Boomerang you were basically right first time. The ASI errors john_tullamarine refers to are basically due to compressibility effects in the pitot tube.
EAS equates to the dynamic pressure that acts upon the airframe.
At sea level this pressure will be the same as the pressure felt in the pitot tube.
At high altitudes and speeds the pressure felt in the pitot tube will be slightly higher than the pressure acting on the airframe. The increase in pressure is due to compressibility effects in the pitot tube. This is the pressure felt by the ASI and of course is known as IAS/CAS. Therefore relative to EAS, CAS will over read at altitude.
EAS is therefore CAS corrected for compressibility. Your ASI reads CAS but the Vmo pointer represents the corresponding CAS for the maximum allowable EAS, hence the difference at altitude.
Last edited by max motor; 15th June 2002 at 10:35.