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) ?
10W, It would be rare for a 600nm leg to be flown below 18000'. the Longer legs would be above TA and we would be using FLs exactly as we do now.
I'd
rather not lose 1 or 2 levels in my hold when the **** hits the fan, because if it does, we (intermediate ACC/Terminal) get the beating, not the APP sector.
Currently Heathrow holding looses 70 and possibly 80 if the pressure does not allow separation from 6,000', hopefully this problem will be removed by a higher TA
One issue that would need to be looked at is the hold protected area which becomes larger with altitude and markedly bigger above 140 , this tends to discourage airspace designers from holding higher. Us pilots want to hold as high as possible to stay out of turbulence and save fuel and I hate sitting at 70 when the only plane in the hold.