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How do you test a transponder?

Posted: Tue Jun 03, 2014 12:27 am
by miket-nyc
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

Re: How do you test a transponder?

Posted: Tue Jun 03, 2014 1:31 am
by ct4me
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.

Re: How do you test a transponder?

Posted: Tue Jun 03, 2014 6:04 am
by Jack Tyler
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.

Re: How do you test a transponder?

Posted: Tue Jun 03, 2014 2:08 pm
by garbageman
You will also need an altitude encoder if you fly inside a "Mode C" area.

Re: How do you test a transponder?

Posted: Tue Jun 03, 2014 6:17 pm
by Merlinspop
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?

Re: How do you test a transponder?

Posted: Mon Jun 09, 2014 9:41 am
by MrMorden
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.

Re: How do you test a transponder?

Posted: Mon Jun 09, 2014 11:32 am
by drseti
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

Re: How do you test a transponder?

Posted: Mon Jun 09, 2014 11:59 am
by Merlinspop
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!

Re: How do you test a transponder?

Posted: Mon Jun 09, 2014 3:14 pm
by designrs
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. :lol:

Re: How do you test a transponder?

Posted: Sun Jul 06, 2014 8:54 am
by CTLSi
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