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Ethiopian Airlines Crash/ B737MAX grounding
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Please tell me the trim tab isn't actuated by a long threaded shaft attached to a motor with a 'nut' on the shaft attached to the tab?
Cutting power would just lock something like that it in the wrong position. You would need something like an explosive bolt to decouple such a mechanism in an emergency.
No, it's not at all like that, there are 2 wheels, about 12" diameter, either side of the centre console, and there is a fold out handle in each wheel to enable manual winding.
They are then linked via cables to a winding system, the problem being that it requires a massive number of turns of the wheel to make a significant change to the trim of the aircraft, the motors move the wheels at a very high revolution rate compared to what can be done winding them by hand, and if the aircraft is significantly out of trim, the control column loads are significant, which can require both hands of the flying pilot to be on the yoke to apply sufficient force to overcome the out of trim load, leaving the non flying pilot to operate the handle and wind as quickly as possible.
example
It's a very short video, but this gives an idea of how fast the wheel moves if the motor is in control. Now imagine that the motor has been running for several seconds, and you now have to manually undo what it's done, while at the same time, performing check list work to try and analyse and resolve the problem that's threatening to crash the aircraft.
Not easy, or quick, or the sort of thing that the non flying pilot is prepared for, and to make it worse, the flying pilot is probably more than fully committed to just trying to keep the control yoke in about the right place to maintain stable flight, and constantly trying to overcome the out of trim forces.Shore, if it was easy, everybody would be doin it.😁
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Irish Steve wrote: »No, it's not at all like that, there are 2 wheels, about 12" diameter, either side of the centre console, and there is a fold out handle in each wheel to enable manual winding.
They are then linked via cables to a winding system, the problem being that it requires a massive number of turns of the wheel to make a significant change to the trim of the aircraft, the motors move the wheels at a very high revolution rate compared to what can be done winding them by hand, and if the aircraft is significantly out of trim, the control column loads are significant, which can require both hands of the flying pilot to be on the yoke to apply sufficient force to overcome the out of trim load, leaving the non flying pilot to operate the handle and wind as quickly as possible.
example
It's a very short video, but this gives an idea of how fast the wheel moves if the motor is in control. Now imagine that the motor has been running for several seconds, and you now have to manually undo what it's done, while at the same time, performing check list work to try and analyse and resolve the problem that's threatening to crash the aircraft.
Not easy, or quick, or the sort of thing that the non flying pilot is prepared for, and to make it worse, the flying pilot is probably more than fully committed to just trying to keep the control yoke in about the right place to maintain stable flight, and constantly trying to overcome the out of trim forces.
Thanks Irish Steve for a great post. I can only imagine how difficult it must be for the pilots to manage this process whilst in the critical take off and climb phase of the Ethiopian flight. My long held admiration of pilots continues.0 -
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Irish Steve wrote: »No, it's not at all like that, there are 2 wheels, about 12" diameter, either side of the centre console, and there is a fold out handle in each wheel to enable manual winding.
They are then linked via cables to a winding system, the problem being that it requires a massive number of turns of the wheel to make a significant change to the trim of the aircraft, the motors move the wheels at a very high revolution rate compared to what can be done winding them by hand, and if the aircraft is significantly out of trim, the control column loads are significant, which can require both hands of the flying pilot to be on the yoke to apply sufficient force to overcome the out of trim load, leaving the non flying pilot to operate the handle and wind as quickly as possible.
example
It's a very short video, but this gives an idea of how fast the wheel moves if the motor is in control. Now imagine that the motor has been running for several seconds, and you now have to manually undo what it's done, while at the same time, performing check list work to try and analyse and resolve the problem that's threatening to crash the aircraft.
Not easy, or quick, or the sort of thing that the non flying pilot is prepared for, and to make it worse, the flying pilot is probably more than fully committed to just trying to keep the control yoke in about the right place to maintain stable flight, and constantly trying to overcome the out of trim forces.
That's not an entirely accurate representation of the automatic trim system.
There are two servo motors installed on the base of the screwjack, the main electric trim servo motor is larger than the autopilot trim servo motor.
What you're seeing is the main electric trim motor trimming the stab (via the pickle switches). The auto pilot trim servo motor doesn't trim as fast as that and doesn't trim through the same range. I can't remember the exact figures but the autopilot trim servo can trim it X number of degrees nose down and y number of degrees nose up but the main electric trim motor can trim it a few degrees further in either direction and the manual trim wheels can trim it even further in each direction, it's all controlled by limit switches.
Next time you're on a 737 (on the ground) trim it forward or aft to the stops and then see how much further you can wind it on manually.
Then try engaging the autopilot on the ground, push the control column forward and watch how much slower the autopilot trim servo trims the stab.0 -
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https://www.youtube.com/watch?v=cQirIH_DuAs
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