Walked through the avionics bay on a 737-800 yesterday and noticed something weird about the DME antenna coax routing

The cable had a sharp 90 degree bend right behind the rack mount and I'm wondering if that's a common install issue you guys see or if this bird was just put together on a Monday morning, has anyone else had problems with signal degradation from bad coax bends?

3 comments

3 Comments

sean_barnes2420d ago

I feel you man, that 90 degree bend on the coax is a REALLY common spot for issues. I've seen it on at least three 737s where the cable was pinched right behind the rack mount like that, and it always messes with the signal. The problem is the center conductor gets stressed and can break internally, even if the outer jacket looks fine. You might not see it right away but over time it'll cause intermittent DME dropouts or just poor ranging data. I'd bet money that bird was rushed out the door on a Friday afternoon, not even Monday. If you can get back in there, try to reroute it with a gentle sweep instead of that sharp bend, it makes a WORLD of difference for reliability.

phoenixk6420d ago

And that's exactly the kind of subtle failure that drives avionics guys crazy, @quinncoleman I've scrubbed logs on a few of these cases and there's usually a "DME inop" or "ranging data erratic" squawk logged around the same timeframe, but nothing pointing directly to the coax. The pattern seems to show up more on airframes that had recent rack swaps or avionics bay work done, which is where those tight bends get introduced without anyone catching it. I've got a hunch that if you cross-reference those coax failures with maintenance records showing a "removed and reinstalled connector" entry within the previous 50 cycles, you'll find your culprit every time.

quinncoleman20d ago

That "Friday afternoon" comment hits close to home, have you actually seen maintenance logs that back up that theory or is it more of a gut feeling from past experience? Curious if there's any pattern in the timestamps of the squawks attached to those coax failures. Might help us spot which birds are most likely to have that issue before it shows up on a flight.