Indicated airspeed
Encyclopedia
Indicated airspeed 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.
, 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 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.
Ref: Pilot's Notes for Tempest V
Sabre IIA
Engine - Air Ministry
A.P.2458C-PN
and/or ground speed
(GS) using the following method:
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.
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
read directly from the airspeed indicator
Airspeed indicator
The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot.- Use :...
on an aircraft, driven by the pitot-static system
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 static port, and the pitot-static instruments...
. IAS is directly related to calibrated airspeed
Calibrated airspeed
Calibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...
(CAS), which is the IAS corrected for instrument and installation errors
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 accurate...
.
An aircraft's indicated airspeed in knots is typically abbreviated KIAS for "Knots-Indicated Air Speed" (vs. KCAS for calibrated airspeed
Calibrated airspeed
Calibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...
and KTAS for true airspeed
True airspeed
True airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
).
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
Density altitude
Density altitude is the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground...
. 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
Transonic
Transonic speed is an aeronautics term referring to the condition of flight in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above the speed of sound in the range of Mach 0.8 to 1.2, i.e. 600–900 mph...
or supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
speeds.
IAS and V speeds
Unless an aircraft is at sea level under International Standard Atmosphere conditions (15°C, 1013 hPaPascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
, 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
V speeds
In aviation, V-speeds are standard terms used to define airspeeds important or useful to the operation of all aircraft including fixed-wing aircraft, gliders, autogiros, helicopters, and dirigibles...
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
Hawker Tempest
The Hawker Tempest was a British fighter aircraft primarily used by the Royal Air Force in the Second World War. The Tempest was an improved derivative of the Hawker Typhoon, and one of the most powerful fighter aircraft used during the war....
Sabre IIA
Napier Sabre
The Napier Sabre was a British H-24-cylinder, liquid cooled, sleeve valve, piston aero engine, designed by Major Frank Halford and built by Napier & Son during WWII...
Engine - Air Ministry
Air Ministry
The Air Ministry was a department of the British Government with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964...
A.P.2458C-PN
IAS and navigation
For navigation, it is necessary to convert IAS to TASTrue airspeed
True airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
and/or ground speed
Ground speed
Ground speed is the speed of an aircraft relative to the ground. Information displayed to passengers through the entertainment system often gives the aircraft groundspeed rather than airspeed....
(GS) using the following method:
- correct IAS to calibrated airspeedCalibrated airspeedCalibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...
(CAS) using an aircraft-specific correction table; - correct CAS to true airspeedTrue airspeedTrue airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
(TAS) by using Outside Air Temperature (OAT), Pressure-altitude and CAS on an E6B flight computer or equivalent functionality on most GPSGlobal Positioning SystemThe Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
s; - convert TAS to ground speedGround speedGround speed is the speed of an aircraft relative to the ground. Information displayed to passengers through the entertainment system often gives the aircraft groundspeed rather than airspeed....
(GS) by allowing for the effect of windWindWind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
.
With the advent of Doppler radar
Doppler radar
A Doppler radar is a specialized radar that makes use of the Doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been...
navigation and, more recently, GPS
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
receivers, with other advanced navigation equipment that allows pilots to read ground speed
Ground speed
Ground speed is the speed of an aircraft relative to the ground. Information displayed to passengers through the entertainment system often gives the aircraft groundspeed rather than airspeed....
directly, the TAS calculation in-flight is becoming unnecessary for the purposes of navigation estimations.
TAS
True airspeed
True airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
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 airspeedEquivalent airspeedEquivalent airspeed is the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed and altitude at which the aircraft is flying. In low-speed flight, it is the speed which would be shown by an 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 airspeedTrue airspeedTrue airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
(TAS) by allowing for differences in density altitudeDensity altitudeDensity altitude is the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground...
.
See also
- Acronyms and abbreviations in avionicsAcronyms and abbreviations in avionics-A:*ACARS: Aircraft Communications Addressing and Reporting System.*ACAS: Airborne Collision Avoidance System.*ACP: Audio Control Panel.*ACS: Audio Control System.*ADAHRS: Air Data and Attitude Heading Reference System.*ADC: Air Data Computer....
- True airspeedTrue airspeedTrue airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
- Equivalent airspeedEquivalent airspeedEquivalent airspeed is the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed and altitude at which the aircraft is flying. In low-speed flight, it is the speed which would be shown by an airspeed...
- Calibrated airspeedCalibrated airspeedCalibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...