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.
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When a pilot adjusts the barometric scale using the Kollsman window, the data sent to the transponder is not affected, and therefore ATC isn’t going to see a dramatic change in the transponder reading. However, if you are off altitude, perhaps flying at 2,000 feet indicated because that’s what controllers assigned, and ATC shows you at 1,500, they will likely give you a low altitude alert and an updated altimeter setting to get you back where you should be.
“Correcting” is the pilot’s response as they make the change.
Just an FYI, the sneaky reset of the altimeter is a CFI trick to teach you to listen to the automated weather to verify the altimeter setting before entering the pattern. Many student pilots have landed with the altimeter not quite right.
That being said, it is a proud day when the learner recognizes they are slightly too low or too high in the pattern as they know the sight picture and then double-check the altimeter to fix it.
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