33 ms·

Some comments based on my understanding of the situation. I'm an engineer and I'm interested in the study of engineering failures. So, I have been following t

by nas 8y ago

Some comments based on my understanding of the situation. I'm an engineer and I'm interested in the study of engineering failures. So, I have been following this. Initially I was thinking the blame was being unfairly placed on the MCAS system. However, now I understand more, I suspect it is a serious safety problem. The reason is as you say, MCAS runaway (e.g. due to a failed AoA sensor) does not behave like a normal runaway trim (e.g. continuous trim movement). That is key. Watching Youtube videos was helpful in understanding what the pilot experiences. I initially thought the pilot should easily notice the trim moving because of the large trim wheel beside them. However, in normal takeoff, the trim is being adjusted automatically and so pilots learn to ignore it changing. Second, I think with the 737 MAX, the sound made by the trim wheel has been changed (removed)? Using the stab trim cutout switches is a drastic measure (occurs as nearly last item on memory checklist) and, as I understand it, the only way to stop a misbehaving MCAS from doing its bad work. If the pilots are even a little slow in figuring out what's happening, I can easily imagine that leading to disaster. Also, if they to realize the problem and flick the switches off, manually cranking the trim wheel the other way takes time and it takes some serious physical strength to do it. I can imagine that if the MCAS moves the trim fully the wrong way, the pilots might not be able to fix it quickly enough.

msbarnett 8y ago
> The reason is as you say, MCAS runaway (e.g. due to a failed AoA sensor) does not behave like a normal runaway trim (e.g. continuous trim movement). That is key. Watching Youtube videos was helpful in understanding what the pilot experiences. I initially thought the pilot should easily notice the trim moving because of the large trim wheel beside them. However, in normal takeoff, the trim is being adjusted automatically and so pilots learn to ignore it changing. Yeah, that's the 737's Speed Trim system. Even before the MCAS, automatic trim adjustments to adjust for the '37s behaviour at different speeds were a constant thing in the cockpit. It seems like one possibility here is that pilots may be mistaking or unable to distinguish (since there's no real differentiation) the MCAS' trim adjustments from normal Speed Trim adjustments -- and keep in mind a speed warning is going off the entire time they're fighting the MCAS, due to the malfunctioning AOA, so they may be primed to think "Speed Warning + Automated Trim periodically occurring == Speed Trim" and over-focus on the stall & speed warnings and ignore the trim component. > Also, if they to realize the problem and flick the switches off, manually cranking the trim wheel the other way takes time and it takes some serious physical strength to do it. I can imagine that if the MCAS moves the trim fully the wrong way, the pilots might not be able to fix it quickly enough. It's actually worse than that. If the AOA sensor is misbehaving, then it will never show a recovery after the MCAS applies an automatic trim adjustment, leading to the MCAS continuing to apply trim adjustments until the trim is at maximum. At this point, the aircraft is basically in a nose dive, and if they hit the trim cutout, it can be physically impossible to manually trim the aircraft An Avionics engineer discusses the issue here: https://www.satcom.guru/2019/03/aoa-vane-must-have-failed-boeing-fix.html https://www.satcom.guru/2019/03/aoa-vane-must-have-failed-bo... > The standard response to just hit the stabilizer cutout switches and manually trim is actually flawed. If the nose has been pushed down by significant mistrim (nose down stabilizer, nose up elevator), and as airspeed increases, it may not be possible to trim the stabilizer manually nose up without letting the elevator go to a neutral position. The reality, under the MCAS runaway scenario, trimming nose up immediately stops MCAS as well as trims the stabilizer back towards an in-trim position. At that point, you would be best off to cutout the stabilizer. > Many flight crews may not know that you have to relax the elevator to manually trim the stabilizer if the loads is too high. It also mentions that the techniques needed to relieve the forces enough to make trimming manually possible are not necessarily covered in Boeing's current manuals -- because they completely ignored the possibility of the MCAS trimming to max downwards.

by nas 8y ago

msbarnett 8y ago

> The reason is as you say, MCAS runaway (e.g. due to a failed AoA sensor) does not behave like a normal runaway trim (e.g. continuous trim movement). That is key. Watching Youtube videos was helpful in understanding what the pilot experiences. I initially thought the pilot should easily notice the trim moving because of the large trim wheel beside them. However, in normal takeoff, the trim is being adjusted automatically and so pilots learn to ignore it changing. Yeah, that's the 737's Speed Trim system. Even before the MCAS, automatic trim adjustments to adjust for the '37s behaviour at different speeds were a constant thing in the cockpit. It seems like one possibility here is that pilots may be mistaking or unable to distinguish (since there's no real differentiation) the MCAS' trim adjustments from normal Speed Trim adjustments -- and keep in mind a speed warning is going off the entire time they're fighting the MCAS, due to the malfunctioning AOA, so they may be primed to think "Speed Warning + Automated Trim periodically occurring == Speed Trim" and over-focus on the stall & speed warnings and ignore the trim component. > Also, if they to realize the problem and flick the switches off, manually cranking the trim wheel the other way takes time and it takes some serious physical strength to do it. I can imagine that if the MCAS moves the trim fully the wrong way, the pilots might not be able to fix it quickly enough. It's actually worse than that. If the AOA sensor is misbehaving, then it will never show a recovery after the MCAS applies an automatic trim adjustment, leading to the MCAS continuing to apply trim adjustments until the trim is at maximum. At this point, the aircraft is basically in a nose dive, and if they hit the trim cutout, it can be physically impossible to manually trim the aircraft An Avionics engineer discusses the issue here: https://www.satcom.guru/2019/03/aoa-vane-must-have-failed-boeing-fix.html https://www.satcom.guru/2019/03/aoa-vane-must-have-failed-bo... > The standard response to just hit the stabilizer cutout switches and manually trim is actually flawed. If the nose has been pushed down by significant mistrim (nose down stabilizer, nose up elevator), and as airspeed increases, it may not be possible to trim the stabilizer manually nose up without letting the elevator go to a neutral position. The reality, under the MCAS runaway scenario, trimming nose up immediately stops MCAS as well as trims the stabilizer back towards an in-trim position. At that point, you would be best off to cutout the stabilizer. > Many flight crews may not know that you have to relax the elevator to manually trim the stabilizer if the loads is too high. It also mentions that the techniques needed to relieve the forces enough to make trimming manually possible are not necessarily covered in Boeing's current manuals -- because they completely ignored the possibility of the MCAS trimming to max downwards.