Ask the Captain: What determines a plane’s altitude? How does it account for high elevation?

What determines an airplane’s altitude? is it relative to sea level or the land over which it is flying? For instance, when I am flying over Denver, Colorado, does the official altitude at which I am flying decrease by 5,280 feet?

– Mark, Colorado

Most airplanes fly above Mean Sea Level altitudes (MSL), the altimeter would read 5,280 feet in Denver. They are corrected for barometric pressure. There are radio altimeters that the altitude Above Ground Level (AGL), which are used for low visibility approaches.

When flying above 18,000 feet in the U.S., airplanes express altitude in terms of Flight Levels, which have a standard altimeter setting of 29.92 inches of Mercury. Pilots then describe the altitude by the first three digits. For example, 18,000 feet is FL180, 29,000 feet is FL290, 41,000 feet is FL410 and so on.

Using a standard altimeter setting avoids the constant resetting of the altimeter as the airplane flies into high or lower pressure. Additionally, having all the aircraft using the same altimeter setting ensures proper vertical separation.

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