The National Transportation Safety Board, in an interim report, said that a laboratory examination of intact sections of the roof found rivet holes on one layer of the plane’s skin did not line up properly with an underlying layer. The board also said that it found paint from the exterior of the plane had bled through into the inside. Experts said that suggested the aluminum skin had not been properly bound together, leading to premature damage from fatigue.
The board, as is its practice, did not draw any conclusions about the cause of the rupture, which occurred at 34,000 feet. It will not do so until research is complete and its five members receive a report from the staff, something that will probably not happen for months. But outside experts said that the 15-year-old Boeing 737 probably left the factory with flaws.
“It means the assembly was wrong, it means the wrong tools were used, it means they were careless in drilling the holes, and maybe the drill was dull,” said John J. Goglia, an aircraft maintenance expert who is a former member of the safety board.
Robert W. Mann Jr., an aviation industry expert in Port Washington, N.Y., said such flaws were unusual. “The key issue is whether this was systemic,” he said. “ Why weren’t the parts rejected?”
Boeing, in a statement, said it would not speculate about the cause of the incident but that “we remain fully engaged with the investigation.”
The safety board said it was also examining the five other Southwest planes that were found to have cracks. Those five planes and the one that ripped open all had about 40,000 cycles of takeoffs and landings. After the Southwest incident, Boeing said it did not expect that these models of 737s needed to be inspected before at least 60,000 cycles.
In an emergency order days after the incident, the Federal Aviation Administration ordered airlines flying those planes to check for cracks at 30,000 cycles.
The six planes were delivered by Boeing from 1994 to 1996. Boeing said it had completed a worldwide inspection of nearly 80 percent of 190 similar 737s and found no other problems.
An F.A.A. official involved in the investigation, who asked not to be identified because the agency had not taken a formal position, said it was too soon to know whether the agency’s inspection order would have detected bad riveting. But the official added that the results of that inspection did not show that there was any generic problem. In fact, it was possible that the Southwest airplane had a one-of-a-kind problem, the official said.
If the rivet holes on the two pieces of aluminum being fastened together did not line up right, that would mean they were egg-shaped instead of round, Mr. Goglia said. As the two pieces of metal were pulled in opposite directions when a plane is pressurized and depressurized, round holes would spread the forces evenly around the circumference of the hole. But if the hole is egg-shaped, he said, “they’re concentrated in one spot.”
The aviation industry is well acquainted with cracks developing around rivets as airplanes age. In April 1988, an Aloha Airlines plane peeled open almost like a sardine can, resulting in new inspection requirements. But that plane had 89,000 takeoffs and landings.
Hans J. Weber, owner of Tecop International, an aviation consulting firm in San Diego, said that manufacturing flaws were rare. “This is a real puzzle,” he said. “I am not fully satisfied with the explanation. The manufacturing of aluminum airplanes is very well understood.”
Mr. Mann said he was concerned about the paint. “These are not small defects that you could have wicking of the liquids,” Mr. Mann said. “Paint is not a thin substance. It is pretty substantial.”
If the parts were not the perfect shape as they came to the manufacturing plant, “that creates the necessity to redrill, which creates the ovalization,” Mr. Mann said, leading the parts to wear.
Analysts pointed out that there had been several problems in 1990s with planes that had been miswired or misassembled.
With major manufacturers like Boeing, the F.A.A. usually uses company employees to act as its designee in carrying out inspections, although it intermittently reviews their work. Mr. Mann said that in effect, “the F.A.A. designates the manufacturer as their own judge and jury.”
Matthew L. Wald reported from Washington and Jad Mouawad from New York.
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