Originally Posted by
eckhard
I don’t know about the 737-max, but on the “classic” -300 and -400, we had a “Control-column mounted Stab Trim cut-out Override” switch. Its function was to enable continued use of the stab trim switches after a stuck or jammed elevator event.
The “Control-column cut-out” was a device to enable the pilot to stop a stab runaway by applying opposite elevator. The system would recognise the opposite input and remove power from the stab motors.
The downside of this system was that in the event of a stuck elevator, the cut-out would prevent the pilot from moving the stab. The “Override” switch was there to disable the system in this event, thus restoring stab control to the pilot, notwithstanding the stuck elevator.
The switch was mounted on the aft pedestal and its function became a favourite question of mine during route checks. Few pilots knew what it was for.
Anyway, I wonder if this system (or its malfunction) may have been a factor in these accidents?
eckhard - 737 column cutout switches prevent stab commands from pilot wheel mounted pitch trim switches or STS from moving the stabilizer in the direction opposite the column. As you mention, there are separate, manually controlled, cutout override switches that allow the crew to trim the stabilizer opposite the column to mitigate a jammed column. I don't see how these would play a role in an event where pitch control is compromised and the pilot is commanding stabilizer trim in the same direction as the column. No cutout function in that event and thus no need/function for the cutout override switch.