But It Works When It’s on the Ground!?

How many times have you heard this story — a plane comes out of major maintenance and things don’t work quite right. That was the case with Warren’s plane, a classic that was as easy on the eye as it was to fly. The problem was that Warren’s transponder wasn’t transponding quite the way it needed to, and that was costing him big bucks.

The Avionics Editor from Private Pilot Magazine happened to wander into the local EAA Chapter meeting, and the floor was opened up for questions on avionics. Warren’s question quickly came up, and he described the symptoms quite well. “When I am on the ground, the transponder works fine,” he noted. “ATC says they can see me, and read my codes, but the minute I lift off, they lose the signal.” Worse yet, Warren noted that he had been in to one avionics shop for an hour already, and they hadn’t been able to solve the problem.

The editor asked Warren what the transponder did when ATC couldn’t see it. Warren only tested the unit during the day in VFR weather, but he didn’t remember seeing the Reply or Interrogate lamp flashing. That gave the editor the clue he needed — he told Warren that he had a loose wire, most likely his power or ground, or a bad rack to avionics connection.

Troubleshooting isn’t really hard, you just have to know how things work. A transponder “replies” to an “interrogation” signal sent by ATC radar. When everything works right, ATC sends a signal and your transponder replies with its squawk code (and altitude on Mode C equipped planes)… and a bunch of other stuff on Mode S equipped aircraft.

In Warren’s case, ATC didn’t see a thing when he was in the air, but could see him on the ground. Looking at the standard forces that the wing places on the aircraft structure, the editor deduced that as the plane was lifting off, the load being placed on the wires was enough to pull the wire out of the pin socket or break the radio to rack connection. When he landed, the change in load was enough to make the wire regain contact.

Since the panel was recently worked on, our erstwhile editor offered to provide free troubleshooting to his EAA friend. The plane arrived at the local FBO for an annual, and the transponder head was carefully removed from its rack. From the clearances that could be viewed, it was quickly seen that the rack was properly installed, and that all fingers (pin connectors) were in good contact with one another.

The editor then slid under the panel, and gently pulled on each wire going into the back of the transponder rack. The second wire he pulled on popped free with hardly any force at all. The problem was diagnosed, all it took was a trip back to the avionics shop to properly land the wire in the right terminal and the problem was solved.

The road to hell is paved with good intentions, which is part of what happened here. Warren had worked hard with his local A&P to clean up all the old wires hanging around in his airplane, and to be honest, had done an excellent job. However, a bad crimp from over a decade ago was waiting in his panel. When he neatly tied up the wires to the back of the transponder, the stress of the new tie wrap was enough to pull the bad crimp to the breakaway point, and every time he took off it did just that, blacking out his transponder.

IF AVIONICS FAILURE STRIKES, KEEP GOOD NOTES! Was it warm or cold when it happened? What work has been recently performed? When is the LAST TIME you knew this was working properly, and how did you know? Exactly what happened, at what time? Does the problem seem to “fix itself” in flight, and if so, when?

The quality of the notes you keep will be inversely proportional to the size of your troubleshooting bill.

BOTTOM LINE: If you can answer those questions, you will be able to help your avionics shop quickly diagnose and troubleshoot your problems. Not everyone has an avionics editor who lives nearby… and is lucky enough to have him or her look in to their problems for a free troubleshooting session! Being able to answer the questions is the first step toward understanding how your airplane is working ‘behind the scenes.’ And the paper you use up by taking notes is a lot less expensive than the fancy green paper you’ll have to fork over to pay for extensive troubleshooting of your under-explained problems.