Indicated airspeed
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Aircraft display an Indicated Airspeed (abbreviated IAS) on an instrument called an airspeed indicator. Indicated airspeed will differ from true airspeed ("TAS") at air densities other than some reference density. Air density is affected by temperature, atmospheric moisture content (humidity), and pressure altitude.
Principles of operation
The airspeed indicator is a calibrated pressure gauge that responds to the difference in two pressures. One pressure, the pitot pressure, is obtained by a forward facing open tube. In addition to the normal atmospheric pressure at the altitude, an additional "ram" pressure is obtained from the conversion of the kinetic energy of the (relative) moving airstream to potential energy within the tube. The second pressure, called static pressure, is taken from a carefully positioned static port where the pressure does not vary with aircraft speed or attitude, but only with altitude. (The static port also provides pressure for the altimeter and variometer.)
Use in aircraft operation
Indicated airspeed is useful in aircraft operation as in straight and level flight and at a given weight that the aircraft will always stall at the same indicated airspeed, regardless of its true airspeed. Note that a stall can occur in any indicated airspeed, because it doesn't depend on speed but on the angle of attack, so for a given angle of bank, or a pull out, either of which increase the g factor, the stall will occur at a higher indicated airspeed, but this IAS will be constant regardless of density altitude (except under icing conditions). Other important speed points such as maximum speed with flaps and maximum structural airspeed (if less than the speed of sound) are also relative to IAS and are marked on the dial card with colored lines.
Use in aircraft navigation
For purposes of aircraft navigation via dead reckoning (without constant ground reference) the indicated airspeed must be converted to true airspeed and this must then be adjusted for estimated wind conditions to obtain the speed over the ground (groundspeed). Using these factors in combination with the direction the aircraft is pointing (heading) the aircraft's path over the ground (track) may be computed, typically by using a special purpose graphic computer.