The national Transportation Safety Board staff suggested yesterday the Chicago DC10 disaster was the result of a complex sequence of events that included failure of vital flight controls and warning systems caused by maintenance-induced damage to the airplane.
The staff appeared to place the larger burden of blame for the May 25 accident on American Airlines, which flew and maintained the ill-fated plane, rather than McDonnell Douglas, which built the plane and designed the part that was damaged during maintenance. The companies have been pointing fingers at each other almost from the day of the crash which, with 273 deaths, was the nations worst air disater.
The staff suggestion is only part of a working document from which the full safety board will make a final determination of "probable cause" of the accident. The board recessed until this morning after a day of deliberations yesterday.
In preliminary discussion on probable cause yesterday, board member Francis McAdams indicated he will push for a more-equal split. "As I view it," he said, "the cause is improper maintenance procedures and the vulnerbility of the pylon design."
The pylon, a heavy structure that holds an engine to the DC10's wing, became the center of attention for accident investigators within 24 hours of the crash. There has been little debate, after some early confusion, that the accident occurred after the pylon and the engine it supported fell off the left-wing of American Airlines Flight 191 as it was taking off from Chicago's O'Hare International Airport.
The engine and pylon rotated up and over the wing, ripping loose hydraulic connections and electrical wiring and warning systems that might have told the pilot what was wrong with his airplane.
Because of severed control cables and lack of hydraulic fluid, the slats on the front edge of the left wing retracted while those on the right wing remained extended. That created a condition known as "asymmetrical slats" and meant the right wing had more ability to lift the plane than did the left wing.
The plane rolled the left -- a predictable reaction -- and plunged to the ground. It had been airborne 31 seconds.
The sequence started when the pylon and engine separated. Subsequent investigation and metallurgical examinations revealed there had been a 10-inch crack in a flange of the aft support, one of three points where the pylon was connected to the wing. Under the stress of takeoff, the aft support broke, and the engine and pylon pivoted forward and over the wing.
The 10-inch crack, investigators now believe, was caused during heavy maintenance work on the pylon. The pylon and engine had been removed as a unit so a bearing could be replaced. During removal and replacement, the flange apparently was struck and cracked.
Similar damage was discovered on other DC10s in inspections following the Chicago accident. In each case the cracks were traced to the maintenance procedure.
The argument between American and McDonnell Douglas has been over whether McDonnell Douglas knew, and therefore tacitly approved, of the procedure of removing the engine and pylon together. McDonnell Douglas, in instructions it sent to customers, suggested the engine and pylon be removed and replaced separately -- which would have reduced the possibility of a long crack.
Investigators have found that McDonnell Douglas experts offered guidance to Continental Airlines on how to repair two such cracks when flanges had been damaged.
"I believe we're saying," board staff member William Hendricks said, "that McDonnell Douglas had a moral obligation to investigate this [the cause or the cracks]."
The board also discussed the role of the Federal Aviation Administration in certifying the DC10 pylon when it declared the jumbo jet airworthy. The board also tentatively decided the flight crew had no opportunity to save the airplane, partly because instruments were disabled and there was nothing to tell them of the dangerous slat condition. Had they known about the condition, they might have been able to save the plane, board members said. t