{"id":31503411,"date":"2026-11-06T17:00:29","date_gmt":"2026-11-06T16:00:29","guid":{"rendered":"https:\/\/migflug.com\/afterburner\/safe-life-fail-safe-design-damage-tolerance-f-111-comet-25-571\/"},"modified":"2026-11-06T17:01:23","modified_gmt":"2026-11-06T16:01:23","slug":"safe-life-fail-safe-design-damage-tolerance-f-111-comet-25-571","status":"publish","type":"post","link":"https:\/\/migflug.com\/afterburner\/it\/safe-life-fail-safe-design-damage-tolerance-f-111-comet-25-571\/","title":{"rendered":"Safe-Life vs Fail-Safe Design: What the Comet and F-111 Taught"},"content":{"rendered":"<style>.et_pb_title_container h1.entry-title { padding-top: 40px !important; }<\/style>\r\n<!-- mfsh:top -->\r\n\r\n<style>\r\n.mfsh-trigger{display:inline-flex;align-items:center;gap:10px;height:44px;padding:0 18px;margin:0 0 26px;border:2px solid 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The culprit was a flaw about an inch long, forged into a steel fitting at the factory and missed repeatedly by inspectors during manufacture.<\/p><p>That accident did more than ground a fleet. It finished the argument over how aircraft structures should be designed, and it pushed the US Air Force to the philosophy that now underpins both military and civil structural rules: damage tolerance. To understand why, you have to know the two ideas that came before it.<\/p>\r\n\r\n<div style=\"background:#f4f6f9;border-left:4px solid #5C91FF;padding:18px 22px;margin:8px 0 28px\"><p style=\"margin:0 0 8px;font-weight:700;color:#0d1117\">Quick Facts<\/p><ul style=\"margin:0;padding-left:18px;line-height:1.7\"><li><strong>Safe-life:<\/strong> Part is retired before a detectable fatigue crack is expected<\/li><li><strong>Fail-safe:<\/strong> Structure carries load after one element fails<\/li><li><strong>Damage tolerance:<\/strong> Assumes flaws exist; inspections catch them before they become critical<\/li><li><strong>Key civil rule:<\/strong> 14 CFR 25.571, damage-tolerance and fatigue evaluation<\/li><li><strong>Key military standard:<\/strong> MIL-STD-1530, first issued September 1972<\/li><li><strong>USAF ASIP origin:<\/strong> B-47 fatigue break-ups of 1958<\/li><li><strong>F-111 accident:<\/strong> 22 December 1969, Nellis AFB, wing pivot fitting<\/li><li><strong>F-111 cold proof test:<\/strong> -40\u00b0F, to clear D6AC steel wing carry-through<\/li><\/ul><\/div>\r\n\r\n<h2 style=\"padding-top:22px\">Safe-life: retire it before it cracks<\/h2>\r\n<p>The oldest answer to fatigue is the simplest. Work out, by analysis and test, how many load cycles a part can take before a crack is likely to appear, divide that life by a generous scatter factor, and throw the part away before it gets there. That is safe-life design.<\/p>\r\n<p>Its weakness is that it assumes the part starts perfect. A safe-life component is designed so that, with an extremely low accepted level of risk, it will never form a detectable fatigue crack during its service life, which usually means running it at stresses well below what the material could carry. The classic example is the helicopter rotor blade: it sees so many cycles that a crack could grow to critical length within a single flight, before any inspection could find it.<\/p>\r\n<p>For transport aircraft, safe-life survives today as the fallback. Under 14 CFR 25.571(c), damage-tolerance compliance is not required for particular structure if the applicant establishes that it is impractical, and the regulation then sets a safe-life standard instead.<\/p>\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #5C91FF;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><div><em>&ldquo;This structure must be shown by analysis, supported by test evidence, to be able to withstand the repeated loads of variable magnitude expected during its service life without detectable cracks. Appropriate safe-life scatter factors must be applied.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>14 CFR 25.571(c)<\/strong> &mdash; Code of Federal Regulations, Fatigue (safe-life) evaluation (eCFR, current as of 4 November 2026)<\/div><\/div><\/div>\r\n\r\n<h2 style=\"padding-top:22px\">Fail-safe: assume something breaks<\/h2>\r\n<p>The <a href=\"https:\/\/migflug.com\/afterburner\/de-havilland-comet-metal-fatigue-square-windows-investigation-1954\/\">de Havilland Comet<\/a> disasters of 1954 shook the industry. G-ALYP crashed into the Mediterranean off Elba in January, and G-ALYY went down near Naples in April. A complete Comet fuselage was then pressurised again and again in a water tank built for the purpose at Farnborough, until it split after 3,057 flight cycles, 1,221 real and 1,836 simulated.<\/p>\r\n\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0 8px\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/skw2TZAIlCI\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div><p style=\"font-size:13px;color:#777;text-align:center;margin:0 0 24px;font-style:italic\">Boeing's full-scale fatigue test of the 777X: the modern descendant of the Comet water tank, cycling a complete airframe through simulated flights.<\/p>\r\n\r\n<p>Wikipedia's account of the inquiry notes that afterwards aircraft were designed to fail-safe or safe-life standards. Fail-safe design accepts that a part may break and asks a different question: can the rest of the structure carry the load until the damage is found? The answer is multiple load paths, so the failure of one spar cap or one skin panel does not bring the aircraft down, and crack-stopping features that keep a running crack from tearing across the whole structure.<\/p>\r\n<p>The idea is far older than the jet age. Wikipedia's article on damage tolerance opens its history with a note Leonardo da Vinci wrote when describing his flying machine.<\/p>\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #5C91FF;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><div><em>&ldquo;In constructing wings one should make one chord to bear the strain and a looser one in the same position so that if one breaks under the strain, the other is in the position to serve the same function.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>Leonardo da Vinci<\/strong> &mdash; Describing his flying machine, as quoted in Wikipedia's Damage tolerance article<\/div><\/div><\/div>\r\n\r\n<h2 style=\"padding-top:22px\">The B-47 and the birth of ASIP<\/h2>\r\n<p>The US Air Force learned the same lesson in its own way. The Damage Tolerant Design Handbook, the Air Force-sponsored reference on the subject, traces its Aircraft Structural Integrity Program to the <a href=\"https:\/\/migflug.com\/aircraft\/b-47-stratojet\/\">Boeing B-47 Stratojet<\/a>. Designed as a high-altitude bomber, it was flown from 1957 in punishing low-level training missions, including toss-bombing manoeuvres, and its real loading spectrum changed dramatically.<\/p>\r\n<p>On 13 March 1958, two B-47Bs broke up in flight in separate incidents, with failures starting in the lower wing skin, and three more in-flight accidents followed while those investigations were still running. The Air Force ran three concurrent fatigue test programmes, at Boeing Wichita, Douglas Tulsa and the NACA laboratory at Langley, and the results became the keystone of ASIP.<\/p>\r\n<p>Even then the lessons came hard. During the full-scale fatigue test of the <a href=\"https:\/\/migflug.com\/aircraft\/f-105-thunderchief\/\">F-105D<\/a> at Wright Field, the main wing carry-through frame failed at less than 20 per cent of one lifetime, and a redesigned frame later failed at 116 per cent of one lifetime against a requirement for four.<\/p>\r\n<h2 style=\"padding-top:22px\">The F-111 and the end of the argument<\/h2>\r\n<p>Then came the <a href=\"https:\/\/migflug.com\/aircraft\/f-111-aardvark\/\">F-111<\/a>. Cracks in its D6AC steel wing attachment structure had already appeared during ground fatigue testing in 1968, according to the Defense Systems Information Analysis Center. On 22 December 1969, the left wing pivot fitting of F-111 No. 94 failed during a 4.0 g pull-up at Nellis Air Force Base, and the aircraft and its crew of two were lost.<\/p>\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #5C91FF;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><div><em>&ldquo;The resulting investigation during 1970 revealed the cause to be the catastrophic fracture of the D6ac high strength steel outer wing pivot fitting due to the presence of a manufacturing-introduced one inch surface crack that had been missed repeatedly by NDI during fabrication\u2026&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>D. Forney<\/strong> &mdash; AFRL-ML-WP-TR-2006-4145, as quoted by the Defense Systems Information Analysis Center, June 2019<\/div><\/div><\/div>\r\n\r\n<p>The fitting had been through production inspection. That did not matter, because the flaw was there from the start, near a vertical reinforcing rib where production inspections could not see it. A safe-life calculation that assumes a perfect part had nothing to say about a part that was never perfect.<\/p>\r\n\r\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\/11\/safe-life-fail-safe-design-damage-tolerance-f-111-comet-25-5-img1.jpg\" alt=\"AFTI F-111 research aircraft in flight over mountains\" 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 same F-111A airframe in 1985 as the AFTI\/F-111 with mission adaptive wings. The type's variable-sweep wings hung on pivot fittings forged from D6AC steel. Photo: NASA Dryden Flight Research Center (public domain, via DVIDS)<\/figcaption><\/figure>\r\n\r\n<p>The recovery was unlike anything before it. The Damage Tolerant Design Handbook calls the follow-up the largest single material investigation ever, focused on D6AC steel, and says the aircraft were released to fly again on the basis of a cold-proof test combined with the best non-destructive inspection then available. The F-111s were loaded on the ground at minus 40 degrees Fahrenheit to clear the D6AC wing carry-through and its attached components.<\/p>\r\n<p>The logic of a proof test is brutal and elegant. If a part survives a load higher than anything it will see in service, any flaw it contains must be smaller than the size that would have broken it. Fracture mechanics can then estimate how long that largest possible flaw would take to grow to a dangerous size, and the inspection interval is set inside that window. The handbook notes that proof testing works best on high-strength steels and other materials that fracture in a brittle, plane-strain manner, and that this F-111 programme is the notable exception to its otherwise limited use on whole airframes.<\/p>\r\n\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0 8px\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/AXtuEK_1Z54\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div><p style=\"font-size:13px;color:#777;text-align:center;margin:0 0 24px;font-style:italic\">A symposium talk on full-scale fatigue testing of T-38 wings, the kind of ASIP work that keeps ageing military fleets flying.<\/p>\r\n\r\n<h2 style=\"padding-top:22px\">Damage tolerance becomes the rule<\/h2>\r\n<p>A Scientific Advisory Board convened for the F-111 investigation recommended that a damage-tolerant design methodology be used for all future weapon systems. In September 1972 those concepts went into MIL-STD-1530, Aircraft Structural Integrity Program, Airplane Requirements, which also required each aircraft type to have a force structural maintenance plan. MIL-A-83444, Airplane Damage Tolerance Requirements, followed in July 1974, and MIL-STD-1530A in 1975.<\/p>\r\n\r\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\/11\/safe-life-fail-safe-design-damage-tolerance-f-111-comet-25-5-img2.jpg\" alt=\"AFTI F-111 banking above Rogers Dry Lake\" 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 AFTI\/F-111 banks toward Rogers Dry Lake and Edwards Air Force Base in 1986. Photo: NASA Dryden Flight Research Center (public domain, via DVIDS)<\/figcaption><\/figure>\r\n\r\n<p>Damage tolerance turns the old assumptions inside out. It starts by assuming that flaws exist in the structure from day one and that they will grow with use. The designer must show that the largest flaw that could plausibly escape manufacture will not grow to critical size before a scheduled inspection has had a realistic chance of finding it, and the structure must keep enough residual strength in the meantime.<\/p>\r\n<p>Civil rules took the same route. Today 14 CFR 25.571 requires inspection thresholds for single-load-path structure, and for fail-safe structure whose failure might go unnoticed, to be based on crack growth from an initial flaw of the maximum probable size that manufacturing or service could leave behind. The rule also demands a limit of validity, backed by full-scale fatigue test evidence, during which widespread fatigue damage will not occur.<\/p>\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #5C91FF;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><div><em>&ldquo;An evaluation of the strength, detail design, and fabrication must show that catastrophic failure due to fatigue, corrosion, manufacturing defects, or accidental damage, will be avoided throughout the operational life of the airplane.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>14 CFR 25.571(a)<\/strong> &mdash; Code of Federal Regulations, Damage-tolerance and fatigue evaluation of structure (eCFR, current as of 4 November 2026)<\/div><\/div><\/div>\r\n\r\n<h2 style=\"padding-top:22px\">Three philosophies, one airframe<\/h2>\r\n<p>No modern aircraft uses just one of these ideas. A helicopter's rotor blades may be safe-life parts on a damage-tolerant airframe. An airliner's fuselage is built with fail-safe multiple load paths, then managed under a damage-tolerance inspection programme with a published limit of validity. Military fleets run the whole package through ASIP, from the first full-scale fatigue test to the last flight.<\/p>\r\n\r\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\/11\/safe-life-fail-safe-design-damage-tolerance-f-111-comet-25-5-img3.jpg\" alt=\"F-35 airframe covered in safety netting inside a structural test rig\" 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\">An F-35 test airframe inside BAE Systems' structural test rig at Brough, Yorkshire, where a full lifetime of durability loading was applied. Photo: BAE Systems, via Institution of Mechanical Engineers<\/figcaption><\/figure>\r\n\r\n<p>Those full-scale tests have grown enormous. At BAE Systems' site in Brough, Yorkshire, a 350-tonne rig with more than 2,500 strain gauges and 160 loading actuators flew an F-35A airframe through 8,000 hours, one complete lifetime, of simulated flight, according to the Institution of Mechanical Engineers.<\/p>\r\n<p>Afterburner has looked elsewhere at <a href=\"https:\/\/migflug.com\/afterburner\/why-aircraft-are-riveted-not-welded-damage-tolerance\/\">why aircraft are riveted rather than welded<\/a>, and at the <a href=\"https:\/\/migflug.com\/afterburner\/ultimate-load-test-static-test-wing-limit-load-boeing-airbus\/\">ultimate load test<\/a> that proves a wing's static strength. Damage tolerance is the third leg: not how strong the structure is on the day it is built, but how safe it stays once the cracks arrive, as they always do.<\/p>\r\n<p><em>Sources: eCFR (14 CFR 25.571), Damage Tolerant Design Handbook (AFGROW\/LexTech), Defense Systems Information Analysis Center, Wikipedia, Institution of Mechanical Engineers, NASA, DVIDS<\/em><\/p>\r\n<!-- mfsh:bottom -->\r\n\r\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>\r\n\r\n<!-- \/mfsh:bottom -->\r\n\r\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>\r\n\r\n\r\n<section class=\"mfq\"><h2>Frequently Asked Questions<\/h2><div class=\"qa\"><details open><summary>What is safe-life design in aircraft structures?<\/summary><div class=\"a\">Safe-life design retires a part before it is expected to develop a detectable fatigue crack. Engineers establish its life by analysis and test, apply scatter factors and replace it at that limit. It suits parts that are hard to inspect or that crack too fast to catch, such as helicopter rotor blades, but it assumes the part starts without flaws.<\/div><\/details><details><summary>What is fail-safe design in aircraft structures?<\/summary><div class=\"a\">Fail-safe design accepts that a structural element may fail and makes sure the remaining structure can still carry the loads until the damage is found. It relies on multiple load paths and crack-arresting features. The approach spread after the de Havilland Comet accidents of 1954, which were traced to fatigue cracking in the pressurised fuselage.<\/div><\/details><details><summary>What is damage tolerance in aircraft design?<\/summary><div class=\"a\">Damage tolerance assumes that flaws exist in a structure from manufacture and will grow in service. Designers must show that the largest flaw likely to escape detection cannot reach critical size before scheduled inspections have a realistic chance to find it, while the structure keeps enough residual strength. It is required by 14 CFR 25.571 and USAF MIL-STD-1530.<\/div><\/details><details><summary>Why did the F-111 lose a wing in 1969?<\/summary><div class=\"a\">On 22 December 1969 an F-111's left wing pivot fitting failed during a 4.0 g pull-up at Nellis Air Force Base, killing both crew. Investigators found a manufacturing flaw about an inch long in the D6AC steel fitting that inspections had repeatedly missed. The fleet was cleared to fly again after cold proof testing at minus 40 degrees Fahrenheit.<\/div><\/details><details><summary>What is the Aircraft Structural Integrity Program?<\/summary><div class=\"a\">The Aircraft Structural Integrity Program, or ASIP, is the US Air Force framework for keeping airframes structurally safe from design to retirement. It grew out of the B-47 fatigue break-ups of 1958. After the F-111 accident its requirements went into MIL-STD-1530 in September 1972, adding damage-tolerance methods and a structural maintenance plan for each fleet.<\/div><\/details><details><summary>What does 14 CFR 25.571 require?<\/summary><div class=\"a\">14 CFR 25.571 requires transport-category airplane makers to show that catastrophic failure from fatigue, corrosion, manufacturing defects or accidental damage will be avoided throughout the operational life. It sets damage-tolerance evaluation as the default, safe-life only where that is impractical, and requires a limit of validity backed by full-scale fatigue testing against widespread fatigue damage.<\/div><\/details><\/div><\/section>\r\n\r\n\r\n<script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[{\"@type\":\"Question\",\"name\":\"What is safe-life design in aircraft structures?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Safe-life design retires a part before it is expected to develop a detectable fatigue crack. Engineers establish its life by analysis and test, apply scatter factors and replace it at that limit. 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It sets damage-tolerance evaluation as the default, safe-life only where that is impractical, and requires a limit of validity backed by full-scale fatigue testing against widespread fatigue damage.\"}}]}<\/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\/why-aircraft-are-riveted-not-welded-damage-tolerance\/\">A Welded Fuselage Would Be Stronger. That Is Why Nobody Builds One<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/ultimate-load-test-static-test-wing-limit-load-boeing-airbus\/\">Ultimate Load Test: Why Boeing and Airbus Bend Wings to Breaking<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/how-long-does-an-airliner-last-flight-hours-cycles-metal-fatigue\/\">How Long Does an Airliner Last? Flight Hours, Cycles and Fatigue<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/de-havilland-comet-metal-fatigue-square-windows-investigation-1954\/\">The de Havilland Comet: The Beautiful Jet That Taught Us How Aircraft Break<\/a><\/p><\/div>\r\n","protected":false},"excerpt":{"rendered":"<p>Safe-life, fail-safe and damage tolerance are three answers to one question: what happens when metal cracks? The Comet, the B-47 and the F-111 forced each change.<\/p>","protected":false},"author":23,"featured_media":31503271,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"editor_notices":[],"footnotes":""},"categories":[665,664],"tags":[],"class_list":["post-31503411","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aviation-world","category-military-aviation"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Safe-Life vs Fail-Safe Design and Damage Tolerance | MiGFlug<\/title>\n<meta name=\"description\" content=\"Safe-life, fail-safe design and damage tolerance explained: the Comet, the B-47, the F-111 cold proof test, 14 CFR 25.571 and USAF MIL-STD-1530.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" 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