6 ms·

The cutoff switches actually disconnect the power to the trim motor, so this is unlikely unless Boeing changed something else they've told no-one about. More l

The cutoff switches actually disconnect the power to the trim motor, so this is unlikely unless Boeing changed something else they've told no-one about. More likely is that the pilots turned the electric trim back on because as the aircraft gained airspeed (a normal thing when dealing with unreliable airspeed indications), the manual trim was not effective enough. This has been tried in the simulator: https://leehamnews.com/2019/04/03/et302-used-the-cut-out-switches-to-stop-mcas/#more-29790 https://leehamnews.com/2019/04/03/et302-used-the-cut-out-swi... Generally, the manual trim and elevator can both overcome a nose-down trimmed stabilizer at the speeds seen just after takeoff. But if you don't rapidly trim back, and you let the airspeed build, you're now in a tricky situation. The manual trim is now too hard to move, increasing back pressure on the yoke is needed to counter the trim, and you don't want to throttle back, because the pitch up from the engines is all that's keeping you from diving. At this point, really your only course of action might be to re-enable electric trim. This does lead to the worrying question as to whether a runaway nose-down trim in an older 737 just after takeoff could result in the same outcome.

inferiorhuman 8y ago
This does lead to the worrying question as to whether a runaway nose-down trim in an older 737 just after takeoff could result in the same outcome. Sure could but there are parts of the MAX that make it worse. The uncommanded stab trim movement on the MAX came about because of faulty angle-of-attack data. The bad AoA data meant that the pilots were faced with a "indicated airspeed disagree" warning (and on at least one of the Lion Air flights an "elevator feel system non-op" warning). The supposition here by MentourPilot is that the IAS disagree checklist dictated that the pilots increase their speed to the point where trimming manually is difficult or impossible. On an NG or Classic I don't think there's anything that would cause both IAS disagree and uncommanded stabilizer trim.
- mhandley 8y ago
  I agree that on the NG, IAS disagree and an uncommanded stabilizer trim are much less likely to occur together. But if you did have an uncommanded nose-down trim close to the ground, you still wouldn't want to reduce throttle because that would cause a pitch-down, so it's possible you'd still end up in the same place. You'd have fewer distractions though, I agree. Anyway, as far as I know, it's never happened, and the NG has a generally good safety record.
  - inferiorhuman 8y ago
    More to the point IAS disagree and uncommanded trimming happening from the same, single failure is new to the MAX (I think). If the same thing happened on an NG it would be due to multiple failures (which is both more terrifying and less likely).
  - mannykannot 8y ago
    Prior to MCAS, most causes of trim runaway could be stopped by countermanding it with the control column, but, because MCAS is intended to avoid a pilot-commanded (though presumably accidental) stall, this is not the case for MCAS: "[MCAS intervention] can be stopped by the pilot counter-trimming on the yoke or by him hitting the CUTOUT switches on the center pedestal. It’s not stopped by the pilot pulling the yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors. This would negate why MCAS was implemented, the pilot pulling so hard on the yoke that the aircraft is flying close to stall." [1] [my emphasis.] So it seems we may have several issues here: 1) Prior to MCAS, trim stab cutout was almost the last-ditch option against trim runaway (there was one more: grab the trim wheels and physically prevent it), but it became the only way to deal with an MCAS-commanded runaway (other than grabbing the trim wheel.) 2) Boeing did not reveal this prior to the inquiry into the Lion Air crash, despite the fact that it introduced a new experience of trim runaway that differed from what pilots had been trained to expect. 3) The Boeing / FAA response to the Lion Air inquiry was an Airworthiness Directive that basically said "pay attention to what we said before about handling trim runaway" (though, whether or not Boeing / FAA made a point of it, pilots may well have become aware of how MCAS runaway differed from their prior training -- that seems to be the case for the Ethiopian Airlines captain, who did use the cutout switches.) 4) No one seems to have connected the dots to realize that one cause of MCAS failure (AofA sensor failure) would also cause other problems, for which the prescribed mitigation procedure made dealing with the consequent trim runaway very difficult, especially when close to the ground. I would be interested in learning, from one of the 737 pilots here, how frequently the 'grab the trim wheel' situation is presented in simulator training. [1] https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/ https://leehamnews.com/2018/11/14/boeings-automatic-trim-for...
treis 8y ago
> The manual trim is now too hard to move, increasing back pressure on the yoke is needed to counter the trim, and you don't want to throttle back, because the pitch up from the engines is all that's keeping you from diving. At this point, really your only course of action might be to re-enable electric trim That doesn't jive with the Indonesian Air crash or how MCAS works. MCAS makes repeated minor adjustments and the pilots can compensate for them. Which the Indonesian pilots did 20 some times before inexplicably stopping. You can see it pretty clearly in this image: https://spectrum.ieee.org/image/MzE4Mjg2OQ.jpeg https://spectrum.ieee.org/image/MzE4Mjg2OQ.jpeg So I don't see how the Ethiopian air gets into a situation where they can't manually trim. They should have been able to get back to a neutral stabilizer position, disabled electronic control, and manually trim from there.
- inferiorhuman 8y ago
  So I don't see how the Ethiopian air gets into a situation where they can't manually trim. The Ethiopian flight was flying dramatically faster than a typical flight at that altitude would. If they followed the IAS disagree checklist that would explain the increased air speed as the general idea is to give the airplane enough thrust avoid a stall until you figure out which side has the correct air speed. Higher airspeed = more difficult to manually trim. They should have been able to get back to a neutral stabilizer position, disabled electronic control, and manually trim from there. They were barely 1,000 ft above the ground (if that). You could easily lose 1,000 ft fiddling with the elevator to unload the stabilizer to manually trim it into a safe position. The Lion Air crew had far more altitude to work with (they were at 6,000-7,000 ft at their highest IIRC). FWIW I think whoever's downvoting some of these comments is really hindering the discourse here. I think what you've posted is factually wrong, but not worthy of a downvote because it mostly seems to be wrong due to missing pieces from the puzzle (and not out of malice).
  - treis 8y ago
    >Higher airspeed = more difficult to manually trim. The point, which I think you missed, is that you don't need to manually trim. Look at the image I linked to. It's MCAS down 2.5 degrees. Pilots up 2.5 degrees. MCAS down 2.5 degrees. Pilots up 2.5 degrees. It's a repeated cycle of the pilots getting trim to where they want it and the MCAS kicking in to nose them down. I suppose if you wait until the MCAS has trimmed all the way you might find yourself in an unrecoverable situation. But I'm not sure how that would happen and doesn't reflect what we saw in the Indoneisa Air crash.

inferiorhuman 8y ago

This does lead to the worrying question as to whether a runaway nose-down trim in an older 737 just after takeoff could result in the same outcome. Sure could but there are parts of the MAX that make it worse. The uncommanded stab trim movement on the MAX came about because of faulty angle-of-attack data. The bad AoA data meant that the pilots were faced with a "indicated airspeed disagree" warning (and on at least one of the Lion Air flights an "elevator feel system non-op" warning). The supposition here by MentourPilot is that the IAS disagree checklist dictated that the pilots increase their speed to the point where trimming manually is difficult or impossible. On an NG or Classic I don't think there's anything that would cause both IAS disagree and uncommanded stabilizer trim.

mhandley 8y ago
I agree that on the NG, IAS disagree and an uncommanded stabilizer trim are much less likely to occur together. But if you did have an uncommanded nose-down trim close to the ground, you still wouldn't want to reduce throttle because that would cause a pitch-down, so it's possible you'd still end up in the same place. You'd have fewer distractions though, I agree. Anyway, as far as I know, it's never happened, and the NG has a generally good safety record.
- inferiorhuman 8y ago
  More to the point IAS disagree and uncommanded trimming happening from the same, single failure is new to the MAX (I think). If the same thing happened on an NG it would be due to multiple failures (which is both more terrifying and less likely).
- mannykannot 8y ago
  Prior to MCAS, most causes of trim runaway could be stopped by countermanding it with the control column, but, because MCAS is intended to avoid a pilot-commanded (though presumably accidental) stall, this is not the case for MCAS: "[MCAS intervention] can be stopped by the pilot counter-trimming on the yoke or by him hitting the CUTOUT switches on the center pedestal. It’s not stopped by the pilot pulling the yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors. This would negate why MCAS was implemented, the pilot pulling so hard on the yoke that the aircraft is flying close to stall." [1] [my emphasis.] So it seems we may have several issues here: 1) Prior to MCAS, trim stab cutout was almost the last-ditch option against trim runaway (there was one more: grab the trim wheels and physically prevent it), but it became the only way to deal with an MCAS-commanded runaway (other than grabbing the trim wheel.) 2) Boeing did not reveal this prior to the inquiry into the Lion Air crash, despite the fact that it introduced a new experience of trim runaway that differed from what pilots had been trained to expect. 3) The Boeing / FAA response to the Lion Air inquiry was an Airworthiness Directive that basically said "pay attention to what we said before about handling trim runaway" (though, whether or not Boeing / FAA made a point of it, pilots may well have become aware of how MCAS runaway differed from their prior training -- that seems to be the case for the Ethiopian Airlines captain, who did use the cutout switches.) 4) No one seems to have connected the dots to realize that one cause of MCAS failure (AofA sensor failure) would also cause other problems, for which the prescribed mitigation procedure made dealing with the consequent trim runaway very difficult, especially when close to the ground. I would be interested in learning, from one of the 737 pilots here, how frequently the 'grab the trim wheel' situation is presented in simulator training. [1] https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/ https://leehamnews.com/2018/11/14/boeings-automatic-trim-for...

treis 8y ago

> The manual trim is now too hard to move, increasing back pressure on the yoke is needed to counter the trim, and you don't want to throttle back, because the pitch up from the engines is all that's keeping you from diving. At this point, really your only course of action might be to re-enable electric trim That doesn't jive with the Indonesian Air crash or how MCAS works. MCAS makes repeated minor adjustments and the pilots can compensate for them. Which the Indonesian pilots did 20 some times before inexplicably stopping. You can see it pretty clearly in this image: https://spectrum.ieee.org/image/MzE4Mjg2OQ.jpeg https://spectrum.ieee.org/image/MzE4Mjg2OQ.jpeg So I don't see how the Ethiopian air gets into a situation where they can't manually trim. They should have been able to get back to a neutral stabilizer position, disabled electronic control, and manually trim from there.

inferiorhuman 8y ago
So I don't see how the Ethiopian air gets into a situation where they can't manually trim. The Ethiopian flight was flying dramatically faster than a typical flight at that altitude would. If they followed the IAS disagree checklist that would explain the increased air speed as the general idea is to give the airplane enough thrust avoid a stall until you figure out which side has the correct air speed. Higher airspeed = more difficult to manually trim. They should have been able to get back to a neutral stabilizer position, disabled electronic control, and manually trim from there. They were barely 1,000 ft above the ground (if that). You could easily lose 1,000 ft fiddling with the elevator to unload the stabilizer to manually trim it into a safe position. The Lion Air crew had far more altitude to work with (they were at 6,000-7,000 ft at their highest IIRC). FWIW I think whoever's downvoting some of these comments is really hindering the discourse here. I think what you've posted is factually wrong, but not worthy of a downvote because it mostly seems to be wrong due to missing pieces from the puzzle (and not out of malice).
- treis 8y ago
  >Higher airspeed = more difficult to manually trim. The point, which I think you missed, is that you don't need to manually trim. Look at the image I linked to. It's MCAS down 2.5 degrees. Pilots up 2.5 degrees. MCAS down 2.5 degrees. Pilots up 2.5 degrees. It's a repeated cycle of the pilots getting trim to where they want it and the MCAS kicking in to nose them down. I suppose if you wait until the MCAS has trimmed all the way you might find yourself in an unrecoverable situation. But I'm not sure how that would happen and doesn't reflect what we saw in the Indoneisa Air crash.