Equivalent airspeed - Definition and Overview

Equivalent airspeed (EAS) is the airspeed which represents the same dynamic pressure at altitude that would be generated by flying at the corresponding true airspeed (TAS) at sea level. It is useful for predicting aircraft handling, aerodynamic loads, stalling etc.

<math>EAS = TAS \times \sqrt{\frac{actual\ air\ density}{standard\ air\ density}}<math>

Where 

standard air density is 1.225 <math>kg/m^3<math>.

EAS can also be obtained from the aircraft mach number and static air pressure.

<math>EAS = asl \times M \times \sqrt{P_0 \over P_{SL}}<math>

Where 

asl is the standard speed of sound at 15C (661.47 knots)

M is Mach number,

<math>P_0<math> is static air pressure,

<math>P_{SL}<math> is standard sea level pressure (1013.25 hPa)

At sea level EAS is the same as calibrated airspeed (CAS). At high altitude, EAS may be obtained from CAS by correcting for compressibility error.