Yeah, I've heard the supersonic track explanation before, but why then do you sometimes see the term on the milan sigmet for example pertaining to FL390 and below. Firstly there are no supersonic tracks in milan FIR, secondly almost all pax jets can reach FL390, and higher, so surely a general sigmet would be issued for all traffic at those levels. Thanks for the replies though, guys, I'm going for a lie down.
