RVSM
Reduced Vertical Separation Minimum allows aircraft between 29,000 and 41,000 ft to be separated vertically by 1,000 ft instead of 2,000 ft, doubling the usable flight levels. It is standard in nearly all the world's airspace, and an aircraft must have certified altimetry and autopilot equipment plus an operator approval to fly in it. An aircraft without RVSM approval is restricted to below 29,000 ft, where fuel burn is much higher and weather worse, which cripples a jet's range and comfort. When buying an older jet, confirm the RVSM approval is valid; when chartering, it is a given on any properly operated aircraft.