Critical Mach Number has nothing (directly) to do with
altitude.
Mach Number has everything to do Temperature,
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Lower critical mach number may be defined as the highest
subsonic mach number below which the entire flow over it is
subsonic. vijayaprakaash
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Critical Mach Number, or Mcr, refers to the lowest Mach number
that air going over and around an aircraft can reach the speed of
sound. If an airplane goes over its Mcr, handling might be greatly
affected.
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At high speeds and altitudes
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The critical Mach number will increase significantly