airspeed

Airspeed is the speed of an aircraft relative to the surrounding air. It is distinct from groundspeed, which is the speed over the ground, and from the aircraft’s velocity through space. Airspeed is a fundamental parameter for flight safety and performance, influencing stall margins, control effectiveness, engine operation, and structural loads. It is typically measured with pitot-static instruments.

Airspeed is described by several related quantities. Indicated airspeed (IAS) is the raw reading from the airspeed indicator, and is not corrected for instrument or installation errors. Calibrated airspeed (CAS) is IAS corrected for instrument errors and installation effects. Equivalent airspeed (EAS) corrects CAS for compressibility to reflect the dynamic pressure in sea‑level standard air. True airspeed (TAS) is CAS corrected for air density and temperature, representing speed through the actual airmass; TAS increases with altitude for the same CAS. Mach number is TAS divided by the local speed of sound and is a key parameter at higher speeds.

Measurement and corrections: airspeed indicators derive dynamic pressure from the ram air entering a pitot tube, comparing stagnation pressure to static pressure. Instrument and position errors, as well as compressibility effects at higher speeds, are addressed by moving from IAS to CAS and EAS.

Relationships and uses: airspeed changes with altitude because air density and temperature vary; IAS remains relatively stable in standard atmosphere for a given flight, while TAS increases with altitude. Airspeed governs stall protection, aerodynamic efficiency, and engine and fuel planning. Wind affects groundspeed but not airspeed directly; groundspeed is TAS adjusted for wind along the flight path.