In aviation, a flight level (FL) is an aircraft's altitude as determined by a pressure altimeter using the International Standard Atmosphere. A flight level is a surface of constant atmospheric pressure used to define an aircraft’s vertical position relative to other aircraft when flying above the transition altitude. 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.
Background Flight levels are used to ensure safe vertical separation between aircraft. Historically, altitude has been measured using a pressure 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.See also Level Bust for related causes and consequences Flight levels require defining altitudes based on a standard altimeter setting. All aircraft operating at flight levels set their pressure altimeters to assume a sea-level pressure of 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 literal altitude above sea level.
Definition A flight level (FL) is described by a number indicating the nominal pressure altitude in hundreds of feet. Therefore, a nominal pressure altitude of 32,000 feet is referred to as "flight level 320". In metre altitudes the format is "xx,000 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. In radio communications, FL290 would be stated as "flight level two nine(r) zero", indicating a nominal altitude of 29,000 feet.
Transition altitude 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. This altimeter setting is referred to by the Q code QNH (barometric pressure adjusted to sea level). The current local QNH 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 ascending 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 stated as a flight level (FL) instead of altitude.
In the United States and the southern domestic airspace of Canada, the transition altitude is . In most other countries, the transition altitude varies and can be as low as . 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 and changed the transition level from FL130 to FL150.
The transition level (TL) is the lowest flight level above the transition altitude. The table below shows the transition level according to transition altitude and . When descending below the transition level, the pilot starts to refer to altitude of the aircraft by setting the altimeter to the QNH for the region or airfield.
+ Table for determining transition level (TL) Aerodrome in hectopascals Transition altitude (TA) in feet 3,000 ft(900 m) 4,000 ft(1200 m) 5,000 ft(1500 m) 6,000 ft(1850 m) 1050–1060 FL030 FL040 FL050 FL060 1032–1049 FL035 FL045 FL055 FL065 1014–1031 FL040 FL050 FL060 FL070 995–1013 FL045 FL055 FL065 FL075 977–994 FL050 FL060 FL070 FL080 959–976 FL055 FL065 FL075 FL085 940–958 FL060 FL070 FL080 FL090
The transition layer is the airspace between the transition altitude and the transition level.
According to these definitions the transition layer is thick. Aircraft are not normally assigned to fly at the transition level as this would provide inadequate separation from traffic flying on QNH at the transition altitude. Instead, the lowest usable flight level is the transition level plus 500 ft.
However, in some countries, such as Norway and the United Kingdom for example, the transition level is determined by adding a buffer of minimum (depending on QNH) to the transition altitude. Therefore, aircraft may be flying at both transition level and transition altitude, and still be vertically separated by at least . In those areas the transition layer will be thick, depending on QNH.
In summary, the connection between transition altitude (TA), transition layer (TLYR), and transition level (TL) is
TL = TA + TLYR
Semicircular/hemispheric rule The semicircular rule (also known as the hemispheric rule) applies, in slightly different version, to IFR flights in the UK inside controlled airspace and generally in the rest of the world. The standard rule defines an East/West track split:
Eastbound – Magnetic track 000 to 179° – odd thousands (FL 250, 270, etc.) Westbound – Magnetic track 180 to 359° – even thousands (FL 260, 280, etc.)
At FL 290 and above, if reduced vertical separation minima (RVSM) are not in use, 4,000 ft intervals are used to separate same-direction aircraft (instead of 2,000 ft intervals below FL 290), and only odd flight levels are assigned, independent of the direction of flight: Eastbound – Magnetic track 000 to 179° – odd flight levels (FL 290, 330, 370, etc.) Westbound – Magnetic track 180 to 359° – even flight levels (FL 310, 350, 390, etc.) Conversely, RVSM equipped aircraft are able to continue separation in 2,000 ft intervals as outlined in the semicircular rules. Both non-RVSM and RVSM equipped aircraft use a separation of 4,000 ft above FL 410.
Countries where the major airways are oriented north/south (e.g., New Zealand; Italy; Portugal) have semicircular rules that define a North/South rather than an East/West track split.
In Italy, France, Portugal and recently also in Spain (AIP ENR 1.7-3), for example, southbound traffic uses odd flight levels; in New Zealand, southbound traffic uses even flight levels. In Europe commonly used International Civil Aviation Organization (ICAO) separation levels are as per the following table:
+ Vertical separation of VFR and IFR flights Magnetic route figure of merit (FOM) 0° to 179° 180° to 359° IFR VFR IFR VFR FL feet FL feet FL feet FL feet 010 1,000 020 2,000 030 3,000 035 3,500 040 4,000 045 4,500 050 5,000 055 5,500 060 6,000 065 6,500 070 7,000 075 7,500 080 8,000 085 8,500 090 9,000 095 9,500 100 10,000 105 10,500 110 11,000 115 11,500 120 12,000 125 12,500 130 13,000 135 13,500 140 14,000 145 14,500 150 15,000 155 15,500 160 16,000 165 16,500 170 17,000 175 17,500 180 18,000 185 18,500 190 19,000 195 19,500 200 20,000 210 21,000 220 22,000 230 23,000 240 24,000 250 25,000 260 26,000 270 27,000 280 28,000 290 29,000 310 31,000 330 33,000 350 35,000 370 37,000 390 39,000 410 41,000 430 43,000 450 45,000 470 47,000 490 49,000 510 51,000
Quadrantal rule The quadrantal rule is defunct. It was used in the United Kingdom but was abolished in 2015 to bring the UK in line with the semi-circular rule used around the world.
The quadrantal rule applied to IFR flights in the UK both in and outside of controlled airspace except that such aircraft may be flown at a level other than required by this rule if flying in conformity with instructions given by an air traffic control unit, or if complying with notified en-route holding patterns or holding procedures notified in relation to an aerodrome. The rule affected only those aircraft operating under IFR when in level flight above 3,000 ft above mean sea level, or above the appropriate transition altitude, whichever is the higher, and when below FL195 (19,500 ft above the 1013.2 hPa datum in the UK, or with the altimeter set according to the system published by the competent authority in relation to the area over which the aircraft is flying if such aircraft is not flying over the UK.)
The rule was non-binding upon flights operating under visual flight rules (VFR).
Minimum vertical separation between two flights abiding by the UK Quadrantal Rule is 500 ft (note these are in geopotential foot units). The level to be flown is determined by the magnetic track of the aircraft, as follows:Rules of the Air Regulations 2007 (No. 734), rule 34, table 1. Available from the UK Statute Law Database.
Magnetic track 000 to, and including, 089° – odd thousands of feet (FL070, 090, 110 etc.) Magnetic track 090 to, and including, 179° – odd thousands plus 500 ft (FL075, 095, 115 etc.) Magnetic track 180 to, and including, 269° – even thousands of feet (FL080, 100, 120 etc.) Magnetic track 270 to, and including, 359° – even thousands plus 500 ft (FL085, 105, 125 etc.)
Reduced vertical separation minima (RVSM)
Reduced vertical separation minima (RVSM) reduces the vertical separation between FL290 and FL410. This allows aircraft to safely fly more optimum routes, save fuel and increase airspace capacity by adding new flight levels. Only aircraft that have been certified to meet RVSM standards, with several exclusions, are allowed to fly in RVSM airspace. It was introduced into the UK in March 2001. On 20 January 2002, it entered European airspace. The United States, Canada and Mexico transitioned to RVSM between FL 290 and FL 410 on 20 January 2005, and Africa on 25 September 2008.
Track 000 to 179° – odd thousands (FL 290, 310, 330, etc.) Track 180 to 359° – even thousands (FL 300, 320, 340, etc.) At FL 410 and above, 4,000 ft intervals are resumed to separate same-direction aircraft and only odd Flight Levels are assigned, depending on the direction of flight: Track 000 to 179° – odd flight levels (FL 410, 450, 490, etc.) Track 180 to 359° – even flight levels (FL 430, 470, 510, etc.)
Metre flight levels The International Civil Aviation Organization (ICAO) has recommended a transition to using the International System of Units since 1979 Council action in pursuance of Assembly Resolution A22-18 adopted 23 March 1979: [..]to cover all aspects of air and ground operations; provision of standardized system of units based on the SI; identification of non-SI units permitted for use in international civil aviation; provision for termination of the use of certain non-SI units. with a recommendation on using metres (m) for reporting flight levels.International Civil Aviation Organization - International Standards and Recommended Practices - Annex 5 to the Convention on International Civil Aviation - Units of Measurement to be Used in Air and Ground Operations Fifth Edition - July 2010 China, Mongolia, Russia and many CIS countries have used flight levels specified in metres for years. Aircraft entering these areas normally make a slight climb or descent to adjust for this, although Russia and some CIS countries started using feet above transition altitude and introduced RVSM at the same time on 17 November 2011.