{"id":14460,"date":"2025-06-07T16:14:06","date_gmt":"2025-06-07T10:44:06","guid":{"rendered":"https:\/\/ibexaviation.com\/pilot-training\/?p=14460"},"modified":"2025-06-09T19:31:36","modified_gmt":"2025-06-09T14:01:36","slug":"air-speed-indicator","status":"publish","type":"post","link":"https:\/\/ibexaviation.com\/pilot-training\/air-speed-indicator\/","title":{"rendered":"Air Speed Indicator (ASI)"},"content":{"rendered":"\n<h2>Air Speed Indicator<\/h2>\n\n\n\n<h3>Purpose of Air Speed Indicator<\/h3>\n\n\n\n<ul><li>Air Speed Indicator is designed to detect dynamic pressure<\/li><li>ASI measures dynamic pressure and displays it as air speed<\/li><li>Units of measurement are normally Knots or Kilometers per hour<\/li><\/ul>\n\n\n\n<h3>Principle of Air Speed Indicator<\/h3>\n\n\n\n<ul><li>ASI senses Pitot pressure and static pressure<\/li><li>Pitot pressure is a combination of dynamic and static<\/li><li>Dynamic pressure is obtained by subtracting static pressure<\/li><li>Dynamic pressure = Pitot pressure \u00e2\u20ac\u201c Static pressure<\/li><li>Formula for Dynamic pressure = \u00c2\u00bd density x velocity2<\/li><\/ul>\n\n\n\n<h3>Construction of ASI<\/h3>\n\n\n\n<ul><li>Static pressure is provided to the air tight case from a static source<\/li><li>Pitot pressure is made to enter a expandable thin graduated capsule<\/li><li>Pitot pressure varies with change in air speed<\/li><li>Static pressures cancels out providing dynamic pressure<\/li><li>The capsule therefore expands as a factor of dynamic pressure<\/li><li>This capsule movement is transmitted by a temperature compensated magnifying linkage, to a pointer, indicating air speed of the aircraft<\/li><\/ul>\n\n\n\n<h3>Critical Speed Markings<\/h3>\n\n\n\n<ul><li>V no : Normal Operating Speed Range<\/li><li>V ne : Red Line Speed: Never Exceed Speed<\/li><li>V yse : Blue Line Speed: Best Single Engine Climb Speed<\/li><li>V lo : Maximum Landing Gear Operating Speed<\/li><\/ul>\n\n\n\n<h3>Critical Speed Markings<\/h3>\n\n\n\n<ul><li>V le : Maximum Speed With Landing Gear Extended<\/li><li>V s0 : Stall Speed<\/li><li>Minimum Steady Flight Speed With Flaps and Undercarriage Down<\/li><li>V s1 : Stall Speed<\/li><li>Minimum Steady Flight Speed in a Specified Configuration<\/li><\/ul>\n\n\n\n<h3>Colour Codes for Speed Range<\/h3>\n\n\n\n<ul><li>White arc refers to flap operating speed range<\/li><li>Flap operating stall speed (V so ) to Maximum flap operating speed (V fe )<\/li><li>Green arc refers to normal operating speed range (Vno)<\/li><li>Yellow arc refers to cautionary speed range<\/li><li>Normal operating speed (V no) to Never exceed speed (V ne)<\/li><\/ul>\n\n\n\n<h3>Instrument and Position Errors<\/h3>\n\n\n\n<ul><li>Instrument Errors are caused due to manufacturing imperfections<\/li><li>Position error is caused due to suction due turbulent airflow near static vent<\/li><li>Instrument and Position errors are tabulated in a correction card<\/li><\/ul>\n\n\n\n<h3>Manoeuvre Induced Errors<\/h3>\n\n\n\n<ul><li>Manoeuvre induced errors are caused due to change in angle of attack<\/li><li>Manoeuvre induced errors result in Time lag during Air speed indications<\/li><\/ul>\n\n\n\n<h3>Variation of Atmospheric Density<\/h3>\n\n\n\n<ul><li>Density error results from variation of actual and calibrated density of air<\/li><li>Dynamic Pressure depends on both speed of aircraft as well as air density<\/li><li>Density varies with temperature &amp; pressure, which in turn varies with Altitude<\/li><\/ul>\n\n\n\n<h3>Density Error<\/h3>\n\n\n\n<ul><li>ASI is calibrated to read airspeeds correctly at air density of 1225 gm\/m3<\/li><li>This density prevails only at ISA conditions<\/li><li>Dry air (zero humidity)<\/li><li>MSL pressure of 1013.25 hPa<\/li><li>MSL temperature of 15 deg C<\/li><li>In all other conditions, the ASI reads incorrect speeds called density error<\/li><\/ul>\n\n\n\n<h3>Compressibility Error<\/h3>\n\n\n\n<ul><li>Compressibility error occurs due to compression of air at high speeds<\/li><li>Air is a compressible fluid and compression is significant at high speeds<\/li><li>ASI is calibrated only for air at its normal state and not when compressed<\/li><li>Error due to compression are significant at speeds greater than 300 Knots<\/li><li>Compression errors in air speed have to be corrected above 300 knots<\/li><\/ul>\n\n\n\n<h3>Indicated and Calibrated (Rectified) Air Speed<\/h3>\n\n\n\n<ul><li>Indicated air speed (IAS) is the speed indicated by the Air speed indicator<\/li><li>It is a factor of dynamic pressure as measured by the instrument<\/li><li>Calibrated (CAS) is obtained from Indicated air speed (IAS)<\/li><li>Calibrated (CAS) is also called Rectified air speed (RAS)<\/li><\/ul>\n\n\n\n<h3>Calculating Calibrated (Rectified) Air Speed<\/h3>\n\n\n\n<ul><li>IAS corrected for instrument &amp; position errors provides CAS or RAS<\/li><li>Correction card table in flight manual is used for applying the corrections<\/li><li>These corrections could be applied manually or by Air data computer<\/li><\/ul>\n\n\n\n<h3>Equivalent Air Speed<\/h3>\n\n\n\n<ul><li>Equivalent air speed (EAS) is obtained from Calibrated air speed (CAS)<\/li><li>CAS corrected for compressibility error provides EAS<\/li><li>Compressibility error correction is mandatory at speeds above 300 knots<\/li><li>EAS can be calculated using Flight Computer<\/li><li>EAS = CAS + Compressibility error correction<\/li><\/ul>\n\n\n\n<h3>True Air Speed<\/h3>\n\n\n\n<ul><li>True air speed (TAS) is obtained from Equivalent air speed (EAS)<\/li><li>TAS is obtained from EAS by applying Density error correction<\/li><li>Air density reduces with increase in altitude<\/li><\/ul>\n\n\n\n<h3>Variation of True Air Speed<\/h3>\n\n\n\n<ul><li>Density error increases with increase in altitude<\/li><li>At a constant IAS, TAS increases with increase in altitude<\/li><li>At a constant TAS, IAS decreases with increase in altitude<\/li><li>TAS is same as IAS at Mean sea level under ISA conditions<\/li><\/ul>\n\n\n\n<h3>Effect of Pitot Line Blockage in Level Flight<\/h3>\n\n\n\n<ul><li>Change in pitot pressure will not be sensed if pitot line is blocked<\/li><li>ASI will not register any change of speed<\/li><li>Indication remains constant for sometime and slowly reduces to zero speed<\/li><\/ul>\n\n\n\n<h3>Effect of Pitot Line Blockage during Climb and Descent<\/h3>\n\n\n\n<ul><li>Considering an aircraft maintaining steady speed<\/li><li>During descent, ASI under reads due to increase in static pressure<\/li><li>During Climb, ASI over reads due to decrease in static pressure<\/li><\/ul>\n\n\n\n<h3>Blockage in Static Line during Level Flight<\/h3>\n\n\n\n<ul><li>Change in Static Pressure will not be sensed if static line is blocked<\/li><li>Static Pressure in the instrument case will remain constant<\/li><li>ASI would read correctly if there is no change in altitude<\/li><\/ul>\n\n\n\n<h3>Blockage in Static Line in Climb and Descent<\/h3>\n\n\n\n<ul><li>Considering an aircraft maintaining steady speed<\/li><li>During descent, ASI over reads due to increase in static component<\/li><li>During climb, ASI under reads due to decrease in static component<\/li><\/ul>\n\n\n\n<h3>Leaks in Pitot System<\/h3>\n\n\n\n<ul><li>Leak in pitot line reduces the pitot pressure sensed by the system<\/li><li>Naturally, Air speed indicator would under read<\/li><\/ul>\n\n\n\n<h3>Leaks in Static System<\/h3>\n\n\n\n<ul><li>Leaks outside pressurized hull in static line does not cause any error<\/li><li>Static pressure will be correctly sensed by the instrument<\/li><li>Leaks inside pressurized hull in static line will sense cabin pressure as static<\/li><li>Cabin altitude in normally expected to be higher than actual static pressure<\/li><li>ASI would under read due to lesser capsule expansion<\/li><\/ul>\n\n\n\n<h3>Error Summary<\/h3>\n\n\n\n<ul><li>Considering that the aircraft is maintaining same speed<\/li><li>If pitot line is blocked, ASI under reads in descent<\/li><li>IAS reduces as altitude decrease in the altimeter<\/li><li>If static line is blocked, ASI over reads in descent<\/li><li>IAS increase as altitude decrease in the altimeter<\/li><\/ul>\n\n\n\n<h2>Share this Page<\/h2>\n","protected":false},"excerpt":{"rendered":"<p>Air Speed Indicator Purpose of Air Speed Indicator Air Speed Indicator is designed to detect dynamic pressure ASI measures dynamic pressure and displays it as air speed Units of measurement are normally Knots or Kilometers per hour Principle of Air Speed Indicator ASI senses Pitot pressure and static pressure Pitot pressure is a combination of dynamic and static Dynamic pressure is obtained by subtracting static pressure Dynamic pressure = Pitot pressure \u00e2\u20ac\u201c Static pressure Formula for Dynamic pressure = \u00c2\u00bd density x velocity2 Construction of ASI Static pressure is provided&hellip;<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"disable_featured_image":false},"categories":[345],"tags":[],"_links":{"self":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14460"}],"collection":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/comments?post=14460"}],"version-history":[{"count":0,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14460\/revisions"}],"wp:attachment":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/media?parent=14460"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/categories?post=14460"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/tags?post=14460"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}