jackscrew assembly acme nut threads of Alaska Airlines – Mcdonnel Douglas MD83 (N-963AS) flight AS261

Alaska Airlines flight AS261 crashed on January 31, 2000, about 1621 Pacific standard time, Alaska Airlines, Inc., flight AS261, a McDonnell Douglas MD-83, N963AS : it crashed into the Pacific Ocean about 2.7 miles north of Anacapa Island, California.

The 2 pilots, 3 cabin crew members, 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.

Alaska Airlines flight AS261 was near inverted, rolling through -150° with an airplane-nose-down pitch angle of -9°

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 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 Airlinesí extended lubrication interval and the Federal Aviation Administration (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 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.