Definition
Flight Level 180, an altitude of 18,000 feet referenced to the standard altimeter setting of 29.92 inches of mercury. FL 180 is the lowest flight level in the contiguous United States and marks the floor of Class A airspace, where all aircraft must operate under instrument flight rules with altimeters set to 29.92.
Plain English
An altitude of 18,000 feet measured using a fixed altimeter setting (29.92) instead of the local one. It is the bottom of the high-altitude airspace where everyone must fly on an instrument flight plan.
Context Anchor
Seen in instrument procedures, air traffic control clearances, altitude assignments, and discussions of U.S. high-altitude airspace.
Derivation
Flight Level' is a term used to describe altitudes flown using a standard pressure setting rather than the local one. The number 180 represents the altitude in hundreds of feet, so FL 180 means 18,000 feet. The system avoids confusion at high altitudes where small differences in local pressure across a wide area would otherwise cause aircraft to be at slightly different true altitudes for the same indicated reading.
Why Pilots Care
Above this level all aircraft use the same pressure reference, ensuring consistent vertical separation regardless of local weather.
Grounding Statement
At FL 180, aircraft are using the same altimeter reference, so their indicated altitudes can be compared consistently.
Intuition Check
FL 180 does not mean exactly 18,000 feet above sea level or above the ground. It means the altimeter reads 18,000 feet when set to the standard pressure setting.
Example Sentence 1
Climbing through FL 180, the pilot set the altimeter to 29.92 and reported level at the assigned flight level.
Example Sentence 2
ATC assigned FL 180 as the cruising level for the cross-country leg.