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Indicated airspeed (IAS) is the
airspeed In aviation, airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: * Indicated airspeed ("IAS"), what is read on an airspeed gauge connected to a Pitot-static system; * Calibrated ...
of an aircraft as measured by its
pitot-static system A pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a s ...
and displayed by the
airspeed indicator The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometers per hour (km/h), knots (kn), miles per hour (MPH) and/or meters per second (m/s). The recommendation by ICAO is to use km ...
(ASI). This is the pilots' primary airspeed reference. This value is not corrected for installation error,
instrument error Instrument error refers to the error of a measuring instrument, or the difference between the actual value and the value indicated by the instrument. There can be errors of various types, and the overall error is the sum of the individual errors. ...
, or the actual encountered air density, being instead calibrated to always reflect the adiabatic
compressible flow Compressible flow (or gas dynamics) is the branch of fluid mechanics that deals with flows having significant changes in fluid density. While all flows are compressible, flows are usually treated as being incompressible when the Mach number (the r ...
of the
International Standard Atmosphere The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide a ...
at sea level. It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measure
dynamic pressure In fluid dynamics, dynamic pressure (denoted by or and sometimes called velocity pressure) is the quantity defined by:Clancy, L.J., ''Aerodynamics'', Section 3.5 :q = \frac\rho\, u^2 where (in SI units): * is the dynamic pressure in pascals ( ...
. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know the density, it is by design calibrated to assume the sea level standard atmospheric density when calculating airspeed. Since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably from
true airspeed The true airspeed (TAS; also KTAS, for ''knots true airspeed'') of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Tr ...
(TAS), the relative velocity between the aircraft and the surrounding air mass.
Calibrated airspeed Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error. When flying at sea level under International Standard Atmosphere conditions (15 °C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equi ...
(CAS) is the IAS corrected for instrument and
position error Position error is one of the errors affecting the systems in an aircraft for measuring airspeed and altitude. It is not practical or necessary for an aircraft to have an airspeed indicating system and an altitude indicating system that are exactly ...
.Clancy, L.J. (1975), ''Aerodynamics'', Section 3.9, Pitman Publishing Limited, London. An aircraft's indicated airspeed in knots is typically abbreviated ''KIAS'' for "
Knots A knot is a fastening in rope or interwoven lines. Knot may also refer to: Places * Knot, Nancowry, a village in India Archaeology * Knot of Isis (tyet), symbol of welfare/life. * Minoan snake goddess figurines#Sacral knot Arts, entertainme ...
-Indicated Air Speed" (vs. ''KCAS'' for
calibrated airspeed Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error. When flying at sea level under International Standard Atmosphere conditions (15 °C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equi ...
and ''KTAS'' for
true airspeed The true airspeed (TAS; also KTAS, for ''knots true airspeed'') of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Tr ...
). 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 In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', p. 486. Aviation Supplies & Academics, 1997. This occurs when t ...
. These speeds, in true airspeed terms, vary considerably depending upon
density altitude The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as a ...
. 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 Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transoni ...
or supersonic speeds.


Calculation

Indicated airspeed measured by pitot-tube can be approximately expressed by the following equation delivered from
Bernoulli's equation In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after the Swiss mathematic ...
. :IAS \approx \sqrt 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 In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...
. The compression effects can be corrected by use of
Poisson constant In thermal physics and thermodynamics, the heat capacity ratio, also known as the adiabatic index, the ratio of specific heats, or Laplace's coefficient, is the ratio of the heat capacity at constant pressure () to heat capacity at constant volu ...
. This compensation corresponds to
equivalent airspeed Equivalent airspeed (EAS) is calibrated airspeed (CAS) corrected for the compressibility of air at a non-trivial Mach number. It is also the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same a ...
(EAS). :u = \sqrt where: * u is indicated airspeed in m/s, * p_t is stagnation or total pressure in pascals, * p_s is static pressure in pascals, * \rho is fluid density in kg/m^3, and * \ \gamma\, 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 °C, 1013
hPa HPA may refer to: Organizations * Harry Potter Alliance, a charity * Halifax Port Authority, Canada * Hamburg Port Authority, Germany * Hawaii Preparatory Academy, a school in Hawaii, US * Health Protection Agency, UK * Heerespersonalamt, the Ger ...
, 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 In aviation, V-speeds are standard terms used to define airspeeds important or useful to the operation of all aircraft. These speeds are derived from data obtained by aircraft designers and manufacturers during flight testing for aircraft type- ...
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 In aviation, hot and high is a condition of low air density due to high ambient temperature and high airport elevation. Air density decreases with increasing temperature and altitude. The lower air density reduces the power output from the airc ...
airfield will use the same IAS to fly the aircraft at the correct approach and landing speeds as he would 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 In aviation, V-speeds are standard terms used to define airspeeds important or useful to the operation of all aircraft. These speeds are derived from data obtained by aircraft designers and manufacturers during flight testing for aircraft type- ...
(abbreviated ''V''NE), is specified at several differing altitudes in faster aircraft's operating manuals, as shown in the sample table below. Ref: ''Pilot's Notes for Tempest V Sabre IIA Engine'' -
Air Ministry The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964. It was under the political authority of the Secretary of State ...
A.P.2458C-PN


IAS and navigation

For navigation, it is necessary to convert IAS to TAS and/or
ground speed Ground speed is the horizontal speed of an aircraft relative to the Earth’s surface. It is vital for accurate navigation that the pilot has an estimate of the ground speed that will be achieved during each leg of a flight. An aircraft diving ve ...
(GS) using the following method: * correct IAS to
calibrated airspeed Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error. When flying at sea level under International Standard Atmosphere conditions (15 °C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equi ...
(CAS) using an aircraft-specific correction table; * correct CAS to
true airspeed The true airspeed (TAS; also KTAS, for ''knots true airspeed'') of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Tr ...
(TAS) by using Outside Air Temperature (OAT), Pressure-altitude and CAS on an
E6B The E6B flight computer is a form of circular slide rule used in aviation and one of the very few analog calculating devices in widespread use in the 21st century. They are mostly used in flight training, because these flight computers have b ...
flight computer or equivalent functionality on most
GPS The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
s; * convert TAS to
ground speed Ground speed is the horizontal speed of an aircraft relative to the Earth’s surface. It is vital for accurate navigation that the pilot has an estimate of the ground speed that will be achieved during each leg of a flight. An aircraft diving ve ...
(GS) by allowing for the effect of
wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ho ...
. With the advent of
Doppler radar A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the fr ...
navigation and, more recently,
GPS The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
receivers, with other advanced navigation equipment that allows pilots to read
ground speed Ground speed is the horizontal speed of an aircraft relative to the Earth’s surface. It is vital for accurate navigation that the pilot has an estimate of the ground speed that will be achieved during each leg of a flight. An aircraft diving ve ...
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 Equivalent airspeed (EAS) is calibrated airspeed (CAS) corrected for the compressibility of air at a non-trivial Mach number. It is also the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same a ...
(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 The true airspeed (TAS; also KTAS, for ''knots true airspeed'') of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Tr ...
(TAS) by allowing for differences in
density altitude The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as a ...
. On large jet aircraft the IAS is by far the most important speed indicator. Most aircraft speed limitations are based on IAS, as IAS closely reflects dynamic pressure. TAS is usually displayed as well, but purely for advisory information and generally not in a prominent location. Modern jet airliners also include
ground speed Ground speed is the horizontal speed of an aircraft relative to the Earth’s surface. It is vital for accurate navigation that the pilot has an estimate of the ground speed that will be achieved during each leg of a flight. An aircraft diving ve ...
(GS) and
Machmeter A Machmeter is an aircraft pitot-static system flight instrument that shows the ratio of the true airspeed to the speed of sound, a dimensionless quantity called Mach number. This is shown on a Machmeter as a decimal fraction. An aircraft fly ...
. Ground speed shows the actual speed that the aircraft uses compared to the ground. This is usually connected to a
GPS The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
or similar system. Ground speed is just a pilot aid to estimate if the flight is on time, behind or ahead of schedule. It is not used for takeoff and landing purposes, since the imperative speed for a flying aircraft always is the speed against the wind. The Machmeter is, on subsonic aircraft, a warning indicator. Subsonic aircraft must not fly faster than a specific percentage of the speed of sound. Usually passenger airliners do not fly faster than around 85% of speed of sound, or Mach 0.85. Supersonic aircraft, like the
Concorde The Aérospatiale/BAC Concorde () is a retired Franco-British supersonic airliner jointly developed and manufactured by Sud Aviation (later Aérospatiale) and the British Aircraft Corporation (BAC). Studies started in 1954, and France an ...
and military fighters, use the Machmeter as the main speed instrument with the exception of take-offs and landings. Some aircraft also have a taxi speed indicator for use on the ground. Since the IAS often starts at around (on jet airliners), pilots may need extra help while taxiing the aircraft on the ground. Its range is around .


See also

*
Acronyms and abbreviations in avionics Below are abbreviations used in aviation, avionics, aerospace and aeronautics. A B C D E F G H I J K L M N N numbers (turbines) O P Q R S T U V V speeds W X Y Z See also * List of avia ...
* ICAO recommendations on use of the International System of Units *
Air speed In aviation, airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: * Indicated airspeed ("IAS"), what is read on an airspeed gauge connected to a Pitot-static system; * Calibrated a ...
*
Calibrated airspeed Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error. When flying at sea level under International Standard Atmosphere conditions (15 °C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equi ...
*
Equivalent airspeed Equivalent airspeed (EAS) is calibrated airspeed (CAS) corrected for the compressibility of air at a non-trivial Mach number. It is also the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same a ...
*
Flight instruments Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with data about the flight situation of that aircraft, such as altitude, airspeed, vertical speed, heading and much more other crucial information in f ...
*
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
*
True airspeed The true airspeed (TAS; also KTAS, for ''knots true airspeed'') of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Tr ...


References


Bibliography

* Gracey, William (1980)
"Measurement of Aircraft Speed and Altitude"
(11 MB), NASA Reference Publication 1046. {{DEFAULTSORT:Indicated Airspeed Airspeed