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>It seems the electric trim switches (on the control column) override the MCAS commands. ...but not if you follow the procedure set out by Boeing and reiterate

>It seems the electric trim switches (on the control column) override the MCAS commands. ...but not if you follow the procedure set out by Boeing and reiterated in an FAR published after the Lion Air crash. The ET302 pilots recognized that they were dealing with MCAS failure, and followed the procedure, including toggling the trim cutout switches. On 737 NGs, one could turn off trim automation while allowing electrical control from the yoke, but on the Max, the latter was also disabled when these switches were toggled [1]. I don't think this way in which the Max differs from the NG came out until later. This created a second problem - the trim wheels are very hard to turn manually under the load created by using the elevator to counteract the mis-trim. The procedure in this case is to unload the stabilizer jack-screw by pitching down, but the airplane was not high enough for that. This might have been apparent if pilots had trained for an MCAS failure, which brings us to a question I had in another thread: Were there any simulators that realistically simulated MCAS failure, or were the simulator manufacturers and operators as much in the dark about MCAS as were pilots and airlines? [1] https://www.seattletimes.com/business/boeing-aerospace/boeing-altered-key-switches-in-737-max-cockpit-limiting-ability-to-shut-off-mcas/ https://www.seattletimes.com/business/boeing-aerospace/boein...

WalterBright 7y ago
What you're saying does not contradict that the trim switches override the MCAS. Hence, trim could be restored by using the trim switches and throwing the cutouts when not using the trim switches. I read about this in the newspaper, it wasn't a secret. I do not have inside information. Again, if I was an MCAS pilot I'd be very interested in following what happened with the Lion Air crash, to ensure it wouldn't happen to me. I'd still follow the procedures set out by Boeing - but when they didn't work I'd do what did work. It's a little hard to discern from the erratic newspaper reports, but during the time the pilots were fighting the MCAS they were using the trim switches and surely would have noticed that those switches overrode the MCAS. The reason we have pilots, and not a fully automated airplane, is we need them to think outside of the procedures when the procedures aren't working. This is not impossible as the pilots in the flight prior to the LA crash did have MCAS runaway and did successfully deal with it.
- mannykannot 7y ago
  Do you recall when you learned that the trim switches could be used to override MCAS (so long as you did not toggle the trim cutouts, if flying a Max?) I don't believe the switch issue was raised in the FAR. While we certainly hope that pilots will be able to extemporize a solution to a situation, even if that involves going beyond or against the documented procedure, and while it is certainly possible that there was a course of action that could have saved ET302 (though the success of the Lion Air pilots does not necessarily prove that, as there were differences), I do not accept the implication that if such a course of action existed, there is nothing to be concerned about. Reliance on pilots' problem-solving skills should be a last resort, not a planned-for solution, and those skills certainly are not helped by withholding information about aircraft systems.
  - WalterBright 7y ago
    It was in one of the Seattle Times articles about it after the LA crash. I've never seen it since, although if you read the sequence of events it's pretty clear that's the case, as the pilots had been overriding the MCAS commands with the trim switches. > I do not accept the implication that if such a course of action existed, there is nothing to be concerned about. I did not suggest that. Nearly all crashes are the result of a string of failures, including these crashes, and each point needs to be corrected. > and those skills certainly are not helped by withholding information about aircraft systems. After the LA crash, that was no longer the case. I reiterate that my information comes from the newspaper. If I was a MAX pilot you bet I'd be much more diligent in finding things out, if only just to save my own hide. I further find it very strange that after the prior LA flight successfully dealt with the MCAS failure, that the airplane was still deemed flight-worthy and nobody even bothered to inform the next flight crew? There's plenty of failure that needs to be dealt with all around with those accidents.
    - mannykannot 7y ago
      With regard to why the Lion Air airplane was deemed flight-worthy, we have to take into consideration that, because MCAS had not been disclosed, some important information was missing from the analysis. It is unfortunate that the maintenance crew incorrectly thought that they had fixed the sensor, and perhaps felt that was enough to deal with the problem, but they may not have known how aggressively this undisclosed system could mis-trim the airplane, and the pilots certainly did not (it was even a surprise to many at the FAA involved in the Max certification.) Maybe the crew of the earlier Lion Air flight were lucky and had the airplane temporarily back in trim at the point where the deadheading pilot recognized that they had an ongoing trim runaway situation, for which toggling the trim cutout switches was the documented response [1]. I think it is unreasonable to suggest that the EL302 pilots were negligently unprepared. After using the trim cutout switches to disable MCAS, they found they were unable to correct the trim manually, so they did exactly what you suggested in your first post: they re-engaged the electric trim and attempted, unsuccessfully, to re-trim the airplane before it crashed. This suggests a familiarity with the AD issued after the Lion Air inquiry. Maybe if they had not switched the trim off initially, they would have prevailed, but the message from Boeing after the Lion Air crash was that its pilots were at fault because they failed to use the cutout switches. Given that Boeing was still holding to the line that MCAS failure should be handled as if it were just another case of trim runaway, and that the procedure for that was the same as on previous variants, the pilots could reasonably consider themselves to be well-prepared for the situation - they now knew about the problem, and they had trained for recovering from trim runaway on NG simulators. The AD issued after the Lion Air inquiry says "Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT", but it says nothing about this differing from the NG, where one can disable faulty inputs while retaining trim-switch control. It also says "If relaxing the control column causes the trim to move, set stabilizer trim switches to CUTOUT" - and it seems quite possible that, in the case of MCAS runaway, relaxing the control column could coincide with the trim continuing to move (and the crew is not in a position to determine causation), at which point, using the cutout switches is prescribed. Finally, this checklist [1] says "If the runaway continues after the autopilot is disengaged: STAB TRIM CUTOUT switches (both) [to] CUTOUT." None of this says anything directly about getting the airplane back in trim before toggling the cutout switches, if that is what is necessary to regain control if this happens close to the surface. [1] https://www.washingtonpost.com/the-boeing-737-max-8-aircraft-runaway-stabilizer-checklist/2b14d1c4-bace-48f2-994b-cb1a75f860c5_note.html https://www.washingtonpost.com/the-boeing-737-max-8-aircraft...

WalterBright 7y ago

What you're saying does not contradict that the trim switches override the MCAS. Hence, trim could be restored by using the trim switches and throwing the cutouts when not using the trim switches. I read about this in the newspaper, it wasn't a secret. I do not have inside information. Again, if I was an MCAS pilot I'd be very interested in following what happened with the Lion Air crash, to ensure it wouldn't happen to me. I'd still follow the procedures set out by Boeing - but when they didn't work I'd do what did work. It's a little hard to discern from the erratic newspaper reports, but during the time the pilots were fighting the MCAS they were using the trim switches and surely would have noticed that those switches overrode the MCAS. The reason we have pilots, and not a fully automated airplane, is we need them to think outside of the procedures when the procedures aren't working. This is not impossible as the pilots in the flight prior to the LA crash did have MCAS runaway and did successfully deal with it.

mannykannot 7y ago
Do you recall when you learned that the trim switches could be used to override MCAS (so long as you did not toggle the trim cutouts, if flying a Max?) I don't believe the switch issue was raised in the FAR. While we certainly hope that pilots will be able to extemporize a solution to a situation, even if that involves going beyond or against the documented procedure, and while it is certainly possible that there was a course of action that could have saved ET302 (though the success of the Lion Air pilots does not necessarily prove that, as there were differences), I do not accept the implication that if such a course of action existed, there is nothing to be concerned about. Reliance on pilots' problem-solving skills should be a last resort, not a planned-for solution, and those skills certainly are not helped by withholding information about aircraft systems.
- WalterBright 7y ago
  It was in one of the Seattle Times articles about it after the LA crash. I've never seen it since, although if you read the sequence of events it's pretty clear that's the case, as the pilots had been overriding the MCAS commands with the trim switches. > I do not accept the implication that if such a course of action existed, there is nothing to be concerned about. I did not suggest that. Nearly all crashes are the result of a string of failures, including these crashes, and each point needs to be corrected. > and those skills certainly are not helped by withholding information about aircraft systems. After the LA crash, that was no longer the case. I reiterate that my information comes from the newspaper. If I was a MAX pilot you bet I'd be much more diligent in finding things out, if only just to save my own hide. I further find it very strange that after the prior LA flight successfully dealt with the MCAS failure, that the airplane was still deemed flight-worthy and nobody even bothered to inform the next flight crew? There's plenty of failure that needs to be dealt with all around with those accidents.
  - mannykannot 7y ago
    With regard to why the Lion Air airplane was deemed flight-worthy, we have to take into consideration that, because MCAS had not been disclosed, some important information was missing from the analysis. It is unfortunate that the maintenance crew incorrectly thought that they had fixed the sensor, and perhaps felt that was enough to deal with the problem, but they may not have known how aggressively this undisclosed system could mis-trim the airplane, and the pilots certainly did not (it was even a surprise to many at the FAA involved in the Max certification.) Maybe the crew of the earlier Lion Air flight were lucky and had the airplane temporarily back in trim at the point where the deadheading pilot recognized that they had an ongoing trim runaway situation, for which toggling the trim cutout switches was the documented response [1]. I think it is unreasonable to suggest that the EL302 pilots were negligently unprepared. After using the trim cutout switches to disable MCAS, they found they were unable to correct the trim manually, so they did exactly what you suggested in your first post: they re-engaged the electric trim and attempted, unsuccessfully, to re-trim the airplane before it crashed. This suggests a familiarity with the AD issued after the Lion Air inquiry. Maybe if they had not switched the trim off initially, they would have prevailed, but the message from Boeing after the Lion Air crash was that its pilots were at fault because they failed to use the cutout switches. Given that Boeing was still holding to the line that MCAS failure should be handled as if it were just another case of trim runaway, and that the procedure for that was the same as on previous variants, the pilots could reasonably consider themselves to be well-prepared for the situation - they now knew about the problem, and they had trained for recovering from trim runaway on NG simulators. The AD issued after the Lion Air inquiry says "Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT", but it says nothing about this differing from the NG, where one can disable faulty inputs while retaining trim-switch control. It also says "If relaxing the control column causes the trim to move, set stabilizer trim switches to CUTOUT" - and it seems quite possible that, in the case of MCAS runaway, relaxing the control column could coincide with the trim continuing to move (and the crew is not in a position to determine causation), at which point, using the cutout switches is prescribed. Finally, this checklist [1] says "If the runaway continues after the autopilot is disengaged: STAB TRIM CUTOUT switches (both) [to] CUTOUT." None of this says anything directly about getting the airplane back in trim before toggling the cutout switches, if that is what is necessary to regain control if this happens close to the surface. [1] https://www.washingtonpost.com/the-boeing-737-max-8-aircraft-runaway-stabilizer-checklist/2b14d1c4-bace-48f2-994b-cb1a75f860c5_note.html https://www.washingtonpost.com/the-boeing-737-max-8-aircraft...