Flight level

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.

Background

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.[1]

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.

Definition

Flight levels are described by a number, which is this nominal altitude (or, 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 two or 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." 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.[2] 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.

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
1014–1031 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 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.[3]

On 25 November 2004 the Civil Aviation Authority of New Zealand raised New Zealand's transition altitude from 11,000 feet to 13,000 feet and changed the transition level from FL130 to FL150.[4]

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.

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 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:[5]

• 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.)

The UK will abolish the quadrantal rule on 2nd April 2015 and adopt the semi-cicular rule to align with other countries abroad.[6][7]

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; France) have semicircular rules that define a North/South rather than an East/West track split.

In Italy, France 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
Magnetic route figure of merit (FOM)
0° to 179° 180° to 359°
VFR IFR VFR IFR
FL feet FL feet FL feet FL feet
N/A N/A 010 1,000 N/A N/A 020 2,000
N/A N/A 030 3,000 N/A N/A 040 4,000
35 3,500 050 5,000 45 4,500 060 6,000
55 5,500 070 7,000 65 6,500 080 8,000
75 7,500 090 9,000 85 8,500 100 10,000
95 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 N/A N/A 220 22,000
N/A N/A 230 23,000 N/A N/A 240 24,000
N/A N/A 250 25,000 N/A N/A 260 26,000
N/A N/A 270 27,000 N/A N/A 280 28,000
N/A N/A 290 29,000 N/A N/A 310 31,000
N/A N/A 330 33,000 N/A N/A 350 35,000
N/A N/A 370 37,000 N/A N/A 390 39,000
N/A N/A 410 41,000 N/A N/A 430 43,000
N/A N/A 450 45,000 N/A N/A 470 47,000
N/A N/A 490 49,000 N/A N/A 510 51,000

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 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.

• 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.

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

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":

 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

The flight levels below apply to 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 "幺两六" or "幺万两千六百米" for 12,600m. RVSM implement in China at 1600 UTC 21 Nov 2007. 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° 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

On 5 September 2011, the government of the Russian Federation issued a decree №743,[8] 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.[9]