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A
Flight Level (
FL) is a standard nominal
altitudeAltitude is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object...
of an
aircraftAn aircraft is a vehicle which is able to fly by being supported by the air, or in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil An aircraft is a vehicle which is able to fly by being supported...
, in hundreds of feet. This altitude is calculated from a world-wide fixed pressure datum of 1013.25
hPaThe pascal is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of force per unit area, defined as one newton per square metre...
(29.921
inHgInches of mercury, inHg, or "Hg is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports and aviation in the United States, but is considered somewhat outdated elsewhere....
), the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either
above mean sea levelThe term above mean sea level refers to the elevation or altitude of any object, relative to the average sea level datum. AMSL is used extensively in radio by engineers to determine the coverage area a station will be able to reach...
or
above ground levelIn aviation and atmospheric sciences, an altitude is said to be above ground level when it is measured with respect to the underlying ground surface. This is as opposed to above mean sea level , or in broadcast engineering, height above average terrain...
.
Background
Historically, altitude has been measured using a pressure
altimeterAn altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...
, which is essentially a calibrated
barometerA barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather...
. An altimeter measures air pressure, which decreases with increasing altitude, and from the pressure calculates and displays the corresponding altitude. To display altitude above sea level, a pilot must recalibrate 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. If this is not done, two aircraft could be flying at the same
altitudeAltitude is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object...
even though their altimeters appear to show that they are at considerably different altitudes. This is a critical safety issue.
Flight levels solve this problem by defining altitudes based on a standard pressure. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure. Flight levels are described by a number, which is this nominal altitude ("
pressure altitudeIn aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. This setting – 1013.25 hPa, equivalent to 1013.25 millibar , or 29.92 inches Hg – is equivalent to the air pressure at mean sea level in the International Standard Atmosphere...
") in feet, divided by 100. Therefore an apparent altitude of, for example, 32,000 feet is referred to as "flight level 320". To avoid collisions between two aircraft due to their being at the same altitude, their 'real' altitudes (compared to ground level, for example) are not important; it is the difference in altitudes that determines whether they might collide. This difference can be determined from the air pressure at each craft, and does not require knowledge of the local air pressure on the ground.
Flight levels are usually designated in writing as
FLxxx, where
xxx is a one- to three-digit number indicating the pressure altitude in units of 100 feet. In radio communications, FL290 would be pronounced as "flight level two niner zero", in most jurisdictions. The phrase "flight level" makes it clear that this refers to the standardized pressure altitude.
Transition altitude
Whilst 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 true height of an aircraft relative to an object on the ground needs to be known. The pressure setting to achieve this is called
QNHQNH is a Q code. Most commonly referred to as "Quasi-Non-Hydrostatic". It is a pressure setting used by pilots, air traffic control and low frequency weather beacons to refer to the barometric altimeter setting which will cause the altimeter to read altitude above mean sea level within a certain...
or "altimeter setting" and is available from various sources, including
air traffic controlAir traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
and the local
METARMETAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....
-issuing station.
The
transition altitude (TA) is the height at which aircraft change from the use of altitude 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. Above the TA, the aircraft altimeter pressure setting is normally adjusted to the standard pressure setting of 1013.25
hectopascalsThe pascal is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of force per unit area, defined as one newton per square metre...
(millibars) or 29.92 inches of mercury and aircraft altitude will be expressed as a flight level.
Table for determining transition level
QNH
(in millibars) | | Transition altitude (in feet) |
|---|
| 3,000 | 4,000 | 5,000 | 6,000 | 18,000 |
|---|
| 1032-1050 |
FL25 |
FL35 |
FL45 |
FL55 |
FL175 |
| 1014-1032 |
FL30 |
FL40 |
FL50 |
FL60 |
FL180 |
| 996-1013 |
FL35 |
FL45 |
FL55 |
FL65 |
FL185 |
| 978-995 |
FL40 |
FL50 |
FL60 |
FL70 |
FL190 |
| 960-977 |
FL45 |
FL55 |
FL65 |
FL75 |
FL195 |
| 943-959 |
FL50 |
FL60 |
FL70 |
FL80 |
FL200 |
In the USA and Canada, the transition altitude is 18,000ft. In Europe, the transition altitude varies and can be as low as 3,000ft. There are discussions to standardise the transition altitude within the Eurocontrol area.
On 25 November 2004 the
Civil Aviation Authority of New ZealandThe Civil Aviation Authority of New Zealand is the government agency tasked with establishing civil aviation safety and security standards in New Zealand....
raised New Zealand's transition altitude from 11 000 feet to 13 000 feet and changing the transition level from flight level 130 to flight level 150.
The
transition level is the lowest flight level above the transition altitude. The table on the right shows the transition level according to transition altitude and QNH. When descending below the transition level, the pilot starts to refer to
altitudeAltitude is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object...
of the aircraft by setting the altimeter to the
QNHQNH is a Q code. Most commonly referred to as "Quasi-Non-Hydrostatic". It is a pressure setting used by pilots, air traffic control and low frequency weather beacons to refer to the barometric altimeter setting which will cause the altimeter to read altitude above mean sea level within a certain...
for the region or airfield. Note that the transition level is, by definition, less than 500ft above the transition altitude. Aircraft are not normally assigned to fly at the transition level as this does not guarantee separation from other traffic flying (on QNH) at the transition altitude; the lowest usable flight level is the transition level plus 500 feet.
The
transition layer is the airspace between the transition altitude and the transition level.
In some countries - eg.
NorwayNorway , officially the Kingdom of Norway, is a country in Northern Europe occupying the western portion of the Scandinavian Peninsula, as well as Jan Mayen and the Arctic archipelago of Svalbard under the Spitsbergen Treaty...
- the transition level is determined including a buffer of minimum 1000 ft (depending on QNH) to the transition altititude. Therefore aicrafts may be flying at both transition level and transition altitude, and still be vertically separated by at least 1000 ft. In those areas the transition layer will be a given vertical distance between 1000 and 1500 ft, depending on QNH.
Quadrantal rule
(This rule applies to IFR flights in the UK outside controlled airspace and is advised for VFR flights above 3,000 ft AMSL outside controlled airspace; few other countries adopt this rule) Flight levels are 500 ft apart, but to further ensure the separation of aircraft, aircraft travelling in different directions in level flight (i.e. not climbing or descending) below FL 245 (24,500 ft) are required to adopt flight levels according to the
quadrantal rule, as follows:
- Magnetic Track 000 to 089° - odd thousands of feet (FL 70, 90, 110 etc)
- Magnetic Track 090 to 179° - odd thousands + 500 ft (FL 75, 95, 115 etc)
- Magnetic Track 180 to 269° - even thousands of feet (FL 80, 100, 120 etc)
- Magnetic Track 270 to 359° - even thousands + 500 ft (FL 85, 105, 125 etc)
Semicircular/Hemispheric rule
(Versions of this apply to IFR in the UK inside controlled airspace and generally in the rest of the world)
The
semicircular rule (also known as the hemispheric rule) applies, in slightly different version, in all of the world, including in the UK inside controlled airspace.
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 MinimaReduced Vertical Separation Minima or Minimum is an aviation term used to describe the reduction of the standard vertical separation required between aircraft flying at levels between FL290 and FL410 from 2,000 feet to 1,000 feet...
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, depending on 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.)
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, for example, southbound traffic uses odd flight levels.
Reduced Vertical Separation Minima
(In the U.S. and Canada, the foregoing information applies to flights under
instrument flight rules (IFR)Instrument flight rules are regulations and procedures for flying aircraft by referring only to the aircraft instrument panel for navigation. Even if nothing can be seen outside the cockpit windows, an IFR-rated pilot can fly while looking only at the instrument panel...
. Different altitudes will apply for aircraft flying under
visual flight rules (VFR)Visual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than Basic VFR Weather Minimums, as specified in the rules of the...
above 3000 ft AGL.)
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Reduced Vertical Separation MinimaReduced Vertical Separation Minima or Minimum is an aviation term used to describe the reduction of the standard vertical separation required between aircraft flying at levels between FL290 and FL410 from 2,000 feet to 1,000 feet...
or RVSM reduces the vertical separation above FL 290 from 2,000 ft to 1,000 ft. This allows aircraft to safely fly more optimum routes, gain fuel savings and increase airspace capacity by adding six 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 the 20th January 2002, it entered European airspace. The United States, Canada and Mexico transitioned to RVSM between FL 290 and FL 410 on the 20th of January, 2005, and Africa on the 25th of 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, 2,000 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
China, Mongolia, Russia and many
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countries use flight levels specified in metres. Aircraft entering these areas normally make a slight climb or descent to adjust for this.
The flight levels below apply to Russia, Mongolia, North Korea, 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":
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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.
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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.
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China
The flight levels in China, excluding Hong Kong, Macau and Taiwan, are:
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True 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)
and every 1,200 metres thereafter.
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True 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)
and every 1,200 metres thereafter.
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