How is equivalent airspeed (EAS) defined?

Equivalent airspeed (EAS) is defined as calibrated airspeed (CAS) that has been corrected for compressibility effects. This definition is important because EAS provides a more accurate assessment of the aerodynamic forces acting on an aircraft as it operates at varying altitudes and speeds, particularly at higher speeds where compressibility effects become significant.

When an aircraft flies at high speeds, especially near or above the speed of sound, the air pressure and density changes affect how the aircraft interacts with the surrounding air. By using EAS, pilots and engineers can better understand the actual aerodynamic performance, regardless of the altitude at which the aircraft is flying. This makes it a crucial parameter in both flight planning and performance evaluation.

In contrast, the other options either describe different concepts related to airspeed or do not provide the necessary context for understanding the unique aspects of EAS. Understanding EAS involves recognizing how true airspeed and pressure play a role in the performance of an aircraft, emphasizing its importance in aviation safety and efficiency.