Flight Levels
Flight levels (FL) are used by air traffic controllers to simplify the vertical separation of aircraft and one exists every 1000 feet relative to an agreed pressure level. Above a transitional altitude, which varies from country to country, the worldwide arbitrary pressure datum of 1013.25 millibar or the equivalent setting of 29.92 inches of mercury is entered into the altimeter and altitude is then referred to as a flight level. The altimeter reading is converted to a flight level by removing the trailing two zeros: for example, 29000 feet becomes FL290. When the pressure at sea level is by chance the international standard then the flight level is also the altitude. To avoid confusion, below the transition altitude, height is referred to as a numeric altitude, for example 'climb flight level 250' or 'descend 5000 feet'.
Airways have a set of associated standardized flight levels (sometimes called the "flight model") which must be used when on the airway. On a bi-directional airway, each direction has its own set of flight levels. A valid flight plan must include a legal flight level at which the aircraft will travel the airway. A change in airway may require a change in flight level.
In the USA and Canada, for eastbound (heading 0–179 degrees) IFR flights, the flight plan must list an "odd" flight level in 2000 foot increments starting at FL190 (i.e., FL190, FL210, FL230, etc.); Westbound (heading 180–359 degrees) IFR flights must list an "even" flight level in 2000 foot increments starting at FL180 (i.e., FL180, FL200, FL220, etc.). However, Air Traffic Control (ATC) may assign any flight level at any time if traffic situations merit a change in altitude.
Aircraft efficiency increases with height. Burning fuel decreases the weight of an aircraft which may then choose to increase its flight level to further improve fuel consumption. For example an aircraft may be able to reach FL290 early in a flight, but step climb to FL370 later in the route after weight has decreased due to fuel burn off.
- RVSM
Read more about this topic: Flight Plan
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