Editing Flight level (section)

==Background==
Flight levels are used to ensure safe vertical separation between aircraft. Historically, altitude has been measured using an  [[altimeter]], essentially a calibrated [[barometer]]. An altimeter measures ambient air pressure, which decreases with increasing altitude following the [[barometric formula]]. It displays the corresponding altitude. If aircraft altimeters were not calibrated consistently, then two aircraft could be flying at the same altitude even though their altimeters appeared to show that they are at different altitudes.<ref>See also [http://www.skybrary.aero/index.php/Level_Bust Level Bust] for related causes and consequences</ref> Flight levels require defining altitudes based on a standard altimeter setting. All aircraft operating at flight levels set 1013&nbsp;hPa or 29.92&nbsp;inHg. On the descent when descending through the published transition level, the altimeter is set to the local surface pressure, to display the correct altitude above sea level.