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Electric trim question Max and others

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Electric trim question Max and others

Old 25th Mar 2019, 21:01
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Electric trim question Max and others

In the airplane I fly sometimes, there is an emergency trim interrupt switch on the yoke. If I mash it down, the trim won't run *as long as I hold it*, but whatever issue was running it before would do it again as soon as I let go if the autopilot was not the culprit or had decided to ignore the disconnect request. So when it decided to go nuts once or twice, the routine was to mash the button with your left thumb and pull the trim breaker with your right hand and revert to manual trim. Is the 737 Max anything like this?
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Old 25th Mar 2019, 22:49
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Not sure about the MAX but the -800 has stab trim cutout switches on the throttle quadrant. Flip them and it disconnects the electric trim.
There is also a manual trim wheel with a physical connection to the stabilizer which you can use to trim with.

Not sure how MCAS affect this though
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Old 26th Mar 2019, 02:00
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If you push trim buttons on MAX it will stop MCAS for 5 seconds. Then it will start moving again. As was already said if you want to prevent it from doing anything you have to switch off STAB TRIM CUTOUT switches on the throttle quadrant.
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Old 26th Mar 2019, 09:57
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It has the same as the 800 but renamed.... but same stuff
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Old 26th Mar 2019, 09:58
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Originally Posted by Rarife View Post
If you push trim buttons on MAX it will stop MCAS for 5 seconds. Then it will start moving again.
IF it still senses high alpha.
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Old 28th Mar 2019, 10:53
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I know some people are saying that the Stab Trim Cutout switches near the quadrant, cut the circuit to the stabaliser... But is this just the signal circuitry, or all d.c. power to the Screwjack as well.
I thought the Screwjack contains a transistorised power supply circuit that produces an a.c. supply for the brushless motors, which are required to turn the screwjack nut..
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Old 29th Mar 2019, 03:32
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Originally Posted by scifi View Post
I know some people are saying that the Stab Trim Cutout switches near the quadrant, cut the circuit to the stabiliser... But is this just the signal circuitry, or all d.c. power to the Screwjack as well.
I thought the Screwjack contains a transistorised power supply circuit that produces an a.c. supply for the brushless motors, which are required to turn the screwjack nut..
The stab trim electric motor is a 270 volt DC motor. However, the aircraft supplies 3 phase 115 volt AC power to the actuator assembly in the tail. Power is converted from AC to DC. The autopilot and the column trim switches provide a smaller dc voltage to the control relays in the actuator assembly. The cutout switches on the quadrant stop the smaller DC signals from reaching the trim switches and also stop (via a big relay) AC power from reaching the actuator assembly.

There are relays and power converters in the tail assembly, but unless the converter is solid state, I can't imagine there are too many transistors there.
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