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NACO Flight Level graphic
NACO Flight Level graphic

A Flight Level (FL) is a standard nominal altitude of an aircraft, referenced to a world-wide fixed pressure datum of 1013. Altitude is the Elevation of a point or object from a known level or datum (plural data 25 hPa (29. 921 inHg), the average sea-level pressure. Inches of mercury, inHg or "Hg is a measuring unit for Pressure. It is not necessarily the same as the aircraft's true altitude above mean sea level. The term above mean sea level ( AMSL) refers to the Elevation (on the ground or Altitude (in the Air) of any object relative to the

Contents

Background

Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter is an instrument used to measure the Altitude of an object above a fixed level History The first barometer is thought to have been built unintentionally by Gasparo Berti, sometime between 1640 and 1643 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, two aircraft could be flying at the same height even though their altimeters show 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 then assigned a number, which is this nominal altitude ("pressure altitude") in feet, divided by 100. In Aviation, pressure altitude is the indicated altitude when an Altimeter is set to an agreed baseline pressure setting A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit Therefore an apparent altitude of, for example, 32,000 feet is referred to as "flight level 320. " To avoid collisions between two planes, their real altitudes are not important; 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.

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 is a standardized pressure altitude.

Transition altitude

At low altitudes the true height of an aircraft relative to an object on the ground needs to be known. For this reason, the transition altitude (TA) is defined. Transition altitude is the maximum Altitude at which an aircraft should use an altimeter pressure setting indicating height above mean sea level (QNH. When operating at or below the TA, aircraft altimeters are set to show the 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-issuing station. QNH is a Q code. It is a pressure setting used by pilots, Air traffic control (ATC and Low frequency Weather beacons to refer to Air traffic control ( ATC) is a service provided by ground-based controllers who direct Aircraft on the ground and in the air METAR (METeorological Aviation Report is a format for reporting Weather information

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. 25 hPa. 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 3,000 ft and a QNH of 996, the Transition Level is FL35; equivalent to an altitude of less than 3,100 ft. (See Manual of Air Traffic Services Part 1 Appendix A)

Flights above transition altitude being directed by air traffic control are generally assigned flight levels. Air traffic control ( ATC) is a service provided by ground-based controllers who direct Aircraft on the ground and in the air The vertical region extending from the TA to the TL is known as the transition layer.

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:

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 define 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 (instead of 2,000 ft intervals below FL 290), and only odd flight levels are assigned, depending on the direction of flight:

Countries where the major airways are oriented north/south (e. g. New Zealand; France; Italy; Portugal) have semicircular rules that define a North/South rather than an East/West track split. In France, 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). Different altitudes will apply for aircraft flying under visual flight rules (VFR) above 3000 ft AGL. )

Main article: Reduced Vertical Separation Minima

Reduced Vertical Separation Minima or RVSM reduces the vertical separation above FL 290 from 2,000 ft to 1,000 ft. Reduced Vertical Separation Minima or Minimum (RVSM is an Aviation term used to describe the reduction of the standard vertical separation required between aircraft Reduced Vertical Separation Minima or Minimum (RVSM is an Aviation term used to describe the reduction of the standard vertical separation required between aircraft 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 the 24th of January, 2002. The United States, Canada and Mexico transitioned to RVSM between FL 290 and FL 410 on the 20th of January, 2005.

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:

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.

The flight levels below apply to Russia, Mongolia, North Korea, Kyrgyzstan, Kazakhstan, 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 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.

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.

China

The flight levels in China, excluding Hong Kong, Macau and Taiwan, are:

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.

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.

See also

Flight planning is the process of producing a Flight plan to describe a proposed Aircraft flight

Dictionary

flight level

-noun

  1. Any of several internationally agreed standard altitudes that are based on atmospheric pressure rather than on true height
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