When flying above the transition altitude (3000' in the UK), unless flying under the control of ATC; Fighter Controllers or RN Ship-borne controllers, then all ac should be flown at a Flight Level (FL), with 1013Mb set on the altimeter, that is linked with their magnetic heading. Below FL 245, if you are flying between 360 and 089 degrees then you fly at an odd FL eg FL50; between 090 and 179 an odd FL + 500' eg FL55; between 180 and 269 an even FL eg FL60; between 270 and 359 an even FL + 500' eg FL65. Above FL 245, FLs are separated by 1000'. Generally, although the Reduced Vertical Separation Minima (RVSM) status of ac can change things considerably (if I've blown your mind talking about RVSM then ignore it!), if you are flying between (I think) 360 and 179 then you fly an odd FL eg FL250; if you are flying between 180 and 359 then you fly at an even FL eg FL260. In this way we achieve vertical separation for everyone and we don't have conflictions.
Oh no, sorry. That only works if we don't want to climb or descend. But it's a starting point.
mk,
boacs link is accurate only to a point. The 4000\' separation above FL290 is not accurate. If you want to go that far in depth just ask!
Regards,
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