Originally Posted by
Uplinker
Two rockers don't actually give any particular advantage, because if one jams, the other won't do anything, even if it still moves.
Let's say the left switch jams up (AND). When the pilot releases the AND input the right switch will return to center and trim will stop. The other switch did something - It stopped the trim.
Pilot doesn't notice the left switch is stuck and provides a second AND input. Trim responds as commanded. Again the other switch did something. It allowed the required trim change.
Then pilot need ANU trim and tries to move both switches down. He now notices that the left switch is in the wrong place and can't be moved and uses alternate trim. In this case the other switch did not do anything.
All the types I have worked on had two trim switches. I don't know the reason for changing to a single switch.