We all know Concorde went at Mach 2 at FL600, but were there instances (for the press, certification, etc) that you went supersonic considerably closer to the deck? and what issues (if any) did that bring up?
As Christian says, she could go to Mach 1 at about FL280. I can't remember the exact values, but we would have gone a little way beyond that at that sort of FL when establishing the manouvre limits (that set the CG envelope in this region), Vd/Md and for flutter clearance. Wouldn't have hung about there though!
Generally speaking the region from say 0.98 to 1.2M is one to pass through and be thankful!
CliveL