If the TA were raised to 18K or whatever what altimeter setting would everyone use beneath it?
QNH, fool, everyone cries!
Yeah, right. But which QNH? Regional? Near the border between 'em which regional? Local airfield? Local large airfield? Local LARS? Any one you please?
This might work in the vast empty spaces of the U S of A where for the most part there is only one city airport for hundreds of miles but in crowded UK you're going to end up with scores of en-route aircraft in close proximity all on different altimeter settings depending on who they are talking to or where they just came from, so how is that compatible with air safety? It works in the 3-4000ft predominantly VFR layer we currently have but I winder how it would work for mid level transit traffic on IFR flights.