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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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1 Answer 1

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.

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Yes, I saw this about the aftermath: aviation.stackexchange.com/questions/9/… –  CJBS Apr 21 at 23:15
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? –  CJBS Apr 21 at 23:16
@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. –  StallSpin Apr 21 at 23:19
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?") –  voretaq7 Apr 21 at 23:20
@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. –  StallSpin Apr 22 at 19:18

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