Upgraded to the Garmin GFC 600 autopilot in my shop's 172 last month and the old wiring harness threw off the heading calibration for three days.

Turned out a previous repair had swapped two pins in the connector block and nobody ever caught it until the new unit flagged the mismatch, has anyone else run into hidden wiring issues after a modern avionics swap?

2 comments

2 Comments

oliverbailey17d ago

Pulled the same hair out with a G5 install last year where the magnetometer kept giving random heading errors. Found out the previous guy had stuffed a ferrous screwdriver in the panel cavity behind the glareshield and forgot it. It was just sitting there messing with the magnetic field for months. Also had a case where a shop used a non-shielded wire for the ARINC 429 bus on a GFC 500 install because they ran out of the right spec and thought it wouldn't matter. Took two weeks of chasing ghosts until someone checked the cable type with a micrometer. Little stuff like that gets buried in old airframes and only shows up when the new gear actually has decent error checking.

ryanhayes17d ago

Actually the ARINC 429 thing is a little different than most people think. Standard ARINC 429 wire is shielded twisted pair, but the twist pitch and insulation thickness actually matter a lot more than the shield itself for signal integrity. I've seen guys use regular twisted pair thinking it's fine, but the impedance mismatch causes reflection errors that look like random data dropouts. That two week ghost chase sounds about right though. The real kicker is that most digital avionics these days can actually show you the bit error rate on a diagnostic page if you know where to look. That would have saved them a week right there. Still, nobody ever reads the manual until they're desperate.