A flight level (FL) is specific barometric pressure, expressed as a nominal altitude in hundreds of feet. The pressure is computed assuming an International standard sea-level pressure datum of 1013.25 hPa (29.92 inHg), and therefore is not necessarily the same as the aircraft's true altitude either above mean sea level or above ground level.
Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures air pressure, which decreases with increasing altitude following the barometric formula, and from the surrounding's 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 altitude 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—of aircraft flying at the same altitude, though their altimeters indicate different barometric heights—by defining altitudes based on a standardised air pressure at sea-level. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure.
A second advantage of flying at a consistent flight level, rather than true altitude, is that an aircraft's aerodynamic and engine performance depend on air pressure, so it is convenient to trim a plane to operate well at a particular air pressure, and not have to adjust it for a considerable time.
Flight levels are described by a number, which is this nominal altitude ("pressure altitude") in feet, divided by 100, while being a multiple of 500 ft, therefore always ending on 0 or 5. 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 
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 true height of an aircraft relative to an object on the ground needs to be known. The pressure setting to achieve this is called QNH or "altimeter setting" and 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 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 hectopascals (millibars) or 29.92 inches of mercury and aircraft altitude will be expressed as a flight level.
|Transition altitude (in feet)|
In the United States and Canada, the transition altitude is 18,000 ft. In Europe, the transition altitude varies and can be as low as 3,000 ft. There are discussions to standardise the transition altitude within the Eurocontrol area.
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 altitude of the aircraft by setting the altimeter to the QNH for the region or airfield. Note that the transition level is, by definition, less than 500 ft 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 ft.
The transition layer is the airspace between the transition altitude and the transition level.
In some countries, e.g., Norway, the transition level is determined including a buffer of minimum 1000 ft (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 1000 ft. In those areas the transition layer will be a given vertical distance between 1000 ft and 1500 ft, depending on QNH.
Quadrantal rule 
This rule applies 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 affects 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 is non-binding upon flights operating under 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:
- Magnetic Track 000 to, and including, 089° – odd thousands of feet (FL70, 90, 110 etc.)
- Magnetic Track 090 to, and including, 179° – odd thousands plus 500 ft (FL75, 95, 115 etc.)
- Magnetic Track 180 to, and including, 269° – even thousands of feet (FL80, 100, 120 etc.)
- Magnetic Track 270 to, and including, 359° – even thousands plus 500 ft (FL85, 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 Minima 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 and Portugal, for example, southbound traffic uses odd flight levels; in New Zealand, southbound traffic uses even flight levels. In Europe are commonly used ICAO separation levels as per the following table:
|VERTICAL SEPARATION OF VFR AND IFR FLIGHTS|
|FOM 0° TO 179°||FOM 180° TO 359°|
Reduced Vertical Separation Minima 
(In the U.S. and Canada, the foregoing information 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 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 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 
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.
The flight levels below apply to 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":
and every 2,000 metres thereafter.
and every 2,000 metres thereafter.
Flight levels in Russian Federation 
On 5 September 2011, the government of the Russian Federation issued a decree №743, 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.
See also 
- Learn more about Level Bust and related causes and consequences
- "CAP 410 Manual of Flight Information Services" (PDF). UK Civil Aviation Authority. CAP410. Retrieved 25 February 2013.
- "A Common European Transition Altitude; An ATC perspective" (PDF). Eurocontrol. Retrieved 27 February 2008.
- "Part 91, Amendment 12" (PDF). Civil Aviation Authority of New Zealand. Retrieved 4 February 2009.
- Rules of the Air Regulations 2007 (No. 734), rule 34, table 1. Available from the UK Statute Law Database.
- "Постановление Правительства РФ от 05.09.2011 N 743". Консультант Плюс. Retrieved 29 September 2011.