Spent 3 years chasing ground loops that weren't there

Was on a Gulfstream GIV in Savannah last month tracing a flickering NAV display. Pulled out the O-scope like always, saw noise, started ripping out bonding straps and checking every ground point. After 8 hours of nothing, the senior guy walks over, looks at the waveform, says "that's not noise, that's the AC ripple from your probe being too close to the inverter." Moved the probe 6 inches and the signal was clean. I had been fighting phantom problems for years because I never considered my test equipment placement. Anyone else find out they were creating their own troubleshooting headaches?

2 comments

2 Comments

the_jordan1d ago

benk26 exactly, coiled ground leads are just antennas. Learned that one on a King Air baffling myself for a day.

benk261d ago

Wait, you were using an unshielded probe on a GIV? Those inverters put out some nasty common mode noise at 400Hz, and if you're not using a differential probe or at least a properly shielded coaxial cable, you're basically turning your scope lead into an antenna. Had the same thing happen on a Challenger 604 last spring - spent two days chasing a phantom fluctuation on the EICAS only to realize my probe ground lead was coiled up near the PSU. The noise floor in those Gulfstream avionics bays is no joke either, especially if someone left the APU inverter running during troubleshooting.