Definition
Flight levels are altitudes expressed in hundreds of feet above the standard datum plane (29.92 inches Hg), used at and above 18,000 feet in the United States. They are written as FL followed by three digits — for example, FL250 means 25,000 feet with the altimeter set to 29.92. Because all aircraft at these altitudes use the same standard pressure setting, their indicated altitudes remain consistently spaced relative to each other, which is essential for vertical separation in the high-altitude structure.
Plain English
Above 18,000 feet, pilots stop setting their altimeters to local pressure and instead all set them to the same standard value. The altitudes shown on the altimeter from that point up are called flight levels. FL250 means the altimeter reads 25,000 feet when set to standard pressure.
Context Anchor
Seen in instrument flying, high-altitude operations, air traffic control clearances, and pressure-altitude discussions.
Derivation
The term 'flight level' was chosen to distinguish these standardized pressure-based altitudes from true altitude above the ground or sea level. They are 'levels' in the sense of layers in the atmosphere, defined by pressure rather than by physical height.
Why Pilots Care
Maintains consistent vertical separation between aircraft when local pressure variations would otherwise create unsafe altitude differences.
Grounding Statement
A flight level is an altitude reference based on air pressure, not a direct measurement of height above the ground.
Intuition Check
Do not read “flight levels” as simply “altitudes where airplanes fly.” In this context, flight levels are specific pressure-based altitude levels using the standard altimeter setting.
Example Sentence 1
Climbing through 17,500 feet, the pilot reset the altimeter to 29.92 and reported level at FL230.
Example Sentence 2
After passing 18,000 feet the pilot reset the altimeter to 29.92 and reported level at flight level one nine zero.