Equivalent airspeed
Equivalent airspeed (EAS) is calibrated airspeed (CAS) corrected for the compressibility of air at a non-trivial Mach number. It is also the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed (TAS) and altitude at which the aircraft is flying. In low-speed flight, it is the speed which would be shown by an airspeed indicator with zero error. It is useful for predicting aircraft handling, aerodynamic loads, stalling etc. formula_1 where: formula_2 is actual air density. formula_3 is standard sea  level density (1.225 kg/m or 0.00237 slug/ft). EAS is a function of dynamic pressure. formula_4 where: formula_5 is dynamic pressure formula_6 EAS can also be obtained from the aircraft Mach number and static pressure. formula_7 where: formula_8 is , the standard speed of sound at 15 °C formula_9 is Mach number formula_10 is static pressure formula_11 is standard sea level pressure (1013.25 hPa) Combining the above with the expression for Mach number gives EAS as a function of impact pressure and static pressure (valid for subsonic flow): formula_12 where: formula_13 is impact pressure. At standard sea level, EAS  is the same as calibrated airspeed (CAS) and true airspeed (TAS). At any other altitude, EAS may be obtained from CAS by correcting for compressibility error. The following simplified formula allows calculation of CAS from EAS: formula_14 where: pressure ratio: formula_15 formula_16 and formula_17 are airspeeds and can be measured in knots, km/h, mph or any other appropriate unit. The above formula is accurate within 1% up to Mach 1.2 and useful with acceptable error up to Mach 1.5. The 4th order Mach term can be neglected for speeds below Mach 0.85.