What I learned in ground school and faithfully regurgitated it in the oral exam:
Calibrated airspeed (CAS) is indicated airspeed corrected for instrument errors, position error (due to incorrect pressure at the static port) and installation errors.
What I occasionally ask an instructor or a more knowledgeable pilot (which is most of them) is:
What good is CAS? What important piece of information does it give you that IAS and TAS doesn't? Where in a POH do you find the factors to allow you to solve for CAS? Take the controls and let me find my reading glasses and dig out my whiz wheel from the bag behind the seat and I'll get you TAS and even our Mach number if you really want them, but I can't get you CAS and don't know why I'd want to.
Now, I know that CAS is an important number for those of us in the Sport Pilot world, as the upper limit for speed of a airplane that meets the LSA definition is 120kts CAS at max continuous power in standard atmosphere and at sea level.
So is CAS just important during the engineering and certification phases and after that, just a useless bit of minutia?
No one has ever given me a really good answer. It'd be nice to have one less rattle in the cranial void.
Thanks!
