Remembering the time a DME display went haywire over Lake Havasu

I was troubleshooting a King KDM 706 in a Cessna 210 about 10 years ago and the thing started showing 999 knots on the ground. Turned out a ground wire had corroded inside the connector (weird spot, right?). Has anyone else had a cryptic failure trace back to a simple bad ground like that?

2 comments

2 Comments

casey_ward511mo ago

Did you check if the DME antenna cable was sharing a bundle with any high-power stuff? I've seen a similar weird readout in a Baron where the DME ground was fine but the coax was too close to a comm antenna feed line. The RF coupling made the unit think it was seeing reflections from a target moving at mach speeds. It only showed up when the com was keyed, so it was maddening to find. Fixed it by rerouting the coax about 6 inches away and adding some ferrite clamps. Grounds are always the first suspect, but sometimes the interference path is sneakier than a bad wire.

the_lucas1mo ago

Hang on though - those ferrite clamps probably weren't doing much at DME frequencies. They're usually specced for lower bands and can actually mess with the signal if they're not the right type. Most guys I know just use them for common mode noise on audio lines, not UHF stuff.