Phoenix:
The five-foot hole that ripped open midflight Friday in the roof of a Southwest Airlines jet and terrified passengers on their way to Sacramento from Phoenix was caused by fatigue cracks in the aluminum underskin of the lap joints, the National Transportation Safety Board said Sunday.
In addition, Southwest Airlines said that inspections had detected subsurface cracks in the bodies of two other Boeing 737 jetliners similar to those found on that flight.
"Based on this incident and the additional findings, we expect further action from Boeing and the F.A.A. for operators of the 737-300 fleet worldwide," Mike Van de Ven, Southwest's executive vice president and chief operating officer, said in a statement Sunday evening.
The airline said it has retested and cleared 19 other planes and planned to finish testing 58 more by Tuesday. Southwest had canceled about 300 daily flights on Saturday and Sunday after the problem on Friday.
Robert L. Sumwalt, a member of the board, said in a telephone interview from Arizona that the fatigue would not have been apparent on a visual inspection of the lap joints in the fuselage of the Sacramento flight, and that current regulations did not require high-tech techniques like ultrasound that might have detected the hidden cracks.
A fuselage section is being flown to the board's labs in Washington for a more comprehensive metallurgical examination.
According to Federal Aviation Administration records, the airline identified and fixed 21 cracks in the fuselage of the plane 11 months ago during a scheduled inspection that lasted more than a week. Outside airline maintenance specialists say such fatigue cracks are not uncommon in older jets.
Southwest Airlines has a history of maintenance problems. In 2008, the F.A.A. proposed a $10.2 million penalty, later reduced to $7.5 million, for Southwest's failure to do mandatory inspections for fuselage fatigue cracking on some of its Boeing 737s.
A report that year by the inspector general of the Transportation Department agreed with a whistle-blower's complaint that an F.A.A. supervisor had been too cozy with Southwest. The report found "serious lapses in F.A.A.'s air carrier oversight."
The Southwest plane involved in the incident on Friday, a 15-year-old Boeing 737-300 carrying 118 passengers, had almost reached 35,000 feet when passengers were frightened by a gunshotlike explosion and the sight of a gaping hole in the cabin ceiling behind the left wing. Some people reported feeling the dizziness that occurs during a swift loss of cabin pressure. Oxygen masks were released and at least two people passed out as the pilot guided the plane to an emergency landing at Yuma Marine Corps Air Station in Arizona. No one was seriously injured.
Mr. Sumwalt, a former pilot, said the hole -- through which sunlight was visible -- was about two feet behind the left wing on the upper part of the fuselage. An inspection found hairline cracks emanating from each of the rivet holes in the joints on the roof.
"We have clear evidence that the skin separated along the rivet line," he said. "The preliminary on-site inspection reveals fatigue along the entire fracture surface."
He said that the cracks were in the underskin of the joint -- where two sections of the 737's skin overlap and are riveted together. He said that federal regulations did not require more sophisticated inspections of the joints. But the board's finding will go to the F.A.A., which can require such inspections.
When asked about the new finding, Ian Gregor, a spokesman for the F.A.A., referred to an earlier statement that said "the F.A.A. is working closely with the N.T.S.B., Southwest Airlines and Boeing to determine what actions may be necessary."
The airline's own in-depth inspection of the plane in March 2010 revealed 10 cracks in parts of the frame and 11 cracks in the "stringer clips," which help secure the aircraft skin, according to Service Difficulty Reports listed on the F.A.A. Web site.
They were all repaired, the reports said. At that time, the plane had 45,944 flight hours.
A total of 288 Boeing 737-300s are flying for airlines in the United States and 931 worldwide, according to the F.A.A. Southwest said the 737-300s were the oldest planes in its fleet. Boeing, in a statement, said it had seen no reason to take fleetwide action involving the planes. The company said it was monitoring all of its in-service planes and helping Southwest and the N.T.S.B.
Douglas Clark, an airplane maintenance specialist with Expert Aviation Consulting, an Indianapolis business not involved with the investigation, said, "It's amazing it didn't rip open further." Metal fatigue, he said, "has been something that has plagued the industry for years."