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 altitude from an aircraft's transponder, which reports pressure altitude (referenced to 29.92 Hg) and is unaffected by the pilot's Kollsman window setting.
Although adjusting the Kollsman window doesn't change what ATC directly sees, they will detect if the pilot is flying at an incorrect indicated altitude compared to the actual pressure altitude and will issue alerts or updated altimeter settings.
The CFI's altimeter reset trick is a common teaching method designed to train pilots to verify altimeter settings (e.g., from automated weather) and develop an accurate visual understanding of altitude in the pattern.

See a mistake? Contact us.

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.