However, whenever the aircraft is operating above flight level 450, the pilot, or one of the pilots, seated at the controls of the aircraft must wear an oxygen mask that is properly fitted and supplying oxygen
That would have meant that an N-reg Concorde PIC w
The rule in FAA-land is F410 which does affect bizjet operators. We're trying to change it, but it doesn't effect the airlines, so the FAA isn't interested.
GF