Under what conditions are IAS and CAS approximately the same?

Indicated Airspeed (IAS) and Calibrated Airspeed (CAS) are approximately the same under specific flight conditions, particularly at cruise and higher airspeeds. At these conditions, the effects of air density and temperature variations on airspeed measurement are minimized.

During cruise, aircraft operate at relatively smooth and consistent airflows, where compressibility effects become more pronounced at high airspeeds but are still manageable. The calibration corrections that distinguish IAS from CAS primarily arise from deviations in pressure readings due to instrument error and airflow disruptions around the aircraft; these errors are less impactful during stable flight at cruise.

At lower airspeeds and under varying atmospheric conditions, such as not following Standard International Atmosphere (ISA) norms, the discrepancies between IAS and CAS can be more substantial due to increased sensitivity to changes in airflow and temperature. Thus, under cruise settings with higher airspeeds, IAS approaches CAS more closely, supporting the assertion that these measurements become nearly equivalent in those specific flight regimes.