3
$\begingroup$

When flying instrument approaches in a rented plane, Approach called out an altitude discrepancy between the assigned altitude which is being flown using the altimeter and the altitude being reported by the Mode 3/C transponder. Back at the hangar, the question is: does the aircraft have or need an encoding altimeter interfaced to the transponder to report altitude, is it built into some transponders; or was the Approach controller using altitude as reported by the radar if it has height finding capabilities?

$\endgroup$
  • 1
    $\begingroup$ Possible duplicate of How are airspace violations detected? $\endgroup$ – fooot Dec 25 '17 at 18:05
  • $\begingroup$ Seems that they compared their primary radar altitude (active measurement) with what your transponder reported (pressure altitude). Taking into account the measurement error from the radar and the pressure they determined that the two values were still far enough off to notify you of the difference. $\endgroup$ – user7241 Dec 25 '17 at 22:34
4
$\begingroup$

does the aircraft have or need an encoding altimeter interfaced to the transponder to report altitude, is it built into some transponders

In order to squawk mode C, you need an encoding altimeter connected to your transponder and the transponder needs to be set to transmit mode C. Does that transponder have a build-in encoding altimeter? That requires a check through the logs and/ or manuals. In my aircraft they are separate, in yours I have no idea.

or was the Approach controller using altitude as reported by the radar if it has height finding capabilities?

Maybe. It depends upon the specifics of the airport. If your transponder's altimeter was calibrated incorrectly or otherwise bad (extremely difficult for you as the pilot to verify in a rental aircraft during preflight) then they may have been trusting your transponder for your altitude data, which was incorrect. If they were using radar with vertical capabilities (precision approach radar is, depending upon your viewpoint of part 91 vs part 121, either relatively rare or everywhere you might need it where I live) they may have been comparing that to your transponder's altitude reported.

All of this depends upon if you checked your altimeter using the current local setting on the runway threshold prior to takeoff. Have I always done that? Yes. Does everyone? No. If your panel altimeter checked out, you used the proper setting during the approach, and your transponder is telling them you are at the wrong altitude the transponder's altimeter appears to be the problem.

$\endgroup$
  • 1
    $\begingroup$ If the ATIS is old and a front is passing through, it could be enough for a 50 or 100' difference between what the field is reporting as the altimeter setting and what the automated system ATC uses is reporting. $\endgroup$ – JScarry Dec 25 '17 at 20:07
  • 1
    $\begingroup$ Very true, I keep forgetting that I am spoiled with one minute ASOS updates at my home field. However, if that is the case then everyone's altitude should be off by the same amount and typically ATC will announce a new altimeter setting on the approach frequency if that happens. $\endgroup$ – RudyB Dec 26 '17 at 2:54
  • $\begingroup$ @RudyB: ...assuming, of course, that there aren't any localised spikes or dips in the atmospheric pressure around the airport. $\endgroup$ – Sean Feb 10 at 3:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.