Definition
Flight conducted at or above 18,000 feet MSL in the United States, where altitudes are referenced as Flight Levels (FL) using a standard altimeter setting of 29.92 inches of mercury rather than local altimeter settings. At and above this altitude, all aircraft set the same pressure datum so they share a common vertical reference for separation.
Plain English
Flying high enough that everyone resets their altimeter to the same standard setting and reports altitude as a Flight Level (like FL310 for 31,000 feet) instead of a regular altitude in feet.
Context Anchor
Seen in IFR altitude selection, clearances, enroute charts, and high-altitude instrument procedures.
Derivation
"Flight level" came into use because at high altitudes, aircraft no longer reference local barometric pressure. They all use one standard pressure (29.92 in Hg), so the number reported isn't a true altitude above sea level — it's a level in the flight environment. Calling it a "flight level" rather than an "altitude" signals that distinction.
Why Pilots Care
Proper flight level operations maintain vertical separation between aircraft in high-altitude controlled airspace and prevent altimeter errors from local pressure variations.
Analogy
It is like everyone agreeing to use the same ruler before measuring. The ruler may not match the exact local ground pressure below you, but because everyone is using the same one, the spacing between aircraft stays consistent.
Intuition Check
Do not read “level” as meaning the airplane is flying straight and level. In this phrase, “flight level” means a numbered high-altitude pressure level based on the standard altimeter setting.
Example Sentence 1
Climbing through 18,000 feet, the pilot reset the altimeter to 29.92 and reported level at FL230, transitioning into flight level operations.
Example Sentence 2
ATC cleared the aircraft for flight level operations at FL350 to maintain separation from opposing traffic.