Equivalent Airspeed

Equivalent Airspeed (EAS) is calibrated airspeed corrected for the compressibility of air at high speeds. At low altitudes and low speeds, the difference between CAS and EAS is negligible. However, as airspeed increases (particularly above 200 knots) and altitude rises, compressibility effects become significant and EAS diverges from CAS.

The airspeed hierarchy is: IAS → CAS → EAS → TAS. EAS is important because it represents the true dynamic pressure experienced by the aircraft — the actual aerodynamic loads on the structure. This makes EAS the relevant airspeed for structural design limits and flutter analysis.

In practice, pilots rarely work directly with EAS. Air data computers handle the conversions automatically, and cockpit instruments display IAS (or CAS in some systems) and Mach number. Flight engineers and performance analysts use EAS in detailed performance calculations and structural assessments.