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Flight level
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Flight level
In aviation, a flight level (FL) is an aircraft's altitude as determined by a pressure altimeter using the International Standard Atmosphere. It is expressed in hundreds of feet or metres. The altimeter setting used is the ISA sea level pressure of 1013 hPa or 29.92 inHg. The actual surface pressure will vary from this at different locations and times. Therefore, by using a standard pressure setting, every aircraft has the same altimeter setting, and vertical clearance can be maintained during cruise flight.
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. Flight levels require defining altitudes based on a standard altimeter setting. All aircraft operating at flight levels set 1013 hPa or 29.92 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.
Flight levels are described by a number, which is the nominal altitude, or pressure altitude, in hundreds of feet, and a multiple of 100 ft. Therefore, a pressure altitude of 32,000 ft (9,800 m) is referred to as "flight level 320". In metre altitudes the format is xx000 metres.
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 (30 m). In radio communications, FL290 would be stated as "flight level two nine(r) zero".
While use of a standardised pressure setting facilitates separation of aircraft from each other, it does not provide the aircraft's actual altitude above sea level. Below the Transition level (which varies worldwide), the altimeter is set to the local altimeter setting, which can be directly compared to the known elevation of the terrain. The pressure setting to achieve this varies with local atmospheric pressure. It is called QNH ("barometric pressure adjusted to sea level"), or "altimeter setting", the current local value is available from various sources, including air traffic control and the local airport weather frequency or a METAR-issuing station.
The transition altitude (TA) is the altitude above sea level at which aircraft change from the use of local pressure to the use of standard pressure. When operating at or below the TA, aircraft altimeters are usually set to show the altitude above sea level. Above the TA, the aircraft altimeter pressure setting is changed to the standard pressure setting of 1013 hectopascals (equivalent to millibars) or 29.92 inches of mercury, with the aircraft altitude will be stated as a flight level instead of altitude.
In the United States and Canada, the transition altitude is 18,000 ft (5,500 m). In Europe, the transition altitude varies and can be as low as 3,000 ft (900 m). There are discussions to standardize the transition altitude within the Eurocontrol area. In the United Kingdom, different airports have different transition altitudes, between 3000 and 6000 feet.
On 25 November 2004 the Civil Aviation Authority of New Zealand raised New Zealand's transition altitude from 11,000 to 13,000 feet (3,400 to 4,000 m) and changed the transition level from FL130 to FL150.
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Flight level
In aviation, a flight level (FL) is an aircraft's altitude as determined by a pressure altimeter using the International Standard Atmosphere. It is expressed in hundreds of feet or metres. The altimeter setting used is the ISA sea level pressure of 1013 hPa or 29.92 inHg. The actual surface pressure will vary from this at different locations and times. Therefore, by using a standard pressure setting, every aircraft has the same altimeter setting, and vertical clearance can be maintained during cruise flight.
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. Flight levels require defining altitudes based on a standard altimeter setting. All aircraft operating at flight levels set 1013 hPa or 29.92 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.
Flight levels are described by a number, which is the nominal altitude, or pressure altitude, in hundreds of feet, and a multiple of 100 ft. Therefore, a pressure altitude of 32,000 ft (9,800 m) is referred to as "flight level 320". In metre altitudes the format is xx000 metres.
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 (30 m). In radio communications, FL290 would be stated as "flight level two nine(r) zero".
While use of a standardised pressure setting facilitates separation of aircraft from each other, it does not provide the aircraft's actual altitude above sea level. Below the Transition level (which varies worldwide), the altimeter is set to the local altimeter setting, which can be directly compared to the known elevation of the terrain. The pressure setting to achieve this varies with local atmospheric pressure. It is called QNH ("barometric pressure adjusted to sea level"), or "altimeter setting", the current local value is available from various sources, including air traffic control and the local airport weather frequency or a METAR-issuing station.
The transition altitude (TA) is the altitude above sea level at which aircraft change from the use of local pressure to the use of standard pressure. When operating at or below the TA, aircraft altimeters are usually set to show the altitude above sea level. Above the TA, the aircraft altimeter pressure setting is changed to the standard pressure setting of 1013 hectopascals (equivalent to millibars) or 29.92 inches of mercury, with the aircraft altitude will be stated as a flight level instead of altitude.
In the United States and Canada, the transition altitude is 18,000 ft (5,500 m). In Europe, the transition altitude varies and can be as low as 3,000 ft (900 m). There are discussions to standardize the transition altitude within the Eurocontrol area. In the United Kingdom, different airports have different transition altitudes, between 3000 and 6000 feet.
On 25 November 2004 the Civil Aviation Authority of New Zealand raised New Zealand's transition altitude from 11,000 to 13,000 feet (3,400 to 4,000 m) and changed the transition level from FL130 to FL150.
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