FullWings, No you're not barking up the wrong tree, what you say is correct. The discussion here is not so much about a change in Mach and CAS following a temperature change (at a constant Pressure Height), but about the FAA reference originally provided by Rivet gun, which indicated that -
the Mach number changes, even though the indicated airspeed may not change,
which I believe to be incorrect. My assertion is that although both Mach and CAS will change with the temperature change (a thrust and an inertia effect), once stabilised again, BOTH Mach and CAS will have returned to their original values.
enicalyth (G'day Enny) raises valid points, as usual, but I think that he's set one of his cats amongst the pigeons. Gamma (may I refer to it as Y) is certainly variable, but as the radical ((Y-1)/Y) and it's inverse is used for instrument calibration purposes for both the Airspeed indication and the Mach meter, any environmental variation in gamma will affect both indications. In Rivet gun's last post he has quite rightly stated - "flight instrument calibration equations are based on ISA which assumes inter alia that air is a ideal gas and that gamma = 1.4". Thus, although both may be somewhat in error due to environmental variation from the calibration standard, the relationship between IAS and Mach will be the same. One will not change whilst the other remains unaffected.
Sorry to awaken a thread in the 11th day of it's hibernation, work, the curse of the Prooning classes, has taken me to far flung pest holes during the course of an interesting discussion.
Regards,
Old Smokey