Which airspeed concept accounts for compressibility effects at higher speeds?

• A. Equivalent Airspeed (EAS)
• B. Indicated Airspeed (IAS)
• C. Calibrated Airspeed (CAS)
• D. True Airspeed (TAS)

Answer: (A)

When air speeds get high, air becomes noticeably compressible and the pressure readings used by airspeed indicators no longer map neatly to speed in a simple 1/2 ρ V^2 way. Equivalent Airspeed is the speed we use to account for that compressibility because it translates the measured dynamic pressure into the speed that would produce the same pressure if the flow were incompressible and the air were at sea level density. In other words, EAS represents the actual dynamic pressure effects (including compressibility) in a way that’s comparable across different altitudes and conditions. It’s the speed concept that makes those compressibility effects explicit and comparable, which is why it’s the best choice here.

Calibrated or indicated airspeed are still tied to the instrument readings and don’t directly adjust for compressibility, true airspeed is the actual speed through the air and varies with density and altitude but doesn’t normalize for the compressibility correction, so they don’t address the compressibility influence in the same way.