Originally Posted by
Uplinker
Back in the day when the pitch trim was a simple switch controlling an electric motor, and aircraft were electrical and hydro-mechanical only; two switches wired in series and two rockers were a sensible way of preventing trim runaways over a single basic switch arrangement, since any basic electrical switch can potentially collapse internally or weld on.
I don't dispute that simple series switch configurations may be used on some aircraft but I don't know of any. More typically each of the switches in the pair controls a separate function and both function states have to agree for trim to run. E.g. -
MD-80 uses an electric trim motor controlled by two switches. One switch controls the motor drive relays and the other switch controls the brake solenoid. Trim requires motor to be powered and the brake to be released.
MD-11 uses hydraulic trim motors controlled by two switches. One switch controls the solenoid valve directional spool and the other switch controls the solenoid valve shut-off spool. Trim requires both spools to be energized for trim to run.