Definition
FL 240 is a flight level corresponding to an indicated altitude of 24,000 feet when the altimeter is set to the standard pressure setting of 29.92 inches of mercury (1013.2 hPa). Flight levels are used at and above 18,000 feet MSL in the United States to provide consistent vertical separation between aircraft regardless of local pressure variations.
Plain English
FL 240 means the aircraft is flying at what its altimeter reads as 24,000 feet, with every aircraft up there using the same standard pressure setting so they all measure altitude the same way.
Context Anchor
Seen in instrument clearances, route planning, and en route altitude assignments for aircraft operating in the flight levels.
Derivation
The 'FL' stands for Flight Level. The number that follows represents hundreds of feet, so FL 240 means 24,000 feet. The term came into use because, above a certain altitude, it became impractical for every aircraft to keep adjusting its altimeter to local pressure, so a shared reference was adopted.
Why Pilots Care
It provides standardized vertical separation between aircraft on the same airway regardless of local pressure variations.
Intuition Check
Do not read FL 240 as ordinary “24,000 feet above the ground.” It means the 24,000-foot pressure level with the altimeter set to 29.92.
Example Sentence 1
Example Sentence 2
At FL 240 the pilot set the altimeter to 29.92 and confirmed the assigned altitude on the en route chart.