= Calibrated airspeed
{title2=$V_{\rm CAS}$}
\b[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.<Glider Flying Handbook by Federal Aviation Administration>[Chapter 4, printed pp. 4-4–4-5, Figure 4-7 and Calibrated Airspeed]{ref}
Represent a specified correction chart by
$$
V_{\rm CAS}=V_{\rm IAS}+\Delta V(V_{\rm IAS},\text{configuration}).
$$
The correction can depend on the operating condition. In an original invented chart, suppose indicated $20$ and $40\,\mathrm{m/s}$ correspond to calibrated $22$ and $41\,\mathrm{m/s}$. Linear interpolation over this stipulated interval gives
$$
V_{\rm CAS}=3.0\,\mathrm{m/s}+0.95V_{\rm IAS},
$$
so an indication of $30\,\mathrm{m/s}$ maps to \b[$31.5\,\mathrm{m/s}$]. Neither a constant $2\,\mathrm{m/s}$ 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 $\rho_0V_{\rm CAS}^2/2$, 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.
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