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One thing I don't understand is how the pilots can override the operation of the electric trim system by holding the manual trim wheels and yet the electric tri

One thing I don't understand is how the pilots can override the operation of the electric trim system by holding the manual trim wheels and yet the electric trim appears to be able to overcome airloads on the jackscrew that the pilots can't. In light aircraft the design of autopilot servos normally includes a clutch so that the pilot can override the servos. But that limits the force that the servo can apply. I guess it could have some kind of torsion sensing system which cuts the power to the trim motor but I've been unable to find this detailed anywhere.

janoc 7y ago
Well, the "Stabilizer wheel - grasp and hold" item of the procedure doesn't necessarily mean the crew will be able to override it. That's the last ditch option on the checklist, when everything else has failed, even turning the motors off (there could be e.g. a short circuit powering the motor). There is no clutch on the 737, AFAIK, because the forces are such that it wouldn't work. Here is a good video by an actual 737 captain explaining how the runaway stab trim procedure works and what you have to do. https://www.youtube.com/watch?v=xixM_cwSLcQ https://www.youtube.com/watch?v=xixM_cwSLcQ And this simulator video shows both how the 737 horizontal stabilizer is controlled and what kind of forces are on the trim wheel and control column when severely out of trim: https://www.youtube.com/watch?v=aoNOVlxJmow https://www.youtube.com/watch?v=aoNOVlxJmow Should give you an idea what were the Ethiopian pilots facing when unable to use the electric trim, plane trimmed nose heavy by MCAS and at high speed ...
lutorm 7y ago
My impression is that, at least partly, it's a result of the trim wheel being connected through a very long mechanical linkage to the tail, while the motor is right there. The pilots also don't have great mechanical leverage to turn a fairly small wheel in the direction they have to.
rasz 7y ago
From what I understand disabling MCAS kills electric trim and leaves you with this https://youtu.be/aoNOVlxJmow?t=11m59s https://youtu.be/aoNOVlxJmow?t=11m59s

janoc 7y ago

Well, the "Stabilizer wheel - grasp and hold" item of the procedure doesn't necessarily mean the crew will be able to override it. That's the last ditch option on the checklist, when everything else has failed, even turning the motors off (there could be e.g. a short circuit powering the motor). There is no clutch on the 737, AFAIK, because the forces are such that it wouldn't work. Here is a good video by an actual 737 captain explaining how the runaway stab trim procedure works and what you have to do. https://www.youtube.com/watch?v=xixM_cwSLcQ https://www.youtube.com/watch?v=xixM_cwSLcQ And this simulator video shows both how the 737 horizontal stabilizer is controlled and what kind of forces are on the trim wheel and control column when severely out of trim: https://www.youtube.com/watch?v=aoNOVlxJmow https://www.youtube.com/watch?v=aoNOVlxJmow Should give you an idea what were the Ethiopian pilots facing when unable to use the electric trim, plane trimmed nose heavy by MCAS and at high speed ...

lutorm 7y ago

My impression is that, at least partly, it's a result of the trim wheel being connected through a very long mechanical linkage to the tail, while the motor is right there. The pilots also don't have great mechanical leverage to turn a fairly small wheel in the direction they have to.

rasz 7y ago

From what I understand disabling MCAS kills electric trim and leaves you with this https://youtu.be/aoNOVlxJmow?t=11m59s https://youtu.be/aoNOVlxJmow?t=11m59s