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Indicated airspeed
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<h2 id="calculation">Calculation</h2> <p>Indicated airspeed measured by <a href="/facts/Pitot_tube/5xSGoYJf">pitot-tube</a> can be approximately expressed by the following equation delivered from <a href="/facts/Bernoulli%2527s_equation/Y7urHvPr">Bernoulli's equation</a>. </p> I A S ≈ 2 ( p t − p s ) ρ ( 0 ) {\displaystyle IAS\approx {\sqrt {\frac {2(p_{t}-p_{s})}{\rho (0)}}}} <p>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 <a href="/facts/Compressibility/eePznleN">Compressibility</a>. </p><p>The compression effects can be corrected by use of <a href="/facts/Poisson_constant/jFcMaukb">Poisson constant</a>. This compensation corresponds to <a href="/facts/Equivalent_airspeed/MUakxf01">equivalent airspeed</a> (EAS). </p> u = 2 γ γ − 1 p s ρ [ ( p t p s ) γ − 1 γ − 1 ] {\displaystyle 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]}}} <p>where: </p> <ul><li> u {\displaystyle u} is indicated airspeed in m/s,</li> <li> p t {\displaystyle p_{t}} is <a href="/facts/Stagnation_pressure/Tv1lrdQ8">stagnation</a> or total pressure in pascals,</li> <li> p s {\displaystyle p_{s}} is static pressure in pascals,</li> <li> ρ ( 0 ) {\displaystyle \rho (0)} is standard atmosphere fluid density in k g / m 3 {\displaystyle kg/m^{3}} at sea level, and</li> <li> γ {\displaystyle \ \gamma \,} is the <a href="/facts/Heat_capacity_ratio/jFcMaukb">specific heat capacity ratio</a> (≈1.401 for air<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a>).</li></ul> <h2 id="ias-vs-cas">IAS vs CAS</h2> <p>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 <a href="/facts/Pascal_(unit)/Am1To2Qm">hPa</a>, 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 <a href="/facts/Calibrated_airspeed/aXAAGA8m">CAS (Calibrated Airspeed).</a> 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. </p> <h2 id="ias-and-v-speeds">IAS and V speeds</h2> <p>The aircraft's pilot manual usually gives critical <a href="/facts/V_speeds/9R6xcg7V">V speeds</a> 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 <a href="/facts/Hot_and_high/oR0It1qk">hot and high</a> 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. </p><p>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 <a href="/facts/Vne/9R6xcg7V">never exceed speed</a> (abbreviated <i>V</i>NE), is specified at several differing altitudes in faster aircraft's operating manuals, as shown in the sample table below. </p> <p>Ref: <i>Pilot's Notes for <a href="/facts/Hawker_Tempest/p5SuQfG3">Tempest V</a> <a href="/facts/Napier_Sabre/3cVux5Sl">Sabre IIA</a> Engine</i> - <a href="/facts/Air_Ministry/C0mh6SyH">Air Ministry</a> A.P.2458C-PN </p> <h2 id="ias-and-navigation">IAS and navigation</h2> <p>For navigation, it is necessary to convert IAS to <a href="/facts/True_airspeed/aHdFdAjl">TAS</a> and/or <a href="/facts/Ground_speed/Dq0vsMyP">ground speed</a> (GS) using the following method: </p> <ul><li>correct IAS to <a href="/facts/Calibrated_airspeed/aXAAGA8m">calibrated airspeed</a> (CAS) using an aircraft-specific correction table;</li> <li>correct CAS to <a href="/facts/True_airspeed/aHdFdAjl">true airspeed</a> (TAS) by using <a href="/facts/Outside_air_temperature/p2hYzhSx">Outside Air Temperature (OAT)</a>, Pressure-altitude and CAS on an <a href="/facts/E6B/eL8dGaCv">E6B</a> flight computer or equivalent functionality on most <a href="/facts/Global_Positioning_System/DKVLnzF3">GPSs</a>;</li> <li>convert TAS to <a href="/facts/Ground_speed/Dq0vsMyP">ground speed</a> (GS) by allowing for the effect of <a href="/facts/Wind/Nt0QIu5B">wind</a>.</li></ul> <p>With the advent of <a href="/facts/Doppler_radar/4a6nL2lA">Doppler radar</a> navigation and, more recently, <a href="/facts/Global_Positioning_System/DKVLnzF3">GPS</a> receivers, with other advanced navigation equipment that allows pilots to read <a href="/facts/Ground_speed/Dq0vsMyP">ground speed</a> directly, the TAS calculation in-flight is becoming unnecessary for the purposes of navigation estimations. </p><p><a href="/facts/True_airspeed/aHdFdAjl">TAS</a> is the primary method to determine aircraft's cruise performance in manufacturer's specs,<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> speed comparisons and pilot reports. </p> <h2 id="other-airspeeds">Other airspeeds</h2> <p>From IAS, the following speeds can also be calculated: </p> <ul><li>convert CAS to <a href="/facts/Equivalent_airspeed/MUakxf01">equivalent airspeed</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;</li> <li>convert EAS to <a href="/facts/True_airspeed/aHdFdAjl">true airspeed</a> (TAS) by allowing for differences in <a href="/facts/Density_altitude/GRHyTDVo">density altitude</a>.</li></ul> <p>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. </p><p>Modern jet airliners also include <a href="/facts/Ground_speed/Dq0vsMyP">ground speed</a> (GS) and <a href="/facts/Machmeter/4uQFp1EM">Machmeter</a>. Ground speed shows the actual speed that the aircraft uses compared to the ground. This is usually connected to a <a href="/facts/GPS/DKVLnzF3">GPS</a> 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. </p><p>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. <a href="/facts/Supersonic/D3VpzmlC">Supersonic</a> aircraft, like the <a href="/facts/Concorde/TAMElD6g">Concorde</a> and military fighters, use the Machmeter as the main speed instrument with the exception of take-offs and landings. </p><p>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). </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Acronyms_and_abbreviations_in_avionics/2rTYW6jT">Acronyms and abbreviations in avionics</a></li> <li><a href="/facts/International_Civil_Aviation_Organization/TzrL1mLr">ICAO recommendations on use of the International System of Units</a></li> <li><a href="/facts/Air_speed/BbJIjqSs">Air speed</a></li> <li><a href="/facts/Calibrated_airspeed/aXAAGA8m">Calibrated airspeed</a></li> <li><a href="/facts/Equivalent_airspeed/MUakxf01">Equivalent airspeed</a></li> <li><a href="/facts/Flight_instruments/S0tKK3jl">Flight instruments</a></li> <li><a href="/facts/Global_Positioning_System/DKVLnzF3">Global Positioning System</a></li> <li><a href="/facts/True_airspeed/aHdFdAjl">True airspeed</a></li></ul> <h3>Bibliography</h3> <ul><li>Gracey, William (1980), <a href="http://apps.dtic.mil/dtic/tr/fulltext/u2/a280006.pdf">"Measurement of Aircraft Speed and Altitude"</a> <a href="https://web.archive.org/web/20210926222531/https://apps.dtic.mil/dtic/tr/fulltext/u2/a280006.pdf">Archived</a> 2021-09-26 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> (11 MB), NASA Reference Publication 1046.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Environmental technical manual (PDF). Vol. I, Procedures for the Noise Certification of Aircraft (3rd ed.). International Civil Aviation Organization. 2018. p. 19. ISBN 978-92-9258-369-9. 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. <a href="978-92-9258-369-9" target="_blank">978-92-9258-369-9</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"Chapter 8, Flight Instruments" (PDF). Pilot's Handbook of Aeronautical Knowledge. U.S. Department of Transportation, 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. <a href="https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf" target="_blank">https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Chapter 8, Flight Instruments" (PDF). Pilot's Handbook of Aeronautical Knowledge. U.S. Department of Transportation, 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. <a href="https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf" target="_blank">https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Environmental technical manual (PDF). Vol. I, Procedures for the Noise Certification of Aircraft (3rd ed.). International Civil Aviation Organization. 2018. p. 19. ISBN 978-92-9258-369-9. 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. <a href="978-92-9258-369-9" target="_blank">978-92-9258-369-9</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Clancy, L.J. (1975), Aerodynamics, Section 3.9, Pitman Publishing Limited, London. ISBN 0-273-01120-0 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Kermode, A.C.,Mechanics of Flight, 8th Edition – page 64. Longman Group Limited, London ISBN 0-582-23740-8 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"Air - Specific Heat Ratio". The Engineering ToolBox. 2003. Archived from the original on 12 July 2022. Retrieved 26 December 2022. <a href="https://www.engineeringtoolbox.com/specific-heat-ratio-d_602.html" target="_blank">https://www.engineeringtoolbox.com/specific-heat-ratio-d_602.html</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"Chapter 8, Flight Instruments" (PDF). Pilot's Handbook of Aeronautical Knowledge. U.S. Department of Transportation, 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. <a href="https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf" target="_blank">https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> </ol>