This explanation courtesy of "Eye in the Sky"
With effect from 24th January 2002 RVSM will be implemented in most European airspace and has already been in force on the North Atlantic and UK airspace for some years.
In the early days of high altitude flight it was discovered that pressure altimeters were relatively inaccurate at high altitude and therefore 2000ft separation was used above FL290. In recent years the advent of Air Data Computers has meant that height keeping is much more accurate now, and it was decided to increase capacity on the NAT tracks by reducing separation to 1000ft. When this was successful the UK domestic airspace followed suit. From 24/1/02 all traffic between FL290 and FL410 will have to be RVSM compliant or it will be excluded.
In the old days, we used levels 2000ft apart in each direction. Therefore Eastbound levels were FL290, 330, 370, 410 etc. Westbound were FL310, 350, 390 etc. Under RVSM, Eastbound levels are FL290, 310, 330, 350, 370, 390 and 410. Westbound levels are FL300, 320, 340, 360, 380, 400. Above FL410 it reverts to 2000 ft separation, so the next usable level is FL430 Westbound, FL450 Eastbound.
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