Stall speed changes with altitude.

[MauleSkinner]

Mach .69 seems like way to low of an airspeed.

Depends on the airplane...for a Falcon 10 it is...for a Citation it's not...for a Beechjet it's about right for the climb.

At FL400 (Max is FL410) I don't get any buffet above the correct AOA for stall buffet and this correlates to a specifc speed at a certain weight. I am usually paying attention to AOA here and not trying to get a specific speed or mach number. On my next functional check flight I will get these numbers for you.

As I said, if your theory were correct for all cases I would expect buffet (from the low speed condition supersonic flow) at an angle of attack less then that for actual low speed stall buffet and I don't experience that.

Again, it depends on the airplane...I've never seen it in the Hawker, although we do have charts for it. The airplane just becomes a serious slug long before you get to that speed/AOA (still well above "stall") that I've never been tempted to get close.

Fly safe!

David