Intermittent Transponder Problem

Fearless Tower

Fearless Tower

Touchdown! Greaser!
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Fearless Tower
Most of the time, my xpdr appears to be working fine. It was just checked out/certified a couple months ago during the most recent annual.

However, recently I have had a couple flights where ATC was not picking up my xpdr (I'm based at a class D airport underneath the San Diego Class B ).

The most recent was yesterday departing for an IFR flight. SoCal departure was not picking me up and cycling the xpdr didn't seem to help. I cancelled while I was still in VMC and returned to the airport. While talking to tower, they mentioned that they started to pick me up at 2000' (1500 AGL).

Before I call the avionics guys and pay them $85 and hour to troubleshoot, does anyone have any ideas what the problem might be?

Does this sound like an antenna issue, xdpr power issue, or is the box about to die a horrible death?
 
I might look around for loose connections before having them repaired (by an authorised person);)
 
Fearless Tower said:
Most of the time, my xpdr appears to be working fine. It was just checked out/certified a couple months ago during the most recent annual.

However, recently I have had a couple flights where ATC was not picking up my xpdr (I'm based at a class D airport underneath the San Diego Class B ).

The most recent was yesterday departing for an IFR flight. SoCal departure was not picking me up and cycling the xpdr didn't seem to help. I cancelled while I was still in VMC and returned to the airport. While talking to tower, they mentioned that they started to pick me up at 2000' (1500 AGL).

Before I call the avionics guys and pay them $85 and hour to troubleshoot, does anyone have any ideas what the problem might be?

Does this sound like an antenna issue, xdpr power issue, or is the box about to die a horrible death?
Click to expand...

What brand/model xpndr? Try cleaning the antenna with solvent. And if you can gain access, loosen then retighten the antenna mounting screws as well as the antenna cable connection. It may also be that your xpndr's frequency is off a bit although if it was bench checked I would expect the frequency would have been tweaked then.
 
gismo said:
What brand/model xpndr? Try cleaning the antenna with solvent. And if you can gain access, loosen then retighten the antenna mounting screws as well as the antenna cable connection. It may also be that your xpndr's frequency is off a bit although if it was bench checked I would expect the frequency would have been tweaked then.
Click to expand...
It's a NARCO AT150

Thanks for the ideas....I'll check the antenna and the xpdr connections for looseness. The other thing I did not mention is that the airplane was painted about 10 hrs ago and right after the transponder check was done.....I am wondering if there might be related 'complications'.
 
Have a look, if it's not painted maybe they removed it and painted all around the mounting point, causing an iffy ground.

Just spit balling but the info is worth what you spent on it!
 
Skylane81E said:
Just spit balling but the info is worth what you spent on it!
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No problem - to me it is worth much more than what I spent on it!

I've often found that if I can narrow the problem down, it is much easier on the techs (and easier on my wallet) than if I just call the shop and tell them to 'check out xpdr - sometimes it works....sometimes it doesn't'
 
Fearless Tower said:
It's a NARCO AT150

Thanks for the ideas....I'll check the antenna and the xpdr connections for looseness. The other thing I did not mention is that the airplane was painted about 10 hrs ago and right after the transponder check was done.....I am wondering if there might be related 'complications'.
Click to expand...

Try spinning all the knobs through a few cycle, my AT 150 had the opposite problem in that it was showing on the radar TWICE. Avionics dude told me the AT150s wafers get dirty or something and to spin the knobs through a few cycles, I did it and no problems since. Money back guarantee.
 
Well, I think I may have narrowed down the problem.....freakin' paintshop painted the antenna. It is pretty obvious that they didn't even attempt to mask it off, just sprayed it right along with the belly.

So, what is the best way to address the antenna? Can it be cleaned up with paint solvent, or do I need to replace the antenna?
 
bartmc said:
Try spinning all the knobs through a few cycle, my AT 150 had the opposite problem in that it was showing on the radar TWICE. Avionics dude told me the AT150s wafers get dirty or something and to spin the knobs through a few cycles, I did it and no problems since. Money back guarantee.
Click to expand...
I think that kind of problem (dirty switches) is more likely to cause the wrong Mode A code to be transmitted rather than none at all. Of course ATC can set their display to hide any transponder codes that aren't supposed to be there so that could mean the controller wouldn't see your plane. So it might be worth asking the controller if he can see any other codes at your approximate location when he says you've disappeared.
 
gismo said:
So it might be worth asking the controller if he can see any other codes at your approximate location when he says you've disappeared.
Click to expand...

In this case they aren't picking me up in the first place, or at least they aren't picking me up until I am considerably higher than the rest of the traffic.
 
Fearless Tower said:
Well, I think I may have narrowed down the problem.....freakin' paintshop painted the antenna. It is pretty obvious that they didn't even attempt to mask it off, just sprayed it right along with the belly.

So, what is the best way to address the antenna? Can it be cleaned up with paint solvent, or do I need to replace the antenna?
Click to expand...

What kind of paint?

Solvent is unlikely to do you much good. Paint remover may be a bad idea.

Blade antenna or the little lollypop style?

I would look at mechanically removing the paint - as in remove the antenna and take it to a wire wheel (assuming the little lollypop). But, for a certificated airplane that is probably not legal for you to do yourself. You could resort to something like scraping with a pocket knife if it is hard to remove the antenna and/or the paint shop is a long way away. Otherwise, you might call the shop and try to push them into buying you a new one - that may be the only option for a blade antenna.
 
Capt. Geoffrey Thorpe said:
What kind of paint?

Solvent is unlikely to do you much good. Paint remover may be a bad idea.

Blade antenna or the little lollypop style?

I would look at mechanically removing the paint - as in remove the antenna and take it to a wire wheel (assuming the little lollypop). But, for a certificated airplane that is probably not legal for you to do yourself. You could resort to something like scraping with a pocket knife if it is hard to remove the antenna and/or the paint shop is a long way away. Otherwise, you might call the shop and try to push them into buying you a new one - that may be the only option for a blade antenna.
Click to expand...
It's the little lollypop style
 
Fearless Tower said:
It's the little lollypop style
Click to expand...

So it's is all metal except for the insulator at the base (which would not be good to put paint remover on, I wouldn't think). Plus less area to have to clean up.

The insulator is the white part:

AV22.jpg


I have no idea what is and is not legal for you to do yourself in this case.

They really don't cost very much compared to most aircraft parts...
 
Fearless Tower said:
Well, I think I may have narrowed down the problem.....freakin' paintshop painted the antenna. It is pretty obvious that they didn't even attempt to mask it off, just sprayed it right along with the belly.

So, what is the best way to address the antenna? Can it be cleaned up with paint solvent, or do I need to replace the antenna?
Click to expand...


Maybe the paint shop should replace the antenna.

Worth a shot.
 
The rod type antenna is very sensitive to contaminants and ice. This is due to the lack of protection of the radiating element (rod) and the surrounding structure and insulator. Best antenna type is the blade type which has a plastic enclosure protecting the radiating element. Besides checking antenna connections clean the transponder tray contacts by removing the transponder and spraying contact cleaner on the contacts. Over half of the intermitent avionics problem are due to contact oxidation. Also check for any connectors between the encoder and the transponder.

José
 
Fearless,

Buddy snapped off my XPDR antenna at the 6y9 fly-in. (that's my story anyway). A new one was like $17 from AS or one of those places. At that price it's not worth trying to clean the painted one. It took maybe 10 minutes to install.

Also, at that price, it's not worth seeing if the paint shop will replace if they're not on your home field.
 
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Skylane81E said:
Maybe the paint shop should replace the antenna.

Worth a shot.
Click to expand...

The FARs (part 43) are silent on the legality of a pilot removing an antenna for cleaning. That specific task isn't listed in Appendix A which would indicate that an A&P signoff is required but a pilot is allowed to paint an aircraft (except for balanced control surfaces) and I'd think that removing antennas would be required for that so this might be a grey area. I sure wouldn't have any qualms about loosening the nut from the inside (a pilot is definitely allowed to R&R the floorboards on most airplanes) and cleaning the metal for better contact before tightening it back up.
 
The paint shop left the antennas on the aircraft? Really? That sucks.

You mentioned silver paint. That's metallic silver, right?

They've basically probably shorted your antenna's fed element to the frame/"ground" through the metallic paint.

Not good for the transmitter at all. Hard to say without photos.

I'd get 'em to replace the antenna and pay someone to install it correctly checking to make sure the overspray isn't causing a "grounding" problem at the base

Geez. Dumb.
 
denverpilot said:
The paint shop left the antennas on the aircraft? Really? That sucks. .
Click to expand...
They left most of the antennas on for painting, but masked them off....all except the little xpdr antenna on the belly that is.

denverpilot said:
You mentioned silver paint. That's metallic silver, right?

They've basically probably shorted your antenna's fed element to the frame/"ground" through the metallic paint.
Click to expand...
Yep, metallic flake.....lovely....just lovely.
 
Well, apparently it is not the antenna that is the real problem.....shop took a look at it yesterday and apparently when you turn it on, the xpdr is replying on its own without being interrogated. So basically it is too busy talking to hear and properly respond to the interrogation. Kind of like my 4 year old.

Anyone ever had that happen (I mean the xpdr thing....not parenting a 4 year old)?
 
Is it posible that it's replying to the reflection of it's reply?

Still seems like it should have something to do with the paint as it's what changed.

However I'm not an avionics geek
 
The metallic flake paint could easily be setting up a standing wave that triggers the transponder to reply. Clean the antenna first, you know that needs to be done anyway - then troubleshoot further.
 
I'll have to ask the shop, but I believe he said that he determined this bench testing the xpdr.
 
Fearless Tower said:
I'll have to ask the shop, but I believe he said that he determined this bench testing the xpdr.
Click to expand...
If the transponder is generating unsolicited replies on the bench, either the interrogation response threshold is set too low or the receiver front end has gotten noisy. Either one means the unit needs service at a certified repair station licensed to work on it (of course you could always replace it). BTW if that behavior (continuous replies) only occurs in the airplane, it's possible that an external noise source is causing the problem. I know I could always get my KT76 to reply pretty much continuously by operating my miniature shop vac near the transponder antenna.
 
LearDriver said:
I think you may want to clean your antenna. We used to get a lot of oil blow by on the belly that would cause residual buildup that would cause weird transponder issues.
Click to expand...
I'm having a hard time seeing how buildups on aircraft antennae could affect the performance of a transponder on the bench of a repair station.
 
Well step one is to fix what you know is wrong, so the antenna should be done anyway.

Now if it was on the bench then there is more to the story but it doesn't make the painted antenna any less of a problem.
 
Agreed. Yes, bench-fix the radio but I've seen a lot of commercial and Amateur radio gear damaged by bad antenna installations of all varieties.

It'd suck to fix the transponder only to damage it again hooking it to a shorted antenna. :(
 
And if it is determined that the painting of the antenna caused the failure of the Xpdr, then I think it's time for a talk with the paint shop, because they should have known better than to paint the antenna. And it would obviously be a lot more than $17 to fix or replace. Because of this, you'll want to document everything so you can reach an informed agreement with the paint shop. (read: they can't claim they didn't paint the antenna.) Hopefully fixing the xpdr will be a trivial and inexpensive exercise and with a new antenna it'll work like new.
 
FWIW, avionics shop fixed the issue with the box and they were able to clean the paint off the antenna. Flew it today and appeared to be squawking fine.....waiting for the next gremlin to show up......
 
Congrats! Virtual radio maintenance via Internet Web Board, complete! (Oh, and the shop who actually did the work... Heh heh.)

:rofl:
 
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