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 aircraft altitude from the transponder, which references pressure altitude (29.92 Hg) and is unaffected by the pilot's altimeter setting in the Kollsman window.
While an incorrect altimeter setting doesn't directly change the transponder's output to ATC, flying at a significantly different indicated altitude than expected can trigger ATC low altitude alerts and prompt them to provide the correct setting.
The CFI's altimeter reset is a common teaching method designed to train student pilots to always verify altimeter settings, such as from automated weather, and develop an accurate visual "sight picture" for altitude.

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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.