Special Reports

Heavy wear found on jackscrew in '97

WASHINGTON - Alaska Airlines determined in 1997 that critical parts of the aircraft that crashed Jan. 31 should be replaced but reversed itself a day later when further tests showed the parts fell within maximum tolerances, the National Transportation Safety Board said Sunday.

The board, in a late-night statement, emphasized there was no determination whether this had any bearing on the crash of Alaska Airlines Flight 261, which dived into the Pacific off Los Angeles, killing 88 people. However, the information is significant enough to spark a major review of all the airline's maintenance records on the accident aircraft, a McDonnell Douglas twin jet MD-83.

The parts in question, the jackscrew and a gimbal nut that fit together to raise and lower the horizontal stabilizer, have been found to have evidence of damage that might have occurred before the crash and that could have come from grinding between the two as the mechanism moved.

Meanwhile, eight Alaska Airlines jetliners remained grounded Sunday with tail equipment problems that may be linked to the crash of Flight 261. Other carriers said most of their planes were back in the air.

The Federal Aviation Administration last week ordered inspections of nearly 1,100 planes, focusing on the jackscrew.

Airlines had reported finding jackscrew problems in 22 planes by midday Sunday. They were given until today to complete the checks of all MD-80 series, MD-90s, DC-9s and Boeing 717s.

Two of Alaska Airlines' grounded planes have metal shavings around the jackscrews, a problem FAA officials are most concerned about. The other six, and many of the 14 planes from other airlines, have a gritty residue around the jackscrews that may prove to be from normal wear.

The crew of Flight 261 reported serious problems with the stabilizer, which changes angle to keep the plane steady and level in flight, as they flew from Puerto Vallarta, Mexico, to San Francisco and Seattle.

As the crew prepared for an emergency landing in Los Angeles, the plane suddenly dived 17,900 feet in about one minute, plunging into the ocean about 40 miles offshore.

Sources close to the investigation said earlier that preliminary examination of the jackscrew and the nut indicated the nut apparently was worn far beyond limits and therefore might have been able to slip up and down over the jackscrew. If that proves to be the case, it is possible that the stabilizer made a sudden movement that ripped out mechanical stops intended to hold it within certain angles and possibly either pushed the plane into a dive or put the entire tail surface into an aerodynamic stall, which would have the same effect.

The board said that the Sept. 29, 1997, tests on the aircraft came at the airline's maintenance base in Oakland, Calif. Mechanics at that base are under investigation by the U.S. attorney's office and the Department of Transportation's office of inspector general for allegedly falsifying maintenance records, although there is no indication yet that might have been the case with the MD-83 that crashed.

The plane was undergoing the second-most exhaustive maintenance check, designated C-5. The heaviest, a D check, strips a plane to its frame and rebuilds it.

The board said maintenance records show that an "end-play" check of the jackscrew and nut found that the nut "has maximum allowable end-play limit," meaning it was worn almost to its limit and needed to be replaced.

However, one day later, Sept. 30, maintenance records show that "this initial planned action was re-evaluated," and the assembly was reinspected. That test, and five other identical tests, showed that the nut was within tolerances and the results were "signed off" by an Alaska Airlines maintenance inspector.

The board's statement emphasized that the safety board "will further investigate all maintenance activity associated with the stabilizer trim system, from date of manufacture to the date of the accident."

The Associated Press contributed to this report.

  Comments