Airspeed

Airspeed describes an aircraft's motion relative to the surrounding air. When the kinematic magnitude is intended, it is the true airspeed,It differs from groundspeed whenever wind changes the relative motion. A ground-station velocity alone cannot establish the aerodynamic airspeed.^[ref]

An airspeed instrument normally infers motion from pressure. At low Mach number, for an aligned Pitot-static tube in the regime of the Bernoulli equation,Here $p_0$ is stagnation pressure, $p$ is static pressure and $\rho$ is air mass density. For a stipulated difference of $245\mathrm{Pa}$, $\rho =1.225kg/{\mathrm{m}}^3$ gives $V=20m/s$; the same difference at $0.6125kg/{\mathrm{m}}^3$ gives $28.28m/s$. The instrument senses a pressure difference, so interpreting its reading requires a density model and an instrument calibration.^[ref]

Indicated airspeed is the displayed reading before instrument and installation corrections. Calibrated airspeed includes those corrections. True airspeed expresses actual motion through the air, rather than simply relabeling either reading. The FAA distinguishes these quantities in its instrument chapter.^[ref]

The square-root calculation above assumes an appropriate low-speed stopping relation. At appreciable Mach number, compressibility changes that relation; a blocked passage or displaced static pressure source introduces a different measurement problem. Report which airspeed convention and corrections accompany a performance curve. Comparing speeds without that information can confound a change of air mass density with a change in the aircraft's loading. Functioning ports and known corrections are part of the inference, not extra measurements of groundspeed.