Indicated airspeed
 |
This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2009) |
Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. The IAS varies considerably from the true airspeed depending upon atmospheric conditions and configuration of the aircraft and even upon how the aircraft is being flown. Calibrated airspeed (CAS) is the IAS corrected for instrument and installation errors at a standard air pressure and temperature, the International Standard Atmosphere.
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).
The IAS is an important value for the pilot because it is the indicated speeds which are specified in the aircraft pilot's manual for such important values as the stall speed and various airframe structurally limited speeds. These speeds, in true airspeed terms, vary considerably depending upon density altitude; but the aircraft will always stall at the same indicated airspeed, regardless of density altitude.
Furthermore the IAS is specified in some regulations and by air traffic control to pilots since the airspeed indicator displays that speed (by definition) and it is the pilot's primary airspeed reference in an aircraft when operated below transonic or supersonic speeds.
IAS and V speeds
The following paragraph is wrong. IAS should not be referred to at all in it. Only CAS should be mentioned. It is calibrated airspeed which, at conditions of the International Standard Atmosphere, is the true airspeed. IAS is merely what the ASI, a mere instrument, and an inaccurate one, says.
Unless an aircraft is at sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) and no wind, the IAS will not be the actual speed an aircraft is moving in reference to the ground; however, because the air density affects 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 density decreases. This leads to an apparent falling-off of airspeed at higher altitudes. For this and other reasons never exceed speeds (abbreviated VNE) are often given at several differing altitudes in some aircraft's operating manuals, the VNE IAS figure falling as height is increased, as shown in the sample table below.
| Diving below | mph 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
For navigation, it is necessary to convert IAS to TAS and/or ground speed (GS) using the following method:
- correct IAS to calibrated airspeed (CAS) using an aircraft-specific correction table;
- correct CAS to true airspeed (TAS) by using Outside Air Temperature (OAT), Pressure-altitude and CAS on an E6B flight computer or equivalent functionality on most GPSs;
- convert TAS to ground speed (GS) by allowing for the effect of wind.
With the advent of Doppler radar navigation and, more recently, GPS receivers, with other advanced navigation equipment that allows pilots to read ground speed directly, the TAS calculation in-flight is becoming unnecessary for the purposes of navigation estimations.
TAS is the primary method to determine aircraft's cruise performance in manufacturer's specs, speed comparisons and pilot reports.
Other Airspeeds
From IAS, the following speeds can also be calculated:
- convert CAS to equivalent airspeed (EAS) by allowing for compressibility effects (not necessary at slow speed or low altitude); EAS is used by aircraft engineers and some very high-altitude flying aircraft such as the U-2 and the SR-71;
- convert EAS to true airspeed (TAS) by allowing for differences in density altitude.
