- The speed of an aircraft relative to its surrounding air mass. The term air speed can mean any one of the following: indicated air speed, calibrated air speed, equivalent air speed, and true air speed. Indicated air speed (indicated air speed) or IAS is the uncorrected reading obtained from the air speed indicator. This is the speed used in pilot/controller communications under the general term air speed. Calibrated air speed (calibrated air speed), or CAS, is the aircraft relative to its surrounding air mass. Equivalent air speed (equivalent air speed), or EAS, is the calibrated air speed corrected for compressibility-of-air error. True air speed (true air speed), or TAS, is the equivalent air speed corrected for density altitude (i.e., variation in pressure and temperature). True air speed is used in flight planning and the en route portion of flight. True air speed is not shortened to “air speed” in pilot/controller communications.
Aviation dictionary. 2014.