I read the description of a pitch trim that failed to opeate.  But the logical step from the nuisance "it doesn't move when I want it to" to the hazardous "it moves when I don't want it to" isn't quite so logical.  Failing one way just isn't as likely as failing the other way.

1)  Why would the motor fail to run?  It's either not getting any power, or the motor is broken.  This can happen due to a wire connection failue ("broken wire") anywhere in the chain from the power bus to the ground.  Count the connections -- there could be as many as a dozen or more.  A pin-socket connector has three junctions -- the wire to the pin, the pin to the socket, and the socket to the wire.  The junction between the wire and the switch is another, and so forth.  A continuity break anywhere in the loop will cause the trim to not operate.

2)  Why would the motor run uncommanded?  Either the switch has failed (mechanically broken so that the contacts touch) or it is welded shut.  There's really no other way for it to happen.  We don't really put enough power through the switch to weld it, so it's really only a physically broken switch that would cause it.  Yeah, it can happen, but I wouldn't lose sleep over it.  Check it during the preflight and the likelihood of it failing in the next hour is pretty small.

- Rob Wolf