Alaska Airlines Flight 261: How a Worn Jackscrew Caused the Dive

This Mentour Pilot episode on Alaska Airlines Flight 261 spends its first half on the machinery and the maintenance history, because, as the narrator says, the accident began long before the flight. The second half follows the flight using the cockpit voice recording.

The stabiliser and the jackscrew

On the MD-80 the whole horizontal stabiliser, about 40 feet wide and mounted on top of the tail fin, moves to trim the aircraft in pitch. It is driven by a single jackscrew turning through an acme nut. The narrator explains the redundancy built around it: two trim motors, a standby switch, electrical limits and mechanical stops. The screw and nut themselves had no backup. They were designed to be lubricated regularly and checked for wear, because the nut threads are softer than the screw and wear down by design.

Stretched intervals

The video traces how Alaska Airlines extended its lubrication interval over the years, with each extension approved by the FAA, until it reached a calendar limit that worked out to about 2,550 flight hours. It also covers the end play check, the measurement that shows how worn the nut is, which was stretched to about every 9,550 flight hours. The narrator describes evidence from the investigation that the grease was not always applied properly, including a mechanic's description of how long the job took him.

A jammed stabiliser

Captain Ted Thompson, who had flown more than 17,750 hours, and First Officer Bill Tansky, 57, flew the aircraft out of Puerto Vallarta. During the climb the stabiliser stopped responding, and the autopilot disconnected. The crew flew by hand, pulling on the controls, and as fuel burned off less force was needed. The cockpit voice recording covers only the last 30 minutes, so the narrator reconstructs the earlier part from flight data. The crew talked to company maintenance and dispatch about diverting to Los Angeles, and the narrator discusses the dispatcher's mention of delays at LAX.

The first dive

As the crew worked through the trim systems again, the stabiliser moved beyond its normal nose-down limit and the aircraft dived. The first officer asked, "What are you doing?" The two pilots together recovered with very large control forces. The narrator praises the captain's short announcement to the passengers and the first officer's argument for not touching the trim again while the aircraft was still flying. They planned to descend over the sea to test the aircraft at low speed before landing.

The end

At 16:19 the remaining threads failed and the stabiliser moved to an extreme nose-down position. The aircraft pitched over and rolled inverted. The captain tried to keep it flying upside down, and the engines stalled in the extreme airflow. The aircraft struck the ocean north of Anacapa Island.

The investigation

The jackscrew was recovered from the seabed with the nut's threads worn away. The narrator summarises the NTSB's findings on lubrication, the interval extensions and the lack of a fail-safe design, and notes that the two pilots were posthumously awarded the Air Line Pilots Association's Gold Medal for heroism. For other accidents rooted in maintenance, see American Airlines Flight 191 and Japan Air Lines Flight 123.