I think you will find that even at sea level high TAS will generate significant pressure(and therefore density) errors. And yet,if you set your wizzwheel to ISA msl values it shows CAS equal to TAS at all speeds, and the Compressibility Correction at zero. Why is this?
Well, I've been told that the ASI makes allowance for compressibility using factors at ISA msl values but can't account for errors everywhere and that what we see on the wizzwheel as the compressibility correction at, say FL200 is actually a limited correction, the difference between the full correction and the ISA msl value
Any comment?