On January 31, 2000, about 1621 Pacific standard time, Alaska Airlines, Inc., flight 261, a McDonnell Douglas MD-83, N963AS, crashed into the Pacific Ocean about 2.7 miles north of Anacapa Island, California. The 2 pilots, 3 cabin crewmembers, and 83 passengers on board were killed, and the airplane was destroyed by impact forces. Flight 261 was operating as a scheduled international passenger flight under the provisions of 14 Code of Federal Regulations Part 121 from Lic Gustavo Diaz Ordaz International Airport, Puerto Vallarta, Mexico, to Seattle-Tacoma International Airport, Seattle, Washington, with an intermediate stop planned at San Francisco International Airport, San Francisco, California. Visual meteorological conditions prevailed for the flight, which operated on an instrument flight rules flight plan.
The National Transportation Safety Board determines that the probable cause of this accident was a loss of airplane pitch control resulting from the in-flight failure of the horizontal stabilizer trim system jackscrew assembly's acme nut threads. The thread failure was caused by excessive wear resulting from Alaska Airlines' insufficient lubrication of the jackscrew assembly.
Contributing to the accident were Alaska Airlines' extended lubrication interval and the Federal Aviation Administration's (FAA) approval of that extension, which increased the likelihood that a missed or inadequate lubrication would result in excessive wear of the acme nut threads, and Alaska Airlines' extended end play check interval and the FAA's approval of that extension, which allowed the excessive wear of the acme nut threads to progress to failure without the opportunity for detection. Also contributing to the accident was the absence on the McDonnell Douglas MD-80 of a fail-safe mechanism to prevent the catastrophic effects of total acme nut thread loss.
The safety issues discussed in this report include lubrication and inspection of the jackscrew assembly, extension of lubrication and end play check intervals, jackscrew assembly overhaul procedures, the design and certification of the MD-80 horizontal stabilizer trim control system, Alaska Airlines' maintenance program, and FAA oversight of Alaska Airlines. Safety recommendations are addressed to the FAA.
As a result of the investigation of the Alaska Airlines flight 261 accident, the National Transportation Safety Board makes the following recommendations to the Federal Aviation Administration:
Issue a flight standards information bulletin directing air carriers to instruct pilots that in the event of an inoperative or malfunctioning flight control system, if the airplane is controllable they should complete only the applicable checklist procedures and should not attempt any corrective actions beyond those specified. In particular, in the event of an inoperative or malfunctioning horizontal stabilizer trim control system, after a final determination has been made in accordance with the applicable checklist that both the primary and alternate trim systems are inoperative, neither the primary nor the alternate trim motor should be activated, either by engaging the autopilot or using any other trim control switch or handle. Pilots should further be instructed that if checklist procedures are not effective, they should land at the nearest suitable airport. (A-02-36)
Direct all certificate management offices to instruct inspectors to conduct surveillance of airline dispatch and maintenance control personnel to ensure that their training and operations directives provide appropriate dispatch support to pilots who are experiencing a malfunction threatening safety of flight and instruct them to refrain from suggesting continued flight in the interest of airline flight scheduling. (A-02-37)
As part of the response to Safety Recommendation A-01-41, require operators of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to remove degraded grease from the jackscrew assembly acme screw and flush degraded grease and particulates from the acme nut before applying fresh grease. (A-02-38)
As part of the response to Safety Recommendation A-01-41, require operators of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 17 series airplanes, in coordination with Boeing, to increase the size of the access panels that are used to accomplish the jackscrew assembly lubrication procedure. (A-02-39)
Establish the jackscrew assembly lubrication procedure as a required inspection item that must have an inspector's signoff before the task can be considered complete. (A-02-40)
Review all existing maintenance intervals for tasks that could affect critical aircraft components and identify those that have been extended without adequate engineering justification in the form of technical data and analysis demonstrating that the extended interval will not present any increased risk and require modifications of those intervals to ensure that they (1) take into account assumptions made by the original designers, (2) are supported by adequate technical data and analysis, and (3) include an appropriate safety margin that takes into account the possibility of missed or inadequate accomplishment of the maintenance task. In conducting this review, the Federal Aviation Administration should also consider original intervals recommended or established for new aircraft models that are derivatives of earlier models and, if the aircraft component and the task are substantially the same and the recommended interval for the new model is greater than that recommended for the earlier model, treat such original intervals for the derivative model as "extended" intervals. (A-02-41)
Conduct a systematic industrywide evaluation and issue a report on the process by which manufacturers recommend and airlines establish and revise maintenance task intervals and make changes to the process to ensure that, in the future, intervals for each task (1) take into account assumptions made by the original designers, (2) are supported by adequate technical data and analysis, and (3) include an appropriate safety margin that takes into account the possibility of missed or inadequate accomplishment of the maintenance task. (A-02-42)
Require operators to supply the Federal Aviation Administration (FAA), before the implementation of any changes in maintenance tasks intervals that could affect critical aircraft components, technical data and analysis for each task demonstrating that none of the proposed changes will present any potential hazards, and obtain written approval of the proposed changes from the principal maintenance inspector and written concurrence from the appropriate FAA aircraft certification office. (A-02-43)
Pending the incorporation of a fail-safe mechanism in the design of the Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 horizontal stabilizer jackscrew assembly, as recommended in Safety Recommendation A-02-49, establish an end play check interval that (1) accounts for the possibility of higher-than-expected wear rates and measurement error in estimating acme nut thread wear and (2) provides for at least two opportunities to detect excessive wear before a potentially catastrophic wear condition becomes possible. (A-02-44)
Require operators to permanently (1) track end play measures according to airplane registration number and jackscrew assembly serial number, (2) calculate and record average wear rates for each airplane based on end play measurements and flight times, and (3) develop and implement a program to analyze these data to identify and determine the cause of excessive or unexpected wear rates, trends, or anomalies. The Federal Aviation Administration (FAA) should also require operators to report this information to the FAA for use in determining and evaluating an appropriate end play check interval. (A-02-45)
Require that maintenance facilities that overhaul jackscrew assemblies record and inform customers of an overhauled jackscrew assembly's end play measurement. (A-02-46)
Require operators to measure and record the on-wing end play measurement whenever a jackscrew assembly is replaced. (A-02-47)
Require that maintenance facilities that overhaul Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series airplanes' jackscrew assemblies obtain specific authorization to perform such overhauls, predicated on demonstrating that they possess the necessary capability, documentation, and equipment for the task and that they have procedures in place to (1) perform and document the detailed steps that must be followed to properly accomplish the end play check procedure and lubrication of the jackscrew assembly, including specification of appropriate tools and grease types; (2) perform and document the appropriate steps for verifying that the proper acme screw thread surface finish has been applied; and (3) ensure that appropriate packing procedures are followed for all returned overhauled jackscrew assemblies, regardless of whether the assembly has been designated for storage or shipping. (A-02-48)
Conduct a systematic engineering review to (1) identify means to eliminate the catastrophic effects of total acme nut thread failure in the horizontal stabilizer trim system jackscrew assembly in Douglas DC-9 (DC-9), McDonnel Douglas MD-80/90 (MD-80/90), and Boeing 717 (717) series airplanes and require, if practicable, that such fail-safe mechanisms be incorporated in the design of all existing and future DC-9, MD-80/90, and 717 series airplanes and their derivatives; (2) evaluate the horizontal stabilizer trim systems of all other transport-category airplanes to identify any designs that have a catastrophic single-point failure mode and, for any such system; (3) identify means to eliminate the catastrophic effects of that single-point failure mode and, if practicable, require that such fail-safe mechanisms be incorporated in the design of all existing and future airplanes that are equipped with such horizontal stabilizer trim systems (A-02-49)
Modify the certification regulations, policies, or procedures to ensure that new horizontal stabilizer trim control system designs are not certified if they have a single-point catastrophic failure mode, regardless of whether any element of that system is considered structure rather than system or is otherwise considered exempt from certification standards for systems. (A-02-50)
Review and revise aircraft certification regulations and associated guidance applicable to the certification of transport-category airplanes to ensure that wear-related failures are fully considered and addressed so that, to the maximum extent possible, they will not be catastrophic. (A-02-51)
Previously Issued Recommendations Resulting From This Accident Investigation
As a result of the Alaska Airlines flight 261 accident investigation, the Safety Board issued the following safety recommendations to the FAA on October 1, 2001:
Require the Boeing Commercial Airplane Group to revise the lubrication procedure for the horizontal stabilizer trim system of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to minimize the probability of inadequate lubrication. (A-01-41)
On June 14, 2002, the Safety Board classified Safety Recommendation A-01-41 "Open-Acceptable Response."
Require the Boeing Commercial Airplane Group to revise the end play check procedure for the horizontal stabilizer trim system of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to minimize the probability of measurement error and conduct a study to empirically validate the revised procedure against an appropriate physical standard of actual acme screw and acme nut wear. This study should also establish that the procedure produces a measurement that is reliable when conducted on-wing. (A-01-42)
On June 14, 2002, the Safety Board classified Safety Recommendation A-01-42 "Open-Acceptable Response."
Require maintenance personnel who lubricate the horizontal stabilizer trim system of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to undergo specialized training for this task. (A-01-43)
On June 14, 2002, the Safety Board classified Safety Recommendation A-01-43 "Open-Unacceptable Response." system of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to undergo specialized training for this task. This training should include familiarization with the selection, inspection, and proper use of the tooling to perform the end play check. (A-01-44)
On June 14, 2002, the Safety Board classified Safety Recommendation A-01-44 "Open-Unacceptable Response."
Before the implementation of any proposed changes in allowable lubrication applications for critical aircraft systems, require operators to supply to the FAA technical data (including performance information and test results) demonstrating that the proposed changes will not present any potential hazards and obtain approval of the proposed changes from the principal maintenance inspector and concurrence from the FAA applicable aircraft certification office. (A-01-45)
On July 29, 2002, the Safety Board classified Safety Recommendation A-01-45 "Closed-Acceptable Action."
Issue guidance to principal maintenance inspectors to notify all operators about the potential hazards of using inappropriate grease types and mixing incompatible grease types. (A-01-46)
On July 29, 2002, the Safety Board classified Safety Recommendation A-01-46 "Closed-Acceptable Action."
Survey all operators to identify any lubrication practices that deviate from those specified in the manufacturer's airplane maintenance manual, determine whether any of those deviations involve the current use of inappropriate grease types or incompatible grease mixtures on critical aircraft systems and, if so, eliminate the use of any such inappropriate grease types or incompatible mixtures. (A-01-47)
On June 14, 2002, the Safety Board classified Safety Recommendation A-01-47 "Open-Acceptable Response."
Within the next 120 days, convene an industrywide forum to disseminate information about and discuss issues pertaining to the lubrication of aircraft components, including the qualification, selection, application methods, performance, inspection, testing, and incompatibility of grease types used on aircraft components. (A-01-48)
On June 14, 2002, the Safety Board classified Safety Recommendation A-01-48 "Open-Acceptable Response."