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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.

Calibrated airspeed is indicated airspeed corrected for instrument and installation errors. It remains a pressure-based speed convention; correcting the indicator does not by itself convert the reading into true airspeed. The FAA uses an airspeed-calibration chart to distinguish the indicated and corrected values.^[ref]

Represent a specified correction chart byThe correction can depend on the operating condition. In an original invented chart, suppose indicated $20$ and $40m/s$ correspond to calibrated $22$ and $41m/s$. Linear interpolation over this stipulated interval givesso an indication of $30m/s$ maps to $31.5m/s$. Neither a constant $2m/s$ correction nor a universal percentage reproduces both chart endpoints. Extrapolation outside the measured interval is an additional assumption.

At low Mach number, corrected pressure difference is approximately ${\rho }_0{V}_{\mathrm{CAS}}^2/2$, with ${\rho }_0$ the standard reference air mass density. The example's corrected pressure estimate is $(31.5/30{)}^2=1.1025$ times the estimate obtained by treating the indication as calibrated. The corresponding correction to an inferred aerodynamic force at fixed coefficient and area is therefore not merely the $5\%$ speed correction.

Instrument testing checks the mapping from applied pressures to the reading. Installed-system work also addresses static-pressure position error: a port can sample a local pressure different from the intended free-stream static pressure. The correction must accompany its configuration and reference conditions. To obtain true airspeed, account for ambient conditions as well. At appreciable Mach number, compressibility also separates calibrated and equivalent airspeed; the low-speed square-root density conversion must not be silently extended to that regime.

True airspeed is the magnitude of an aircraft's velocity relative to the surrounding air,It determines the relative motion used in aerodynamic models, while groundspeed concerns horizontal travel over the ground. Indicated airspeed and calibrated airspeed describe stages in a pressure-instrument reduction, rather than another reference frame.^[ref]

For an aligned low-Mach number pressure measurement in the regime of the Bernoulli equation,An equivalent airspeed uses the same dynamic pressure with a standard reference mass density, $V_{\mathrm{EAS}}=\sqrt{2q/{\rho }_0}$. HenceOnly where the low-speed pressure relations justify $V_{\mathrm{CAS}}\simeq V_{\mathrm{EAS}}$ may calibrated speed be substituted in this last expression without a separate compressibility correction.

For an original low-speed example, use $V_{\mathrm{CAS}}\simeq 30m/s$, ${\rho }_0=1.225kg/{\mathrm{m}}^3$ and ambient $\rho =0.800kg/{\mathrm{m}}^3$. The corrected difference is approximately $551.25\mathrm{Pa}$, giving $V_{\mathrm{TAS}}\simeq 37.12m/s$. At a fixed pressure difference, halving ambient mass density increases the inferred true airspeed by $\sqrt{2}$, rather than doubling it. This follows from the square-root dependence, not a wind correction.

Obtaining the ambient mass density can require independent static pressure and thermodynamic temperature measurements, with a suitable equation of state. For fixed $q$, a small relative density error changes the inferred speed by approximately $\delta V/V=-\delta \rho /(2\rho )$. Wind information is needed separately to predict the ground track. A single ground-travel speed cannot supply all of these quantities. The pressure inference also depends on clear passages and valid stopping-flow assumptions.^[ref]