Investigation Summary
The MD-80 series uses a T-tail design where the entire horizontal stabilizer moves to provide trim. This movement is driven by a jackscrew assembly. During the flight, the Acme nut threads inside this assembly stripped away entirely. This occurred because the grease intended to lubricate the screw had been depleted, leading to metal-on-metal wear. The NTSB found that Alaska Airlines had extended the interval between lubrication tasks and end-play checks (a measurement of wear) without adequate technical justification. When the threads finally failed, the stabilizer was no longer held in place by the jackscrew, allowing it to flip beyond its aerodynamic limits. The pilots fought to maintain control for approximately 35 minutes after the initial jam. They attempted to divert to Los Angeles, but during their troubleshooting, the remaining mechanical constraints failed, leading to a final, unrecoverable nose-down pitch.
Final Conclusions
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. The failure was caused by excessive wear resulting from Alaska Airlines' insufficient lubrication of the jackscrew assembly. Contributing factors included the airline's extended lubrication interval and the FAA's approval of those extensions, as well as the lack of a fail-safe mechanism in the MD-80 design to prevent catastrophic loss of control if the Acme nut threads failed.
Video Analysis
The Maintenance Failure of Alaska Airlines Flight 261 — This video examines the 2000 crash of an MD-83 caused by a stripped jackscrew assembly. It details the maintenance oversight and lubrication failures that led to the loss of horizontal stabilizer control.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
Pilot-in-command experience on MD-80s.
First Officer
All recorded hours were as a first officer on the MD-80.
Systems & Failure Modes
Horizontal Stabilizer Jackscrew
A large screw and nut assembly that moves the horizontal stabilizer to trim the aircraft's pitch.
Acme Nut
The internal component of the jackscrew assembly whose threads failed due to lack of lubrication.
Interesting Facts
- 01The aircraft had logged 26,584 flight hours and 14,315 cycles at the time of the accident.
- 02The Acme nut threads were found to be completely stripped, leaving the jackscrew free to move through the nut.
- 03Alaska Airlines had extended the lubrication interval for the jackscrew from 600 hours to 2,500 hours.
- 04Both pilots were posthumously awarded the ALPA Gold Medal for Heroism for their attempts to fly the aircraft while inverted.
Safety Actions & Advisories
Jackscrew Lubrication Intervals
Source ↗The NTSB recommended that the FAA significantly reduce the maximum allowable interval between jackscrew lubrications and end-play checks for all DC-9 and MD-80 series aircraft.
Maintenance Oversight Reform
Source ↗The investigation led to stricter FAA oversight of airline maintenance programs, specifically regarding the extension of service intervals for critical flight components.
Known Controversies & Unanswered Questions
Dispatcher Pressure to Continue
Reference ↗The NTSB investigation revealed that Alaska Airlines dispatchers repeatedly suggested the crew continue to San Francisco (SFO) despite the jammed stabilizer. The crew expressed concern about flying over populated areas with a compromised aircraft, but dispatchers focused on the operational impact of a diversion to Los Angeles (LAX).