This crops up again
Mach Number is a number, it has nothing to do with speeds which have you left you confused and wishing to "know" which box to tick.
It is normal to use CAS in climb until "cross-over" after which Mach Number is used. The point at which CAS and Mach Number "match" is the crossover. That point is essentially temperature dependent.
Say alt 28000ft is ISA +10, then OAT is -30*C; use your whirly-wheel to deduce TAS is 504kts and TAT is 3.5*C; as a further exercise use the same wheel and settings to show that CAS is 330kts. The two exercises are mutually explanatory.
In other words if PAlt is 28000ft, and OAT is -30*C then Mach Number is 0.83...... don't think of "ace-ing the interview" until you can swop the "wheel" numbers or the electronic calc version in your sleep. You are trying to quote numbers without understanding the underlying physics.
Best of Luck!!
Last edited by Dr Syntax; 18th Jun 2013 at 18:53.
Reason: spelling, grammar