Definition
A standard altitude expressed in hundreds of feet above the 29.92 inches of mercury (1013.2 hPa) reference pressure setting. For example, FL 250 means an indicated altitude of 25,000 feet when the altimeter is set to 29.92. Flight levels are used at and above 18,000 feet MSL in the United States, where all aircraft set their altimeters to the standard pressure setting so that vertical separation between aircraft is consistent regardless of local pressure.
Plain English
A way of stating altitude in the high-altitude environment, written as FL followed by three digits that represent the altitude in hundreds of feet, with every aircraft using the same standard pressure setting on the altimeter.
Context Anchor
Seen in high-altitude instrument flying, air traffic clearances, and discussions of reduced vertical separation minimum airspace.
Derivation
Flight Level is a procedural term adopted internationally to distinguish altitudes flown on the standard pressure setting from altitudes flown on a local altimeter setting. The shorthand FL plus three digits keeps radio calls short and unambiguous.
Why Pilots Care
At and above 18,000 feet, pilots must set 29.92 and report altitudes as flight levels. Forgetting to switch to or from the standard setting at the transition altitude can cause significant errors in true altitude and break separation from other traffic.
Grounding Statement
A flight level gives aircraft a shared vertical reference by having them use the same standard altimeter setting.
Intuition Check
FL does not simply mean “feet above sea level” using the local altimeter setting. It means a pressure-based level using the standard setting of 29.92 inches of mercury.
Example Sentence 1
Example Sentence 2
RVSM allows properly equipped aircraft to use flight levels 290 through 410 with 1,000-foot spacing.