Alaska Airlines Flight 261 , a McDonnell Douglas MD-83 aircraft, experienced a fatal accident on January 31, 2000 in the Pacific Ocean about 2.7 miles (4.3 km) north of Anacapa Island, California. The two pilots, three cabin crewmembers, and 83 passengers on board were killed—the highest death toll of any aviation accident involving a McDonnell Douglas MD-83 ever, and the aircraft was destroyed. Alaska 261 was a scheduled international passenger flight from Lic. Gustavo Díaz Ordaz International Airport (PVR), Puerto Vallarta, Mexico, to Seattle-Tacoma International Airport (SEA), with an intermediate stop planned at San Francisco International Airport (SFO).
The subsequent investigation by the National Transportation Safety Board determined that inadequate maintenance led to excessive wear and catastrophic failure of a critical flight control system during flight. The probable cause was stated to be "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 thread failure was caused by excessive wear resulting from Alaska Airlines's insufficient lubrication of the jackscrew assembly."
Accident flight
Passengers
47 of the passengers were bound for Seattle and 3 of the crew members were based in Seattle. Most of the other passengers were bound for San Francisco. Of the passengers, 1 was Mexican and 1 was British; the rest were Americans.
Of the occupants, at least 35, including 12 employees, were connected to Alaska Airlines or Horizon Air in some manner, leading many airline employees to mourn for the losses in the crash.
Notable passengers
- Jean Gandesbery, author of Seven Mile Lake , died with her husband Robert Gandesbery
- Morris Thompson, Commissioner of the Bureau of Indian Affairs in the late 1960s and early 1970s, died with his wife Thelma and daughter Sheryl.
- Tom Stockley, wine writer for the Seattle Post-Intelligencer .
- Cynthia Oti, a financial talk show host at San Francisco's KSFO-AM.
Initial flight segment
Alaska 261 departed from Puerto Vallarta at 1:37 p.m. PST, and climbed to its intended cruising altitude of flight level 310 (31,000 ft). Approximately 2 hours into the flight, the flight crew, consisting of captain Ted Thompson and first officer William "Bill" Tansky, first contacted the airline's dispatch and maintenance control facilities in SeaTac, Washington, and on a shared company radio with operations and maintenance facilities at Los Angeles International Airport (LAX) discussed a jammed horizontal stabilizer and a possible diversion to LAX. The jammed stabilizer prevented operation of the trim system, which normally would make slight adjustments to the flight control surfaces to keep the plane stable in flight. At their cruising altitude and speed the position of the jammed stabilizer required the pilots to pull on their controls with approximately 10 pounds (44N) of force to keep level. Neither the flight crew, nor company maintenance, were able to determine the cause of the jam. Repeated attempts to overcome the jam with the primary and alternate trim systems were unsuccessful
During this time the flight crew had several discussions with the company dispatcher about whether to divert to Los Angeles, or continue on as planned to San Francisco. Ultimately the pilots chose to divert. Later the NTSB found that while "the flight crew's decision to divert the flight to Los Angeles...was prudent and appropriate", nonetheless "Alaska Airlines dispatch personnel appear to have attempted to influence the flight crew to continue to San Francisco...instead of diverting to Los Angeles." Cockpit Voice Recorder (CVR) transcripts indicate that the dispatcher was concerned about the effect on the schedule ("flow") should the flight divert.
First dive and recovery
At 4:09 p.m., the flight crew was able to unjam the horizontal stabilizer with the primary trim system, however, upon being freed, it quickly moved to an extreme "nose-down" position, forcing the aircraft into a dive. Alaska 261 went from about 31,500 feet to between 23,000 and 24,000 feet in around 80 seconds. Both pilots struggled together to regain control of the aircraft, and only by exerting a pulling force of 130 to 140 pounds (59.1 to 63.6 kg) on the controls were the flight crew able to arrest the 6,000 foot-per minute descent of the aircraft and stabilize themselves at approximately 24,400 feet.
Alaska 261 informed Air Traffic Control (ATC) of their control problems. After the flight crew stated their intention to land at LAX, ATC inquired if they wanted to proceed to a lower altitude in preparation for approach. The captain replied: "I need to get down to about ten, change my configuration, make sure I can control the jet and I'd like to do that out here over the bay if I may." Later, during the public hearings into the accident, the request by the pilot not to overfly populated areas was specifically commended by NTSB board members. During this time the flight crew considered, and rejected, any further attempts to correct the runaway trim. They proceeded to descend to a lower altitude and start to configure the aircraft for landing at LAX.
Second dive and crash
Beginning at 4:19 p.m., the CVR recorded the sounds of at least four distinct "thumps", followed 17 seconds later by an "extremely loud noise". The aircraft rapidly pitched over into a dive. Several aircraft in the vicinity had been alerted by ATC to maintain visual contact with the stricken jet and they immediately contacted the controller. One pilot radioed "that plane has just started to do a big huge plunge,"; another reported, "Yes sir, ah, I concur he is, uh, definitely in a nose down, uh, position descending quite rapidly." ATC then tried to contact Alaska 261. The crew of a Skywest airliner reported "He's, uh, definitely out of control" Although the CVR captured the co-pilot saying "Mayday", no radio communications were received from the flight crew during the final event.
The CVR transcript shows the pilots' continuous attempts for the duration of the dive to regain control of the aircraft. At one point, unable to raise the nose, they attempted to fly the aircraft "upside-down". However the aircraft was beyond recovery; it descended inverted and nose-down about 18,000 feet for 81 seconds, a descent rate exceeding 13,300 feet per minute before hitting the ocean at high speed. At this time, pilots from aircraft flying in the same area reported in, with one SkyWest Airlines pilot saying, "and he's just hit the water," meaning the plane had plunged into the ocean. Another reported, "Yeah sir, he, uh, he, uh, hit the water, he's, uh, down." Everyone on board died when the plane struck the water, and the aircraft was destroyed upon impact.
Investigation
Wreckage recovery and analysis
Using side-scan sonar, remotely operated vehicles, and a commercial fishing trawler, workers recovered about 85% of the fuselage (including the tail section) and a majority of the wings. In addition, both engines, as well as the Flight Data Recorder (FDR) and CVR were retrieved. All wreckage was unloaded at Port Hueneme, California for examination and documentation. Both the horizontal stabilizer trim system jackscrew (also referred to as "acme screw"), and the corresponding acme nut, which the jackscrew turns through, were retrieved. As the jackscrew rotates it moves up or down through the (fixed) acme nut. This up and down motion moves the horizontal stabilizer for the trim system. The jackscrew was found with metallic filaments wrapped around it; these were later determined to be remnants of the threads from the acme nut.
Later analysis estimated that 90% of the threads in the acme nut had been previously worn away, and that they were then completely sheared off during the accident flight. Once the threads failed, the horizontal stabilizer assembly was then subject to aerodynamic forces that it could not withstand, and ultimately failed. Based on the time since the last inspection of the jackscrew assembly, the NTSB determined that the wear had occurred at a much faster than average rate (0.012 inch per 1,000 flight hours, when the expected wear was 0.001 inch per 1,000 flight hours). The NTSB considered a number of potential reasons for this excessive wear, including the substitution by Alaska Airlines (with the approval of the aircraft manufacturer Boeing) of Aeroshell 33 grease instead of the previously approved lubricant, Mobilgrease 28. The use of Aeroshell 33 was found not to be a factor in this accident. Insufficient lubrication of the components was also considered as a reason for the wear. Examination of the jackscrew and acme nut revealed that no effective lubrication was present on these components at the time of the accident. Ultimately the lack of lubrication, and resulting excessive wear of the threads, were determined to be the direct causes of the accident.
NTSB Animation: Longitudinal Trim System Description and Failure Sequence (.wmv file)
Identification of passengers
The following indicators were used to identi
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