Indicated airspeed
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Indicated airspeed (IAS) is the
This is the pilots' primary airspeed reference.This value is not corrected for installation error, instrument error, or the actual encountered air density,[2] being instead calibrated to always reflect the adiabatic compressible flow of the International Standard Atmosphere at sea level.[1]
It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measure dynamic pressure. 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 (TAS), the relative velocity between the aircraft and the surrounding air mass. Calibrated airspeed (CAS) is the IAS corrected for instrument and position error.[3]
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
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
Calculation
Indicated airspeed measured by
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
where:
- is indicated airspeed in m/s,
- is stagnation or total pressure in pascals,
- is static pressure in pascals,
- is standard atmosphere fluid density in at sea level, and
- is the specific heat capacity ratio (≈1.401 for air[5]).
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, 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 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
Diving below | IAS mph |
IAS km/h |
30,000 ft (9,100 m) | 370 | 595 |
25,000 ft (7,600 m) | 410 | 660 |
20,000 ft (6,100 m) | 450 | 725 |
15,000 ft (4,600 m) | 490 | 790 |
10,000 ft (3,000 m) | 540 | 870 |
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,[2] 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.
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
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.
Some aircraft also have a taxi speed indicator for use on the ground. Since the IAS often starts at around 74–93 km/h (40–50 kn) (on jet airliners), pilots may need extra help while taxiing the aircraft on the ground. Its range is around 0–93 km/h (0–50 kn).
See also
- Acronyms and abbreviations in avionics
- ICAO recommendations on use of the International System of Units
- Air speed
- Calibrated airspeed
- Equivalent airspeed
- Flight instruments
- Global Positioning System
- True airspeed
References
- ^ OCLC 1031377368. Archived(PDF) from the original on 2022-12-22. Retrieved 2022-12-26.
Indicated airspeed. The aircraft velocity as measured by a pitot-static airspeed system calibrated to reflect standard atmosphere adiabatic compressible flow at sea level uncorrected for airspeed system errors.
- ^ FAA, Flight Standards Service. 2016. p. 8. Archived(PDF) from the original on 20 December 2022. Retrieved 26 December 2022.
Indicated airspeed (IAS)—the direct instrument reading obtained from the ASI, uncorrected for variations in atmospheric density, installation error, or instrument error.
- ISBN 0-273-01120-0
- ISBN 0-582-23740-8
- ^ "Air - Specific Heat Ratio". The Engineering ToolBox. 2003. Archived from the original on 12 July 2022. Retrieved 26 December 2022.
Bibliography
- Gracey, William (1980), "Measurement of Aircraft Speed and Altitude" Archived 2021-09-26 at the Wayback Machine (11 MB), NASA Reference Publication 1046.