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 surface pressure of 1013 hPa or (29.92 inHg). The actual surface pressure may 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.^[1]

Scale comparison of some flight level systems

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.^[2] 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^[3] are described by a number, which is the nominal altitude, or pressure altitude, in hundreds of feet, and a multiple of 500 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 Flight Level 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".

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. 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.^[4] 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, aircraft altitude will be stated as a flight level.

In the United States and Canada, the transition altitude is 18,000 ft (5,500 m).^[5] 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.^[6] In the United Kingdom, different airports have different transition altitudes, between 3000 and 6000 feet.^[7]

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.^[8]

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 f (= 900 m)	4,000 f (= 1200 m)	5,000 f (= 1500 m)	6,000 f (= 1850 m)	18,000 f (= 5500 m)
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 (0–150 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,^[9] 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^[10]

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

Quadrantal rule

The quadrantal rule is defunct.^[11] 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.^[12][13]

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:^[14]

• 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)

Main article: Reduced vertical separation minima

In the U.S. and Canada, the information in this section applies to flights under instrument flight rules (IFR). Different altitudes will apply for aircraft flying under visual flight rules (VFR) above 3000 ft AGL.

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° – odd 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^[15][16] with a recommendation on using metres (m) for reporting flight levels.^[17] 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":

Track 180 to 359° 600 m (2,000 ft) 1,200 m (3,900 ft) 1,800 m (5,900 ft) 2,400 m (7,900 ft) 3,000 m (9,800 ft) 3,600 m (11,800 ft) 4,200 m (13,800 ft) 4,800 m (15,700 ft) 5,400 m (17,700 ft) 6,000 m (19,700 ft) 6,600 m (21,700 ft) 7,200 m (23,600 ft) 7,800 m (25,600 ft) 8,600 m (28,200 ft) 9,600 m (31,500 ft) 10,600 m (34,800 ft) 11,600 m (38,100 ft) 13,100 m (43,000 ft) 15,100 m (49,500 ft) and every 2,000 metres thereafter.

Track 000 to 179° 900 m (3,000 ft) 1,500 m (4,900 ft) 2,100 m (6,900 ft) 2,700 m (8,900 ft) 3,300 m (10,800 ft) 3,900 m (12,800 ft) 4,500 m (14,800 ft) 5,100 m (16,700 ft) 5,700 m (18,700 ft) 6,300 m (20,700 ft) 6,900 m (22,600 ft) 7,500 m (24,600 ft) 8,100 m (26,600 ft) 9,100 m (29,900 ft) 10,100 m (33,100 ft) 11,100 m (36,400 ft) 12,100 m (39,700 ft) 14,100 m (46,300 ft) 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 (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.

Track 180 to 359° 600 m (2,000 ft) 1,200 m (3,900 ft) 1,800 m (5,900 ft) 2,400 m (7,900 ft) 3,000 m (9,800 ft) 3,600 m (11,800 ft) 4,200 m (13,800 ft) 4,800 m (15,700 ft) 5,400 m (17,700 ft) 6,000 m (19,700 ft) 6,600 m (21,700 ft) 7,200 m (23,600 ft) 7,800 m (25,600 ft) 8,400 m (27,600 ft) 9,200 m (30,100 ft) 9,800 m (32,100 ft) 10,400 m (34,100 ft) 11,000 m (36,100 ft) 11,600 m (38,100 ft) 12,200 m (40,100 ft) 13,100 m (43,000 ft) 14,300 m (46,900 ft) and every 1,200 metres thereafter.

Track 000 to 179° 900 m (3,000 ft) 1,500 m (4,900 ft) 2,100 m (6,900 ft) 2,700 m (8,900 ft) 3,300 m (10,800 ft) 3,900 m (12,800 ft) 4,500 m (14,800 ft) 5,100 m (16,700 ft) 5,700 m (18,700 ft) 6,300 m (20,700 ft) 6,900 m (22,600 ft) 7,500 m (24,600 ft) 8,100 m (26,600 ft) 8,900 m (29,100 ft) 9,500 m (31,100 ft) 10,100 m (33,100 ft) 10,700 m (35,100 ft) 11,300 m (37,100 ft) 11,900 m (39,100 ft) 12,500 m (41,100 ft) 13,700 m (44,900 ft) 14,900 m (48,900 ft) and every 1,200 metres thereafter.

Flight levels in Russian Federation and North Korea

On 5 September 2011 the government of the Russian Federation issued decree No.743,^[18] 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:

Track 180 to 359° FL (metres) (feet) 20 600 2000 40 1200 4000 60 1850 6000 80 2450 8000 100 3050 10000 120 3650 12000 140 4250 14000 160 4900 16000 180 5500 18000 200 6100 20000 220 6700 22000 240 7300 24000 260 7900 26000 280 8550 28000 300 9150 30000 320 9750 32000 340 10350 34000 360 10950 36000 380 11600 38000 400 12200 40000 430 13100 43000 470 14350 47000 510 15550 51000

Track 000 to 179° FL (metres) (feet) 10 300 1000 30 900 3000 50 1500 5000 70 2150 7000 90 2750 9000 110 3350 11000 130 3950 13000 150 4550 15000 170 5200 17000 190 5800 19000 210 6400 21000 230 7000 23000 250 7600 25000 270 8250 27000 290 8850 29000 310 9450 31000 330 10050 33000 350 10650 35000 370 11300 37000 390 11900 39000 410 12500 41000 450 13700 45000 490 14950 49000

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.^[19]

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.^[20] Most other airports still^{[as of?]} use QFE.

Unlike Russia, North Korea uses metres below the TL based on QNH.

See also

• ICAO recommendations on use of the International System of Units
• Flight planning
• Gillham code

References

1. Wragg, David W. (1973). A Dictionary of Aviation (first ed.). Osprey. p. 133. ISBN 9780850451634.
2. See also Level Bust for related causes and consequences
3. Federal Aviation Administration (29 March 2018). "Pilot/Controller Glossary". Aeronautical Information Manual (PDF). p. PCG F−3. Retrieved 1 March 2019.
4. "CAP 410 Manual of Flight Information Services" (PDF). UK Civil Aviation Authority. CAP410. Retrieved 25 February 2013.^{[permanent dead link‍]}
5. "14 CFR § 91.121 - Altimeter settings".
6. "A Common European Transition Altitude; An ATC perspective" (PDF). Eurocontrol. Archived from the original (PDF) on 26 September 2013. Retrieved 3 April 2014.
7. "IAA Transition Altitude Consultation". 17 January 2016.
8. "Part 91, Amendment 12" (PDF). Civil Aviation Authority of New Zealand. Archived from the original (PDF) on 6 March 2016. Retrieved 4 February 2009.
9. "Eurocontrol specifications for harmonized Rules for Operational Air Traffic (OAT) under Instrument Flight Rules (IFR) inside controlled Airspace of the ECAC Area (EUROAT)" (PDF). Eurocontrol. p. 63.
10. "Application of Separation Minima" (PDF). International Civil Aviation Organization. November 2020. Archived from the original (PDF) on 2 January 2022. Retrieved 10 October 2021.
11. "SERA Implementation | UK Civil Aviation Authority". Archived from the original on 7 August 2019. Retrieved 19 September 2019.
12. "Standardised European Rules of the Air – UK Civil Aviation Authority". Civil Aviation Authority. Retrieved 20 August 2016.
13. Dave Drake, CAA Project Lead for SERA. "SERA – all you need to know" (PDF). flyontrack.co.uk. Retrieved 20 August 2016.
14. Rules of the Air Regulations 2007 (No. 734), rule 34, table 1. Available from the UK Statute Law Database.
15. International Civil Aviation Organization - Assembly Resolutions in Force (as of 8 October 2010) - Doc 9958 - Published by authority of the Secretary General
16. 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.
17. 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
18. "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.
19. "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.
20. Selleck, Declan (22 February 2017). "Big change: Russia finally moving to QNH". International Ops 2021. Retrieved 18 July 2021.