Disagreeing with the 'always bench test first' rule

I was at a hangar in Dallas last month and heard a senior tech tell a new guy to always pull the LRU and bench test it before checking the wiring. I get the logic, but on a recent King Air 350 job, that approach would have wasted half a day. The fault code pointed to the transponder, but I spent 10 minutes with the manual and found a known issue with a specific connector pin on the main avionics rack. It was a corroded pin, not a bad box. Sometimes the quickest path is to trust the fault tree and look at the system, not just swap parts. When do you guys decide to skip the bench and go straight to the aircraft wiring?

4 comments

4 Comments

mason5311mo agoMost Upvoted

Totally get that. The "always bench first" rule is for people who don't know how to read a schematic. Your King Air story is perfect. If the fault tree or service bulletin points to a specific connector or ground point, you'd be a fool not to check that wire first. I only grab the box if the diagnostics are vague or point straight to an internal failure.

daniel_carter1mo ago

That old Gulfstream SB saved us a week once.

gray31416d ago

Exactly. The manual is just a guess from an engineer who wasn't there. Saw a G1000 where the code said ADC, but it was a chafed wire bundle behind the panel. Swapped two boxes before finding it. Sometimes the book is right, sometimes your gut is. You gotta know when to follow the steps and when to look past them.

emma_jones1mo agoTop Commenter

That Dallas tech gave the only safe advice for a new guy. Last year we had a Cessna Citation come back three times for the same autopilot fault because a guy kept chasing wires. It was a bad internal board the whole time. You can't always trust a fault code or a manual, they just give you a place to start. Pulling the unit first is the only way to be sure you aren't wasting time on a wild goose chase. A half day saved once could mean three days lost on the next job.