Flight level explained

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]

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 aircraftaltimeters 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. When approaching an airport, after descending through a standard 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, while being a multiple of 500 ft. Therefore, a pressure altitude of 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 100feet. 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 .^[5] In Europe, the transition altitude varies and can be as low as . 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 NaNfeet 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

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 NaNfeet 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 (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° – 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.)

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)

See main article: Reduced vertical separation minima.

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)
• 17,600 m (57,700 ft)
• 20,600 m (67,500 ft)
• 24,200 m (79,300 ft)
• 28,400 m (93,175 ft)
• 33,200 m (20.63 mi)
• 38,600 m (23.98 mi)
• 44,600 m (27.71 mi)
• 51.2 km (31.81 mi)
• 58.4 km (36.28 mi)
• 66.2 km (41.13 mi)
• 74.8 km (46.48 mi)
• 84.4 km (52.44 mi)
• 95 km (59.03 mi)
• 100 km (62.14 mi)

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 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

Notes and References

1. Book: Wragg, David W. . A Dictionary of Aviation . 9780850451634 . first . Osprey . 1973 . 133.
2. See also Level Bust for related causes and consequences
3. Book: Aeronautical Information Manual . Federal Aviation Administration . Federal Aviation Administration . Pilot/Controller Glossary . PCG F−3 . 29 March 2018 . 1 March 2019.
4. Web site: CAP 410 Manual of Flight Information Services . UK Civil Aviation Authority . CAP410 . 25 February 2013 .
5. Web site: 14 CFR § 91.121 - Altimeter settings.
6. Web site: A Common European Transition Altitude; An ATC perspective . Eurocontrol . 3 April 2014 . dead . https://web.archive.org/web/20130926112255/http://www.eurocontrol.int/sites/default/files/content/documents/nm/airspace/airspace-atmprocedures-cta-atc-perspective.pdf . 26 September 2013 . dmy-all .
7. Web site: IAA Transition Altitude Consultation . 17 January 2016 .
8. Web site: Part 91, Amendment 12 . . 4 February 2009 . https://web.archive.org/web/20160306222005/https://www.caa.govt.nz/rules/Amendments/Part_091_040705_Amendment_11.pdf . 6 March 2016 . dead .
9. Web site: Eurocontrol specifications for harmonized Rules for Operational Air Traffic (OAT) under Instrument Flight Rules (IFR) inside controlled Airspace of the ECAC Area (EUROAT) . Eurocontrol . Eurocontrol . 63 . en .
10. Web site: November 2020. Application of Separation Minima. dead. 10 October 2021. International Civil Aviation Organization. 2 January 2022. https://web.archive.org/web/20220102215351/https://www.icao.int/EURNAT/EUR%20and%20NAT%20Documents/NAT%20Documents/NAT%20Documents/NAT%20Doc%20008%20-%20NAT%20ASM/NAT%20Doc%20008%20%20(EN)%20-%20Edition%2001,%20Amd%2010.pdf.
11. Web site: SERA Implementation | UK Civil Aviation Authority . 19 September 2019 . 7 August 2019 . https://web.archive.org/web/20190807012711/http://www.caa.co.uk/commercial-industry/airspace/rules-of-the-air/sera-implementation/ . dead .
12. Web site: Standardised European Rules of the Air – UK Civil Aviation Authority. Civil Aviation Authority. 20 August 2016.
13. Web site: SERA – all you need to know . Dave Drake, CAA Project Lead for SERA . flyontrack.co.uk . 20 August 2016.
14. Rules of the Air Regulations 2007 (No. 734), rule 34, table 1. Available from the UK Statute Law Database.
15. https://www.icao.int/publications/Documents/9958_en.pdf 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. https://aerosavvy.com/wp-content/uploads/2014/08/an05_cons.pdf 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. Web site: Постановление Правительства РФ от 05.09.2011 N 743. Консультант Плюс. ru. Checked on September 5, 2011: N 743, AMENDING In the federal regulations of air space use RUSSIAN FEDERATION. 29 September 2011. dead. https://web.archive.org/web/20160203091332/http://base.consultant.ru/cons/cgi/online.cgi?req=doc%3Bbase%3DLAW%3Bn%3D119021. 3 February 2016.
19. Web site: 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 . rusaero.aero . 20 November 2011 . 20 August 2016 . 15 April 2016 . https://web.archive.org/web/20160415045050/http://www.rusaero.aero/files/679/aic-2011-11ang.pdf . dead .
20. Web site: Declan . Selleck . 2017-02-22. Big change: Russia finally moving to QNH. 2021-07-18. International Ops 2021. en-US.