Originally Posted by Dplus$
yea but wouldn't you have a greater tas at a higher altitude...
No, at typical cruising levels, cruise is scheduled at a Mach Number, and with decreasing temperatures with increasing Pressure Height, TAS will reduce. Above the tropopause where the temperature is constant, TAS will also be constant.
All of what I've said applies to cruise at a constant Mach number, in these times of cruising at Cost Index speed (now leaning heavily on the Fuel cost side and approaching Maximum Range Cruise), the sum of the Low speed and High speed drag polars around MRC usually indicates an increasing Mach Number with increasing Pressure Height. Below the tropopause this increasing Mach is usually insufficient to completely reverse the TAS reduction trend, but above the tropopause will lead to small TAS increases with increasing Pressure Height.
Regards,
Old Smokey