Originally Posted by gasman
Then a safe way to wire the trim is to have it active only when the auto pilot is disengaged..... unless you HAVE auto trim.
You're just moving the failure point around. You'd have to have a relay in the trim circuit, activated by power to the autopilot servo, and there are ways for *that* to fail, either open or closed. Not to mention that failure of the relay might cause a failure of the autopilot itself (which could be particularly bad if, say, in IMC).
FMEAs are not as simple as just adding more complexity to the system.