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Please don’t flag replies like this; thankfully I was able to vouch for Walt. For some background, Walt and I are routinely brigaded with downvotes on 737 MAX

Please don’t flag replies like this; thankfully I was able to vouch for Walt. For some background, Walt and I are routinely brigaded with downvotes on 737 MAX threads. What he and I have in common, compared to those who engage in such behaviors, is that unlike those we typically are replying to, we both have extensive backgrounds in the aviation field, especially on the programming side. Back on topic, the takeaway that Walt is making and that seems to be missed continually in these threads, is that Lion Air did not lose their plane due to the MAX’s MCAS implementation. Rather, the pilots engaged with the plane in a manner precisely opposite to what procedure calls for. Despite all of the poor calls made by Boeing here, if LA’s pilots had simply reacted as the previous flight’s pilots had, the plane would have made it.

ummonk 5y ago
Why are you bringing up Lion Air pilots when the discussion is about the Ethiopian Airlines flight?
- WalterBright 5y ago
  The first LA crew was not aware of the existence of MCAS, nor did they have the benefit of the Emergency Airworthiness Directive. But they worked the problem and solved it and landed safely.
WalterBright 5y ago
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.

ummonk 5y ago

Why are you bringing up Lion Air pilots when the discussion is about the Ethiopian Airlines flight?

WalterBright 5y ago
The first LA crew was not aware of the existence of MCAS, nor did they have the benefit of the Emergency Airworthiness Directive. But they worked the problem and solved it and landed safely.

WalterBright 5y ago

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.