Editing Indicated airspeed (section)

{{Short description|Displayed on the airspeed indicator on an aircraft}}
{{More citations needed|date=January 2009}}
[[File:Airspeed Indicator.svg|thumb|A basic [[airspeed indicator]] with the indicated airspeed (IAS) indicated in [[knot (unit)|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).]]
[[File:Primary Flight Display.svg|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]]<ref name=":0">{{Cite book |url=https://www.icao.int/environmental-protection/Documents/SGAR_2018_ETM_Vol_I.pdf |title=Environmental technical manual. |publisher=[[International Civil Aviation Organization]] |year=2018 |isbn=978-92-9258-369-9 |edition=3rd |volume=I, Procedures for the Noise Certification of Aircraft |pages=19 |language=en |oclc=1031377368 |quote=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. |access-date=2022-12-26 |archive-url=https://web.archive.org/web/20221222105916/https://www.icao.int/environmental-protection/Documents/SGAR_2018_ETM_Vol_I.pdf |archive-date=2022-12-22 |url-status=live}}</ref> and displayed by the [[airspeed indicator]] (ASI).<ref name=":1">{{cite book |title=Pilot's Handbook of Aeronautical Knowledge |publisher=[[U.S. Department of Transportation]], [[FAA]], Flight Standards Service |year=2016 |pages=8 |language=en |chapter=Chapter 8, Flight Instruments |quote=Indicated airspeed (IAS)—the direct instrument reading obtained from the ASI, uncorrected for variations in atmospheric density, installation error, or instrument error. |access-date=26 December 2022 |chapter-url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf |archive-url=https://web.archive.org/web/20221220052755/https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf |archive-format=PDF |archive-date=20 December 2022 |url-status=live}}</ref> This is the pilots' primary airspeed reference.

This value is not corrected for installation error, [[instrument error]], or the actual encountered [[Density of air|air density]],<ref name=":1" /> being instead calibrated to always reflect the [[Adiabatic process|adiabatic]] [[compressible flow]] of the [[International Standard Atmosphere]] at sea level.<ref name=":0" />

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 [[International Standard Atmosphere|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]].<ref name="LJC3.9">Clancy, L.J. (1975), ''Aerodynamics'', Section 3.9, Pitman Publishing Limited, London. {{ISBN|0-273-01120-0}}</ref>

An aircraft's indicated airspeed in knots is typically abbreviated ''KIAS'' for "[[Knot (unit)|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.<ref>Kermode, A.C.,''Mechanics of Flight'', 8th Edition – page 64. Longman Group Limited, London {{ISBN|0-582-23740-8}}</ref>

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.