Final Report: FAA-approved maintenance procedure cited in fatal accident
MD-80, Anacapa Island, Calif., Jan. 31, 2000–Both pilots, three cabin crewmembers and 83 passengers on board the Alaska Airlines MD-80 were killed when Flight 261 crashed into the Pacific Ocean about 2.7 miles north of Anacapa Island at 4:21 p.m. PST.
It was a scheduled international passenger flight operating under Part 121 from Lic Gustavo Diaz Ordaz International Airport (MMPR) in Puerto Vallarta, Mexico, to Seattle-Tacoma International Airport (SEA), with an intermediate stop planned at San Francisco International Airport (SFO). Weather is not considered a factor as the entire trip was conducted in VMC. The pilots reported a handling problem to ATC and were en route to Los Angeles (LAX) for an emergency landing when the stabilizer failed and the aircraft plunged into the ocean.
The NTSB determined the probable cause 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. Subsequent investigation showed the horizontal stabilizer jackscrew likely jammed shortly after takeoff. The thread failure was caused by excessive wear resulting from Alaska Airlines’ insufficient lubrication of the jackscrew assembly. Contributing to the accident was Alaska Airlines’ extended lubrication interval, which had FAA approval. The NTSB believes that extension increased the likelihood that a missed or inadequate lubrication would result in excessive wear of the acme nut threads.
Also listed as contributing to the accident was the airline’s 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. The NTSB also cited as a contributing factor the lack of a McDonnell Douglas fail-safe mechanism to prevent the catastrophic effects of total acme nut thread loss.