Which airspeed measure accounts for compressibility effects at higher speeds?

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

Answer: (D)

As you go faster, air becomes more compressible and the pressure the aircraft senses doesn’t follow a simple v^2 relationship anymore. The instruments originally measure dynamic pressure to give you reading in indicated airspeed, but that value is affected by installation and instrument factors, which we correct to get calibrated airspeed. To compare aerodynamic performance across different altitudes and speeds, we translate that value further into Equivalent Airspeed. EAS represents the speed in sea-level standard air that would produce the same ram pressure if the flow were incompressible. In other words, it adjusts CAS to account for compressibility, giving a common reference that remains meaningful at higher speeds. The other measures don’t apply this compressibility correction in the same way, so they don’t reflect that effect.