The change from local QNH and SAS 1013 and vice versa should take place in the transition layer, between transition altitude and TL. This is to ensure that all AC will be on same altimeter settings.
Therefore if you are just airborne and the ATCO gives you climb to say FL300, DO NOT set SAS 1013 until you are passing transition layer.
Vice versa, if you are at FL300 and ATCO gives you descend to 3000ft, DO NOT set local QNH until passing TL.
Otherwise we risk having AC on different altimetersettings in the same altitude- or flightlevel-areas which again might result in loss of separation!
Ie, if local QNH is 983 and you set SAS 1013 too early on climb you will actually be 810ft lower that what you should be!
Your altimeter will indicate say 5000ft on the climb but you are actually at 4090ft in relation to all other traffic flying on QNH 983.
So when ATC gives you trafficinfo on traffic at 4500ft, you think you are in the clear but you're not!
Last edited by Somewhere In Time; 5th Sep 2009 at 15:26.