Monday, June 20, 2005
Beating the Transponder Gremlin
Yellowbird's transponder gremlin has had a lengthy but unwelcome residency. He's been there at least since I met Yellowbird in February of 2004. The ferry pilot who delivered her noted some problems with the transponder, despite her just having passed her biannual transponder check the previous month. As Yellowbird and I got to know each other, we sometimes experienced problems with her identity, including incorrect squawk codes, intermittent transponder responses, and occasional mode C errors. Twice I took her to the Radio Shop at Worcester to have things checked, and both times everything checked out fine, but still the problems persisted. It was bad enough that I was hesitant to request radar advisories or fly in class C or B airspace.The last straw came a few weeks ago when we were scolded out of the sky by Bradley approach while practicing ILS approaches. The transponder's antics were annoying them so much that they asked us to land and not bother them again until the transponder was fixed. Back to the shop we went and a temporary replacement transponder was swapped for the bad one. We went up again a few days later for more practice, and this time we only had a faulty mode C reading. That was enough to end that day's lesson, and arrangement were made to get back to Worcester to have the altitude encoder looked at. The weather didn't look good for the rest of the week, but the Transpondoctor was willing to try his best as soon as we got a chance to visit again.
That chance came earlier than expected, as Friday's weather defied a gloomy forecast. We arrived at the shop at the appointed time and Dr. Squawk set up his equipment. This included a test apparatus I had seen before, consisting of a transponder test box with a remote antenna. The antenna was set up close to Yellowbird's transponder antenna below the baggage compartment, and when turned on, it confirmed that the transponder was communicating appropriately. A second piece of test equipment comprised a magic box containing assorted air pumps and valves, as well as the three standard pitot/static instrument: airspeed indicator, altimeter, and vertical speed indicator. This was connected to Yellowbird's static system, and when activated, it would adjust the air pressure to simulate the environment at a desired altitude.
With the transponder transmitting the altitude, the good Doctor worked the magic box to take Yellowbird up to a simulated 3,000 feet. As the altimeter on the magic box ascended, the mode C readout on the transponder test box echoed the altitude. Holding level at 3,000 ft, everything seemed fine. On a hunch, the Doctor began thumping the side of the encoder. Each time he thumped, the mode C readout on the transponder test box would flicker. When he stopped, it read steady at 3,000 ft. He thumped some more, the digital readout flickered again, and after a few seconds, it suddenly dropped to 2,500 ft. and stayed there. The altimeter on the magic box still showed 3,000 ft. It appeared that we had found our gremlin.
A replacement encoder was produced, and a few minutes with a screwdriver saw the transplant completed. Again, the boxes were hooked up and the air pump chugged gently and took us back up to 3,000 ft. Watching the slowly ascending needle and the corresponding digital readout, I was reminded of the scene in Apollo 13 where the mission controllers anxiously watch the telemetry data coming from the crippled Service Module, hoping that the oxygen leak can be stemmed by shutting down the fuel cells.
In our case, we reached 3,000 ft again with no problems. Again, the Transpondoctor thumped. Again the readout flickered. And again it dropped suddenly to 2,500 ft. I knew that Yellowbird wasn't going to be trapped in lunar orbit, but imagining the imminent slow leak of money from my bank account gave me a similar sense of foreboding. But the Transpondoctor, with many years of practical experience, saw something that I hadn't. Below the digital readout on the transponder test box was a group of about a dozen LED lights, apparently one for each wire in the harness connecting the encoder to the transponder. As the altitude changed, each 100 ft. increment was indicated by a different combination of lights as the encoder translated the altitude to a binary code. As he thumped and observed the flickering display, he noticed that one light in particular was prone to cutting out. Crawling under the panel, he probed at the wiring harness until he found the corresponding wire. One tug told him all he needed to know. He disassembled the connector plug at the encoder end of the harness and confirmed that the solder joint had failed leaving only the tight confines within the connecter to keep the wire in contact with the plug. Five minutes with a soldering iron set things to rights, and with the original encoder reinstalled, we ran the test again. This time the altitude held steady throughout even the thumps.
We were back in the air less than an hour after we had arrived. With a freshly soldered connection, Yellowbird seemed a bit more sprightly than usual, and I let her romp around a bit among the scattered cumulus. Over the weekend we spent nearly two and a half hours shooting approaches at Westfield and Westover, with no complaints from Bradley. The loaner transponder has done well so far, but once next month's annual is out of the way, we'll be getting a permanent replacement. Yellowbird at last has a healthy sense of self, and we can confidently contact ATC without fear of rejection. As for the gremlin, who knows where he has been hiding. He was last seen eying a large mallet at an undisclosed Air Force base, apparently up to no good.
