Sport Pilot Talk

I'll need a Mode C transponder in my plane, and one I wanted is for sale used at a decent price from Aircraft Spruce. If I wind up getting it, I could hook it up to a battery and see that the lights go on, etc., but how would you actually test a transponder (or any used instrument, really), without having a flying airplane to put it in?

Mike Taglieri

I think you need specific test equipment to test a transponder. They have to be checked/recert'd every two years. 'Having mine done this week. $80 or so.

Mike, you'll want to use an avionics shop - folks who are able to certify the transponder every two years - and if it doesn't pass, Spruce does have a good return policy. You might want to ask them if that policy covers returned equipment. Last year I bought an 'opened package' product from them and the return policy was the same.

You will also need an altitude encoder if you fly inside a "Mode C" area.

garbageman wrote:You will also need an altitude encoder if you fly inside a "Mode C" area.

(Revealing one of the many holes in my aeronautical knowledge) Do the encoder and transponder have to be tested together as a pair, or can they be tested individually/separately?

If you get on flight following and tell the controller you are testing your transponder, they will tell you what altitude they are showing for you and you can compare. You can also test the ident that way. I heard somebody else do that last time I was on FF.

Merlinspop wrote: Do the encoder and transponder have to be tested together as a pair, or can they be tested individually/separately?

They are tested together, Bruce. The test requirement is in FAR 91.413, which references the procedure in FAR 43 Appendix E, which in turn says:

Measure the automatic pressure altitude at the output of the installed ATC transponder when interrogated on Mode C

drseti wrote:

Merlinspop wrote: Do the encoder and transponder have to be tested together as a pair, or can they be tested individually/separately?

They are tested together, Bruce. The test requirement is in FAR 91.413, which references the procedure in FAR 43 Appendix E, which in turn says:

Measure the automatic pressure altitude at the output of the installed ATC transponder when interrogated on Mode C

Makes sense. Thanks!

Hmm... reminds me of one of my first flight lessons flying an older LSA. (Not at the flight school that I finished up at). We made a little cross country hop to an avionics shop. The instructor explained that the transponder was transmitting altitude that was off by about 300 feet. I had no idea what that meant, what a transponder was for, or how any type of airspace was designated. He just said that the FAA doesn't like it.

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CTLSi wrote:......

The premise of needing a "Trained and Licensed Professional" test transponders/encoders in the aircraft and permanently installed is...The transponder talks to ATC and the controllers make important decisions based on that communicated info. These important decisions they make can potentially bring down an airliner full of passengers.

IT IS IMPERATIVE THAT THE INFORMATION YOUR MAGIC BOX DELIVERS IS ACCURATE.

I had this done 3 times during the ownership of my Certificated helicopter, and I watched the avionics tech do it. Here's what I remember.
He had a portable setup that he could carry out on the ramp.

First, in his shop, on the bench, he hooked up his NIST Certified altimeter which was connected to a NIST certified vacuum pump. Then he connected my encoder. He did a series of climbs and descents using his vacuum pump to simulate altitudes. He compared my encoder's measurement to his altimeter's reading. If it was within the FAA tolerance, he put a dated sticker on my encoder and it was good for 2 more years.

Now to test the XPDR at the helicopter. He took his gear out to my aircraft. It included an interrogator simulator that acted as the FAA Radar. It had an antenna on a camera tripod that he moved around my aircraft to make sure it was working 360 degrees. He checked signal strength. He dialed in many squawk codes to make certain my box was positively identifiable on the code the pilot was told to squawk. He function checked the IDENT feature. If everything was within FAA tolerances, the box got a dated sticker and was good for 2 more years.

Last thing he did was make an entry in the Airframe Logbook that the XPDR FAR 91.413 Check had been done and provided me a bill from a Licensed Avionics Shop. All these things are required for a "real" transponder check. This is a job that cannot even be done by the builder/repairman of an EAB, unless that person is a licensed Avionics Tech.

bryancobb wrote:This is a job that cannot even be done by the builder/repairman of an EAB, unless that person is a licensed Avionics Tech.

Most if what you said is entirely correct, Bryan.
Actually, though, it's not generally a "licensed avionics tech" which does the check (the FAA doesn't even issue such a license), or even necessatily an A&P, but rather an employee of a certified repair station. Those repair stations must have their equipment calibrated anually against NIST-traceable standards, as you mentioned. But they also get audited annually by an FAA inspector. You can't take your transponder to just any shop for its 91.413 check. There are also different test requirements for IFR vs. VFR operation. And yes, this does indeed apply even to experimental aircraft.

miket-nyc wrote:I'll need a Mode C transponder in my plane, and one I wanted is for sale used at a decent price from Aircraft Spruce. If I wind up getting it, I could hook it up to a battery and see that the lights go on, etc., but how would you actually test a transponder (or any used instrument, really), without having a flying airplane to put it in?

Mike Taglieri

If you are purchasing it from a reputable source "Aircraft Spruce" its probably already "TAGGED." I doubt AS would just sell a traded in or otherwise used piece of equipment without taking any measures to its operability. That said, you can either install it or have a shop do it, but you will need a shop to test it with their specialized equipment and sign off on it.

Guys, while all useful information the original post is 5 years old.