Originally Posted by specialrider987
, which would require a change in IAS.
Not so, at least as I understood it.
You are flying a constant flight level and the autothrottle holds a constant Mach number. You fly from a colder to a warmer air mass:
Going to the warmer air the local speed of sound increases, therefore TAS increases.
The warmer air is less dense, which affects the IAS / TAS ratio.
The laws of physics conspire such that these effects cancel each other and IAS remains constant.
Another way of looking at this is to consider the inputs required to an Air Data Computer (or traditional instruments).
To compute IAS (theoretically CAS if we correct for position error) requires only impact pressure (this is the pitot - static differential measured by the traditional "airspeed" capsule).
To compute Mach number requires both impact pressure and absolute static pressure (traditional "altitude" capsule).
So far we have both IAS and Mach number without any input of air temperature. Thus the relation between IAS and Mach number is independant of air temperature.
Now if we wish to compute Static Air Temperature (SAT) and TAS, we require also the Total Air Temperature (TAT) input.