Originally Posted by Rivet Gun
You will find that the EAS is always less than the CAS displayed on your speed tape. The difference is compressibility error. I have not bothered with the table below FL 100 because the compressibility error is small at lower altitudes.
Let's rather say the Airbus is limited by its VMO at lower altitudes, so it cannot run into regions where compressibility is an issue. A faster aircraft would experience the same CAS/EAS disagree at M.78 at Sea Level as at higher altitude.