What is indicated airspeed (IAS) compared to true airspeed (TAS) and equivalent airspeed (EAS)?

• A. TAS is IAS corrected for altitude; EAS is IAS corrected for compressibility; IAS remains the same.
• B. EAS is TAS corrected for altitude and temperature.
• C. IAS is the airspeed relative to the air; TAS is IAS corrected for altitude and temperature; EAS accounts for compressibility effects.
• D. IAS is equivalent to TAS at sea level only; EAS is greater at higher altitude.

Answer: (C)

Indicated airspeed is the airspeed shown by the pitot-static system, reflecting the speed relative to the surrounding air through the dynamic pressure it measures. True airspeed is the actual speed through the air, which changes with altitude and temperature because air density changes; you convert IAS to TAS by correcting for density (altitude and temperature). Equivalent airspeed is the speed that would produce the same dynamic pressure if the air were at sea level standard density, so it accounts for density differences and compressibility effects at higher speeds. In short, IAS is the relative-airspeed reading, TAS is the speed corrected for altitude and temperature, and EAS adjusts for compressibility effects to compare dynamic pressure under standard sea-level conditions.