Flight level


In aviation and aviation meteorology, flight level 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 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. 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 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 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. In radio communications, FL290 would be pronounced as "flight level two nine 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 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 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. Above the TA, the aircraft altimeter pressure setting is normally adjusted to the standard pressure setting of 1013.25 hectopascals 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. In Europe, the transition altitude varies and can be as low as. There are discussions to standardize the transition altitude within the Eurocontrol area.
On 25 November 2004 the Civil Aviation Authority of New Zealand raised New Zealand's transition altitude from and changed the transition level from FL130 to FL150.
The transition level 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.
The transition layer is the airspace between the transition altitude and the transition level.
According to these definitions the transition layer is 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, the transition level is determined by adding a buffer of minimum 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", "transition layer", and "transition level" is
TL = TA + TLYR

Semicircular/hemispheric rule

The semicircular 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:
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, and only odd flight levels are assigned, independent of the direction of flight:
Countries where the major airways are oriented north/south have semicircular rules that define a North/South rather than an East/West track split.
In Italy, France, Portugal and recently also in Spain, 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 separation levels are as per the following table:

Quadrantal rule

This rule is defunct. 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.
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
The rule was non-binding upon flights operating under visual flight rules.
Minimum vertical separation between two flights abiding by the UK Quadrantal Rule is 500 ft. The level to be flown is determined by the magnetic track of the aircraft, as follows:
reduces the vertical separation above FL 290 to 1,000 ft. 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.
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:
The International Civil Aviation Organization has recommended a transition to using the International System of Units since 1979 with a recommendation on using metres for reporting 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.

North Korea, Kyrgyzstan, Kazakhstan, Tajikistan, Uzbekistan, and Turkmenistan

The flight levels below apply to North Korea, Kyrgyzstan, Kazakhstan, Tajikistan and Uzbekistan and 6,000 m or below in Turkmenistan. Flight levels are read as e.g. "flight level 7,500 metres":
;Track 180 to 359°
and every 2,000 metres thereafter.
;Track 000 to 179°
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 in Chinese "一两六" or "一万两千六百米" for 12,600 m.

RVSM implement in China at 16:00 UTC 21 November 2007. In Mongolia at 00:01 UTC 17 November 2011. The aircraft fly 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 30 metres.
;Track 180 to 359°
and every 1,200 metres thereafter.
;Track 000 to 179°
and every 1,200 metres thereafter.

Flight levels in Russian Federation

On 5 September 2011 the government of the Russian Federation issued 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:
;Track 180 to 359°
FL
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
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.
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.