{"id":26685014,"date":"2026-10-05T12:10:02","date_gmt":"2026-10-05T10:10:02","guid":{"rendered":"https:\/\/migflug.com\/afterburner\/airspeed-explained-ias-tas-groundspeed-mach-number\/"},"modified":"2026-10-05T12:10:45","modified_gmt":"2026-10-05T10:10:45","slug":"airspeed-explained-ias-tas-groundspeed-mach-number","status":"publish","type":"post","link":"https:\/\/migflug.com\/afterburner\/airspeed-explained-ias-tas-groundspeed-mach-number\/","title":{"rendered":"Airspeed Explained: IAS, TAS, Groundspeed and Mach"},"content":{"rendered":"\n<style>.et_pb_title_container h1.entry-title { padding-top: 40px !important; }<\/style>\n\n<!-- mfsh:top -->\n\n<style>\n.mfsh-trigger{display:inline-flex;align-items:center;gap:10px;height:44px;padding:0 18px;margin:0 0 26px;border:2px solid #0F1720;background:#fff;color:#0F1720;font-family:inherit;font-size:14px;font-weight:700;line-height:1;letter-spacing:0;text-transform:none;cursor:pointer;box-shadow:none;transition:background .15s,color 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The cockpit of a jet can show four or five different numbers that all claim to be its speed, and at cruising altitude they can differ by hundreds of knots.<\/p>\n<p>None of them is wrong. Each answers a different question, and pilots use each one for a different job. Here is what indicated airspeed, true airspeed, groundspeed and Mach number actually mean, and why a fighter at high altitude can be flying far faster than its airspeed indicator says.<\/p>\n\n<div style=\"background:#f4f6f9;border:1px solid #e2e7ee;padding:18px 22px;margin:22px 0 26px\"><p style=\"margin:0 0 8px;font-weight:700;color:#0d1117\">Quick Facts<\/p><ul style=\"margin:0;padding-left:18px\"><li style=\"margin:4px 0\"><strong>IAS:<\/strong> Indicated airspeed, read from the pitot-static system; the pilot's main reference for flying the aircraft<\/li><li style=\"margin:4px 0\"><strong>CAS and EAS:<\/strong> IAS corrected for instrument and position error, then for compressibility<\/li><li style=\"margin:4px 0\"><strong>TAS:<\/strong> True airspeed, the actual speed through the air mass<\/li><li style=\"margin:4px 0\"><strong>Groundspeed:<\/strong> Speed over the ground, TAS plus or minus the wind<\/li><li style=\"margin:4px 0\"><strong>Mach:<\/strong> Speed as a fraction of the local speed of sound<\/li><li style=\"margin:4px 0\"><strong>Rule of thumb:<\/strong> IAS reads about 2% less than TAS for every 1,000 feet of altitude<\/li><\/ul><\/div>\n\n<h2 style=\"padding-top:22px\">Indicated airspeed: what the wing feels<\/h2>\n<p>The most important number in the cockpit is indicated airspeed, or IAS. It comes from the pitot-static system: a pitot tube facing into the airflow measures total pressure, a static port measures the still air pressure around the aircraft, and the difference between the two is the dynamic pressure of the air rushing past.<\/p>\n<p>The airspeed indicator turns that pressure into a speed, but it has to make an assumption: it is calibrated as if the aircraft were always at sea level in a standard atmosphere. As the aircraft climbs into thinner air, the same speed produces less dynamic pressure, so the indicator reads lower than the aircraft is really going.<\/p>\n<p>That sounds like a flaw, but it is exactly why pilots fly by it. A wing produces lift according to dynamic pressure, and so does the airspeed indicator. An aircraft will stall at the same indicated airspeed for a given configuration, whatever its altitude or true speed. That is why flight manuals publish stall, approach and limit speeds as indicated airspeeds, and why air traffic control gives speed instructions in IAS.<\/p>\n\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/A35XwKW1Ivc\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\n\n<p style=\"font-size:14px;color:#666;font-style:italic\">Boldmethod explains why true airspeed and indicated airspeed are different.<\/p>\n<h2 style=\"padding-top:22px\">Calibrated, equivalent and true<\/h2>\n<p>Between the raw indication and the real speed sit a series of corrections, which pilots remember with the acronym ICE-T. Calibrated airspeed (CAS) is IAS corrected for instrument errors and for the position of the sensors on the airframe. Equivalent airspeed (EAS) also corrects for the compressibility of air, which matters at high speed. True airspeed (TAS) then corrects for the actual air density.<\/p>\n<p>True airspeed is the aircraft's real speed through the air mass around it, and it is what pilots use for navigation and flight planning. The gap grows with altitude. As a rule of thumb, the airspeed indicator reads about 2% less than true airspeed for every 1,000 feet of altitude. An aircraft at 15,000 feet in a standard atmosphere showing 100 knots indicated is actually flying through the air at about 126 knots. Modern aircraft let an air data computer do the sums and simply display TAS.<\/p>\n\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/10\/airspeed-indicator-knots-v-speeds.jpg\" alt=\"An aircraft airspeed indicator showing knots with coloured arcs for V-speeds\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">A classic airspeed indicator, reading in knots. The coloured arcs mark speeds such as the stall, flap and never-exceed limits, all expressed as indicated airspeed. Photo: Ahunt via Wikimedia Commons (public domain).<\/figcaption><\/figure>\n\n<h2 style=\"padding-top:22px\">Groundspeed: what the map sees<\/h2>\n<p>Neither indicated nor true airspeed tells you how fast you are crossing the ground, because the air mass itself is moving. Groundspeed is true airspeed with the wind added or subtracted. An airliner cruising at 480 knots true into a 100-knot jet stream headwind covers the ground at only 380 knots; turn around and it makes 580 knots.<\/p>\n<p>Groundspeed is what decides arrival times and fuel planning, and with GPS it is easy to display. But it tells the pilot nothing about how the wing is doing. A slow light aircraft flying into a headwind almost as strong as its airspeed could show nearly zero groundspeed while flying perfectly happily.<\/p>\n<h2 style=\"padding-top:22px\">Mach: the speed that matters up high<\/h2>\n<p>At high speed and high altitude, another number takes over: the Mach number, the aircraft's true airspeed as a fraction of the local speed of sound. Mach 1 is the speed of sound itself.<\/p>\n<p>The catch is that the speed of sound is not fixed. It depends on air temperature. In the standard atmosphere it is about 661 knots at sea level, but in the cold air above about 36,000 feet it falls to around 573 knots. So Mach 0.85 at cruising altitude is a slower true airspeed than Mach 0.85 at sea level.<\/p>\n<p>Jets fly by Mach number at altitude because the dangerous effects of high speed, shock waves forming on the wing, buffeting and loss of control, depend on how close the airflow gets to the speed of sound. Jet aircraft therefore have two speed limits, a maximum operating speed in knots and a maximum Mach number, and need both an airspeed indicator and a Machmeter.<\/p>\n\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/MC3dujG2Viw\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\n\n<p style=\"font-size:14px;color:#666;font-style:italic\">Why jets switch from knots to Mach number at high altitude.<\/p>\n\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/10\/concorde-cabin-machmeter-0-95-1984.jpg\" alt=\"The cabin Machmeter on Concorde G-BOAB reading Mach 0.95 shortly before going supersonic in 1984\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">The passenger Machmeter on the cabin bulkhead of Concorde G-BOAB reads Mach 0.95 on a flight from London to Cairo in March 1984, moments before the aircraft went supersonic. Photo: M McBey via Wikimedia Commons, CC BY 2.0.<\/figcaption><\/figure>\n\n<h2 style=\"padding-top:22px\">Why a fighter can be faster than it looks<\/h2>\n<p>Put it all together and you get some surprising numbers. A fighter at 40,000 feet may show only a modest indicated airspeed while its true airspeed is far higher, and with a tailwind its groundspeed higher still. Conversely, a jet flying very fast at low level will show a high indicated airspeed, because the dense air is hammering on its pitot tube, even though its Mach number may be lower than it routinely reaches at altitude.<\/p>\n<p>That is why fighter pilots talk about speed in different ways depending on the situation: knots indicated for take-off, landing and manoeuvring, Mach number for high-altitude intercepts and supersonic runs. The aircraft is the same. The question being asked is different.<\/p>\n\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/E5Kj_c53kZo\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\n\n<p style=\"font-size:14px;color:#666;font-style:italic\">Captain Joe's guide to every type of airspeed pilots need to know.<\/p>\n<p>And a compact overview of all five speeds in one place.<\/p>\n<p>IAS, CAS, TAS, groundspeed and Mach, explained by JxJ Aviation.<\/p>\n\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/AeCTY_6nijU\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\n\n<p><em>Sources: Wikipedia (Indicated airspeed, True airspeed, Airspeed indicator, Mach number), Boldmethod, Captain Joe, JxJ Aviation, PilotClimb<\/em><\/p>\n<!-- mfsh:bottom -->\n\n<button type=\"button\" class=\"mfsh-trigger mfsh-bottom\" data-mfsh-placement=\"bottom\" aria-haspopup=\"dialog\"><svg><use href=\"#mfsh-i-share\"\/><\/svg>Share this story<\/button>\n\n<!-- \/mfsh:bottom -->\n\n<style>.mfq{margin:34px 0 8px}.mfq h2{font:26px\/1.3 \"Gilroy semiBold\",Helvetica,Arial,sans-serif;color:#0d1117;padding-top:22px;margin:0 0 6px}.mfq .qa details{border-top:1px solid #5C91FF;margin:0;padding:0}.mfq .qa details:last-of-type{border-bottom:1px solid #e2e7ee}.mfq .qa summary{cursor:pointer;list-style:none;display:flex;justify-content:space-between;align-items:center;gap:16px;padding:22px 26px;font:18px\/1.7 \"Gilroy semiBold\",Helvetica,Arial,sans-serif;color:#0d1117}.mfq .qa summary::-webkit-details-marker{display:none}.mfq .qa summary::after{content:\"+\";font:24px Helvetica,sans-serif;color:#3568e0;flex:0 0 auto}.mfq .qa details[open] summary::after{content:\"\\2013\"}.mfq .qa details:hover summary{color:#3568e0}.mfq .qa .a{font:16px\/1.7 \"Gilroy regular\",Helvetica,Arial,sans-serif;color:#454e5e;padding:0 26px 24px}.mfq .qa .a a{color:#3568e0}@media(max-width:680px){.mfq .qa summary{padding:18px 14px;font-size:17px}.mfq .qa .a{padding:0 14px 20px}}<\/style>\n\n\n<section class=\"mfq\"><h2>Frequently Asked Questions<\/h2><div class=\"qa\"><details open><summary>What is the difference between indicated airspeed and true airspeed?<\/summary><div class=\"a\">Indicated airspeed (IAS) is read from the pitot-static system, which is calibrated for sea-level air density. True airspeed (TAS) is the aircraft's actual speed through the air. As air gets thinner with altitude, IAS reads lower than TAS, by roughly 2% per 1,000 feet.<\/div><\/details><details><summary>Why do pilots fly by indicated airspeed?<\/summary><div class=\"a\">Because a wing's lift depends on dynamic pressure, which is also what the airspeed indicator measures. An aircraft stalls at the same indicated airspeed for a given configuration regardless of altitude, so flight manuals and air traffic control use indicated speeds.<\/div><\/details><details><summary>What is groundspeed?<\/summary><div class=\"a\">Groundspeed is the aircraft's speed over the ground. It is the true airspeed plus a tailwind or minus a headwind. Groundspeed decides arrival times and fuel planning, but says nothing about how the wing is performing.<\/div><\/details><details><summary>What does ICE-T stand for in aviation?<\/summary><div class=\"a\">ICE-T is a memory aid for the types of airspeed: Indicated airspeed, Calibrated airspeed (corrected for instrument and position errors), Equivalent airspeed (corrected for compressibility) and True airspeed (corrected for air density).<\/div><\/details><details><summary>Why is Mach 1 slower at high altitude?<\/summary><div class=\"a\">The speed of sound depends on air temperature. In the standard atmosphere it is about 661 knots at sea level but falls to around 573 knots in the cold air above about 36,000 feet, so the same Mach number means a lower true airspeed at altitude.<\/div><\/details><details><summary>Why do jets use Mach number at high altitude?<\/summary><div class=\"a\">At high speed, effects such as shock waves on the wing, buffeting and control problems depend on how close the airflow is to the speed of sound. Jets therefore have a maximum Mach number as well as a maximum speed in knots, and fly by Mach at cruising altitude.<\/div><\/details><\/div><\/section>\n\n\n<script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[{\"@type\":\"Question\",\"name\":\"What is the difference between indicated airspeed and true airspeed?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Indicated airspeed (IAS) is read from the pitot-static system, which is calibrated for sea-level air density. 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Groundspeed decides arrival times and fuel planning, but says nothing about how the wing is performing.\"}},{\"@type\":\"Question\",\"name\":\"What does ICE-T stand for in aviation?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"ICE-T is a memory aid for the types of airspeed: Indicated airspeed, Calibrated airspeed (corrected for instrument and position errors), Equivalent airspeed (corrected for compressibility) and True airspeed (corrected for air density).\"}},{\"@type\":\"Question\",\"name\":\"Why is Mach 1 slower at high altitude?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"The speed of sound depends on air temperature. 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Jets therefore have a maximum Mach number as well as a maximum speed in knots, and fly by Mach at cruising altitude.\"}}]}<\/script><div style=\"background:#f0f4ff;border-left:4px solid #5C91FF;padding:16px 20px;margin:32px 0 8px\"><p style=\"margin:0 0 8px;font-weight:600;color:#333\">Related Posts<\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/far-91-117-250-knot-speed-limit-below-10000-feet\/\">FAR 91.117: Why Airliners Slow to 250 Knots Below 10,000 Feet<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/mig-21-nose-cone-inlet-shock-cone-explained\/\">MiG-21 Nose Cone Explained: How the Shock Cone Feeds a Mach 2 Jet<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/boeing-747-g-onee-aaib-open-door-rain-unreliable-airspeed\/\">Boeing 747 G-ONEE: An Open Door in the Rain Cost All Airspeed<\/a><\/p><\/div>\n","protected":false},"excerpt":{"rendered":"<p>Share this story Ask a pilot how fast the aircraft is going and you may get a question back: which speed do you mean? The cockpit of a jet can show four or five different numbers that all claim to be its speed, and at cruising altitude they can differ by hundreds of knots. None [&hellip;]<\/p>\n","protected":false},"author":23,"featured_media":26677880,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"editor_notices":[],"footnotes":""},"categories":[665],"tags":[],"class_list":["post-26685014","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aviation-world"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.6 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Airspeed Explained: IAS, TAS, Groundspeed and Mach | Afterburner - MiGFlug&#039;s Magazine<\/title>\n<meta name=\"description\" content=\"Indicated vs true airspeed, groundspeed and Mach number explained: why pilots fly by IAS, why TAS grows with altitude, and why Mach 1 is slower up high.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/migflug.com\/afterburner\/airspeed-explained-ias-tas-groundspeed-mach-number\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Airspeed Explained: IAS, TAS, Groundspeed and Mach | Afterburner - 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