View Full Version : Transponder Alt Calibration

26th Apr 2011, 12:42
I know this is a bit of a lazy question, as i havent yet looked in the documentation for the transponder yet, but at the weekend i was flying and talking to Farnborough and they said at various altitudes my transponder was reporting 300ft over my actual altitude, I had the correct QNH and it was 300ft over at 5000ft and at 1500ft.

So, my lazy question is does anyone have any thoughts or comments on how easy it is to get this recalibrated and what we might need to do? Its a Bendix King KT76a.

Thanks :)

Torque Tonight
26th Apr 2011, 15:40
I'm pretty sure transponder altitude reporting is based on standard pressure setting. If you are flying a QNH you would expect there to be a difference between altimeter and level reported by the transponder. A 300ft error yesterday would suggest that you had QNH 1023 set, or thereabouts, which ties in with yesterday's weather. 5000ft on 1023mb equates to FL047 which would appear 300ft low, not high as you said, to ATC if they were expecting you at FL050. Hmmm.

26th Apr 2011, 16:04
ATC correct transponded altitudes for QNH - if they say your altitude reporting is out it will be - provided of course your altimeter is accurate and had the correct QNH set!

A KT76A does not have an altiidude encoder built in, and thus will be wired in to some sort of altitude encoder such as an AmeriKing. It is this latter unit that will require attention (assuming your altimeter is OK of course!)

26th Apr 2011, 19:16
Just been through a similar thought process after doing a Mode C check with Radar. They had me 200' below my indicated Altimeter reading set to the QNH they gave me.
On the ground when I set the airfield QFE I am at -100', for an elevation of +25'.
Had a look at the altimeter but no way to adjust externally, so probably have to replace it or send it somewhere to be calibrated. Any ideas?

26th Apr 2011, 20:03
Most altimeters can be adjusted externally. There's a little screw adjacent to the knurled knob for setting the Qs. Undoing the screw releases the locking mechanism then twiddle the knob to set the altimeter and then tighten the screw.

26th Apr 2011, 20:17
Whilst adjusting the baro-scale knob from the front access hole may sort a QFE indication error, it may in fact be that the altimeter is out of calibration across it's range and you can't check this without a calibrated pitot-static test set. There are AD's in force covering quite a few common GA altimeters for mis-reading when the baro-adjustment has come un-done. Best to let a qualified instrument shop such as Aircraft Instruments at Breighton or Pandect at Wycombe sort it out.

For the transponder, it may get it's altitude from either an encoding altimeter or a blind encoder. In either case, the output is always referenced to 1013mB, so your altimeter baro setting will have no effect on the transmitted data. Again, calibration requires checking bottom and top of the operating range, so you need a calibrated pitot-static test set and a transponder test set, ensuring of course that you don't transmit above-ground altitudes where they could be picked up by TCAS-equipped aircraft or they could get false TCAS alerts. If you have a Mode S unit, ensure it's in GND mode when doing any testing.

26th Apr 2011, 20:26

Your unit is in tolarance (+/-200ft) so no need to do anything.


26th Apr 2011, 20:43
On most certifed aircraft, the altimeter tolerence at ground level is 2mb - approx 60 feet, so over 100ft is not in calibration. The mode C tolerence is +/- 125ft, so with a 100ft readout (not Mode S with digital encoder which will have a 25ft readout), the altitude should round to the nearest 100ft, thereby a 125ft error will show as 100ft out, whereas 175ft error will show as 200 ft.

Pilot DAR
27th Apr 2011, 02:46
Most altimeters can be adjusted externally. There's a little screw adjacent to the knurled knob for setting the Qs. Undoing the screw releases the locking mechanism then twiddle the knob to set the altimeter and then tighten the screw.

Whoa there....

Though there is an external adjustment for the barometric scale on altimeters, I'm sure that such adjustments are a "maintenance" activity, and require a qualified maintenance release.

If the person with the screwdriver has satisfied themselves that they are entitled to do this work, they should be very weary, if that screw falls out, a part will drop off inside the altimeter, and then it'll have to be disassembled for repair.

If you have succeded in adjusting the altimeter subscale, you probably have not solved the real problem, which could be an adjustment of the encoder is needed. Even if it is an encoding altimeter, you still have not solved it.

The answer to the original question, which I thought would be self evident, is that the aircraft has to go to a qualified instrument/avionics shop. I believe that even so equipped amature built aircraft do, as the air traffic control system does not care who built the aircraft, if it is controlled airspace sqawking an altitude!

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