{"id":29386077,"date":"2026-10-24T17:24:01","date_gmt":"2026-10-24T15:24:01","guid":{"rendered":"https:\/\/migflug.com\/afterburner\/alaska-airlines-261-jackscrew-acme-nut-lubrication-interval\/"},"modified":"2026-10-24T17:24:43","modified_gmt":"2026-10-24T15:24:43","slug":"alaska-airlines-261-jackscrew-acme-nut-lubrication-interval","status":"publish","type":"post","link":"https:\/\/migflug.com\/afterburner\/de\/alaska-airlines-261-jackscrew-acme-nut-lubrication-interval\/","title":{"rendered":"Alaska Airlines 261: How a Lubrication Interval Grew Sevenfold"},"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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On a McDonnell Douglas MD-83, that part was a steel screw turning inside a nut, mounted near the top of the vertical fin.<\/p>\r\n\r\n<p>The screw set the angle of the horizontal stabilizer. Not the elevator, the whole stabilizer: the surface that trims the aircraft in pitch across its entire speed range. There was one screw, one nut, and no second load path. If the nut gave way, nothing else was holding the front of the stabilizer.<\/p>\r\n\r\n<p>On 31 January 2000, after the nut on Alaska Airlines N963AS had worn down to roughly a tenth of its original thread thickness, it gave way. Eighty-eight people died in the Pacific a few miles north of Anacapa Island, California.<\/p>\r\n\r\n<p>The part of this story that matters is not the failure. Wear is not a surprise; the acme nut was designed as a wear component and everyone involved knew it. What matters is how the two maintenance tasks that existed to catch that wear came to be performed so rarely, and the answer is that nobody ever decided to make them rarer. They simply kept riding along inside a package that was itself being extended, year after year, until the lubrication interval had grown roughly sevenfold from the figure the manufacturer first published.<\/p>\r\n\r\n\r\n<div style=\"background:#f3f5f7;border-left:4px solid #5C91FF;padding:18px 22px;margin:28px 0\">\r\n<p style=\"margin:0 0 10px;font-weight:700;font-size:18px;color:#0d1117\">Quick Facts: Alaska Airlines Flight 261<\/p>\r\n<p style=\"margin:4px 0\"><strong>Date:<\/strong> 31 January 2000, about 16:21 Pacific standard time<\/p>\r\n<p style=\"margin:4px 0\"><strong>Aircraft:<\/strong> McDonnell Douglas MD-83, registration N963AS<\/p>\r\n<p style=\"margin:4px 0\"><strong>Route:<\/strong> Puerto Vallarta to San Francisco, continuing to Seattle-Tacoma<\/p>\r\n<p style=\"margin:4px 0\"><strong>On board:<\/strong> two pilots, three cabin crew, 83 passengers. No survivors<\/p>\r\n<p style=\"margin:4px 0\"><strong>Failed part:<\/strong> the horizontal stabilizer trim jackscrew assembly acme nut<\/p>\r\n<p style=\"margin:4px 0\"><strong>Thread loss:<\/strong> approximately 90 per cent of thread thickness before the remainder sheared<\/p>\r\n<p style=\"margin:4px 0\"><strong>End play tolerance:<\/strong> 0.003 to 0.040 inch; the last recorded check read 0.033 inch<\/p>\r\n<p style=\"margin:4px 0\"><strong>Report:<\/strong> NTSB AAR-02\/01, adopted 2002<\/p>\r\n<\/div>\r\n\r\n\r\n<h2 style=\"padding-top:22px\">One screw, one nut<\/h2>\r\n\r\n<p>The MD-80 family inherited its longitudinal trim system from the DC-9. The horizontal stabilizer is hinged at its rear spar and moved at the front by a jackscrew assembly driven by two electric motors, a primary and an alternate. The screw turns. The acme nut, fixed to the structure, does not. As the screw winds through the nut, the leading edge of the stabilizer rises or falls and the aircraft trims nose-down or nose-up.<\/p>\r\n\r\n<p>That is an elegant arrangement and a very old one. It is also, in this particular implementation, a single load path. Douglas addressed that not with redundancy but with margin: the nut was made deliberately over-strength and designated as the wear point, the cheaper and replaceable half of the pair, with the wear to be managed by maintenance rather than designed out.<\/p>\r\n\r\n\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><svg viewBox=\"0 0 1000 500\" width=\"100%\" style=\"display:block;background:#ffffff\" role=\"img\" aria-label=\"How the MD-80 horizontal stabilizer jackscrew and acme nut carry pitch trim, and how thread wear progresses\"><text x=\"18\" y=\"30\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"20\" font-weight=\"700\" fill=\"#0d1117\">One screw, one nut, the whole pitch trim<\/text><text x=\"18\" y=\"50\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13\" fill=\"#454e5e\">Simplified. The acme nut is fixed to the fin structure; turning the jackscrew through it moves the stabilizer leading edge.<\/text><line x1=\"490\" y1=\"70\" x2=\"490\" y2=\"480\" stroke=\"#e2e7ee\" stroke-width=\"2\"\/><polygon points=\"150,450 250,150 370,150 408,450\" fill=\"#f3f5f7\" stroke=\"#cfd6e0\" stroke-width=\"2\"\/><text x=\"279\" y=\"430\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13\" fill=\"#8a93a0\" text-anchor=\"middle\">vertical stabilizer<\/text><rect x=\"70\" y=\"128\" width=\"400\" height=\"14\" fill=\"#5C91FF\"\/><text x=\"90\" y=\"118\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13.5\" font-weight=\"700\" fill=\"#0d1117\">horizontal stabilizer<\/text><circle cx=\"350\" cy=\"135\" r=\"9\" fill=\"#ffffff\" stroke=\"#0d1117\" stroke-width=\"3\"\/><text x=\"366\" y=\"176\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13\" font-weight=\"700\" fill=\"#0d1117\">rear spar pivot<\/text><text x=\"366\" y=\"193\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\">fixed hinge<\/text><rect x=\"265\" y=\"142\" width=\"14\" height=\"160\" fill=\"#3568e0\"\/><line x1=\"259\" y1=\"152\" x2=\"285\" y2=\"152\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"164\" x2=\"285\" y2=\"164\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"176\" x2=\"285\" y2=\"176\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"188\" x2=\"285\" y2=\"188\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"200\" x2=\"285\" y2=\"200\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"212\" x2=\"285\" y2=\"212\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"224\" x2=\"285\" y2=\"224\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"236\" x2=\"285\" y2=\"236\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"248\" x2=\"285\" y2=\"248\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"260\" x2=\"285\" y2=\"260\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"272\" x2=\"285\" y2=\"272\" stroke=\"#ffffff\" stroke-width=\"2\"\/><line x1=\"259\" y1=\"284\" x2=\"285\" y2=\"284\" stroke=\"#ffffff\" stroke-width=\"2\"\/><text x=\"320\" y=\"300\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13.5\" font-weight=\"700\" fill=\"#0d1117\">jackscrew<\/text><text x=\"320\" y=\"317\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\">turns, driven by two motors<\/text><rect x=\"248\" y=\"206\" width=\"48\" height=\"26\" fill=\"none\" stroke=\"#d32f2f\" stroke-width=\"3\"\/><text x=\"238\" y=\"206\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13.5\" font-weight=\"700\" fill=\"#d32f2f\" text-anchor=\"end\">acme nut<\/text><text x=\"238\" y=\"223\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"end\">fixed; the designated wear point<\/text><text x=\"238\" y=\"240\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"end\">no second load path if it fails<\/text><rect x=\"255\" y=\"300\" width=\"34\" height=\"7\" fill=\"#0d1117\"\/><text x=\"238\" y=\"345\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"end\">lower mechanical stop<\/text><text x=\"520\" y=\"100\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"16\" font-weight=\"700\" fill=\"#0d1117\">What was left of the nut threads<\/text><rect x=\"520\" y=\"150\" width=\"390\" height=\"26\" fill=\"#f3f5f7\"\/><rect x=\"520\" y=\"150\" width=\"390\" height=\"26\" fill=\"#5C91FF\"\/><text x=\"520\" y=\"142\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13.5\" font-weight=\"700\" fill=\"#0d1117\">New nut<\/text><text x=\"520\" y=\"194\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\">full thread thickness<\/text><text x=\"922\" y=\"168\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13\" font-weight=\"700\" fill=\"#0d1117\">100%<\/text><rect x=\"520\" y=\"240\" width=\"390\" height=\"26\" fill=\"#f3f5f7\"\/><rect x=\"520\" y=\"240\" width=\"304\" height=\"26\" fill=\"#5C91FF\"\/><text x=\"520\" y=\"232\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13.5\" font-weight=\"700\" fill=\"#0d1117\">At the 0.040 in end play limit<\/text><text x=\"520\" y=\"284\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\">the point a check would condemn it: about 22 per cent worn<\/text><text x=\"922\" y=\"258\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13\" font-weight=\"700\" fill=\"#0d1117\">78%<\/text><rect x=\"520\" y=\"330\" width=\"390\" height=\"26\" fill=\"#f3f5f7\"\/><rect x=\"520\" y=\"330\" width=\"39\" height=\"26\" fill=\"#d32f2f\"\/><text x=\"520\" y=\"322\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13.5\" font-weight=\"700\" fill=\"#0d1117\">Flight 261 acme nut<\/text><text x=\"520\" y=\"374\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\">about 90 per cent worn before the remainder sheared<\/text><text x=\"922\" y=\"348\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13\" font-weight=\"700\" fill=\"#0d1117\">10%<\/text><text x=\"520\" y=\"435\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#8a93a0\">Figures from NTSB AAR-02\/01. End play tolerance was 0.003 to 0.040 inch.<\/text><\/svg><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">The arrangement, simplified, and what was left of the nut threads. The end play check was the only routine measurement that could have revealed the wear.<\/figcaption><\/figure>\r\n\r\n<p>The FAA understood the design the same way. At the Safety Board public hearing, an FAA senior certification engineer was asked how wear had been treated during certification.<\/p>\r\n\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #d32f2f;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7;display:flex;gap:20px;align-items:flex-start\"><div><em>&ldquo;Wear is not considered as a mode of failure for either a systems safety analysis or for structural considerations.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>FAA senior certification engineer<\/strong> &mdash; Testimony at the NTSB public hearing, quoted in NTSB AAR-02\/01<\/div><\/div><\/div>\r\n\r\n<p>That single sentence explains how a component with no backup could be certificated. A systems safety analysis asks what happens if a part breaks. It does not ask what happens if a part slowly disappears. Wear was handled by calling it a maintenance problem and moving on.<\/p>\r\n\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #1565c0;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7;display:flex;gap:20px;align-items:flex-start\"><div><em>&ldquo;Recognized as a wear component which is addressed initially by its robust design and later by in-service maintenance action to control and monitor that wear.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>Douglas design position<\/strong> &mdash; Manufacturer material quoted in NTSB AAR-02\/01<\/div><\/div><\/div>\r\n\r\n<p>Two tasks did that controlling and monitoring. Lubrication, which kept the threads apart. And the end play check, which measured how much of the nut was left.<\/p>\r\n\r\n\r\n<h2 style=\"padding-top:22px\">The measurement that condemns the nut<\/h2>\r\n\r\n<p>End play is axial slop: how far the screw can move up and down inside the nut without turning. A new assembly has almost none. As the nut threads wear, the gap opens, and the measurement grows in direct proportion to the metal that has gone.<\/p>\r\n\r\n<p>The allowable band was 0.003 to 0.040 inch, and the 0.040 limit is where the arithmetic becomes uncomfortable. The Safety Board calculated that an assembly sitting right at that limit has lost only about 22 per cent of its thread thickness. The nut recovered from Flight 261 had lost about 90 per cent.<\/p>\r\n\r\n<p>Think about what that gap means. The inspection that condemns the part fires when roughly a fifth of it has gone. Failure comes when nine tenths has gone. Between those two states the aircraft flies on, and the only thing standing between them is how often somebody carries out the check.<\/p>\r\n\r\n<h2 style=\"padding-top:22px\">Fifteen years of intervals getting longer<\/h2>\r\n\r\n<p>Here is the chronology the Safety Board reconstructed from Alaska Airlines maintenance records, and it is worth following step by step, because no single step looks unreasonable.<\/p>\r\n\r\n\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><svg viewBox=\"0 0 1000 560\" width=\"100%\" style=\"display:block;background:#ffffff\" role=\"img\" aria-label=\"Alaska Airlines jackscrew lubrication and end play check intervals from 1985 to the Flight 261 accident\"><text x=\"18\" y=\"28\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"20\" font-weight=\"700\" fill=\"#0d1117\">Fifteen years of intervals getting longer<\/text><text x=\"18\" y=\"48\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"13\" fill=\"#454e5e\">Alaska Airlines MD-80 jackscrew tasks, in flight hours. Neither task was ever reviewed on its own.<\/text><text x=\"18\" y=\"100\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"16\" font-weight=\"700\" fill=\"#0d1117\">Lubrication of the jackscrew assembly<\/text><text x=\"18\" y=\"118\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\">Douglas Process Standard 3.17-49, issued 1964, specified 300 to 350 flight hours.<\/text><line x1=\"90\" y1=\"280\" x2=\"930\" y2=\"280\" stroke=\"#e2e7ee\" stroke-width=\"1\"\/><text x=\"80\" y=\"284\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12\" fill=\"#8a93a0\" text-anchor=\"end\">0<\/text><line x1=\"90\" y1=\"205\" x2=\"930\" y2=\"205\" stroke=\"#e2e7ee\" stroke-width=\"1\"\/><text x=\"80\" y=\"209\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12\" fill=\"#8a93a0\" text-anchor=\"end\">1,500<\/text><line x1=\"90\" y1=\"130\" x2=\"930\" y2=\"130\" stroke=\"#e2e7ee\" stroke-width=\"1\"\/><text x=\"80\" y=\"134\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12\" fill=\"#8a93a0\" text-anchor=\"end\">3,000<\/text><line x1=\"90\" y1=\"245\" x2=\"199.8039215686278\" y2=\"245\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><rect x=\"86\" y=\"241\" width=\"8\" height=\"8\" fill=\"#5C91FF\"\/><text x=\"96\" y=\"235\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\">700<\/text><line x1=\"199.8039215686278\" y1=\"255\" x2=\"254.70588235294167\" y2=\"255\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><line x1=\"199.8039215686278\" y1=\"245\" x2=\"199.8039215686278\" y2=\"255\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><rect x=\"195.8039215686278\" y=\"251\" width=\"8\" height=\"8\" fill=\"#5C91FF\"\/><text x=\"205.8039215686278\" y=\"245\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\">500<\/text><line x1=\"254.70588235294167\" y1=\"230\" x2=\"419.41176470588334\" y2=\"230\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><line x1=\"254.70588235294167\" y1=\"255\" x2=\"254.70588235294167\" y2=\"230\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><rect x=\"250.70588235294167\" y=\"226\" width=\"8\" height=\"8\" fill=\"#5C91FF\"\/><text x=\"260.70588235294167\" y=\"220\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\">1,000<\/text><line x1=\"419.41176470588334\" y1=\"220\" x2=\"584.117647058825\" y2=\"220\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><line x1=\"419.41176470588334\" y1=\"230\" x2=\"419.41176470588334\" y2=\"220\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><rect x=\"415.41176470588334\" y=\"216\" width=\"8\" height=\"8\" fill=\"#5C91FF\"\/><text x=\"425.41176470588334\" y=\"210\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\">1,200<\/text><line x1=\"584.117647058825\" y1=\"200\" x2=\"693.9215686274528\" y2=\"200\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><line x1=\"584.117647058825\" y1=\"220\" x2=\"584.117647058825\" y2=\"200\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><rect x=\"580.117647058825\" y=\"196\" width=\"8\" height=\"8\" fill=\"#5C91FF\"\/><text x=\"590.117647058825\" y=\"190\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\">1,600<\/text><line x1=\"693.9215686274528\" y1=\"152.5\" x2=\"930\" y2=\"152.5\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><line x1=\"693.9215686274528\" y1=\"200\" x2=\"693.9215686274528\" y2=\"152.5\" stroke=\"#5C91FF\" stroke-width=\"4\"\/><rect x=\"689.9215686274528\" y=\"148.5\" width=\"8\" height=\"8\" fill=\"#5C91FF\"\/><text x=\"699.9215686274528\" y=\"142.5\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\">2,550<\/text><line x1=\"90\" y1=\"280\" x2=\"930\" y2=\"280\" stroke=\"#0d1117\" stroke-width=\"2\"\/><text x=\"90\" y=\"299\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1985<\/text><text x=\"254.70588235294167\" y=\"299\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1988<\/text><text x=\"419.41176470588334\" y=\"299\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1991<\/text><text x=\"584.117647058825\" y=\"299\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1994<\/text><text x=\"693.9215686274528\" y=\"299\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1996<\/text><text x=\"913.5294117647084\" y=\"299\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">2000<\/text><text x=\"18\" y=\"370\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"16\" font-weight=\"700\" fill=\"#0d1117\">End play check, the inspection that measures wear<\/text><text x=\"18\" y=\"388\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\">The 1996 figure is the interval in force on the day of the accident.<\/text><line x1=\"90\" y1=\"530\" x2=\"930\" y2=\"530\" stroke=\"#e2e7ee\" stroke-width=\"1\"\/><text x=\"80\" y=\"534\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12\" fill=\"#8a93a0\" text-anchor=\"end\">0<\/text><line x1=\"90\" y1=\"465\" x2=\"930\" y2=\"465\" stroke=\"#e2e7ee\" stroke-width=\"1\"\/><text x=\"80\" y=\"469\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12\" fill=\"#8a93a0\" text-anchor=\"end\">5,000<\/text><line x1=\"90\" y1=\"400\" x2=\"930\" y2=\"400\" stroke=\"#e2e7ee\" stroke-width=\"1\"\/><text x=\"80\" y=\"404\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12\" fill=\"#8a93a0\" text-anchor=\"end\">10,000<\/text><line x1=\"90\" y1=\"465\" x2=\"254.70588235294167\" y2=\"465\" stroke=\"#d32f2f\" stroke-width=\"4\"\/><rect x=\"86\" y=\"461\" width=\"8\" height=\"8\" fill=\"#d32f2f\"\/><text x=\"96\" y=\"455\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\">5,000<\/text><line x1=\"254.70588235294167\" y1=\"446.8\" x2=\"693.9215686274528\" y2=\"446.8\" stroke=\"#d32f2f\" stroke-width=\"4\"\/><line x1=\"254.70588235294167\" y1=\"465\" x2=\"254.70588235294167\" y2=\"446.8\" stroke=\"#d32f2f\" stroke-width=\"4\"\/><rect x=\"250.70588235294167\" y=\"442.8\" width=\"8\" height=\"8\" fill=\"#d32f2f\"\/><text x=\"260.70588235294167\" y=\"436.8\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\">6,400<\/text><line x1=\"693.9215686274528\" y1=\"405.85\" x2=\"930\" y2=\"405.85\" stroke=\"#d32f2f\" stroke-width=\"4\"\/><line x1=\"693.9215686274528\" y1=\"446.8\" x2=\"693.9215686274528\" y2=\"405.85\" stroke=\"#d32f2f\" stroke-width=\"4\"\/><rect x=\"689.9215686274528\" y=\"401.85\" width=\"8\" height=\"8\" fill=\"#d32f2f\"\/><text x=\"699.9215686274528\" y=\"395.85\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\">9,550<\/text><line x1=\"90\" y1=\"530\" x2=\"930\" y2=\"530\" stroke=\"#0d1117\" stroke-width=\"2\"\/><text x=\"90\" y=\"549\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1985<\/text><text x=\"254.70588235294167\" y=\"549\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1988<\/text><text x=\"419.41176470588334\" y=\"549\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1991<\/text><text x=\"584.117647058825\" y=\"549\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1994<\/text><text x=\"693.9215686274528\" y=\"549\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">1996<\/text><text x=\"913.5294117647084\" y=\"549\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#454e5e\" text-anchor=\"middle\">2000<\/text><line x1=\"917.9215686274495\" y1=\"110\" x2=\"917.9215686274495\" y2=\"530\" stroke=\"#0d1117\" stroke-width=\"2\" stroke-dasharray=\"6 5\"\/><text x=\"909.9215686274495\" y=\"108\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" font-weight=\"700\" fill=\"#0d1117\" text-anchor=\"end\">31 Jan 2000<\/text><text x=\"18\" y=\"552\" font-family=\"Gilroy semiBold, Helvetica, Arial, sans-serif\" font-size=\"12.5\" fill=\"#8a93a0\">Source: NTSB AAR-02\/01, sections 1.6.3.2.1 and 1.6.3.3.4. The 1996 lubrication task was time-controlled at 8 months with no flight-hour limit; 2,550 hours is the NTSB estimate at the utilisation of the day.<\/text><\/svg><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">Every step is a scheduled-maintenance decision that was made about a check package, not about the jackscrew. The dashed line is the accident.<\/figcaption><\/figure>\r\n\r\n<p>In March 1985 the B check came round every 350 flight hours and the jackscrew was lubricated at every other one, so every 700 hours. In March 1987 the B check moved to 500 hours and the lubrication moved to every B check, so the interval actually fell to 500. So far, so attentive.<\/p>\r\n\r\n<p>In July 1988 Alaska abolished the B check altogether and spread its tasks into the A and C checks. Lubrication landed on every eighth A check. A checks were 125 hours apart, so the interval became 1,000 hours. In February 1991 the A check went to 150 hours and the same every-eighth rule made it 1,200. In December 1994 the A check went to 200 hours and the same rule made it 1,600.<\/p>\r\n\r\n<p>Notice what has happened three times in a row. Nobody decided to lubricate the jackscrew less often. Somebody decided to extend the A check, and the jackscrew interval moved because it was expressed as a multiple of the A check.<\/p>\r\n\r\n<p>In July 1996 the task came off the A check entirely and went onto a time-controlled card with a maximum interval of eight months. There was no flight-hour limit attached at all. At the utilisation Alaska was flying at the time, eight months worked out at roughly 2,550 flight hours. In October 1996 the task was combined with the elevator lubrication, still at eight months, and that was the interval in force on the day of the accident.<\/p>\r\n\r\n<p>Against the 300 to 350 flight hours that Douglas Process Standard 3.17-49 had specified when the DC-9 was certificated in 1964, that is an interval about seven times longer.<\/p>\r\n\r\n<p>The end play check travelled the same road. Every other C check, which in 1985 meant 5,000 flight hours. By July 1988, with C checks on a 13-month cycle and no hour limit, about 6,400 hours. In April 1996 the C check went to 15 months and the end play check, still every other one, became every 30 months, or roughly 9,550 flight hours. Among MD-80 operators, Alaska had the second-longest end play interval in the United States.<\/p>\r\n\r\n\r\n<h2 style=\"padding-top:22px\">Nobody ever examined the task<\/h2>\r\n\r\n<p>This is the finding that should have changed how the industry thinks, and it comes in two halves.<\/p>\r\n\r\n<p>The first half is how Alaska justified its 1996 C-check extension. The airline's director of reliability and maintenance programs described the supporting data package at the public hearing.<\/p>\r\n\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #d32f2f;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7;display:flex;gap:20px;align-items:flex-start\"><div><em>&ldquo;That looked at five separate tail numbers [airplanes] that we selected as a stratified sample of the fleet. At that time I believe there were a little over 40 aircraft. We reviewed in detail all of the nonroutine discrepancies that had been written against the aircraft during those C checks.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>Alaska Airlines director of reliability and maintenance programs<\/strong> &mdash; Testimony at the NTSB public hearing, quoted in NTSB AAR-02\/01<\/div><\/div><\/div>\r\n\r\n<p>He went on to confirm that individual tasks tied to the C check, the end play check among them, were not separately considered when the package was extended. The analysis asked whether C checks as a whole had been finding problems. It did not ask whether any one task inside the package needed its own interval. Two lubrication tasks were in fact pulled out and given shorter intervals on the basis of that review; the jackscrew end play check was not one of them. Approval came through about a month later.<\/p>\r\n\r\n<p>The second half is worse, and it concerns the industry-wide maintenance logic rather than one airline. When the MD-80 programme moved to MSG-3, the modern task-based maintenance methodology, the recommended jackscrew lubrication interval was set at the C check, every 3,600 flight hours or 15 months. The FAA's MD-80 maintenance review board chairman testified that the extension was treated as part of a larger C-check package, and that it had not involved a task-by-task analysis of every task the change would affect.<\/p>\r\n\r\n<p>And the Safety Board established two further things about that decision. Douglas's own earlier recommendation of 600 to 900 flight hours was not considered during the MSG-3 process. And Boeing design engineers were neither consulted about the extended interval nor aware of it.<\/p>\r\n\r\n<p>So the number that governed how often the only wear-control task on a single-load-path flight control was performed had been set by a process that did not look at the task, did not look at the manufacturer's earlier advice, and did not involve the people who designed the part.<\/p>\r\n\r\n<h2 style=\"padding-top:22px\">What the last check found<\/h2>\r\n\r\n<p>N963AS had its final end play check at a C check in September 1997. Five measurements were recorded on the maintenance form, all of them 0.033 inch. That is inside the 0.003 to 0.040 band, so the assembly stayed on the aircraft, and under the interval then in force the next check was not due for another thirty months.<\/p>\r\n\r\n<p>Three scheduled lubrications followed that C check. The Safety Board found that the grease on the recovered assembly was not doing its job, and the probable cause turns on that word insufficient rather than on any single missed task.<\/p>\r\n\r\n<p>Between that 0.033 inch reading and the accident, the nut went from a condition the system called acceptable to one in which roughly nine tenths of the thread thickness was gone. Under the interval then in force there was no scheduled opportunity to notice.<\/p>\r\n\r\n\r\n<h2 style=\"padding-top:22px\">Two hours of flying a jammed aeroplane<\/h2>\r\n\r\n<p>The stabilizer stopped responding to both autopilot and manual trim commands as the aircraft climbed through 23,400 feet. What followed is one of the longest sustained emergencies in the record: the crew hand-flew for roughly two hours, working the trim handles and switches, consulting dispatch and maintenance, repeatedly trying the autopilot, which disengaged each time.<\/p>\r\n\r\n<p>When the primary trim motor eventually broke the jammed nut free, the stabilizer ran to its lower mechanical stop and the aircraft entered a dive from 31,050 feet. The crew recovered it at about 24,000 feet, which is an extraordinary piece of flying. They extended the slats and flaps, and then retracted and redeployed them. Redeploying raised the aerodynamic load on the tail, the jackscrew and acme nut separated completely, and the aircraft was no longer controllable in pitch.<\/p>\r\n\r\n<p>The Safety Board's probable cause runs as follows, and every clause in it is load-bearing.<\/p>\r\n\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #0d1117;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7;display:flex;gap:20px;align-items:flex-start\"><div><em>&ldquo;A loss of airplane pitch control resulting from the in-flight failure of the horizontal stabilizer trim system jackscrew assembly acme nut threads. The thread failure was caused by excessive wear resulting from Alaska Airlines insufficient lubrication of the jackscrew assembly.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>National Transportation Safety Board<\/strong> &mdash; Probable cause, NTSB AAR-02\/01<\/div><\/div><\/div>\r\n\r\n<p>Three things were listed as contributing. Alaska Airlines extended lubrication interval and the FAA's approval of it, which increased the likelihood that a missed or inadequate lubrication would produce excessive wear. Alaska Airlines extended end play check interval and the FAA's approval of it, which allowed that wear to progress to failure without the opportunity for detection. And the absence, on the MD-80, of any fail-safe mechanism against total loss of the acme nut threads.<\/p>\r\n\r\n<p>Read those together and the shape of the accident is clear. The lubrication interval determined how fast the nut wore. The end play interval determined whether anyone would find out. The design determined what happened when nobody did.<\/p>\r\n\r\n<h2 style=\"padding-top:22px\">What changed, and how fast<\/h2>\r\n\r\n<p>Eleven days after the accident, on 11 February 2000, the FAA issued AD 2000-03-51. It did not go through a notice of proposed rulemaking. It went out by individual telegram to every known United States owner and operator of the DC-9, MD-88, MD-90-30 and 717-200, and it required action within 72 hours of receipt. It was not printed in the Federal Register until 28 February, seventeen days after it had already taken effect. If you want to know what an airworthiness directive looks like when the regulator has decided it cannot wait, that is it.<\/p>\r\n\r\n<p>What it demanded was simple and immediate: a general visual inspection of the grease on the jackscrew assembly and of the area directly beneath it for metal shavings and flakes, with the assembly replaced before further flight if any were found, plus inspection and lubrication within 650 hours or 72 hours, and end play checks within 2,000 flight hours of the last one and every 2,000 hours thereafter. In July 2000 AD 2000-15-15 superseded it and widened the inspection to metallic particles, slivers and dust, the FAA having received numerous reports of exactly those.<\/p>\r\n\r\n<p>On 6 April 2000 Alaska Airlines changed its jackscrew lubrication interval from eight months to 650 flight hours, and its end play check from a task riding on the C-check cycle to a time-controlled item every 2,000 flight hours.<\/p>\r\n\r\n<p>Set those numbers against the chart above. The lubrication interval went from roughly 2,550 flight hours to 650, a factor of about four. The end play check went from roughly 9,550 flight hours to 2,000, a factor of about five. Neither new figure required a technological advance. Both were available the whole time.<\/p>\r\n\r\n\r\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\/gAYzBJxOeLw\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n<p style=\"font-size:13px;color:#777;text-align:center;margin-top:-12px;font-style:italic\">Mentour Pilot's account of the flight, from an airline pilot's perspective.<\/p>\r\n\r\n<h2 style=\"padding-top:22px\">The part that keeps being relevant<\/h2>\r\n\r\n<p>It would be comfortable to file this as a story about one airline cutting corners. The Safety Board did not file it that way, and neither should anyone else. Alaska's intervals were long, but they were not unique: another operator's MD-80 lubrication interval was 600 flight hours, and the spread across the fleet was wide. The manufacturer's own MSG-3 recommendation had already moved the lubrication task to the C check. Every extension Alaska made was reviewed and accepted by the FAA.<\/p>\r\n\r\n<p>What failed was a way of thinking. A maintenance programme built out of packages will drift, because extending a package is a single decision with a single business case, while the dozens of tasks inside it each have their own physics. Most of those tasks tolerate the drift. One of them was the only thing measuring the wear on a part with no backup.<\/p>\r\n\r\n<p>Modern maintenance programmes are supposed to handle this by analysing tasks individually rather than inheriting intervals from the check they happen to sit in. That is what MSG-3 is for. Flight 261 is the case that shows what happens when the methodology is applied to the package and not to the task.<\/p>\r\n\r\n\r\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\/V2T4RnS91NA\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n<p style=\"font-size:13px;color:#777;text-align:center;margin-top:-12px;font-style:italic\">A stabilizer trim jackscrew assembly on a Boeing 767. Different aircraft, same class of mechanism, and a good look at the hardware.<\/p>\r\n\r\n<p>The jackscrew is still there, on thousands of aeroplanes, doing the same job. It is checked far more often now, and it is checked because of what happened off Anacapa Island on a Monday afternoon in 2000.<\/p>\r\n\r\n\r\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\/7jpvPmj7lx8\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n<p style=\"font-size:13px;color:#777;text-align:center;margin-top:-12px;font-style:italic\">The Smithsonian Channel on the investigation and the maintenance findings.<\/p>\r\n\r\n<p><em>Sources: NTSB Aircraft Accident Report AAR-02\/01, Loss of Control and Impact with Pacific Ocean, Alaska Airlines Flight 261, McDonnell Douglas MD-83, N963AS, 31 January 2000, in particular sections 1.6.3.2.1, 1.6.3.3.4, 1.18.1 and the probable cause; FAA Lessons Learned, McDonnell Douglas MD-83 N963AS; Federal Register document 00-4337, 28 February 2000, and document 00-19671, 8 August 2000.<\/em><\/p>\r\n\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\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 a jackscrew on a plane?<\/summary><div class=\"a\">A jackscrew is a threaded shaft that converts rotation into linear movement. On the McDonnell Douglas MD-80 family it sets the angle of the horizontal stabilizer: the screw turns inside a fixed acme nut near the top of the vertical fin, raising or lowering the stabilizer leading edge to trim the aircraft in pitch.<\/div><\/details><details><summary>What caused the crash of Alaska Airlines Flight 261?<\/summary><div class=\"a\">The NTSB determined the probable cause was loss of pitch control following the in-flight failure of the horizontal stabilizer trim jackscrew assembly acme nut threads. The threads failed through excessive wear caused by insufficient lubrication. Extended lubrication and end play check intervals, approved by the FAA, and the absence of any fail-safe against total thread loss were listed as contributing.<\/div><\/details><details><summary>How worn was the Alaska 261 jackscrew nut?<\/summary><div class=\"a\">Approximately 90 per cent of the acme nut thread thickness had worn away before the remaining threads sheared. For comparison, the NTSB calculated that an assembly at the 0.040 inch end play limit, the point at which an inspection would condemn it, would have lost only about 22 per cent of its thread thickness.<\/div><\/details><details><summary>What is an end play check?<\/summary><div class=\"a\">An end play check measures how far the jackscrew can move axially inside the acme nut without turning. The gap grows in proportion to the nut material that has worn away, so it is the routine measurement that reveals wear. On the MD-80 the allowable range was 0.003 to 0.040 inch.<\/div><\/details><details><summary>How often was the Alaska Airlines jackscrew lubricated?<\/summary><div class=\"a\">At the time of the accident the task was on a time-controlled card with a maximum interval of eight months and no flight-hour limit, which the NTSB estimated at roughly 2,550 flight hours at the utilisation of the day. The original Douglas specification issued in 1964 had called for lubrication every 300 to 350 flight hours.<\/div><\/details><details><summary>Why did the maintenance intervals get so long?<\/summary><div class=\"a\">Because almost nobody ever made a decision about the jackscrew itself. The lubrication task was expressed as a multiple of the A check and the end play check as a multiple of the C check, so each time those packages were extended the jackscrew tasks stretched with them. The NTSB found that individual tasks were not separately analysed, that the earlier Douglas recommendation was not considered during the MSG-3 process, and that Boeing design engineers were neither consulted about the extended interval nor aware of it.<\/div><\/details><details><summary>What changed after Alaska Airlines Flight 261?<\/summary><div class=\"a\">Eleven days after the accident the FAA issued AD 2000-03-51 by telegram to all known US operators of the DC-9, MD-88, MD-90-30 and 717-200, requiring action within 72 hours. It mandated inspections for metal shavings and brought lubrication to 650 flight hours and end play checks to every 2,000 flight hours. AD 2000-15-15 superseded it in July 2000 and widened the inspection to metallic particles, slivers and dust.<\/div><\/details><details><summary>Did the MD-80 stabilizer have a backup if the nut failed?<\/summary><div class=\"a\">No. The acme nut was a single load path, and the NTSB listed the absence of a fail-safe mechanism against total acme nut thread loss as a contributing factor. A senior FAA certification engineer testified that wear was not treated as a failure mode in either the systems safety analysis or the structural assessment, which is how a component with no redundancy came to be certificated.<\/div><\/details><\/div><\/section>\r\n\r\n\r\n\r\n<script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[{\"@type\":\"Question\",\"name\":\"What is a jackscrew on a plane?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"A jackscrew is a threaded shaft that converts rotation into linear movement. On the McDonnell Douglas MD-80 family it sets the angle of the horizontal stabilizer: the screw turns inside a fixed acme nut near the top of the vertical fin, raising or lowering the stabilizer leading edge to trim the aircraft in pitch.\"}},{\"@type\":\"Question\",\"name\":\"What caused the crash of Alaska Airlines Flight 261?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"The NTSB determined the probable cause was loss of pitch control following the in-flight failure of the horizontal stabilizer trim jackscrew assembly acme nut threads. The threads failed through excessive wear caused by insufficient lubrication. Extended lubrication and end play check intervals, approved by the FAA, and the absence of any fail-safe against total thread loss were listed as contributing.\"}},{\"@type\":\"Question\",\"name\":\"How worn was the Alaska 261 jackscrew nut?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Approximately 90 per cent of the acme nut thread thickness had worn away before the remaining threads sheared. For comparison, the NTSB calculated that an assembly at the 0.040 inch end play limit, the point at which an inspection would condemn it, would have lost only about 22 per cent of its thread thickness.\"}},{\"@type\":\"Question\",\"name\":\"What is an end play check?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"An end play check measures how far the jackscrew can move axially inside the acme nut without turning. The gap grows in proportion to the nut material that has worn away, so it is the routine measurement that reveals wear. On the MD-80 the allowable range was 0.003 to 0.040 inch.\"}},{\"@type\":\"Question\",\"name\":\"How often was the Alaska Airlines jackscrew lubricated?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"At the time of the accident the task was on a time-controlled card with a maximum interval of eight months and no flight-hour limit, which the NTSB estimated at roughly 2,550 flight hours at the utilisation of the day. The original Douglas specification issued in 1964 had called for lubrication every 300 to 350 flight hours.\"}},{\"@type\":\"Question\",\"name\":\"Why did the maintenance intervals get so long?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Because almost nobody ever made a decision about the jackscrew itself. The lubrication task was expressed as a multiple of the A check and the end play check as a multiple of the C check, so each time those packages were extended the jackscrew tasks stretched with them. The NTSB found that individual tasks were not separately analysed, that the earlier Douglas recommendation was not considered during the MSG-3 process, and that Boeing design engineers were neither consulted about the extended interval nor aware of it.\"}},{\"@type\":\"Question\",\"name\":\"What changed after Alaska Airlines Flight 261?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Eleven days after the accident the FAA issued AD 2000-03-51 by telegram to all known US operators of the DC-9, MD-88, MD-90-30 and 717-200, requiring action within 72 hours. It mandated inspections for metal shavings and brought lubrication to 650 flight hours and end play checks to every 2,000 flight hours. 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A senior FAA certification engineer testified that wear was not treated as a failure mode in either the systems safety analysis or the structural assessment, which is how a component with no redundancy came to be certificated.\"}}]}<\/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\/airworthiness-directives-14-cfr-part-39-four-types-myth\/\">Airworthiness Directives: The Four Types That Do Not Exist<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/faa-ad-boeing-737ng-horizontal-stabilizer-freeplay-flutter-2026\/\">Boeing 737NG Flutter Risk: FAA Orders Stabilizer Freeplay Checks<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/southern-airways-242-dc-9-highway-landing-new-hope-1977\/\">Southern Airways 242: The DC-9 That Landed on a Highway<\/a><\/p><\/div>\r\n","protected":false},"excerpt":{"rendered":"<p>A single screw and nut set the pitch trim of the MD-83, with no second load path. Nobody ever decided to check them less often. The intervals simply rode along inside maintenance packages that kept getting longer.<\/p>","protected":false},"author":23,"featured_media":29384721,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"editor_notices":[],"footnotes":""},"categories":[665],"tags":[],"class_list":["post-29386077","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 v28.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Alaska Airlines 261 Jackscrew: The Interval That Grew | MiGFlug<\/title>\n<meta name=\"description\" content=\"How the Alaska Airlines 261 jackscrew acme nut wore away to a tenth of its threads while its lubrication and end play intervals quietly grew.\" \/>\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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