upward through the acme nut by aerodynamic loads, causing upward movement of the horizontal stabilizer, resulting in greater airplane-nose-down motion. This upward pulling motion would have continued until the lower mechanical stop on the acme screw contacted the lower surface of the acme nut, preventing further upward motion of the horizontal stabilizer.
Release of the jam allowed the acme screw to pull up through the acme nut, causing the horizontal stabilizer leading edge to move upward, thus causing the airplane to pitch rapidly downward.