Indicated airspeed

Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed (CAS), which is the IAS corrected for instrument and installation errors.

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 directly indicates stall speed and various airframe structurally limited speeds, regardless of density altitude. Furthermore the IAS is specified in regard to airspeed restrictions below certain altitudes since it is the primary speed indicator in an aircraft when operated below transonic or supersonic speeds.

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 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 V_NE) are often given at several differing altitudes in some aircraft's operating manuals, the V_NE 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

IAS and navigation

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.

See also

• Acronyms and abbreviations in avionics
• True airspeed
• Equivalent airspeed
• Calibrated airspeed