Sorry Dan,
But CAS apply also for independant instruments.
The main difference between RAS and CAS is that compressibility is taken into account at Sea Level in ISA conditions for CAS and that RAS does not take into account compressibility at all. The graduations on the indicator are not exactly computed the same way.
In altitude, it is a correction due to variation of compressibility with altitude which is applied to CAS and not the whole compressibility error it the reason why some countries name this "error"pressure error" and not compressibility.
I have post this topic because some instructors are confusing the 2 definitions and I am not sure than some examiners are cleared on this subject. I have seen some ambiguous questions in the exams there is a need for clarification.
