Update: AS Flight 261 Probe Shifts to Boeing (long)

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Seattle Times

Sunday, November 19, 2000, 12:42 a.m. Pacific

Flight 261 probe shifts to Boeing

by Byron Acohido Seattle Times staff reporter

Federal investigators now believe a critical part of Alaska Airlines Flight 261's tail-control mechanism broke off in flight, making it impossible to pull the jet out of its sudden, fatal dive.

The Boeing MD-80, en route from Puerto Vallarta, Mexico, to San Francisco and then Seattle, crashed into the Pacific Ocean in January, killing all 88 aboard.

Analyzing information from the jet's flight-data recorder, a National Transportation Safety Board investigator has found that the only way Flight 261 could have tumbled into its final dive was if a part called the end stop - intended as a fail-safe device to keep the plane's horizontal stabilizer intact - broke off in flight.

Boeing officials have insisted that the end stop broke off a flight-control mechanism called the jackscrew assembly when the plane hit the water, not as it flew.

If the part is found to have broken in flight, rather than on impact, some liability for the crash could shift to Boeing. Such a finding also would raise questions about the design of the part in use on the more than 2,000 MD-80 and DC-9 jets in service worldwide.

Until the past few months, the focus of the crash investigation had been Alaska's maintenance of the jackscrew assembly, which controls the horizontal stabilizer, the winglike structure on the tail that helps the plane fly level, climb and descend.

Investigators are looking into whether the plane's jackscrew assembly was so worn that it should have been replaced three years before the crash.

An Alaska mechanic had ordered it replaced in September 1997, but other mechanics rescinded his order several days later.

When it was recovered after the crash, the jackscrew assembly was damaged in ways that might explain at least the initial control problems that caused Flight 261 to dive suddenly from 31,000 to 24,300 feet. After its pilots regained some control and brought the jet to 18,000 feet, it dived again, this time into the Pacific Ocean just north of Los Angeles.

The jackscrew assembly includes a 2-foot-long threaded shaft attached to the forward edge of the stabilizer. Electric motors at the top of the jackscrew twist it up and down through a stationary "gimbal nut."

As the jackscrew turns through the gimbal nut, it tilts the forward edge of the stabilizer at different angles: a tilt upward pushes the aircraft nose down and vice versa.

The end stop is attached to the bottom of the jackscrew. It prevents the stabilizer's forward edge from tilting up more than 2.2 degrees, the angle at which the stabilizer is designed to produce its maximum downward force on the nose.

The NTSB now believes the stabilizer may have swung up 22 degrees - 10 times the normal maximum.

Working with data downloaded from Flight 261's digital flight-data recorder, NTSB performance analyst Dan Bowers programmed a flight simulator to mirror Flight 261's moment-to-moment position changes during the second dive.

The only way Bowers could re-create the second dive was by tilting the stabilizer up 22 degrees.

And the only way to do that - or even to tilt it a half degree higher than normal - is to separate the jackscrew from the end stop. Given the damage found on the jackscrew, it is possible the structural integrity of Flight 261's stabilizer may have depended entirely on the end stop, rather than on the entire jackscrew mechanism.

That's never supposed to happen. One of the Federal Aviation Administration's cardinal safety tenets - its "no single failure" rule - is that no single aircraft component should be able to fail in a way that could lead to a crash.

In order to win certification of the DC-9/MD-80 stabilizer system in the 1960s, McDonnell Douglas designers had to provide the FAA with analysis showing that the odds of the end stop failing were one in a billion.

Yet salvagers recovered the jackscrew from the ocean floor with the gimbal-nut threads wrapped Slinky-like around it and the end stop missing.

If the safety board concludes the end-stop design was inadequate, it could advise the FAA to make Boeing redesign the end stop and retrofit it on all 2,000 DC-9/MD-80s worldwide.

Two tests, two results

To counter Bowers' findings, Boeing has argued that the end stop remained intact throughout the entire flight and must have broken off as the plane hit the water.

The aircraft maker ran a "failure test" to show that the aerodynamic forces pushing on Flight 261's stabilizer and jackscrew were never strong enough to break off the end stop.

But when Boeing first produced these findings, other investigators protested the test was too simplistic. Heeding those complaints, the NTSB made Boeing redo the test, with added stress forces from multiple directions.

During that Oct. 2 test at Boeing's Huntington Beach, Calif., lab, the end stop broke off, but at a high-enough force that Boeing could still argue the plane would have had to hit the ocean for the end stop to break off. Since then, investigators have been debating what conclusions to draw from Boeing's second test.

In an Oct. 20 letter to NTSB chief investigator Richard Rodriguez, Alaska's Terry Clark, who is coordinating the airline's participation in the NTSB investigation, says components used in Boeing's most-recent test were assembled incorrectly.

Clark called for a retest by independent experts.

"I can only assume that the apparent irregularities in assembling the test . . . were an innocent error because the effect on the test results would be substantial," wrote Clark. "Nevertheless, we believe it calls for a new test to be conducted."

Clark contends Boeing engineers set up the parts being tested so that too much stress was placed on the jackscrew's outer shaft, made of hardened steel, and too little on its core "quill," made of titanium.

In an actual flight, Clark contends, the titanium bears most of the stress because the end stop rides mostly on the quill.

By directing stress to the steel shaft instead of the titanium quill, Boeing's test supported its claim that high loads of force - such as the plane hitting the water - are required to separate the end stop from the quill.

But under Alaska's scenario, much smaller forces - such as those occurring as the pilots were fighting to keep Flight 261 flying level - were all that were needed to separate the end stop from the quill.

"Simulating accident scenarios with such a model will not represent what could have happened in ASA flight 261," Clark wrote of Boeing's test. "The loads involved in the accident can be substantially lower than what was obtained in this test."

The NTSB's Rodriguez declined comment for this story, as did officials at Boeing and Alaska.

"We're in an investigative process right now," said Boeing spokeswoman Liz Verdier. "The only thing we can do is to support the NTSB investigation to keep the industry safe."

Alaska spokesman Jack Evans said the airline "believes that the public has a right to know," but pointed to a safety-board hearing, scheduled Dec. 13-15 in Washington, D.C., as the proper forum to learn more about what happened to Flight 261.

Dives examined separately

With the hearing less than a month away, investigators are drawing a distinction between the plane's first and second dives.

One theory is that the first dive was caused by weakened threads from the jackscrew's gimbal nut stripping off the nut and coiling around the jackscrew. As the threads stripped, the jackscrew slid up until its end stop ran up against the gimbal nut, tilting the stabilizer to its maximum nose down position.

Capt. Ted Thompson, 53, and First Officer Bill Tansky, 47, both military-trained aviators and experienced airline pilots, managed to level the plane after it dived 7,000 feet in one minute. They told air-traffic controllers and airline ground personnel what was going on.

But about nine minutes later, something caused the aircraft to dive again - this time much more dramatically. The plane pitched down 26 degrees per second for several seconds, sending the 80-ton jetliner on its final plunge into the ocean, from an altitude of more than three miles.

In addressing questions about that second dive, investigators are examining whether troubleshooting by the pilots may have generated forces that contributed to the end stop breaking and the nose dropping so rapidly.

At the same time, they're still looking into the role of Alaska's maintenance.

One possibility is that the jackscrew was not adequately lubricated - either because mechanics were lax or the grease Alaska used didn't work. Any excess friction could have accelerated wear of the gimbal nut, perhaps causing the threads to strip.

If the threads hadn't stripped, Flight 261 might never have reached the point where the structural integrity of its stabilizer depended solely on the strength of the end stop.

-- Rachel Gibson (rgibson@hotmail.com), November 19, 2000


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