A clarification - my computing experience at Boeing was writing Fortran programs to solve design problems, not aviation software.
However, at Boeing I spent 3 years working on the stabilizer trim gearbox on the 757. The 757 system is a newer design than the 737, in that it uses a dual drive system connected via a differential gear system rather than having the manual wheels in the cockpit. Nevertheless, the difference is in detail, not concept. Both systems have cutoff switches within easy reach of the pilot, for a damn good reason - to stop uncontrolled stabilizer trim action. While the 757 did not have MCAS, it did have a computer autopilot that could move the stabilizer.
I did some searching online of the MAX trim system, and indeed the electric trim switches override MCAS commands. In all three incidents the pilots did override it and return the trim to normal.
In the first incident, after a couple times, the crew trimmed it to normal and then cutoff the stab trim. Continued the flight and landed without further incident.
In the second, the pilots brought it back to normal 25 times before the final plunge. For whatever reason, they never switched off the trim system.
I haven't got a solid reference to the EA one, but it appears they restored trim twice before the final plunge. They then turned off the trim system in the plunge. They could not turn the manual trim wheels due to the aerodynamic forces. So they turned the trim back on, the MCAS came on again making things worse. Why they did not counter again with the electric trim I do not know. Why they did not turn off the trim when it was in the normal position I do not know. Those are excellent questions for the NTSB to answer.
But what they didn't do was follow the directions in the EAD.
However, at Boeing I spent 3 years working on the stabilizer trim gearbox on the 757. The 757 system is a newer design than the 737, in that it uses a dual drive system connected via a differential gear system rather than having the manual wheels in the cockpit. Nevertheless, the difference is in detail, not concept. Both systems have cutoff switches within easy reach of the pilot, for a damn good reason - to stop uncontrolled stabilizer trim action. While the 757 did not have MCAS, it did have a computer autopilot that could move the stabilizer.
I did some searching online of the MAX trim system, and indeed the electric trim switches override MCAS commands. In all three incidents the pilots did override it and return the trim to normal.
In the first incident, after a couple times, the crew trimmed it to normal and then cutoff the stab trim. Continued the flight and landed without further incident.
In the second, the pilots brought it back to normal 25 times before the final plunge. For whatever reason, they never switched off the trim system.
I haven't got a solid reference to the EA one, but it appears they restored trim twice before the final plunge. They then turned off the trim system in the plunge. They could not turn the manual trim wheels due to the aerodynamic forces. So they turned the trim back on, the MCAS came on again making things worse. Why they did not counter again with the electric trim I do not know. Why they did not turn off the trim when it was in the normal position I do not know. Those are excellent questions for the NTSB to answer.
But what they didn't do was follow the directions in the EAD.