Flight level


In aviation, a flight level 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.

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. 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.

Definition

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 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. In radio communications, FL290 would be stated as "flight level two nine zero".

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, 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, 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 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 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. In Europe, 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 is the lowest flight level above the transition altitude. The table below shows the transition level according to transition altitude and QNH. 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.
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 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", "transition layer", and "transition level" is
TL = TA + TLYR

Semicircular/hemispheric rule

The semicircular 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
  • Westbound – Magnetic track 180 to 359° – even thousands
At FL 290 and above, if Reduced Vertical Separation Minima are not in use, 4,000 ft intervals are used to separate same-direction aircraft, and only odd flight levels are assigned, independent of the direction of flight:
  • Eastbound – Magnetic track 000 to 179° – odd flight levels
  • Westbound – Magnetic track 180 to 359° – even flight levels
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 have semicircular rules that define a North/South rather than an East/West track split.
In Italy, France, Portugal and recently also in Spain, 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 separation levels are as per the following table:

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
The rule was non-binding upon flights operating under visual flight rules.
Minimum vertical separation between two flights abiding by the UK Quadrantal Rule is 500 ft. The level to be flown is determined by the magnetic track of the aircraft, as follows:
  • Magnetic track 000 to, and including, 089° – odd thousands of feet
  • Magnetic track 090 to, and including, 179° – odd thousands plus 500 ft
  • Magnetic track 180 to, and including, 269° – even thousands of feet
  • Magnetic track 270 to, and including, 359° – even thousands plus 500 ft

    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
  • Track 180 to 359° – even thousands
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
  • Track 180 to 359° – even flight levels

    Metre flight levels

The International Civil Aviation Organization has recommended a transition to using the International System of Units since 1979 with a recommendation on using metres for reporting flight levels. 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.

Kyrgyzstan, Kazakhstan, Tajikistan, Uzbekistan, and Turkmenistan

The flight levels below apply to Kyrgyzstan, Kazakhstan, Tajikistan and Uzbekistan and 6,000 m or below in Turkmenistan. Flight levels are read as e.g. "flight level 7,500 metres":
;Track 180 to 359°
  • 600 m
  • 1,200 m
  • 1,800 m
  • 2,400 m
  • 3,000 m
  • 3,600 m
  • 4,200 m
  • 4,800 m
  • 5,400 m
  • 6,000 m
  • 6,600 m
  • 7,200 m
  • 7,800 m
  • 8,600 m
  • 9,600 m
  • 10,600 m
  • 11,600 m
  • 13,100 m
  • 15,100 m
and every 2,000 metres thereafter.
;Track 000 to 179°
  • 900 m
  • 1,500 m
  • 2,100 m
  • 2,700 m
  • 3,300 m
  • 3,900 m
  • 4,500 m
  • 5,100 m
  • 5,700 m
  • 6,300 m
  • 6,900 m
  • 7,500 m
  • 8,100 m
  • 9,100 m
  • 10,100 m
  • 11,100 m
  • 12,100 m
  • 14,100 m
and every 2,000 metres thereafter.

People's Republic of China and Mongolia

The flight levels below apply to Mongolia and People's Republic of China, not including Hong Kong. To distinguish flight levels in feet, flight levels are read without "flight level", e.g. "one two thousand six hundred metres" or for 12,600 m. To distinguish altitude from flight level, "on standard" or "on QNH" would be added during initial clearance, such as "climb 4,800 metres on standard" or "descent 2,400 metres on QNH 1020".
RVSM was implemented in China at 16:00 UTC on 21 November 2007, and in Mongolia at 00:01 UTC on 17 November 2011. Aircraft flying in feet according to the table below will have differences between the metric readout of the onboard avionics and ATC cleared flight level; however, the differences will never be more than thirty metres.
;Track 180 to 359°
  • 600 m
  • 1,200 m
  • 1,800 m
  • 2,400 m
  • 3,000 m
  • 3,600 m
  • 4,200 m
  • 4,800 m
  • 5,400 m
  • 6,000 m
  • 6,600 m
  • 7,200 m
  • 7,800 m
  • 8,400 m
  • 9,200 m
  • 9,800 m
  • 10,400 m
  • 11,000 m
  • 11,600 m
  • 12,200 m
  • 13,100 m
  • 14,300 m
and every 1,200 metres thereafter.
;Track 000 to 179°
  • 900 m
  • 1,500 m
  • 2,100 m
  • 2,700 m
  • 3,300 m
  • 3,900 m
  • 4,500 m
  • 5,100 m
  • 5,700 m
  • 6,300 m
  • 6,900 m
  • 7,500 m
  • 8,100 m
  • 8,900 m
  • 9,500 m
  • 10,100 m
  • 10,700 m
  • 11,300 m
  • 11,900 m
  • 12,500 m
  • 13,700 m
  • 14,900 m
and every 1,200 metres thereafter.