pulsating intercom static in rain

I had a strange problem in my RV10 the other day. Was flying a long IFR XC in heavy rain and an hour into the flight got some static on the intercom. It gradually got louder and turned into a pulsating sound, about every second. A second loud click sound much faster rate then developed. Fortunately I could still hear ATC and they said my transmissions were fine. As this developed my mode C went crazy, altitudes from 1k to 27k. It all went away after sitting in a hanger for a weekend. This is the second time this has happened, only in heavy rain. Static instruments were all fine. I'm thinking some water must be getting in somewhere affecting avionics causing this clock-like noise; but what instrument would cause this and how do I find the leak?? (if there is one.)
Ideas?

Wayne,

What you were probably experiencing was a combination of precipitation static and corona discharge, both common elements affecting VHF communications particularly in heavy rain. Precipitation static is most commonly heard as a continuous harsh type noise. Corona discharge noise is due to a build up of an electrical static charge on the antenna followed by a sudden discharge back into the atmosphere in the form of an audible pop heard on the radio that may repeat through a series of charging and discharging cycles. Your radios and transponder are probably fine.

Regards,

Static wicks? Those may help.

p- static

Thanks. I had thought about static as the cause but the pulsating nature on top of the constant static didn't seem to fit. I had not heard of coronal discharge. That makes sense. Still not clear why the transponder mode C went sporadic. Maybe that was the discharge point.
Wicks may be next. Sounds like a big task for a finished plane, though.

Check those isolation washers...

I ran into this exact set of symptoms in my RV-7.

I tried static wicks, bonding the ailerons and rudder to the airframe, etc. to no avail. It ultimately turned out to be a failure of the isolation washers/grommets installed under the headphone and microphone jacks.

Small cracks had developed in the shoulder washer (the black piece) and as the humidity % increased a ground loop of sorts developed. Fly into precip, or IMC and the static would increase.

New set solved the issue.

Good luck.

B

static

You may have just saved me a lot of time and money. Thank you!

That clicking was your transponder transmission, BTW.

TomVal said:

Wayne,

What you were probably experiencing was a combination of precipitation static and corona discharge, both common elements affecting VHF communications particularly in heavy rain.

Regards,

Click to expand...

From the original post, it sounds like the noise was being generated in the intercom. The intercom is not VHF communication and static issues with the antennas should have no impact on the intercom.

I would be looking for water issues (either wiring or equipment) first. Static discharge could be an issue and I would ask the intercom manufacturer if they have seen this before and ideas for addressing. That said, LOTS of planes fly in the rain without static issues, excluding the common VHF radio issues mentioned. The fact that transponder received and transmitted fine with only altitude anomalies, points to an electronics issue between the transponder and the encorder or EFIS and not static issues with the antenna.

Larry