Aircraft/ Flight

In aviation, a Flight Level is a standard nominal altitude of an aircraft, referenced to a world-wide fixed pressure datum (29.92). It is not necessarily the same as the aircraft's true altitude above mean sea level. more...

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Altitudes are only called by their three-digit flight level above Flight Level 180 (18,000ft).

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.

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.

Read more at Wikipedia.org