Flight level (FL)A Flight Level is an aircraft's altitude at standard air pressure, 1013.25 hPa (29.92 inHg), and is expressed in hundreds of feet e.g. 37,000ft is expressed as FL370.
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Flight levels are then assigned a number which is the apparent altitude (pressure altitude) to the nearest thousand feet, divided by one hundred. Therefore an apparent altitude of 18,000 feet is referred to as Flight Level 180.
Reduced vertical separation minimum (RVSM) is the reduction, from 2,000 feet to 1,000 feet, of the standard vertical separation required between aircraft flying between flight level 290 (29,000 ft) and flight level 410 (41,000 ft).
In the United States, the Federal Aviation Administration (FAA) has the sole authority to regulate all navigable airspace exclusively determining the rules and requirements for its use.
Upper Class E airspace operations refer to those that take place over 60,000 feet above mean sea level (MSL) in the National Airspace System (NAS). Operations in upper Class E airspace have historically been limited due to the challenges faced by conventional fixed wing aircraft in reduced atmospheric density.