Flight level

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In aviation, a flight level (FL) is an aircraft's altitude using the International Standard Atmosphere, expressed in hundreds of feet or metres. The altimeter setting used is the ISA pressure of 1013 hPa or (29.92 inHg). At a flight level, every aircraft has the same altimeter setting; the actual height varies due to a constant pressure. Because every aircraft has the same altimeter reference, vertical clearance is obtained.^[1]