fl420
FL420 is certainly a level, but it is not vertically separated from FL410 nor from FL430.
RVSM airspace tops at FL410
There was a case some time back when 2 aircraft were sitting on top of one another at FL410 and FL420.
It was only when the flights were passed to the next FIR, and a query was received, that it was realised that they had been like that for quite a period of time.
If there is no traffic at FL410 or FL430, then there are no vertical sep issues.
Not to be recommended though
Last edited by beamwidth; 27th August 2008 at 07:32.