Definition
FL 350 is a specific flight level corresponding to an indicated altitude of 35,000 feet when the altimeter is set to the standard pressure of 29.92 inches of mercury (1013.2 hPa). Flight levels are used at and above 18,000 feet MSL in the United States, where all aircraft set their altimeters to the same standard pressure so that vertical separation is maintained relative to a common reference rather than to local sea-level pressure.
Plain English
FL 350 means the aircraft is flying at 35,000 feet on the altimeter, with the altimeter set to a standard pressure setting that all aircraft use up there. It is the level used during cruise on most airline flights.
Context Anchor
Seen in high-altitude clearances, route descriptions, and instrument procedures for aircraft operating in the flight levels.
Derivation
The 'FL' stands for 'Flight Level,' and the number that follows is the altitude in hundreds of feet. So FL 350 = 350 × 100 = 35,000 feet. This shorthand exists because at high altitude every aircraft uses the same standard altimeter setting, so altitudes are reported as flight levels rather than as feet above sea level.
Why Pilots Care
Maintains vertical separation from other traffic in the flight levels.
Intuition Check
Do not read FL 350 as 350 feet. Also, do not treat it as exact height above terrain; it is 35,000 feet on the standard pressure setting.
Example Sentence 1
Example Sentence 2
Cruising at FL 350 provided smooth air and efficient fuel burn for the long leg.