Flight level

In aviation and aviation meteorology, a flight level (FL) is defined as a vertical altitude at standard pressure, nominally expressed in hundreds of feet. The pressure is computed assuming an International standard sea-level pressure datum of 1013.2 hPa (29.92 inHg), and therefore is not necessarily the same as the aircraft's actual altitude either above mean sea level or above ground level.

Flight levels are used to ensure safe vertical separation between aircraft, despite natural local variations in atmospheric air pressure. Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures ambient air pressure, which decreases with increasing altitude following the barometric formula. It then calculates and displays the corresponding altitude.

To display altitude above sea level, a pilot must calibrate the altimeter according to the local air pressure at sea level, to take into account natural variation of pressure over time and in different regions. If this is not done, two aircraft could be flying at the same altitude even though their altimeters appear to show that they are at considerably different altitudes.

Flight levels solve this problem by defining altitudes based on a standard air pressure at sea-level. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure.

Flight levels are described by a number, which is this nominal altitude (or, pressure altitude) in hecto-feet, while being a multiple of 500 ft, therefore always ending on 0 or 5. Therefore, a pressure altitude of, for example, 32,000 feet is referred to as "flight level 320".

Flight levels are usually designated in writing as FLxxx, where xxx is a two or three-digit number indicating the pressure altitude in units of 100 feet. In radio communications, FL290 would be pronounced as "flight level two nine(r) zero." The phrase "flight level" makes it clear that this refers to the standardized pressure altitude.

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