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flight level
A flight level (FL) is specific barometric pressure, expressed as a nominal altitude in hundreds of feet. The pressure is computed assuming an International standard sea-level pressure datum of 1013.25 hPa (29.92 inHg), and therefore is not necessarily the same as the aircraft's true altitude either above mean sea level or above ground level.
==Background==
Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures air pressure, which decreases with increasing altitude following the barometric formula, and from the surrounding's pressure calculates and displays the corresponding altitude.
To display altitude above sea level, a pilot must recalibrate 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.〔Learn more about (Level Bust ) and related causes and consequences〕
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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