Black "Rubberish" Corrosion on Antenna Connector

SoCal 182 Driver

SoCal 182 Driver

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SoCal 182 Driver
I've been having some intermittent transponder problems -- ATC saying they can't see my Mode C, etc. I recently took the plane into the avionics shop for some troubleshooting, and we discovered that the connector on the cable to the antenna looked like this:

24-0503 corroded connector - sized.jpg

On the inside of the fuselage of my 1967 C182K, all around the area where the antenna is mounted, was a black, rubber-ish material. (Unfortunately, I didn't get a picture.) The avionics tech joked that it looked like roofing tar, but another tech said it was used by Cessna back in the day to prevent oil-canning. Whatever it is, apparently it somehow off-gassed (or through some other mechanism), and fouled the connector on the cable to the antenna. I can't think of any other reason the connector would look like it does in the picture. (FYI, the connector is probably about 4 years old. New cabling was installed when I upgraded the transponder for the ADS-B mandate.)

I'd like to clean the area around the antenna and remove some of the black material from the fuselage. Is there a "tried and true" method of removing that stuff?

Thanks!
 
SoCal 182 Driver said:
I'd like to clean the area around the antenna and remove some of the black material from the fuselage. Is there a "tried and true" method of removing that stuff?
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The black tar was a used by Cessna to dampen sound vs oil-canning. No easy way to remove it. Some methods that can make it easier are use heat gun/scraper to remove the excess then follow up with an alkaline stripper or petroleum based solvent. Have also seen people use OTC adhesive removers with limited success as they usually require multiple applications. In some year/models very labor intensive to remove. But I doubt it had anything to do with your connector corrosion. Perhaps ensure antenna is sealed properly to the exterior skin and no other moisture can migrate into the coax connection.
 
It might also be zinc chromate paste. Long gone now, but it was commonly found sealing stuff like windshields and windows and antennas. It's carcinogenic due to the zinc chromate in it.

Ordinary solvent (like "Varsol") should clean it off. Harsher stuff could damage the coax insulation. DON'T scrape it off the fuselage skin with a metal scraper; you'll scratch the skin, both weakening it so that it could crack, and scratches removes the Alclad layer that prevents corrosion.
 
Bell206 said:
Perhaps ensure antenna is sealed properly to the exterior skin and no other moisture can migrate into the coax connection.
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At the time of the ADS-B upgrade, the antenna was replaced. I’m certain it has the proper gasket. That’s why I’m thinking that somehow the soundproofing material is related to this problem.

Maybe one way to deal with this would be to connect the connector to the antenna, and then seal it with a piece of heat shrink. It’s not a standard way of installing a BNC connector, but it might help prevent this problem from occurring in the future.
 
SoCal 182 Driver said:
Maybe one way to deal with this would be to connect the connector to the antenna, and then seal it with a piece of heat shrink. It’s not a standard way of installing a BNC connector, but it might help prevent this problem from occurring in the future.
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You still need to get that connector real clean. In the antenna end, too.
 
SoCal 182 Driver said:
hat’s why I’m thinking that somehow the soundproofing material is related to this problem.
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After 50+ years that tar is pretty inert. Besides you would still need an electrolyte to cause the corrosion even if the tar was involved. My guess is you have a water leak somewhere and that moisture made its way down the cables/coax to the connector especially if its mounted on the belly. Which is a fairly common issue. And any moisture in that connector, you already have the required components for corrosion to form without the tar. And no I wouldn't recommend heat-shrinking the BNC either as that could trap additional moisture as well.
 
Bell206 said:
After 50+ years that tar is pretty inert. Besides you would still need an electrolyte to cause the corrosion even if the tar was involved. My guess is you have a water leak somewhere and that moisture made its way down the cables/coax to the connector especially if its mounted on the belly. Which is a fairly common issue. And any moisture in that connector, you already have the required components for corrosion to form without the tar. And no I wouldn't recommend heat-shrinking the BNC either as that could trap additional moisture as well.
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Cessnas (and most others) have small drain holes in the belly. There should be one on each side of all the bulkheads. I usually found those holes all plugged with dirt and other debris, and frequently saw watermarks revealing that significant puddles had formed at times.
 
SoCal 182 Driver said:
What about some dielectric grease in the connection?
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Check with the avionics shop re that. If it's OK, some good silicone dielectric grease would keep the water out. It's frequently used in other aviation wiring applications, but I'm not sure it's good around RF stuff. It would certainly repel the water.
 
Dan Thomas said:
Cessnas (and most others) have small drain holes in the belly. There should be one on each side of all the bulkheads. I usually found those holes all plugged with dirt and other debris, and frequently saw watermarks revealing that significant puddles had formed at times.
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I spoke with an A&P friend of mine this morning, and he had the same thought. My plane is hangared, and rarely sees rain, but it does get washed several times a year. I'm guessing that the drain holes are plugged, and water has pooled in the belly (as you describe). Next time I'm at the hangar, I'm going to get the creeper out and go under the plane to clear out the drain holes as best as I can from the outside. Then, at the next annual, when everything is opened up, I'll crawl back into the tail and clean the interior dirt and debris.
 
SoCal 182 Driver said:
What about some dielectric grease in the connection?
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I've never used it or seen it used on aircraft coax connections. So I don't know if it will cause any RF issues. Other electrical connections sure but am more inclined to use Stabil 22 vs a dielectric grease unless the OEM specifically called out for it. As mentioned above, I'd look to prevent moisture from getting in to begin with.
 
Lacquer thinner should remove about any petroleum products. Xylene is probably safer than MEK, which is what I used as a kid working in a restoration shop. Maybe thats why my hair fell out ...
 
When the tech cut off the connector, and stripped the coax to put on the new connector, was the shield bright and shiny, or black or otherwise corroded? Asking because it's possible for moisture to get into the cable from the connector, but it's also possible for moisture to get to the connector from the cable.
 
Albany Tom said:
When the tech cut off the connector, and stripped the coax to put on the new connector, was the shield bright and shiny, or black or otherwise corroded? Asking because it's possible for moisture to get into the cable from the connector, but it's also possible for moisture to get to the connector from the cable.
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He cut it back about 4", and my recollection is that the shield was shiny when he went to install the new connector.
 
SoCal 182 Driver said:
He cut it back about 4", and my recollection is that the shield was shiny when he went to install the new connector.
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That's great to hear. So if water got in, it probably came in from the connector. I don't do airplane work, but I've done some radio work. As you know, BNC connectors aren't waterproof. I've not heard of anyone using dielectric grease on them, no idea if that would be a problem. But the water egress problem is probably more between the jacket and shield than anything else. It can wick down the braid. I don't think I'd try to waterproof a connector in an airplane, but I'll repeat I'm not an avionics guy.

To waterproof BNC for outdoor use, though, you'd normally do a wrap of 3M 33 or 88 tape around connector, from cable to the mating connector, and then cover that with self fusing tape, going beyond the 33 at each end. The self-fusing tape is waterproof. But is more than miserable to try to remove, especially after time. The 33 tape is to protect the connector from damage from the self-fusing tape. Regular electrical tape by itself, as others have mentioned, will just trap water inside and make it worse. Same with heatshrink that doesn't have glue in it. Heat shrink with built in glue is common in power splices, and it's great for that, but I've never heard of using it for connectors that need to be removed.

Just a computer and sometimes radio guy that's always lived places where it's wet. And often cold. So probably isn't all that bright...
 
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