13.1.6 Transponder woes

One of the things that I’ve been chasing for about six months is an ADS-B anomaly.

A bit of background for the non-avionics types (skip ahead if you already know this:-) )

ATC (Air Traffic Control) knows where we are by using radar.  But just having dots on a screen isn’t that helpful. Two dots could be side by side but one could be 2 miles above the other. So we have transponders.  These are receivers/transmitters that when “interrogated” by the ATC radar, they respond with a four-digit identifier that that ATC tells us to use and the altitude of the aircraft. That way the radar scope will provide a more three dimensional picture of where the aircraft are and who they are.  It’s been this way for years.

Lately, there’s a new, improved method. Automatic dependent surveillance – broadcast or ADS-B. With this system, the aircraft continuously broadcasts its’ N-Number, altitude, position, speed.  And that’s just half of it.  What I just described is “ADS-B out”. There’s also an “ADS-B in”.  With that, I can see other aircraft, their position, N-Number, altitude and speed on the screen of my iPad or whatever ADS-B compliant device I happen to be using. It’s pretty cool to see that there’s a Delta 717 flying over me at 34,000′ and I look up and I can see the contrails exactly where it says it should be.

Oh yeah, you can also receive weather data such as the current conditions at your destination and weather radar.  This means that I would be able to cancel my $600 a year XM weather subscription.

The ADS-B out part is not mandatory until 2020.  The long time frame is because it cost a lot of money to retrofit existing certified production aircraft. However, when I was building the avionics package, it was a no-brainer that I was going to be ADS-B out compliant.

The Grand Rapids HXr EFIS system supports ADS-B out as does the Garmin GTN625 GPS.  For the transponder there were only two remote control transponders compatible with the Grand Rapids system.  The Trig TT22 and the Sandia STX 165R.  The Sandia was less money, but it was an older “mode-C” transponder (not important for this discussion) while the Trig was the newer “mode-S”. Grand Rapids seemed to prefer the Trig over the Sandia.  And Trig is based in Scotland… So my decision was made.  Puttin’ in a Trig. 😉

Fast forward to present time.

Everything is working fine.  With one minor exception. When I requested an ADS-B performance report (it tells you if all the ADS-B data you’re transmitting is valid and accurate), it showed this one ridiculously small error.  Basically, it was saying that for 3 to 5 seconds on any flight, I was not transmitting the GPS altitude.  What’s really weird is that it showed this error on a 20 minute flight or 2 hour flight. I checked the flight data logs and it showed the correct GPS altitude for every second of the flights.

Here’s a report from a 2 hour flight.


Any parameters out of compliance are colored red.  In this case, four seconds of missing altitude data.

I checked everything I could think of and kept coming up empty for a solution.

Then I contacted Trig.  I figured that since the data seemed to be present and it was the transponder that sent it, that seemed the logical culprit.  Because they’re based in Scotland, there’s a time difference that I had to work around.  But their responses were very prompt. After just a couple of exchanges, I was told that the software on my transponder was version 2.4 and that they were currently at version 2.7 and that before chasing anything else it would be prudent to get my unit up to date.

I asked how to do the update and was told it would have to be done by an authorized Trig dealer.  The cost would be whatever the dealer decided to charge. Or I could ship it to their US distributor and they would do the update for free. 🙂

So in July I pulled the transponder and shipped it to Mid-Continent Instrument and Avionics in Kansas (they’re the Trig distributor for the US). It took one day to do the update and then it was on it’s way back to me.  Did I mention there was no charge for the update?

I installed the updated transponder and took Ann up for her second flight in the Velocity. After I got back home I requested a report on the ADS-B data.  There were a bunch of errors now! I sent an email to Trig and got a response back within an hour that explained I needed to change one of the configuration parameters on the GPS (that’s where the position data comes from).  I made the change and did a quick flight. Because it was a weekday and my home field is in a very active MOA, I checked in with Tyndall Approach to get a squawk code so they could keep the F-22’s from running over me.  After I entered the code, approach advised that they were receiving no replies and they only had a “faint primary return”.  “Primary return” is what shows up on their screen when the only thing they receive is a return from the radar.  No identifier or altitude.  And the reason it was “faint” is because the airplane isn’t metal. So the return from the radar was mostly going through the plane and bouncing off any metal inside.  I power cycled the transponder and tried a couple other things but nothing.

Now the only thing I did between the first and second flight with the new transponder was to change something on the GPS.  It didn’t seem likely that would cause the transponder to stop transmitting.  But because my profession involves troubleshooting, and what I teach is: “ALWAYS suspect the last thing that was done.”. So I changed the GPS configuration back and flew again.

Same thing.

Change the GPS back to the correct configuration and flew again (I know, I know.  But you never know, right?).

So then I figured that it’s got to be the antenna lead. I disconnected the antenna lead at both ends and looped it (center conductor to the shield) and checked the continuity (I had to check it this way because I’m alone and the ends are pretty far apart).  Showed good.  Then I had a thought (always dangerous).  I realized that I got an ADS-B report after the first flight that showed some valid data. Granted some of the data had errors, but there was data.  Which means the transponder was transmitting. That means the antenna was functional. Just to be on the safe side, I requested another ADS-B report. I guess just to see what is looked like. it showed a perfect data capture.  Zero errors.

So I stopped troubleshooting the antenna.

I contact Trig again and after a few exchanges, they had Mid-Continent overnight me a replacement transponder with an unreleased version 2.9 code. It arrived, I installed it and made a test flight. Approach still couldn’t see me.

Back to Trig.  I suggested that maybe we should downgrade back to the last known good configuration.  Once again, Trig had Mid-Continent overnight me another transponder this time with an out of date version of software.  Good old 2.4 code.

Installed it and had to wait for Hurricane Hermine to pass before I could fly. When I did, same problem. So now I’m sitting here with THREE TRANSPONDERS with three different version of code on them and every single one had the same problem.

I’m really frustrated at this point and I’m sure Trig is frustrated and concerned. I would bet that if I asked, Trig would have put an engineer on a plane and flew them to Panama City, FL to help me get this figured out.

But I decided that they had already performed above and beyond the call of duty. I mean, I was sitting here with three transponders and only one was mine!  And they hadn’t asked for so much as a credit card number.  And besides, at this point there was no way that this could be the transponder.

So I called Carl at Monarch Aviation.  He did my transponder certification a few months back.  I was very impressed with his knowledge and willingness to work on experimental aircraft built by rank amateurs.  He said that if I could get over there today at 1pm, he could take look at it.

I run out to the airport and takeoff headed to Defuniak Springs airport. I pull up as Carl is finishing up a transponder check on an trike.  One of those hang gliders with a Rotax engine on the back.  Carl is willing to work on anything!

I had already given him the rundown on the situation. The first thing he wants to do is see if the transponder is transmitting. So I power up the avionics and he fires up his test rig which will send out the interrogation signal the read the reply. “Nothing” he says. Then we disconnect the antenna lead at both ends. I climb underneath where the antenna is and he hands me one of the test leads and has me put in on the center conductor of the antenna wire. “Looks good” he says. “Now on the shield, please.” once again he says “Looks good”.  None of this is any surprise to me (because I already check this, remember?). Then he asks for the test lead I was using. As I’m getting up, he says “There’s your problem.” I get a sinking feeling because I know what he’s found.  I say “center conductor is shorted to the shield, right?”  “Right you are.” is his response.

Normally I would have checked that.  Honestly, I would!  But the antenna was working!  I know this because I’ve got the ADS-B reports which say so.  But I should have checked it.

He then says that it’s going to be shorted at one of the two ends and that no matter which one we pick to re-terminate, it will end up being the other. I tell him that ain’t going to happen. “Why is that?” he asks.  Because the short is on the connector that attaches to the transponder. When he asks how could I know that, I tell him because it’s the only connector that was touched before the problems started.  So he puts on a new TNC connector at the transponder end of the antenna lead.  Then I power up the avionics while he powers up his test unit.

Good signal!!!

There are a some of lessons here:

1) Don’t shortcut troubleshooting procedures because of what you “think” that you know. Whenever you have the slightest suspicion of a cable, troubleshoot it completely. It only takes a second to do a complete check.  If I had done this in the first place, I would have saved a bunch of people a bunch of time.

2) If you are stuck, don’t hesitate calling in a fresh set of eyes. It took Carl all of five minutes to nail this one down.  Partly because he wasn’t influenced by outside information.  Basically, he was unbiased and assumed nothing.

3) If you need a transponder, radio, GPS or audio panel, consider Trig Avionics!  I can not say enough good things about their support. If I ever get back to the homeland of my Great Grandfather, I will be certain to look up Lindsey at Trig support.

The thing that nobody can explain was how ADS-B data was being transmitted out of an antenna that had a lead with a dead short in it.