= Indicated airspeed
{title2=$V_{\rm IAS}$}
\b[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.<Glider Flying Handbook by Federal Aviation Administration>[Chapter 4, printed pp. 4-1–4-2 and 4-4–4-5, pitot-static system and Indicated Airspeed]{ref}
A simple low-<Mach number> instrument model uses a standard reference <mass density> $\rho_0$ and the sensed difference $q_I$:
$$
V_{\rm IAS}\simeq\sqrt{\frac{2q_I}{\rho_0}}.
$$
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 $\rho_0=1.225\,\mathrm{kg/m^3}$, the model shows \b[$19.50\,\mathrm{m/s}$ instead of $20.00\,\mathrm{m/s}$]. 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.<Glider Flying Handbook by Federal Aviation Administration>[Chapter 4, printed p. 4-5, Calibrated Airspeed and pitot obstruction]{ref}
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