Traditionally, airspeed is thought of as how quickly an aircraft is moving through the air. Although this is true, it does not tell the entire story. Aerodynamicists have identified four main types of airspeeds, each of which have uses before and during flight. By having an understanding of what each type of airspeed is measuring, a pilot will have a greater understanding of how aircraft performance is affected by changes in flight conditions.


Airspeed Description
Indicated (IAS) The actual instrument indication of airspeed for a given flight condition (the number of air molecules flowing over the wing in a given time)
Calibrated (CAS) The result of correcting IAS for errors of the instrument and installation location
Equivalent (EAS) The result of correcting the CAS for compressibility effects
True (TAS) The result of correcting EAS for density altitude (the actual speed of air flowing over the wing)


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