National Transportation Safety Board experts established yesterday that the broken bolt widely blamed for last Friday's DC10 jumbo jet crash that killed 274 persons in Chicago snapped after some other structural failure had occured.
"As far as can be ascertained so far, the bolt broke as a result of the accident," Michael Marks, chief metallurgist for the safety board, told the board at a hearing yesterday afternoon. The plane crashed after it dropped and engine on takeoff.
Since the bolt was first discovered missing shortly after the board began its investigation, it has never been clearly established what broke down first in the complex arrangement of bolts, bushings and struts that held the engine and its support pylon to the wing.
Marks told the board that the bolt "had broken in an overstress condition, rather than a fatigue condition." However, fatigued metal - which has grown "tired" through the application of continual stresses - has been found in several areas around the broken bolt in DC10 engine assembly supports.
Moreover, a number of metal fatigue defects, broken bolts, cracked bushings and snapped metal fasteners have been found on DC10 engine mounting assemblies during inspections over the past two days. Those inspections, ordered by FAA Administrator Langhorne Bond after he grounded all domestic DC10s Tuesday have resulted in repairs on 37 of the aircraft, the FAA said. About 100 DC10s have passed inspection or been repaired and are flying again, the FAA said.
When he was asked yesterday if he thought the plane should be grounded again, Bond said he was satisfied that everything had been done that needed to be done, based on knowledge gained so far.
FAA structural and safety specialists met yesterday afternoon with DC10 experts from McDonnell Douglas, the plane's manufacturer, to gain a better understanding of the engine assembly supporting structure.
"There's difficulty in figuring out the sequence of events" of the engine falling off the wing, said FAA spokesman Jerome Doolittle. The left engine fell just as American Airlines flight 191 was lifting off the runway at Chicago's O'Hare International Airport. The plane rose about 600 feet, rolled to its left, fell to ground and exploded. There were no survivors.
Safety board members asked for the metallurgical report during their regular weekly meeting yesterday. James B. King, board chairman, said that the accident raised the question as to whether the DC10 has a "design problem," but that structural engineers would have to answer it.
The death toll in the Chicago crash was raised from 273 to 274 yesterday when investigators at the crash site found the body of an infant. There is concern that the toll could continue to grow because children and infants are not normally listed on flight manifests.
Some airlines also reported yesterday that passengers are reluctant to fly on the DC10, which is flown by eight U.S. airlines.
Anthony George, interviwed by the Associated Press in Los Angeles, said he was alarmed to learn that his children were booked to New York on a DC10. "I put them on a different plane," he said.