Equivalent Airspeed

Equivalent airspeed is the speed that would produce the same aerodynamic dynamic pressure at sea level as the aircraft is experiencing at its current altitude. It comes from the dynamic-pressure relationship q = 1/2 ρV²: if you keep the pressure effect the same but replace the local air density ρ with sea-level density ρ₀, the matching speed becomes EAS = TAS√(ρ/ρ₀). Because density usually decreases with altitude, EAS is lower than TAS at high altitude even though the aircraft may feel the same aerodynamic loads.

Use this when you want to compare aerodynamic loads, lift, drag, or structural effects at different altitudes without being misled by true airspeed alone. It is especially useful in aircraft performance, flight testing, and basic compressibility/airspeed comparisons.

• Using density ratio directly instead of taking its square root
• Confusing equivalent airspeed with indicated airspeed, which are not always the same
• Forgetting that TAS and EAS can differ a lot at altitude because air density is lower