Indicated airspeed
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Indicated airspeed (IAS) is the speed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed (CAS), but includes instrument errors and position error.
An aircraft's indicated airspeed in knots is typically abbreviated KIAS for "Knots-Indicated Air Speed" (vs. KCAS for calibrated airspeed and KTAS for true airspeed).
[edit] IAS and V speeds
Unless an aircraft is at sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) and no wind, the IAS bears little relation to how fast an aircraft is moving in reference to the ground; however, because the air pressure and density affect IAS/CAS and an aircraft's flight characteristics in exactly the same way, IAS and CAS are extremely useful for controlling an aircraft, and the critical V speeds are usually given as IAS.
In aneroid instruments the indicated airspeed drops-off with increasing altitude as air pressure decreases, and this leads to an apparent falling-off of airspeed at higher altitudes. For this reason Vne speeds are often given at several differing altitudes in the aircraft's operating manual, the Vne IAS figure falling as height is increased, as shown in the sample table below.
| Diving below | m.p.h IAS |
| 30,000 ft | 370 |
| 25,000 ft | 410 |
| 20,000 ft | 450 |
| 15,000 ft | 490 |
| 10,000 ft | 540 |
Ref: Pilot's Notes for Tempest V Sabre IIA Engine - Air Ministry A.P.2458C-PN
[edit] IAS and navigation
For navigation, it is necessary to convert IAS to ground speed (GS) using several steps:
- correct IAS to calibrated airspeed (CAS) using an aircraft-specific correction table;
- convert CAS to equivalent airspeed (EAS) by allowing for compressibility effects (not necessary at slow speed or low altitude);
- convert EAS to true airspeed (TAS) by allowing for differences in density altitude; and
- convert TAS to ground speed (GS) by allowing for the effect of wind.
With the advent of GPS receivers and other advanced navigation equipment that allows pilots to read ground speed (and often, TAS) directly, these complex calculations are becoming unnecessary.
