Air speed

Calibrated airspeed

This expression is based on the form of Bernoulli's equation applicable to isentropic compressible flow. CAS is the same as true air speed at sea level standard conditions, but becomes smaller relative to true airspeed as we climb into lower pressure and cooler air. Nevertheless, it remains a good measure of the forces acting on the airplane, meaning stall speeds can be called out on the airspeed indicator. The values for ${\displaystyle p_{0}}$ and ${\displaystyle a_{0}}$ are consistent with the ISA i.e. the conditions under which airspeed indicators are calibrated.