McDonnell Douglas MD-83 (2024)

Alaska Airlines Flight 261, N963AS

Anacapa Island, California

January 31, 2000

On January 31, 2000, about 1621 Pacific standard time, Alaska Airlines Flight 261, a McDonnell Douglas MD-83, N963AS, crashed into the Pacific Ocean about 2.7 miles north of Anacapa Island, California. All 88 people on board were killed and the airplane was destroyed on impact. Flight 261 was a scheduled international passenger flight from Lic Gustavo Diaz Ordaz International Airport (PVR), Puerto Vallarta, Mexico, to Seattle-Tacoma International Airport, Seattle, Washington, with an intermediate stop planned at San Francisco International Airport, San Francisco, California.

The National Transportation Safety Board (NTSB) determined 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's Acme nut threads. The thread failure was caused by excessive wear resulting from Alaska Airline's insufficient lubrication of the jackscrew assembly.

Accident Overview

Alaska Airlines Flight 261 with two pilots, three cabin crew, and 83 passengers departed Puerto Vallarta (PVR), Mexico to Seattle, Washington with a scheduled stop in San Francisco, California.

Visual meteorological conditions prevailed for the flight that operated under an instrument flight rules flight plan.

The airplane was functioning normally during the initial phase of flight, but the horizontal stabilizer stopped responding to autopilot and pilot commands after the airplane passed through 23,400 feet.

The pilots recognized the longitudinal trim system was not functioning but could not determine why.

The longitudinal trim system allows the pilot to stabilize the airplane with zero control forces or allows the autopilot to control pitch. For the MD-83, the system consists of a single jackscrew and gimbal nut, located near the top of the vertical stabilizer, with primary and alternate motors that drive the jackscrew assembly. The jackscrew nut (Acme nut) is fixed, and rotation of the jackscrew through the Acme nut moves the horizontal stabilizer to trim the airplane. The Acme nut is intended as the "wear point" in the system and requires regular lubrication and inspection.

The jackscrew is retained at the lower end by an end cap and at the upper end by a fixed stop.

The airplane takeoff (TO) weight was 136,515 pounds, well below TO and climb limits. The airplane maximum landing weight was 130,000 pounds so the airplane would have been required to remain in flight for 45 minutes in order to burn enough fuel to reach the max landing weight.

The flight crew did not elect to return to PVR. Neither the Alaska Airlines MD-80 Quick Reference Handbook Stabilizer Inoperative nor the company Runaway Stabilizer Emergency Checklist required landing at the nearest suitable airport.

The flight crew manually flew the airplane and contacted Alaska Airlines dispatch and maintenance control in San Francisco when they got close to the U. S. border, approximately two hours after departure.

At altitude and higher cruise speed, manually flying the airplane was not difficult for the crew.

The crew discussed the malfunctioning trim system and current and expected weather at Los Angeles (LAX) and San Francisco (SFO) with Alaska's dispatch and maintenance. Although the crew elected not to return to Puerto Vallarta, they decided to land at LAX rather than continue to SFO. The weather conditions at SFO, prevailing crosswinds, the increased landing speed required, and over flying a suitable airport (i.e., LAX) may have influenced their decision.

After flying manually for two hours, the crew tried to use the autopilot several times and it disengaged each time.

The flight crew was troubleshooting the trim system by using the trim handles and control wheel trim switches while in contact with the dispatch and maintenance personnel. The Alaska Airlines MD-80 Stabilizer Inoperative checklist states, "do not use the autopilot" if the trim systems are inoperative.

While the flight crew was troubleshooting the system, the primary trim motor moved the jackscrew and freed the jammed Acme nut, allowing the horizontal stabilizer to move leading edge up until it reached the lower mechanical stop, initiating a dive from the cruise altitude of 31,050 feet.

The crew made control inputs to bring the nose up, deployed the speed brakes to retard the rapidly increasing airspeed, and recovered from the dive at approximately 24,000 feet.

After the flight crew recovered from the initial dive, the crew deployed the slats and flaps to slow the airplane in preparation for an emergency landing at LAX; the airplane remained stable and controllable.

The slats and flaps were later retracted, and the airspeed increased. The captain then ordered redeployment of the slats and flaps. The co*ckpit Voice Recorder (CVR) recorded a discernible noise; the airplane rapidly pitched nose down in the final dive and crashed into the ocean.

Radar detected several small primary returns consistent with parts of the vertical stabilizer being torn from the airplane.

See NTSB Horizontal Trim System Description and Sequence Failure Animation.

See Alaska 261 Jackscrew Flash Animation.

See Alaska 261 3D flight path Flash Animation.

After the accident, severely worn and sheared remnants of the Acme nut threads were recovered wrapped around the Acme screw. Approximately 90 percent of the thread thickness had worn away before the remainder of the threads sheared off.

When the crew operated the trim system, the jam was freed causing the initial dive. The Acme screw did not completely separate during the initial dive. The pilots knew the longitudinal trim system was jammed but did not know why. The worn threads of the Acme nut had been sheared off and jammed the Acme screw. The redeployment of the flaps and slats resulted in higher aerodynamic loads on the tail surfaces and caused the jackscrew and Acme nut to completely separate, and the final dive.

Accident Memorial

Memorial to the victims of Alaska Flight 261 located at Port Hueneme, California. The 20-foot-long sundial will cast a shadow on the memorial plaque at 4:22 PM each January 31.

Accident Board Findings

View complete text of the National Transportation Safety Board's Aircraft Accident Report which includes Accident Board Findings.

The safety board determined the probable cause of the accident was a loss of airplane pitch control resulting from 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.

While not specifically mentioned in the NTSB findings, the Acme nut grease fitting passage, which allows the grease to reach the jackscrew and Acme nut threads, was found plugged with dry residue.

The NTSB concluded that two major factors contributed to the accident: 1) Alaska Airlines' extended lubrication interval and the Federal Aviation Administration's (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 2) 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. Also contributing to the accident was the absence of a fail-safe mechanism on the MD-83 horizontal stabilizer jackscrew trim system to prevent the catastrophic effects of total Acme nut thread loss. The dual-thread design of the Acme screw nut does not provide redundancy with respect to wear. The design of the jackscrew assembly did not account for the loss of the Acme nut threads as a catastrophic single-point failure mode.

The NTSB viewed the decision not to return to Puerto Vallarta as understandable given that there is no requirement in the Airplane Flight Manual (AFM) to land at the nearest suitable airport following a jammed or malfunctioning horizontal stabilizer. The crew's decision to land at LAX was considered appropriate.

However, the flight crew's use of the autopilot while the horizontal stabilizer was jammed was not appropriate. The crew should not have tried to troubleshoot the system by using the autopilot and trim motors. Once a stable airplane-landing configuration was obtained, landing at LAX should have followed immediately.

Airplane Life Cycle:

Operational
Design / Manufacturing

Accident Threat Categories:

Structural Failure
Lack of System Isolation / Segregation

Groupings:

Accident Common Themes:

Organizational Lapses
Human Error
Flawed Assumptions

Common Themes

Organizational Lapses

Alaska Airlines had extended the recommended maintenance and inspection intervals of the jackscrew assembly, resulting in it becoming unlubricated. A plugged grease fitting and inspection intervals that were beyond those recommended by the airplane manufacturer allowed the condition of the jackscrew to deteriorate to a sufficiently worn state that it failed and resulted in a loss of control of the airplane.

Human Error

The flight crew underestimated the severity of the threat to the airplane. The crew did not recognize the deteriorating flight conditions until it was too late. They spent approximately two hours trouble shooting prior to contacting the airline's dispatch and maintenance control facilities to discuss a jammed horizontal stabilizer and a possible diversion to Los Angeles International Airport

Flawed Assumptions

It was assumed that the horizontal trim system was sufficiently redundant that a failure of a single thread path would not interfere with the operation of the jackscrew or trim system. Ultimately, this assumption was flawed, as a single failure resulted in the structural failure of the horizontal stabilizer.

Certain designs have proven to be less vulnerable to wear and wear-related failures. Ballscrew thread configurations have shown to be more resistant to failures. Many current transport airplanes designs use ballscrew type assemblies for stabilizer trim.

Related Accidents / Incidents

Eastern Airlines Douglas DC-8, New Orleans, Louisiana, February 25, 1964

Eastern Air Lines, Inc., Flight 304, a DC-8, N8607, crashed in Lake Pontchatrain, Louisiana, approximately 19 miles northeast of the New Orleans International Airport, at approximately 0205 CST, February 25, 1964. All 51 passengers and the crew of seven were fatally injured.

The flight, scheduled from Mexico City to New York City, with several intermediate stops, had just departed New Orleans at 0200. Three minutes later the captain acknowledged a request to change radio frequencies, but no further communications were received from the flight. At 0205-40 the radar target associated with Flight 304 had disappeared from the scopes of both the radar controllers who were observing the flight. Moderate to severe turbulence existed in the area at the time of the accident.

The Civil Aeronautics Board determined the probable cause of this accident was the degradation of aircraft stability characteristics in turbulence because of abnormal longitudinal trim component positions.

LESSONS LEARNED

Technical Related Lessons

Single point failure of any critical structure or system should be avoided through design, whenever practical. Where single point failures are unavoidable, robust safety provisions must be in place, including inspection and maintenance intervals, which will preclude the catastrophic failure. (Threat Category: Lack of System Isolation/Segregation)

The jackscrew assembly had functional redundancy; in that it was designed with two independent sets of threads. However, a single failure of the Acme nut threads allowed all the threads in the assembly to be stripped (fail), which freed the jackscrew from the assembly, and resulted in unrestricted motion of the horizontal stabilizer.

Common Theme Related Lessons

Safety strategies that are dependent on multiple load paths should ensure that the load paths are adequately isolated. (Common Theme: Flawed Assumptions)

The horizontal stabilizer jackscrew and nut assembly were designed with two sets of helical threads, intended to operate independently. Both sets of threads were load bearing and supported the flight loads imposed on the horizontal stabilizer. The design intent of the dual threads was to provide redundancy should a single thread path fail. Appropriate isolation of the threads would have allowed the failure of one set of threads without threatening the integrity of the other set of threads. In this accident, the failure of a single thread path resulted in the failure of both paths, resulting in failure of the jackscrew, and the accident.

Deviation from flight crew procedures can have catastrophic results. (Common Theme: Human Error)

The Alaska Airlines checklist procedure associated with the jammed horizontal stabilizer prohibited engagement, or use, of the autopilot. The flight crew, while troubleshooting the jammed stabilizer repeatedly attempted to engage the autopilot. The autopilot would momentarily engage, then disconnect. The repeated momentary engagement of the autopilot may have contributed to the thread failures in the acme nut, allowing the jackscrew to be unrestrained in its movement, resulting in the loss of control.

Certain key maintenance tasks should be identified andemphasized in a manner that ensures required tasks are consistently and correctly accomplished. (Common Theme: Organizational Lapses)

The manufacturer intended that the stabilizer jackscrew receive required regular and consistent lubrication to avoid excessive, accelerated wear. Inadequate lubrication, either by extended maintenance intervals, or improper task performance, caused instances of under lubricated jackscrew assemblies, and in this case, total failure of the jackscrew assembly.

Last updated: Friday, December 16, 2022