Alaska Airlines Flight 261

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Alaska Airlines Flight 261
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Frame from NTSB animation of Alaska Airlines Flight 261
Summary
DateJanuary 31, 2000
TypeMechanical failure
SitePacific Ocean near Anacapa Island, California
Passengers83
Crew5
Fatalities88
Survivors0
Aircraft typeMcDonnell Douglas MD-83
OperatorAlaska Airlines
Tail numberN963AS
Flight originLic. Gustavo Díaz Ordaz International Airport
StopoverSan Francisco International Airport
DestinationSeattle-Tacoma International Airport

Alaska Airlines Flight 261, a McDonnell Douglas MD-83 aircraft, crashed 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, and the airplane 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).[1]

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’ insufficient lubrication of the jackscrew assembly."[1]

2000-01-31 MD83 Alaska Airlines AS CA SFO Mechanical (jackscrews) 1320 88 0

The 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 [2].

Of the occupants, at least 35, including 12 employees[3], were connected to Alaska Airlines or Horizon Air in some manner [4], leading many airline employees to mourn for the losses in the crash.

Notable passengers

Initial flight segment

Alaska 261 departed from PVR at 1:37 p.m. PST, and climbed to its intended cruising altitude of flight level 310 (31,000 feet). 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 SEA, 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.[6] 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 of force to keep level.[6] Neither the flight crew, nor company maintenance, were able to determine the cause of the jam.[6] Repeated attempts to overcome the jam with the primary and alternate trim systems were unsuccessful[6]

During this time the flight crew had several discussions with the company dispatcher about whether to divert to LAX, or continue on as planned to SFO. Ultimately the pilots chose to divert.[6] 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."[6] Cockpit Voice Recorder (CVR) transcripts indicate that the dispatcher was concerned about the effect on the schedule ("flow") should the flight divert.[7]

File:Alaska Airlines Flight 261 path.PNG
Final flight path of Alaska 261

First dive and recovery

At 4:09 p.m., the flight crew were 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.[6] Both pilots struggled together to regain control of the aircraft, and only by exerting a pulling force of 130 to 140 pounds on the controls were the flight crew able to arrest the descent of the aircraft and stabilize themselves at approximately 24,400 feet.[6]

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.[7] 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."[7] 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.[8] 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.[6]

Second dive and crash

NTSB Animation: Final pitch-over and initial portion of the dive (.wmv file)

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.[6] Several aircraft in the vicinity had been alerted by ATC to maintain visual contact with the stricken jet and they immediately contacted the controller.[9] 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".[9] ATC then tried to contact Alaska 261. Although the CVR captured the co-pilot saying "Mayday", no radio communications were received from the flight crew during the final event.[7][9]

The CVR transcript shows the pilots' continuous attempts for the duration of the dive to regain control of the aircraft.[7] At one point, unable to raise the nose, they attempted to fly the aircraft "upside-down".[7] However the aircraft was beyond recovery, and fell approximately 18,000 feet nose-down and inverted for 1 minute, 21 seconds before impacting the ocean at high speed. At this time, pilots from aircraft in the area reported in, one SkyWest pilot saying "and he's just hit the water", meaning the plane had plunged into the ocean. Another reported, saying, "Yeah sir he uh he-uh hit the water, he's uh down". Everyone on board died instantly, and the aircraft was destroyed upon impact.[6]

The 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.[6] 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.[6]

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.[6] 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).[6] 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 be a factor in this accident.[6] 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.[6]

NTSB Animation: Longitudinal Trim System Description and Failure Sequence (.wmv file)

Identification of passengers

The following indicators were used to identify crash victims: [10]

  • Fingerprints
  • Dental records
  • Tattoos
  • Personal items
  • Anthropology examinations

Inadequate lubrication and end play checks

The investigation then proceeded to examine why scheduled maintenance had failed to adequately lubricate the jackscrew assembly. In interviews with the Alaska Airlines SFO mechanic who last performed the lubrication it was revealed that the task took about 1 hour, whereas the aircraft manufacturer estimated the task should take 4 hours.[6] This and other evidence suggested to the NTSB that "the SFO mechanic who was responsible for lubricating the jackscrew assembly in September 1999 did not adequately perform the task."[6] Laboratory tests indicated that the excessive wear of jackscrew assembly could not have accumulated in just the 4 months period between the September 1999 maintenance and the accident flight.[6] Therefore, the NTSB concluded that "more that just the last lubrication was missed or inadequately performed."[6]

In order to monitor wear on the jackscrew assembly a periodic maintenance inspection called an "end play check" was used. The NTSB examined why the last end play check on the accident aircraft in September 1997 did not uncover excessive wear. The investigation found that Alaska Airlines had fabricated tools to be used in the end play check that did not meet the manufacturer's requirements.[6] Testing revealed that the non-standard tools ("restraining fixtures") used by Alaska Airlines could result in inaccurate measurements, and that it was possible that if accurate measurements had been obtained at the time of the last inspection, these measurements would have indicated the excessive wear and the need for the replacement of the affected components.[6]

Extension of maintenance intervals

Between 1985 and 1996 Alaska Airlines progressively increased the period in between jackscrew lubrication as well as end play checks with the approval of the Federal Aviation Administration (FAA).[6] Since each lubrication or end play check subsequently not conducted had represented an opportunity to adequately lubricate the jackscrew or detect excessive wear, the NTSB examined the justification of these extensions. In the case of extended lubrication intervals, the investigation was not able to determine what information, if any, was presented by Alaska Airlines to the FAA prior to 1996.[6] Testimony from a FAA inspector regarding an extension granted in 1996 was that Alaska Airlines submitted documentation from Boeing as justification for their extension.[6]

End play checks were conducted during a periodic comprehensive airframe overhaul process called a "C-check". Testimony from the director of reliability and maintenance programs of Alaska Airlines was that a data analysis package based on the maintenance history of 5 sample airplanes was submitted to the FAA to justify the extended period between C-checks. Individual maintenance tasks (such as the end play check) were not separately considered in this extension.[6] The NTSB found that "Alaska Airlines' end play check interval extension should have been, but was not, supported by adequate technical data to demonstrate that the extension would not present a potential hazard."[6]

FAA oversight

A special inspection conducted by the FAA in April 2000 of Alaska Airlines uncovered widespread significant deficiencies that "the FAA should have uncovered earlier."[6] The investigation concluded that "FAA surveillance of Alaska Airlines had been deficient for at least several years."[6] The NTSB noted that in July 2001, an FAA panel determined that Alaska Airlines had corrected the previously identified deficiencies. However several factors led the Board to question "the depth and effectiveness of Alaska Airlines corrective actions" and "the overall adequacy of Alaska Airlines' maintenance program."[6]

Systematic problems were identified by the investigation in the FAA's oversight of maintenance programs, including inadequate staffing, its approval process of maintenance interval extensions, and the aircraft certification requirements.[6]

Aircraft design and certification issues

The jackscrew assembly was designed with 2 independent threads, each of which was strong enough to withstand the forces placed on it.[6] Maintenance procedures such as lubrication and end play checks were to catch any excessive wear before it progressed to a point of failure of the system. The aircraft designers assumed that at least one set of threads would always be present to carry the loads placed on it, therefore the effects of catastrophic failure of this system were not considered, and no "fail-safe" provisions were needed.

In order for this design component to be approved ("certified") by the FAA without any fail-safe provision, it had to be considered "extremely improbable". This was defined as "having a probability on the order of 1 x 10-9 or less each flight hour."[6] However the accident showed that certain wear mechanisms could affect both sets of threads, and that the wear might not be detected. The NTSB determined that the design of "the horizontal stabilizer jackscrew assembly did not account for the loss of the acme nut threads as a catastrophic single-point failure mode."[6]

Jackscrew design improvement

In 2001, NASA recognized the risk to its hardware (such as the Space Shuttle) attendant upon use of similar jackscrews. An engineering fix developed by engineers of NASA and United Space Alliance promise to make progressive failures easy to see and thus complete failures of a jackscrew almost impossible. [11]

Conclusions

In addition to the probable cause, the NTSB found the following contributing factors:[6]

  • Alaska Airlines' extended lubrication interval and the FAA's approval of that extension, which increased the likelihood that a missed or inadequate lubrication would result in excessive wear of the acme nut threads
  • Alaska Airlines' 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 absence on the McDonnell Douglas MD-80 of a fail-safe mechanism to prevent the catastrophic effects of total acme nut loss
File:Routine maintenance on a MD-80 tailplane.PNG
Routine maintenance of the tail section a Northwest Airlines Douglas DC-9, the predecessor to the MD-80

During the course of the investigation, and later in its final report, the NTSB issued a total of 24 safety recommendations, covering maintenance, regulatory oversight, and aircraft design issues. More than half of these were directly related to jackscrew lubrication and end play measurement.[6] Also included was a recommendation that pilots were to be instructed that in the event of a flight control system malfunction they should not attempt corrective procedures beyond those specified in the checklist procedures, and in particular in the event of a horizontal stabilizer trim control system malfunction the primary and alternate trim motors should not be activated, and if unable to correct the problem through the checklists they should land at the nearest suitable airport.[6]

In NTSB board member John J. Goglia's statement for the final report, which was concurred with by all three other board members,[6] he wrote:

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Aftermath

File:Sundial.jpg
Memorial Sundial

The families of the victims approved the construction of a memorial sundial that was placed at Port Hueneme. The sundial was designed by Santa Barbara artist Bud Bottoms to cast a shadow on a memorial plaque at 4:22 p.m. each January 31st.[12]

For their actions during the emergency, Captain Ted Thompson and First Officer Bill Tansky were awarded the Airline Pilots Association Gold Medal for Heroism, the only time the award has been given posthumously.[13]

Both Boeing and Alaska Airlines eventually conceded liability for the crash, and all but one of the lawsuits brought by surviving family members were settled out-of-court before going to trial.[14] Two victims from Alaska 261 were falsely accused of being the fathers of children in Guatemala in an attempt to gain insurance and settlement money. DNA testing revealed these allegations to be false.[15]

This crash was featured in a 2004 episode of National Geographic Channel's Air Crash Investigation television program (originally known as Mayday and also known as Air Emergency), titled Cutting Corners or Fatal Error.

Seattle Post-Intelligencer, Snopes, and About.com state that an urban legend stating that a pastor's wife led the passengers in prayer is false. [16] [17] [18]

The Ted Thompson/Bill Tansky Scholarship Fund was named after the two flight crew members.

The death of passenger Rodney "Rod" Pearson with his wife Sarah (an Alaska Airlines flight attendant) and their two daughters may have been a critical factor in the demise of the popular Seattle-area restaurant chain Six Degrees, which he co-founded.[19]

The Alaska Airlines flight 261 crash has appeared in various advance fee fraud ("419") email scams. In these scams, a scammer uses the name of someone who died in the crash to lure unsuspecting victims into sending money to the scammer by claiming the crash victim left huge amounts of unclaimed money in a foreign bank account. The names of Morris Thompson and Ronald and Joyce Lake were used in schemes unrelated to them.[20][21]

References

External links

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