The National Transportation Safety Board determined today that the probable cause of the January 31, 2000 Alaska Airlines flight 261 accident was the loss of airplane pitch control resulting from in-flight failure of the horizontal stabilizer trim system jackscrew assembly's acme nut thread. The component failed because of excessive wear resulting from Alaska Airlines' insufficient lubrication of the jackscrew assembly.
Contributing to the accident were (1) Alaska Airlines' extended lubrication interval, and the FAA's approval of that extension, which increased the likelihood that an unperformed or inadequate lubrication would result in excessive wear of the acme nut threads; and (2) 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, the Board said, was the absence on the MD-80 of a fail-safe mechanism to prevent the catastrophic effects of total acme nut thread loss.
In today's final report, the Safety Board issued 16 recommendations to the FAA, including one calling for a review of all existing maintenance intervals for tasks that could affect critical aircraft components and another recommending that the FAA conduct a systematic engineering review of all transport category airplanes to identify means to eliminate the catastrophic effects of a system or structural failure in the horizontal stabilizer trim jackscrew assembly.
On January 31, 2000, Alaska Airlines flight 261, an MD-83, departed Puerto Vallarta, Mexico enroute to Seattle, Washington with a stop in San Francisco. Nearly four hours into the flight, and after several attempts to control the aircraft and perform an emergency landing at Los Angeles International Airport, the aircraft crashed into the Pacific Ocean near Anacapa Island, California. All 88 persons aboard the aircraft died.
In October 2001, as a result of the investigation of this accident, the Safety Board issued eight recommendations to the FAA dealing with issues such as lubrication and inspection procedures, and training requirements for mechanics who inspect horizontal stabilizers on this series of aircraft.
Recommendations issued to the FAA in today's final report include:
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. Pilots should further be instructed that if checklist procedures are not effective, they should land at the nearest suitable airport.
Require operators and maintenance facilities that overhaul jackscrew assemblies to permanently (1) track end play measure according to airplane registration numbers 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 should also require operators and maintenance facilities that overhaul jackscrews assemblies to report this information to the FAA for use in determining and evaluating an appropriate end play check interval.
Require that maintenance facilities that overhaul jackscrew assemblies record and inform customers of an overhaul assembly's end play measurements.
Require operators to measure and record the on-wing end play measurement whenever a jackscrew assembly is replaced.
Establish the jackscrew assembly lubrication procedure as a required inspection item that must have an inspector's signoff before the task be considered complete.
A summary of this report is available now on the NTSB Web site at http://www.ntsb.gov, under "Publications"; the complete report will also be posted at that location in a few weeks. Soon afterwards, printed copies will be available for purchase through the National Technical Information Service.