Expert: Marc Ebelini - 7/10/2011

Question
QUESTION: Marc, we have an encoding altimeter Aerosonic and KXP 750A transponder. I suspect the encoder part is not working properly as we get erroneous  Alt. readouts that ATC reports when in mode C.

Which would be less expensive:
add new blind encoder to work with altimeter and transponder?
is this possinble?
or
send out the encoding altimeter for repair?

Thanking you in advance for any assistance you can provide.

Roger

ANSWER: This is the part I hate about being retied. I loved playing the super sleuth. There are may different reasons for your problem. Unfortuneately they all require test equipment to elimiate the cause. But here are the checks. Let's start with the altimeter. Has it been checked for the proper gray code out. If it has. now we test the transponder for transmitting the input codes properly This has nothing to do we altitudes. Only that whatever code is applied to the set produces the same output codes. (This rare but can be the cause) If the first and second test is okay then do a check on the cable. one open or shorted wire will cause reporting wrong altitude.
Now if the altimeter tested okay and the transponder and cable test okay There is one more thing, the antenna or antenna system. Should the standing wave be too high it has been known to cause ringing in the cavity of the transmitter unit. This results in additional pulses to be transmitted. The old Narco blade antennas were always a problem with cracking and water getting in. Repairing the faulty item should be cheaper than buying a new encoder, besides what if turns out that it was a shorted or open wire at the transponder, the problem would still be there. The same if the problem is in the antenna system. As you can see I was not one to do shotgun troubleshooting. Fortuneately I had all the test gear needed to do what I just said should be done. I assuming that a test with a ramp box verified the problem on the ground. This requires an altimeter static tester and a ramp box. Remember to place the alitimeter on 29.92 so that the altitude matches the box reading. If the box does not read altitude then you will need a gray code chart. Wish I could be of more help. I sold Aerosonic altimeters that were made in Clearwater Florida. They used to send airplanes to me with these problems when dealers were having problems. Guess what? It was always bad wiring, not the altimeter

---------- FOLLOW-UP ----------

QUESTION: Marc, if it is the encoder can we add a blind encoder to existing altimeter? Which would be less expensive: adding a blind unit or sending the altimeter out for repair?

Answer
Must I assume it is the altimeter that is the problem. Sending it back to Aerosonics for repair would likely be cheaper because buying a blind encoder would require some rewiring. Don't know the labor rates in your area but they raise the cost of installing the blind one as compare to just removing the altimeter and reinstalling it. Did you do the tests as I recommended?
Sorry for the delay in responding. Your email did not get placed in my mail box. Marc