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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 tr

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...

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...