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).
This reduction in vertical separation minimum therefore increases the number of aircraft that can fly in a particular volume of controlled airspace.
[citation needed] Efforts to reduce this separation above flight level 290 began almost immediately,[4] but doing so without compromising safety required improvements in altimeters and other equipment, due in part to inherent difficulties in accurately determining and maintaining aircraft altitudes and, therefore, the actual vertical distance between aircraft.
[5] It was not until the 1990s that air data computers (ADCs), altimeters, and autopilot systems became sufficiently accurate to safely reduce the vertical separation minimum.
[6] Thus, RVSM in effect constituted a return to the original procedures established in the mid-1940s, with the notable difference that 500 feet separation is only permitted between IFR and VFR flights (on non-converging tracks).