Originally Posted by
DaveReidUK
And not the aerodynamic load on the stab ?
I believe it is the aerodynamic load causing it, but indirectly. I believe the aerodynamic loads on the stabilizer and the elevator cause increased friction between the nut and the jack screw.
Originally Posted by
Grebe
I think (member berry ) meant to say --Mentour's video it was getting harder **** and harder to trim ANU *** since the 'problem' to correct was severe AND made so by simulating ( MCAS ) which trimmed AND at a faster rate than yoke switch trims ANU. And of course maybe the same problem with the opposite ANU runaway ?
No, I didn't mean that. My original comment is accurate. To replicate the behavior of MCAS the FO was trimming manually AND (because with the flaps up the electric trim can't move the stab AND as much as MCAS can), and it was becoming harder and harder to trim AND. At 2.5 units from full AND trim it was almost impossible to trim further AND. Then, to see what the pilots on the Ethiopian flight might have experienced, the FO briefly tried to revert the AND trim he applied earlier, so he started trimming ANU. That was also impossible. So basically at 2.5 units from full AND the manual trim wheels became stuck, they couldn't be moved in either direction in that simulator.
Originally Posted by
Europa01
I note your reply on the clutch thanks. My logic for my description of the clutch /trim wheel / stab motor went like this:-
If holding the trim wheel against rotation from the stab motor stops HS movement then that is the torque at which the clutch disengages - this will be the maximum torque the stab motor can exert.
In a simple system, where the electric motor would be connected directly to the cable drum, you would be correct. But the electric motor is not connected directly to the cable drum, it's connected to it through the gear box, so there are two torques involved:
- between the aft cable drum and the gearbox (close to 0 if the trim wheels in the cockpit are allowed to move freely, even if the electric motor is active)
- between the electric motor and the gearbox (this one will be high when the electric motor is active, and the aerodynamic load on the stabilizer is large).
What I was saying is that it is the torque between the aft cable drum and the gear box which causes the clutch between the electric motor and the gearbox to disconnect. So, if you don't touch the trim wheels, the clutch shouldn't disconnect. This is designed to give the trim wheels authority over the electric motor. Of course the electric motor has additional protections to prevent it from being over torqued and overheating, but those protections are independent from what I've described above.