Reduced Vertical Separation Minima
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Reduced Vertical Separation Minima or Minimum (RVSM) is an aviation term used to describe the reduction of the standard vertical separation required between aircraft flying at levels from FL290 (29,000 ft.) and FL410 (41,000 ft.) from 2,000 feet to 1,000 feet. This therefore increases number of aircraft that can safely fly in a particular volume of airspace.
Historically, standard vertical separation was 1,000 feet from the surface to FL290, 2,000 feet from FL290 to FL410 and 4,000 feet above this. This was because the accuracy of the pressure altimeter used in aircraft to determine level decreases with height. However over time altimeters have become more accurate and autopilots more adept at maintaining a set level, therefore it became apparent that for many modern aircraft, the 2,000 foot separation was too cautious. It was therefore proposed by ICAO that this be reduced to 1,000 feet.
Between 2002 and 2004 RVSM was implemented in much of Europe, North Africa, Southeast Asia and North America, and over the North Atlantic and Pacific Oceans.
Only aircraft with specially certified altimeters and autopilots may fly in RVSM airspace, otherwise the aircraft must fly lower or higher than the airspace, or seek special exemption from the requirements.
Critics of the change are concerned that by reducing the space between aircraft, RVSM may increase the number of mid-air collisions and near-collisions. In the US this program was known as the Domestic Reduced Separation Minimum (DRVSM).
