Calibrated Airspeed (CAS)

Calibrated airspeed (CAS) is indicated airspeed (IAS) corrected for instrument error and position error. Position error arises because the static port, which measures ambient air pressure, is located in a position on the aircraft where local airflow disturbance causes the measurement to differ slightly from true ambient pressure. CAS is the most accurate speed available without electronic air data processing.

IAS → CAS → EAS → TAS

The progression from raw instrument reading to true speed through the air: IAS (raw reading) → CAS (corrected for instrument and position error) → EAS (corrected for compressibility at high altitude) → TAS (corrected for air density). In general aviation at low altitude and moderate speed, IAS and CAS are nearly identical and the distinction is rarely operationally significant.

When CAS Matters

Aircraft performance charts and limitations published in the POH are typically based on IAS or CAS. V-speeds are given in IAS. For flight planning at higher altitudes, TAS is used for groundspeed calculations. For high-performance or high-altitude operations, EAS becomes relevant for structural load calculations.

Related Terms

See also: IAS, EAS, True Airspeed.