RELATIONSHIPS BETWEEN AIRSPEEDS
Indicated Airspeed (IAS): The speed registered on the Airspeed Indicator.
Calibrated Airspeed (CAS): An accurate measure of dynamic pressure when the aircraft is flying slowly. When IAS is corrected for this ‘position’ or ‘pressure’ error, as it is called, the resultant is Calibrated Airspeed.
Equivalent Airspeed (EAS): An accurate measure of dynamic pressure when the aircraft is flying fast. When the IAS is corrected for ‘position’ AND ‘compressibility’ error, the resultant is Equivalent Airspeed.
True Airspeed (TAS): The speed of the aircraft through the air.
MAXIMUM OPERATING SPEED (VMO)
VMO is a speed that may not be deliberately exceeded in any regime of flight (climb, cruise or descent).
The maximum operating IAS is, however, an EAS because it is a high speed.
Aircraft ‘V’ speeds are CAS, except VMO which is an EAS, but all are presented to the pilot in the Flight Manual as IAS.
Note: On of the advantages related to the combined Mach/Airspeed instrument is the ability to correct for instrument and position errors to give Calibrated Airspeed (CAS) instead of IAS.
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