Teddy Robinson
There have been a number of incidents/accidents where the aircraft was apparently in an auto-flight mode, trimming back to reduce speed, and at some point full power was required, the thrust line / pitch couple kicked in and resulted in a loss of available pitch down authority whilst the stab trim took time to readjust to the demand.
Out of interest, is this switch in any way related to the mitigation of this issue ?
I'm sorry my initial response to your question was short and a bit rude:
No. It's to allow stab trim operation in the case of a jammed elevator.
This situation you talk about is indeed a potential occurence on the 737, but is not related to this switch. The 737 has an annunciator light "STAB OUT OF TRIM" which will illuminate during an auto go-around if the autoflight system has not had sufficient time to trim forward to allow a low altitude capture. In this situation the pilot must disconnect the autopilot and capture the altitude manually. Manual flight allows stronger control inputs than those that limit the autopilot (and faster trim IIRC).
That's according to the manuals - I've never seen it actually happen in either the simulator or the aircraft.
To clarify the jammed elevator scenario, if the elevator is jammed neutral - no problem. If the elevator is jammed aft, you will not be able to use electric trim to pitch down (or prevent further pitch up) unless you activate this switch (and vice versa if the elevator is jammed forward). This should be a memory item in my opinion because the aircraft may be unrecoverable by the time you find this in the NNC, and it's amazing how many guys in my outfit either never knew or have forgotten what this switch does!
Having said that the other pitch options available are thrust and/or manual trim.