Mcrit is constant as it is related to the design of the wing - only. However, the actual speed is lower in high altitudes as the speed of sound also is lower.
The aircraft is limited by two speeds. One is a certain indicated airspeed and the other is Mcrit. The limiting indicated airspeed is not to overstress the airframe and is typically around 330 kts. At mean sea level that would be apprx. mach .50 (330/662 = 0.5 *).
The indicated airspeed decreases with altitude since the air gets thinner. That means that two aicrafts flying at same groundspeed (in no wind), then the one at mean sea level will have a higher indicated airspeed than the one at 40000'. At some point (around 20-25000') the crew will fly a machspeed rather than an indicated. The aircraft will now reach Mcrit (say .92) before 330 kts, therefore Mcrit is now the limiting speed, and the aircraft can now go faster than at mean sea level (.92 as oposed to .50).
Tor
* = note: the indicated 662 is also indicated airspeed, as trueairspeed = indicated at mean sea level.
[This message has been edited by Tor (edited 05 October 2000).]