Originally Posted by selfin
ASIs have been designed to take account of compressibility since the 1930s.
So, assuming zero position and instrument error, the airspeed indicator is showing us EAS even in "high"-speed environment (M > 0.4)? Why even mention it then, if IAS = EAS (assuming IAS = CAS)? Not being an expert on the subject matter, but I hardly doubt that's the truth...
EAS is actually very important, because airspeed indicators get dynamic pressure by substracting static pressure (static port) from total pressure (pitot tube). But this is done under assumption that air is incompressible, which is a lie of course, since every fluid is compressible, it's just the compressibility factor that is different between fluids. In high airspeed environment (beyond spamcan range, around M = 0.3 and upwards) the compressibility effect becomes noticeable, since the air flowing into the pitot tube is compressed and thus the sensed total pressure is higher than it actually is, so the sensed dynamic pressure will be higher, thus the indicated airspeed (IAS) will be higher as well. The effect of compressibility is even worse in high altitude environment, since the density of air is lower and the compressibility factor of air increases.