A stubborn autopilot fault on a Cessna Citation had me stumped for two full shifts

It was a pitch trim runaway that kept coming back after every reset. I finally found the issue by checking the harness behind the panel for chafing, and sure enough, there was a wire rubbing against a bracket. I had to splice in a new section and add some extra loom. Has anyone else had a trim system fault that turned out to be a simple wiring problem like this?

4 comments

4 Comments

spencer7822mo ago

Tbh, chasing a gremlin like that for two days is the worst. I swear half the "complex system faults" I see are just a wire that got tired of its job and decided to rub itself to death on a random bracket. It's always the last place you look, after you've already swapped three perfectly good boxes.

wood.eric2mo ago

Wire that got tired of its job" is the perfect way to put it, man.

angela_harris2mo agoMost Upvoted

Seriously, is a loose wire really that big of a deal?

jakejones23d ago

Wait, did you say you swapped three perfectly good boxes before finding the loose wire? That part doesn't sit right with me. If it's a loose wire chafing against a bracket, swapping boxes wouldn't fix it anyway. The problem is the wire physically rubbing through its insulation, not a bad component. So those boxes were probably fine from the start, which means you wasted a couple of hours and some good hardware for nothing. Next time, do a quick visual walkthrough of the wiring paths before you start swapping parts. More often than not, the gremlin is sitting right there in plain sight, just hiding behind a cable tie.