Does ATC Know When Your Altimeter Is Set Incorrectly?

How transponders, pressure altitude, and controllers spot aircraft flying at the wrong altitude.

Meg Godlewski

Wednesday, September 30, 2026 at 09:00 AM ET

Verified

Edited By:

Travis Tingle

Cockpit of aircraft on final approach [Credit: Meg Godlewski]

Key Takeaways:

ATC reads an aircraft's altitude from its transponder, which uses pressure altitude (referenced to 29.92 Hg) and is not directly affected by the pilot's Kollsman window setting.
While ATC doesn't see the pilot's altimeter setting, a misset altimeter will cause the aircraft to fly at an incorrect indicated altitude, which ATC will identify as a deviation from expected or assigned altitude and prompt an alert.
The CFI's trick of resetting the altimeter is a common teaching method to emphasize the importance of verifying altimeter settings, such as with automated weather, and developing a strong sense of altitude awareness.

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Question: I’m a student pilot with about five hours experience and yesterday the CFI distracted me and, when I looked away, he reset the altimeter so it was a couple of hundred feet off. Of course, I didn’t catch it until after we landed. I know you can get into trouble for flying at the wrong altitude. Does ATC know it when you don’t set the altimeter correctly? I am flying out of a nontowered airport if that makes a difference.

Answer: Air traffic control reads altitude from the aircraft’s transponder. The transponder receives its information from the aircraft’s encoding altimeter, which measures the pressure in reference to 29.92 Hg (inches of mercury), also known as pressure altitude.