This relates directly to a previous thread:
http://www.pprune.org/tech-log/44376...ch-number.html
PPL students are taught that indicated stalling speed remains constant with altitude, but some things they teach PPL students don't hold good at high altitudes and Mach numbers.
So why don't we calibrate our EFIS speed tapes to EAS? It would just be a software mod. There was an argument that mechanical standby airspeed indicators were calibrated CAS so the PFD tape had to read the same as the standby. Nowadays even standby instruments are EFIS so there is no reason why we should not switch to EAS displays.
There would still be a slight increase in stalling speed due to the Mach effect on CLmax, but at least the compressibility error could be eliminated.
CAS was invented as a calibration standard for the mechanical airspeed indicator. As the mechanical ASI is consigned to history, so should be CAS.