Originally Posted by
Dave Therhino
My understanding is that the wire isolation concern is with the potential for 28 VDC hot shorts to either the up or down control signal wires and the arm circuit wire (the circuit that closes the R64 motor power contactor relay). Those control and arm wires are routed together over much of their runs and are co-routed with numerous 28 VDC power wires. Hot shorts to the motor power wires themselves are not the issue.
In that case the arm circuit and thus the cutout would stop the motor. Prior comments were saying that it couldn't be stopped by any reasonable crew action.
The post I replied to said "so it would not be stopped by opening the control wire enable switches on the console."
The cutout switches would do exactly what they are supposed to, disconnecting the power feed via the relay.
And other comments even implied that the trim breakers wouldn't stop the motor. Not sure where the breakers are, but I'll agree they may be difficult to reach quickly, but the cutouts are there specifically to stop runaway/unwanted uncommand movement.
The best I can tell it takes 2 signals to actual move the trim motor. The ARM signal form R1193 (MAIN TRIM ARM) enabled by the thumb switches (the upper set of contacts in the linked drawing) as well as the direction signals that pass through the limit switches.
So 2 control wires, 1 arm and 1 direction, would have to short to power. That just got a lot less likely.
Depending on where R850 (STAB TRIM INTERLOCK) is located on the NG, I might be more concerned about the NG wiring than the MAX.
(That diagram doesn't show how the NG FCC trim signals work)