{"id":14470,"date":"2025-06-07T16:16:24","date_gmt":"2025-06-07T10:46:24","guid":{"rendered":"https:\/\/ibexaviation.com\/pilot-training\/?p=14470"},"modified":"2026-07-26T10:27:10","modified_gmt":"2026-07-26T04:57:10","slug":"machmeter","status":"publish","type":"post","link":"https:\/\/ibexaviation.com\/pilot-training\/machmeter\/","title":{"rendered":"Machmeter"},"content":{"rendered":"<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-1.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<h2>Measurement of Mach Number using Machmeter<\/h2>\n<p>A Mach meter tells pilots how fast the aircraft is flying relative to the speed of sound, rather than just its speed through the air. This becomes especially important at high altitudes and high speeds.<\/p>\n<h3>Local Speed of Sound<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-2.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The speed of sound is not constant and varies with temperature.<\/li>\n<li>Sound travels slower in colder air and faster in warmer air.<\/li>\n<li>The speed of sound decreases with increasing altitude.<\/li>\n<li>Local Speed of Sound (LSS) at sea level in ISA (+15&deg;C) is <strong>661 knots<\/strong>.<\/li>\n<li>Local Speed of Sound (LSS) at 30,000 ft in ISA (-45&deg;C) is <strong>589 knots<\/strong>.<\/li>\n<\/ul>\n<h3>Calculation of Local Speed of Sound<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-3.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li><strong>Formula:<\/strong><\/li>\n<\/ul>\n<p><strong>Local Speed of Sound (knots) = 38.95 \u00d7 \u221a(Absolute Temperature in Kelvin)<\/strong><\/p>\n<h3>Definition of Mach Number<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-4.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li><strong>Mach Number = True Air Speed (TAS) \u00f7 Local Speed of Sound (LSS)<\/strong>.<\/li>\n<li>Mach Number is the ratio of the aircraft&#8217;s speed to the local speed of sound.<\/li>\n<li>It can also be expressed as the ratio between dynamic and static pressures.<\/li>\n<li><strong>Mach 0.85<\/strong> means the aircraft is flying at <strong>85% of the local speed of sound<\/strong>.<\/li>\n<\/ul>\n<h3>Importance of Mach Number<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-5.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Shock waves form when an aircraft approaches the local speed of sound.<\/li>\n<li>These shock waves create handling difficulties.<\/li>\n<li>Lift decreases.<\/li>\n<li>Drag increases.<\/li>\n<li>Pitch changes occur.<\/li>\n<li>Buffeting may occur.<\/li>\n<li>Loss of control effectiveness is possible.<\/li>\n<\/ul>\n<h3>Construction of Machmeter<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-6.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The Machmeter displays True Air Speed as Mach Number by comparing dynamic and static pressures.<\/li>\n<li>It contains two capsules:<\/li>\n<ul>\n<li>Dynamic pressure (airspeed) capsule.<\/li>\n<li>Static pressure (altitude) capsule.<\/li>\n<\/ul>\n<li>The ratio of static and dynamic pressure is compared.<\/li>\n<li>This moves the ratio arm, which drives the ranging arm and pointer.<\/li>\n<\/ul>\n<h2>Errors and Resolution of Errors in a Machmeter<\/h2>\n<h3>Instrument and Position Errors<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-7.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Instrument errors are unavoidable due to manufacturing imperfections.<\/li>\n<li>Errors increase at higher altitudes because pressure changes become smaller.<\/li>\n<li>Position (pressure) error is caused by turbulence around the static source.<\/li>\n<li>Using separate static vents reduces the error.<\/li>\n<li>Advanced pitot-static systems on high-speed aircraft further reduce the error.<\/li>\n<\/ul>\n<h3>Manoeuvre-Induced Errors<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-8.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Short-term fluctuations occur due to changes in pitch or yaw.<\/li>\n<\/ul>\n<h3>Density and Temperature Errors<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-9.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The Machmeter indicates speed as a ratio.<\/li>\n<li>Therefore, it is not affected by density or temperature errors.<\/li>\n<li>Temperature and density changes affect both static and dynamic pressures equally.<\/li>\n<li>Their effects cancel each other.<\/li>\n<\/ul>\n<h3>Compressibility Errors<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-10.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The Machmeter measures the ratio between aircraft speed and the speed of sound.<\/li>\n<li>Therefore, it does not suffer from compressibility errors.<\/li>\n<li>Compressibility depends on the ratio of dynamic to static pressure.<\/li>\n<li>The Machmeter is specifically designed to measure this ratio.<\/li>\n<\/ul>\n<h2>Effect of Blockage and Leaks on Machmeter <\/p>\n<h3>Blockage in Pitot Line<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-11.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>A blocked pitot line prevents changes in pitot pressure from reaching the instrument.<\/li>\n<li>The Machmeter will not register changes in speed.<\/li>\n<li>In level flight, the indication remains constant.<\/li>\n<li>During a constant-speed climb, the Machmeter <strong>over-reads<\/strong>.<\/li>\n<li>During a constant-speed descent, the Machmeter <strong>under-reads<\/strong>.<\/li>\n<\/ul>\n<h3>Blockage in Static Vent<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-12.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>A blocked static system keeps static pressure constant.<\/li>\n<li>Mach indication remains correct during level flight.<\/li>\n<li>During a constant-speed climb, the Machmeter <strong>under-reads<\/strong>.<\/li>\n<li>During a constant-speed descent, the Machmeter <strong>over-reads<\/strong>.<\/li>\n<\/ul>\n<h3>Leaks in Static Line (Outside Pressurised Cabin)<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-13.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>A leak in the pitot line reduces pitot pressure and causes the Machmeter to <strong>under-read<\/strong>.<\/li>\n<li>A leak in the static line outside the pressurised cabin causes no error because correct static pressure is still sensed.<\/li>\n<\/ul>\n<h3>Leaks in Static Line (Inside Pressurised Cabin)<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-14.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>A leak inside the pressurised cabin causes cabin pressure to be sensed instead of outside static pressure.<\/li>\n<li>Cabin pressure is normally higher than actual static pressure.<\/li>\n<li>The Machmeter <strong>under-reads during climb<\/strong>.<\/li>\n<li>The Machmeter <strong>over-reads during descent<\/strong>.<\/li>\n<\/ul>\n<h2>Comparison between IAS, TAS and Mach Number under varied atmospheric conditions <\/p>\n<h3>Constant CAS Climb in Standard Conditions<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-15.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>During a constant CAS climb, True Air Speed (TAS) increases with altitude.<\/li>\n<li>This occurs because air density decreases with altitude.<\/li>\n<li>Density is primarily affected by atmospheric pressure.<\/li>\n<\/ul>\n<h3>Constant TAS Climb in Standard Conditions<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-16.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>During a constant TAS climb, Mach Number increases with altitude.<\/li>\n<li>Local Speed of Sound decreases with altitude.<\/li>\n<li>The speed of sound is lower in colder air.<\/li>\n<\/ul>\n<h3>Climb Planning for High-Performance Aircraft<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-17.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Constant CAS is maintained at lower altitudes.<\/li>\n<li>Constant Mach Number is maintained at higher altitudes.<\/li>\n<\/ul>\n<h3>Descent in Standard Conditions<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-18.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>During a constant Mach descent, TAS increases at lower altitudes.<\/li>\n<li>TAS increases to match the higher Local Speed of Sound.<\/li>\n<li>CAS also increases because air density is higher at lower altitudes.<\/li>\n<\/ul>\n<h3>Descent Plan for High-Performance Aircraft<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-19.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Constant Mach Number is maintained at higher altitudes.<\/li>\n<li>Constant CAS is maintained at lower altitudes.<\/li>\n<li>In ISA conditions:<\/li>\n<\/ul>\n<p><strong>CAS &lt; TAS &lt; Mach Number<\/strong><\/p>\n<h3>Mach Number and TAS in an Isothermal Layer<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-20.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Mach Number remains constant with height.<\/li>\n<li>Temperature remains constant.<\/li>\n<li>The ratio between TAS and Local Speed of Sound remains constant.<\/li>\n<\/ul>\n<h3>Constant TAS Climb and Descent in an Isothermal Layer<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-21.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>During a constant TAS climb, CAS decreases because density decreases.<\/li>\n<li>During a constant TAS descent, CAS increases because density increases.<\/li>\n<\/ul>\n<h3>Constant CAS Climb and Descent in an Isothermal Layer<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-22.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>During a constant CAS climb, TAS increases because density decreases.<\/li>\n<li>During a constant CAS descent, TAS decreases because density increases.<\/li>\n<\/ul>\n<h3>Local Speed of Sound in an Inversion Layer<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-23.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Local Speed of Sound increases with altitude because temperature increases.<\/li>\n<li>During a constant TAS climb, Mach Number decreases.<\/li>\n<li>During a constant TAS descent, Mach Number increases.<\/li>\n<\/ul>\n<h3>Constant CAS Climb in an Inversion Layer<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-24.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>TAS increases with altitude.<\/li>\n<li>Density decreases with altitude.<\/li>\n<li>Atmospheric pressure has a greater effect on density than temperature.<\/li>\n<\/ul>\n<h3>Summary of CAS and TAS<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-25.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>During a constant CAS climb, TAS always increases.<\/li>\n<li>During a constant TAS climb, CAS always decreases.<\/li>\n<li>Pressure has a greater effect on air density than temperature.<\/li>\n<\/ul>\n<h3>Summary of CAS, TAS and Mach Number<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-26.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Pressure has a greater effect on air density than temperature.<\/li>\n<li>During a constant Mach climb, CAS always decreases.<\/li>\n<li>During a constant CAS climb, Mach Number always increases.<\/li>\n<\/ul>\n<h2>Mach\u2013Airspeed Indicator<\/h2>\n<h3>Utility of a Mach\u2013Airspeed Indicator<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter27.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Combines the functions of a Machmeter and an Airspeed Indicator.<\/li>\n<li>An analogue instrument displays Airspeed on a fixed scale and Mach Number on a movable scale.<\/li>\n<li>Digital systems display both Mach Number and Airspeed using an Air Data Computer (ADC).<\/li>\n<\/ul>\n<h3>Uses of Mach\u2013Airspeed Indicator<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-28.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Provides Calibrated (Rectified) Airspeed instead of Indicated Airspeed.<\/li>\n<li>Includes a manually adjustable speed limiter pointer.<\/li>\n<li>Displays:<\/li>\n<ul>\n<li>Maximum Operating Mach Number (M<sub>MO<\/sub>).<\/li>\n<li>Maximum Operating Speed (V<sub>MO<\/sub>).<\/li>\n<\/ul>\n<\/ul>\n<h3>Errors of Mach\u2013Airspeed Indicator<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/flight-instruments\/i9-machmeter-29.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Combines the errors of both the Airspeed Indicator and Machmeter.<\/li>\n<li>Instrument errors.<\/li>\n<li>Position (pressure) errors.<\/li>\n<li>Manoeuvre-induced errors.<\/li>\n<li>Density errors.<\/li>\n<li>Temperature errors.<\/li>\n<li>Compressibility errors.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Measurement of Mach Number using Machmeter A Mach meter tells pilots how fast the aircraft is flying relative to the speed of sound, rather than just its speed through the air. This becomes especially important at high altitudes and high speeds. Local Speed of Sound The speed of sound is not constant and varies with temperature. Sound travels slower in colder air and faster in warmer air. The speed of sound decreases with increasing altitude. Local Speed of Sound (LSS) at sea level in ISA (+15&deg;C) is 661 knots. Local&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,"footnotes":""},"categories":[345],"tags":[],"class_list":["post-14470","post","type-post","status-publish","format-standard","hentry","category-cpl-atpl-flight-instruments"],"_links":{"self":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14470","targetHints":{"allow":["GET"]}}],"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=14470"}],"version-history":[{"count":1,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14470\/revisions"}],"predecessor-version":[{"id":16654,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14470\/revisions\/16654"}],"wp:attachment":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/media?parent=14470"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/categories?post=14470"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/tags?post=14470"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}