Quote:
Originally Posted by sa226
Up at that altitude they wouldn't fly referencing IAS. They would be referencing Mach number instead, and then transition to IAS at a lower altitude.
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I'm not sure I agree with that entirely - at high altitude and high speed there is a very narrow range of indicated airspeeds that an aircraft can operate at - too little IAS, and you're in a stall, even though you're supersonic, and just a little bit too fast, and you encounter big issues with compressibility that can result in abrupt loss of control:
http://en.wikipedia.org/wiki/Coffin_corner_(aviation)
I believe the Concorde had an indicated speed range at 60,000 feet of something like 140 knots. The U-2 had a speed range of 10 knots at altitude, which is to say that you had only a 10 knot difference between stalling and exceeding maximum velocity, resulting in a loss of control in either case. Quite a small margin!
I'm certainly no expert on this though - someone please correct me if I'm wrong.