# Flight level

In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25 hPa (29.92 inHg) at sea level, and therefore is not necessarily the same as the aircraft's actual altitude, either above sea level or above ground level.

## Background

Flight levels are used to ensure safe vertical separation between aircraft, despite natural local variations in atmospheric air pressure. Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures ambient air pressure, which decreases with increasing altitude following the barometric formula. It then calculates and displays the corresponding altitude. If different aircraft's altimeters were not calibrated consistently, then two aircraft could be flying at the same altitude even though their altimeters appeared to show that they were at different altitudes.[1] Flight levels solve this problem by defining altitudes based on a standard air pressure at sea level. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure.

To display true altitude above sea level, a pilot would need to calibrate the altimeter according to the local air pressure at sea level, to take into account natural variation of pressure over time and in different regions.

## Definition

Flight levels[2] are described by a number, which is the nominal altitude, or pressure altitude, in hundreds of feet, while being a multiple of 500 ft, therefore always ending in 0 or 5. Therefore, a pressure altitude of 32,000 ft (9,800 m) is referred to as "flight level 320".

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 pronounced as "flight level two nine(r) 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 height above ground. At low altitudes, the altimeter is commonly set to show the altitude above sea level, which can be directly compared to the known elevation of the terrain. The pressure setting to achieve this varies with weather conditions. It is called QNH ("barometric pressure adjusted to sea level"), or "altimeter setting", and the current local value is available from various sources, including air traffic control and the local METAR-issuing station.

The transition altitude (TA) is the altitude above sea level at which aircraft change from the use of local barometer derived altitudes to the use of flight levels. When operating at or below the TA, aircraft altimeters are usually set to show the altitude above sea level.[3] Above the TA, the aircraft altimeter pressure setting is normally adjusted to the standard pressure setting of 1013.25 hectopascals (which is equivalent to millibars) or 29.92 inches of mercury and aircraft altitude will be expressed as a flight level.

In the United States and Canada, the transition altitude is 18,000 ft (5,500 m).[4] In Europe, the transition altitude varies and can be as low as 3,000 ft (910 m). There are discussions to standardize the transition altitude within the Eurocontrol area.[5] In the United Kingdom, different airports have different transition altitudes, between 3000 and 6000 feet.[6]

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.[7]

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 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.

Table for determining transition level[citation needed]
QNH
(in hectopascals)
Transition altitude (in feet)
3,000 4,000 5,000 6,000 18,000
1032–1050 FL025 FL035 FL045 FL055 FL175
1014–1031 FL030 FL040 FL050 FL060 FL180
996–1013 FL035 FL045 FL055 FL065 FL185
978–995 FL040 FL050 FL060 FL070 FL190
960–977 FL045 FL055 FL065 FL075 FL195
943–959 FL050 FL060 FL070 FL080 FL200

The transition layer is the airspace between the transition altitude and the transition level.

According to these definitions the transition layer is 0–500 feet (0–152 m) 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 for example,[8] the transition level is determined by adding a buffer of minimum 1,000 ft (300 m) (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 1,000 ft (300 m). In those areas the transition layer will be 1,000–1,500 ft (300–460 m) 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° – odd 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[9]
Magnetic route figure of merit (FOM)
0° to 179° 180° to 359°
VFR IFR VFR IFR
FL feet FL feet FL feet FL feet
010 1,000 020 2,000
030 3,000 040 4,000
035 3,500 050 5,000 045 4,500 060 6,000
055 5,500 070 7,000 065 6,500 080 8,000
075 7,500 090 9,000 085 8,500 100 10,000
095 9,500 110 11,000 105 10,500 120 12,000
115 11,500 130 13,000 125 12,500 140 14,000
135 13,500 150 15,000 145 14,500 160 16,000
155 15,500 170 17,000 165 16,500 180 18,000
175 17,500 190 19,000 185 18,500 200 20,000
195 19,500 210 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

The quadrantal rule is defunct.[10] 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.[11][12]

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.)[citation needed]

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:[13]

• 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, gain fuel savings 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° – odd flight levels (FL 430, 470, 510, etc.)

## Metric flight levels

The International Civil Aviation Organization (ICAO) has recommended a transition to using the International System of Units since 1979[14][15] with a recommendation on using metres (m) for reporting flight levels.[16] 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 (where feet is used for FL210 and above). Flight levels are read as e.g. "flight level 7,500 metres":

### 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 (Chinese only available in Chinese airspace).
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.

## Flight levels in Russian Federation and North Korea

On 5 September 2011 the government of the Russian Federation issued decree №743,[17] pertaining to the changes in the rules of use of the country's airspace. The new rules came into force on 17 November 2011, introducing a flight level system similar to the one used in the West. RVSM has also been in force since this date.

The following table is true for IFR flights:

The new system would eliminate the need to perform climbs and descents in order to enter or leave Russian airspace from or to jurisdictions following the Western standard.[18]

From February 2017, Russia is changing to use QNH and Feet below the Transition Level. The first airport to use this is ULLI/St. Petersburg.[19] Most other airports still[as of?] use QFE.

Unlike Russia, North Korea usually uses metric system below TL based on QNH.

## References

2. ^ Federal Aviation Administration (29 March 2018). "Pilot/Controller Glossary". Aeronautical Information Manual (PDF). p. PCG F−3. Retrieved 1 March 2019.
3. ^ "CAP 410 Manual of Flight Information Services" (PDF). UK Civil Aviation Authority. CAP410. Retrieved 25 February 2013.
4. ^
5. ^ "A Common European Transition Altitude; An ATC perspective" (PDF). Eurocontrol. Archived from the original (PDF) on 26 September 2013. Retrieved 3 April 2014.
6. ^ "IAA Transition Altitude Consultation". 17 January 2016.
7. ^ "Part 91, Amendment 12" (PDF). Civil Aviation Authority of New Zealand. Archived from the original (PDF) on 6 March 2016. Retrieved 4 February 2009.
8. ^
9. ^ "Application of Separation Minima" (PDF). International Civil Aviation Organization. November 2020. Archived from the original (PDF) on 2 January 2022. Retrieved 10 October 2021.
10. ^ "SERA Implementation | UK Civil Aviation Authority". Archived from the original on 7 August 2019. Retrieved 19 September 2019.
11. ^ "Standardised European Rules of the Air – UK Civil Aviation Authority". Civil Aviation Authority. Retrieved 20 August 2016.
12. ^ Dave Drake, CAA Project Lead for SERA. "SERA – all you need to know" (PDF). flyontrack.co.uk. Retrieved 20 August 2016.
13. ^ Rules of the Air Regulations 2007 (No. 734), rule 34, table 1. Available from the UK Statute Law Database.
14. ^ International Civil Aviation Organization - Assembly Resolutions in Force (as of 8 October 2010) - Doc 9958 - Published by authority of the Secretary General
15. ^ 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.
16. ^ 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
17. ^ "Постановление Правительства РФ от 05.09.2011 N 743" [Checked on September 5, 2011: N 743, AMENDING In the federal regulations of air space use RUSSIAN FEDERATION] (in Russian). Консультант Плюс. Archived from the original on 3 February 2016. Retrieved 29 September 2011.
18. ^ "About the Transition to the ICAO Vertical Separation System and Reduced Vertical Separation Minimum (RVSM) From FL 290 TO FL 410 Inclusive in the Airspace of the Russian Federation Effective From 17 November 2011" (PDF). rusaero.aero. 20 November 2011. Archived from the original (PDF) on 15 April 2016. Retrieved 20 August 2016.
19. ^ Selleck, Declan (22 February 2017). "Big change: Russia finally moving to QNH". International Ops 2021. Retrieved 18 July 2021.