Flight level
In aviation, a flight level (FL) is an aircraft's altitude as determined by a pressure altimeter using the International Standard Atmosphere.
An altimeter measures ambient air pressure, which decreases with increasing altitude following the barometric formula.
[2] Flight levels require defining altitudes based on a standard altimeter setting.
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 (30 m).
When operating at or below the TA, aircraft altimeters are usually set to show the altitude above sea level.
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 standard rule defines an East/West track split: At FL 290 and above, if Reduced Vertical Separation Minima (RVSM) 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, independent of the direction of flight: Conversely, RVSM equipped aircraft are able to continue separation in 2,000 ft intervals as outlined in the semicircular rules.
Countries where the major airways are oriented north/south (e.g., New Zealand; Italy; Portugal) have semicircular rules that define a North/South rather than an East/West track split.
In Europe commonly used International Civil Aviation Organization (ICAO) separation levels are as per the following table: The quadrantal rule is defunct.
[12][13] 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.
Minimum vertical separation between two flights abiding by the UK Quadrantal Rule is 500 ft (note these are in geopotential foot units).
This allows aircraft to safely fly more optimum routes, save fuel and increase airspace capacity by adding new flight levels.
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.
The flight levels below apply to Mongolia and People's Republic of China, not including Hong Kong.
On 5 September 2011 the government of the Russian Federation issued decree No.743,[18] pertaining to the changes in the rules of use of the country's airspace.
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.
