Plain English Meaning

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.

Full 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.

Context Anchor

Seen in instrument flying, high-altitude operations, , and pressure-altitude discussions.