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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.

Indicated airspeed is the airspeed read directly from an indicator before correcting its instrument and installation errors. It is a displayed measurement, not automatically the aircraft's true airspeed or groundspeed. A conventional Pitot-static tube supplies total and static pressure through separate connections; the indicator converts their difference into a speed reading.^[ref]

A simple low-Mach number instrument model uses a standard reference mass density ${\rho }_0$ and the sensed difference $q_I$:This formula describes the stipulated ideal scale; real instrument errors need separate characterization. In particular, using ${\rho }_0$ does not assert that local air has that mass density.

For an original error example, let the actual total-minus-static difference be $245\mathrm{Pa}$, but suppose the static connection reads $12\mathrm{Pa}$ too high while the total-pressure connection is correct. Then $q_I=233\mathrm{Pa}$. With ${\rho }_0=1.225kg/{\mathrm{m}}^3$, the model shows $19.50m/s$ instead of $20.00m/s$. A positive error in one input has become a negative error in the difference. At lower pressure differences, the same offset has a larger fractional effect on the indicated speed.

A pressure-bench check can apply known differences to the instrument and test its zero and scale. It cannot alone characterize static-pressure position error caused by the ports' location on the flying aircraft. Calibrated airspeed accounts for the established instrument and installation corrections. A blocked port is another failure mode: treating every discrepancy as a correctable constant offset would conceal a changing or trapped pressure signal. The FAA specifically identifies obstruction and instrument deterioration as causes of incorrect indications.^[ref]