Introduced by FAA in January 2005, domestic reduced vertical separation minimum (DRVSM) added six flight levels (FLs) to the high-altitude structure of the National Airspace System (NAS): FL 300, 320, 340, 360, 380 and 400. Aircraft flying in the upper airspace above the 48 contiguous states, Alaska, southern Canada and part of the Gulf of Mexico now maintain 1,000-foot vertical separation, half the distance required before DRVSM rules went into effect. With this change, the United States caught up with Europe, Southeast Asia, Australia and northern Canada, as well as Pacific and Atlantic Ocean regions, where RVSM rules already were in effect. (Because their airspaces are necessarily linked, southern Canada had to wait until the United States adopted RVSM before it could do likewise.)
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