737 Pitch Trim Related Switches
There seems to be some confusion with regard to 737 switches associated with pitch trim. That is very understandable as there are quite a few and it is easy to get them mixed up. Let me take a run at an overview:
Wheel mounted pilot electric pitch trim switches - two pairs of side-by-side switches with one pair mounted on each of the two control wheels for thumb activation
- Spring centered with contacts in both the fwd/push and aft/pull directions.
- Fwd/push commands airplane nose down stabilizer motion via electric stab trim motor
- Aft/pull commands airplane nose up stabilizer motion via electric stab trim motor
- Stabilizer motion requires activation of both switches found on the same wheel in the same direction. This design avoids unintended stabilizer motion for a failure of one switch or a short in one of the two associated wiring circuits.
Column cutout switches - one each in push and pull directions
- Not visible to the crew, hidden within the column mechanism
- One is activated for column push beyond a certain threshold, the other for column pull beyond a certain threshold
- These prevent pilot electric pitch trim in the direction opposite the column (nose up trim inhibited when column is pushed for example)
- These also serve to stop STS stabilizer commands in the direction opposite the column
- These do not stop MCAS stabilizer commands in either direction
Column cutout override switch (not sure of the location(s) or how many there are)
- When in the override position, this causes the column cutout switches to be bypassed allowing electric stabilizer trim in either direction regardless of column position
- These do not have any role with respect to normal operation
- These have no function with respect to MCAS as MCAS stabilizer commands are not subject to the column cutout functionality that is overridden here
Stabilizer cutout switches - on pair of guarded switches located on the lower, aft end of the aislestand
- These are the switches that when activated remove electric power to the stabilizer trim motor and thus disable all electric trim (pilot wheel switches, STS, MCAS, autopilot offload)
- These are the switches that have been most discussed in this thread
- These are the switches that the runaway stabilizer procedure calls for toggling to stop errant stab motion
I hope this helps.
Last edited by FCeng84; 13th Mar 2019 at 03:49.
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