thumb|A basic [[airspeed indicator with the indicated airspeed (IAS) indicated in knots ("Kt" or "Kts" or "KIAS") -- the most common unit of measure for airspeed. Some airspeed indicators in aircraft prior to the mid-1970s indicate in miles per hour plus knots (1 knot = 1.15 mph) or kilometers per hour (1 knot = 1.85 km/h).]]

thumb|A [[primary flight display with the indicated airspeed (IAS) displayed in the form of a vertical "tape" on the left.]]

Indicated airspeed (IAS) is the airspeed of an aircraft as measured by its pitot-static system and displayed by the airspeed indicator (ASI). This is the pilots' primary airspeed reference.

This value is not corrected for installation error, instrument error, or the actual encountered air density,

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 flight manual for such important performance values as the stall speed. These speeds, in true airspeed terms, vary considerably depending upon density altitude. However, at typical civilian operating speeds, the aircraft's aerodynamic structure responds to dynamic pressure alone, and the aircraft will perform the same when at the same dynamic pressure. Since it is this same dynamic pressure that drives the airspeed indicator, an aircraft will always, for example, stall at the published indicated airspeed (for the current configuration) regardless of density, altitude or true airspeed.

Furthermore, the IAS is specified in some regulations, and by air traffic control when directing pilots, since the airspeed indicator displays that speed (by definition) and it is the pilot's primary airspeed reference when operating below transonic or supersonic speeds.

Calculation

Indicated airspeed measured by pitot-tube can be approximately expressed by the following equation delivered from Bernoulli's equation.

:<math>IAS \approx \sqrt{\frac{2 (p_t - p_s)}{\rho(0)</math>

NOTE: The above equation applies only to conditions that can be treated as incompressible. Liquids are treated as incompressible under almost all conditions. Gases under certain conditions can be approximated as incompressible. See Compressibility.

The compression effects can be corrected by use of Poisson constant. This compensation corresponds to equivalent airspeed (EAS).

:<math>u = \sqrt{ \frac{2 \gamma}{\gamma - 1} \frac{p_s}{\rho} \left[\left(\frac{p_t}{p_s}\right)^\frac{\gamma-1}{\gamma}-1 \right] }</math>

where:

  • <math>u</math> is indicated airspeed in m/s,
  • <math>p_t</math> is stagnation or total pressure in pascals,
  • <math>p_s</math> is static pressure in pascals,
  • <math>\rho(0)</math> is standard atmosphere fluid density in <math> kg/m^3</math> at sea level, and
  • <math>\ \gamma\,</math> is the specific heat capacity ratio (≈1.401 for air).

IAS vs CAS

The IAS is not the actual speed through the air even when the aircraft is at sea level under International Standard Atmosphere conditions (15&nbsp;°C, 1013 hPa, 0% humidity). The IAS needs to be corrected for known instrument and position errors to show true airspeed under those specific atmospheric conditions, and this is the CAS (Calibrated Airspeed). Despite this the pilot's primary airspeed reference, the ASI, shows IAS (by definition). The relationship between CAS and IAS is known and documented for each aircraft type and model.

IAS and V speeds

The aircraft's pilot manual usually gives critical V speeds as IAS, those speeds indicated by the airspeed indicator. This is because the aircraft behaves similarly at the same IAS no matter what the TAS is: E.g. A pilot landing at a hot and high airfield will use the same IAS to fly the aircraft at the correct approach and landing speeds as when landing at a cold sea level airfield, even though the TAS must differ considerably between the two landings.

Whereas IAS can be reliably used for monitoring critical speeds well below the speed of sound this is not so at higher speeds. An example: Because (1) the compressibility of air changes considerably approaching the speed of sound, and (2) the speed of sound varies considerably with temperature and therefore altitude; the maximum speed at which an aircraft structure is safe, the never exceed speed (abbreviated V<sub>NE</sub>), is specified at several differing altitudes in faster aircraft's operating manuals, as shown in the sample table below.

{| class="toccolours" border="1" cellpadding="4" style="border-collapse:collapse"

|- bgcolor="#dedede"

|Diving below

|IAS <br/>mph

|IAS <br/> km/h

|-

|

| 370

|595

|-

|

| 410

|660

|-

|

| 450

|725

|-

|- valign="top"

|

| 490

|790

|-

|

| 540

|870

|}

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,