Originally Posted by 10W
I'm not averse to the change, but how does it work in practice in the US ?
Let's say you are flying on Airways over a 600NM leg. You obviously have the departure airfield QNH set first, but what then ? Do en-route ATC give you a regional setting ? Are the 'regions' defined (similar to the UK Regional Pressure Setting regions) ?
In the US, en-route IFR clearances within ATC sectors below transition will include the altimeter setting in use, eg 'route ALB-SAX as filed, maintain 14,000 Stuart altimeter 30.28'. Seems pretty simple, and effectively operates as a regional QNH. VFR traffic operating in the lower airspace have access to local altimeter setting info from Flight Service Stations (FSS), or, of course, from ATC.
As someone said earlier, the US system of 18,000'/Fl180 transition works well and avoids peak workload. The Australian system of 10,000/FL110 does not, and the current UK system is a mess. I would strongly vote for 18,000, and hope it comes into effect Europe-wide.