Flight level

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NACO Flight Level graphic

In aviation, a Flight Level is a standard nominal altitude of an aircraft, referenced to a world-wide fixed pressure datum. It is not necessarily the same as the aircraft's true altitude above mean sea level.

1 Background
2 Transition altitude
3 Quadrantal rule
4 Semicircular rule
5 Reduced Vertical Separation Minima
6 Metric flight levels
7 See also

[edit] Background

To see why flight levels are used, a discussion of the means of measuring altitude is necessary. Historically, altitude has most easily been measured using a pressure altimeter, which is essentially a calibrated barometer — it measures air pressure, which decreases with increasing altitude. To display altitude above sea level, a pilot must recalibrate the altimeter according to the local air pressure from time to time, to take into account natural variation of pressure over time and in different regions. If this isn't done, different aircraft may be flying at different heights even though their altimeters show the same altitude. More critically, different aircraft may be flying at the same height even though their altimeters show different heights. Clearly this is a safety issue.

Flight levels solve this problem by defining altitudes based on a standard pressure of 1013.2 mbar (the equivalent setting of 29.92 inHg is used in U.S. and Canada). All aircraft operating on flight levels calibrate to this same standard setting regardless of the actual sea level pressure. Flight levels are then assigned a number, which is this nominal altitude ("pressure altitude") in feet, divided by 100. Therefore an apparent altitude of 32,000 feet is referred to as flight level 320. Note that an aircraft flying at flight level 320 will usually be at some other actual height above mean sea level than 32,000 feet, but since all other aircraft set their altimeters to a standard pressure, no collision risk arises. To avoid collisions between two planes, their real altitudes are not important, but only the difference between them. This difference solely depends on the air pressure at both planes, and does not require knowledge of the local air pressure on the ground.

[edit] Transition altitude

Flight levels are not used close to the ground, for perhaps obvious reasons — the true height of an aircraft relative to an object on the ground needs to be known. For this reason an altitude called the transition altitude (TA) is defined. When operating at or below the TA, aircraft altimeters are set to show the true altitude above sea level. 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. The TA varies from country to country, and in some cases within a country. It is clearly desirable that it is chosen to be above any obstacles within some distance of the operating area, however the size of this area varies very widely. In the United Kingdom the standard TA outside controlled airspace is 3000 ft, but within controlled airspace it varies and will be specified on the appropriate charts. For example, in the controlled airspace around London Heathrow the transition altitude is 6000 ft, for Manchester it is 5000 ft, for Birmingham 4000 ft. Throughout Germany the TA is 5000 ft, throughout Belgium the TA is 4500ft. There is considerable discussion of standardisation of the TA throughout Europe, or at least a large part of it, but so far no agreement. Throughout the USA and southern Canada the TA is 18000 ft. In other parts of the world other values apply.

The lowest usable flight level above the TA is called the transition level (TL). Because the transition altitude is fixed and the atmospheric pressure varies the TL varies from time to time. It is therefore possible to have a valid flight level of 30 in the UK when the atmospheric pressure is above 1013.2. Note that vertical separation is not guaranteed between an aircraft flying at the transition altitude and one flying at the transitional level. For example in the UK with a transition altitude of 3000' and a QNH of 996 the Transition Level is FL35 equivalent to an altitude of less than 3100'. (See Manual of Air Traffic Services Part 1 Appendix A)

Flights above transition altitude being directed by air traffic control will be assigned flight levels to fly. A vertical region extending from the TA to the TL is known as the transition layer.

[edit] Quadrantal rule

(This 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 feet 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 feet) are required to adopt flight levels according to the quadrantal rule, as follows:

Track 000 to 089° - odd thousands of feet (FL 70, 90, 110 etc)
Track 090 to 179° - odd thousands + 500 (FL 75, 95, 115 etc)
Track 180 to 269° - even thousands of feet (FL 80, 100, 120 etc)
Track 270 to 359° - even thousands + 500 (FL 85, 105, 125 etc)

[edit] Semicircular 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:

Track 000 to 179° - odd thousands (FL 250, 270, etc.)
Track 180 to 359° - even thousands (FL 260, 280, etc.)

At FL 290 and above, 4000 ft. intervals are used to separate same-direction aircraft (instead of 2000 ft. intervals below FL 290), and only odd flight levels are assigned, depending on the direction of flight:

Track 000 to 179° - odd flight levels (FL 290, 330, 370, etc.)
Track 180 to 359° - odd flight levels (FL 310, 350, 390, etc.)

Next time you fly, listen to the captain say what flight level you're at - it will obey this rule according to what direction you are flying in. On the return trip, notice the altitude difference (e.g., FL 290 or FL 330 eastbound, and then perhaps FL 310 or FL 350 westbound).

Countries where the major airways are oriented north/south (e.g. New Zealand and France) have semicircular rules that define a North/South rather than an East/West track split. In France for example southbound traffic uses odd flightlevels.

In the U.S. and Canada, note that 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 AGL.

[edit] Reduced Vertical Separation Minima

Main article: Reduced Vertical Separation Minima

Reduced Vertical Separation Minima or RVSM reduces the vertical separation above FL 290 from 2000 ft. to 1000 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. RVSM went into effect in Europe between FL 290 and FL 410 on January 24, 2002. The United States, Canada and Mexico transitioned to RVSM between FL 290 and FL 410 on January 20, 2005.

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, 4000 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.)

[edit] Metric flight levels

China, Mongolia, Russia and many CIS countries use flight levels specified in metres. Aircraft entering these areas normally make a slight climb or descent to adjust for this.