Dartagnan,
In a high speed jet or turboprop, you have a precomputed changeover altitude, wherby you transition from a constant IAS to Mach mo. in a climb, and vice versa in a decent.
i.e at what altiude will 280 KIAS =M .750. not the other way around .
CAS is used by the computer for TAS computations.
and if you want [doesn't sound applicable to your particular situation] Mach no
I'm a little confused by your question
hope that helps, there has been an immense amount of material written on Pprune recently about Speeds and Mach number try a search and you'll be greatly rewarded
To The Tire Speed Limit And Beyond
rhov
