page_title: Alaska Airlines Flight 261
section_title: Wreckage recovery and analysis
hierarchical_section_title: Alaska Airlines Flight 261 / Investigation / Wreckage recovery and analysis
caption_attribution_description: English: Jackscrew on N963AS of . Image of the horizontal stabilizer trim system jackscrew recovered from Alaska Airlines Flight 261 wreckage; the metallic filaments wrapped around it are remnants of the threads from the acme nut. Image retrieved from NTSB website URL: http://www.ntsb.gov/Events/2000/Aka261/jackscrew.htm
context_page_description: Alaska Airlines Flight 261 was a scheduled international passenger flight from Licenciado Gustavo Díaz Ordaz International Airport in Puerto Vallarta, Jalisco, Mexico, to Seattle–Tacoma International Airport in Seattle, Washington, United States, with an intermediate stop at San Francisco International Airport in San Francisco, California. On January 31, 2000, the aircraft operating the route, a McDonnell Douglas MD-83, crashed into the Pacific Ocean roughly 2.7 miles north of Anacapa Island, California, following a catastrophic loss of pitch control. The accident killed all 88 on board: two pilots, three cabin crew members, and 83 passengers.
  The subsequent investigation by the National Transportation Safety Board determined that inadequate maintenance led to excessive wear and eventual failure of a critical flight control system during flight. The probable cause was stated to be "a loss of airplane pitch control resulting from the in-flight failure of the horizontal stabilizer trim system jackscrew assembly's trapezoidal nut threads. The thread failure was caused by excessive wear resulting from Alaska Airlines' insufficient lubrication of the jackscrew assembly."
context_section_description: Using side-scan sonar, remotely operated vehicles, and a commercial fishing trawler, workers recovered about 85% of the fuselage (including the tail section) and a majority of the wing components.  In addition, both engines, as well as the flight data recorder (FDR) and CVR were retrieved. All wreckage recovered from the crash site was unloaded at the Seabee's Naval Construction Battalion Center Port Hueneme, California, for examination and documentation by NTSB investigators. Both the horizontal stabilizer trim system jackscrew (also referred to as "acme screw") and the corresponding acme nut, which the jackscrew turns through, were found. The jackscrew was constructed from case-hardened steel and is 22 inches (56 cm) long and 1.5 inches (3.8 cm) in diameter. The acme nut was constructed from a softer copper alloy containing aluminum, nickel, and bronze. As the jackscrew rotates, it moves up or down through the (fixed) acme nut, and this linear motion moves the horizontal stabilizer for the trim system. Upon subsequent examination, the jackscrew was found to have metallic filaments wrapped around it, which were later determined to be the remains of the acme-nut thread.
  Later analysis estimated that 90% of the thread in the acme nut had already worn away previously, and that it had finally stripped out during the flight while en route to San Francisco. Once the thread had failed, the horizontal stabilizer assembly was then subjected to aerodynamic forces that it was not designed to withstand, leading to complete failure of the overstressed stabilizer assembly. Based on the time since the last inspection of the jackscrew assembly, the NTSB determined that the acme-nut thread had deteriorated at 0.012 inches (0.30 mm) per 1000 flight‑hours, much faster than the expected wear of 0.001 inches (0.025 mm) per 1000 flight‑hours. Over the course of the investigation, the NTSB considered a number of potential reasons for the substantial amount of deterioration of the nut thread on the jackscrew assembly, including the substitution by Alaska Airlines (with the approval of the aircraft manufacturer McDonnell Douglas) of Aeroshell 33 grease instead of the previously approved lubricant, Mobilgrease 28. The use of Aeroshell 33 was found not to be a factor in this accident. Insufficient lubrication of the components was also considered as a reason for the wear. Examination of the jackscrew and acme nut revealed that no effective lubrication was present on these components at the time of the accident. Ultimately, the lack of lubrication of the acme-nut thread and the resultant excessive wear were determined to be the direct causes of the accident.
caption: Recovered jackscrew –  the spiral "wire" wrapped around the threaded portion is the remnants of the internal screw thread stripped from the acme nut
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