Are airspace violations (e.g. entry to class B without clearance) based on primary radar and/or Mode C transponder, or something else?

I read that Mode C altitude is based on pressure altitude, i.e., set to 29.92" ... but presumably that's adjusted at the ATC facility based on the current pressure before being used for altitude enforcement.

This begs the question, what would stop one (hypothetically), just winding back the altimeter pressure reading to appear to be at a lower altitude?

So to summarize: How are airspace violations detected:

  1. What data input is used?
  2. If Mode C reading is used, is it based on pilot's altimeter?
  3. Would winding back the altimeter make a plane report a lower altitude?
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    $\begingroup$ I busted class D one time by about 0.3 to 0.5 miles and they did not really care. $\endgroup$
    – Keegan
    Commented Jan 3, 2015 at 7:43
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    $\begingroup$ @SpongeBob I probably have too... I'm more concerned with busting class B without a clearance. $\endgroup$
    – CJBS
    Commented Jan 4, 2015 at 7:46

2 Answers 2


The transponder usually uses its own pressure reading, not what is set in the pilot's altimeter. So to prevent cheating as you describe, it is inspected and calibrated every 24 months. Tampering with it would be difficult to do on the fly because you'd have to adjust it based on the current atmospheric conditions and what altitude you want to seem to fly at. But yes, you could, in theory, adjust its readings to broadcast something different.

As far as I'm aware, only major violations are really pursued, or if ATC knows who you are when you commit the violation.

  • 3
    $\begingroup$ Yes, I saw this about the aftermath: aviation.stackexchange.com/questions/9/… $\endgroup$
    – CJBS
    Commented Apr 21, 2014 at 23:15
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    $\begingroup$ Does this mean that primary radar is used for, say Class D (and perhaps C) airspace, which, in the U.S., a Mode C transponder isn't required for? $\endgroup$
    – CJBS
    Commented Apr 21, 2014 at 23:16
  • $\begingroup$ @CJBS I'm not sure what you mean, related to the topic. A lot of Class D's don't even have radar, or they might only be tied in to the radar of a nearby class C. $\endgroup$
    – StallSpin
    Commented Apr 21, 2014 at 23:19
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    $\begingroup$ If you really screwed up (like "cut through the heart of the New York Class B airspace squawking VFR and not talking to anybody") ATC will probably follow you on their scope, determine where you're landing, and you'll receive "the dreaded phone number" to call when you land (either from local ATC, or they'll call the uncontrolled field you landed and ask "Who was that plane that landed at your field at 1320 Zulu?") $\endgroup$
    – voretaq7
    Commented Apr 21, 2014 at 23:20
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    $\begingroup$ @CJBS Airspace doesn't change with air pressure... You adjust the altimeter for pressure variations so that it reads the correct altitude. The only airspace that can change with pressure is Class A, but they don't assign aircraft to the couple-thousand feet in conflict. $\endgroup$
    – StallSpin
    Commented Apr 22, 2014 at 19:18

In the event of a disagreement between the transponder altitude and the pilot's stated altitude ATC is required to go with what you tell them on the radio. So if their screen says you are at 18,000 but you say "no, I'm at 15,000" they go with that number. Therefore, if you do fudge your numbers a bit for whatever reason, ATC will be unable to do anything about it. They will also not issue avoidance vectors to the A340 approaching head-on at 0.8mach - you are, after all, 3000 feet lower than the Airbus.

Regarding the "dreaded phone number to call" - once you have landed and parked you are no longer air traffic and no longer required to follow their instructions. In many cases the FAA cannot issue you a violation without your co-operation, so don't give them any.


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