NTSB: Cracks found in 3 grounded Southwest planes

officials said.

"What we saw with Flight 812 was a new and unknown issue," said Mike Van de Ven, Southwest executive vice president and chief operating officer. He said the airline had complied with federally mandated and Boeing-recommended inspections for the plane.

Federal records show cracks were found and repaired a year ago in the frame of the same Southwest plane.

A March 2010 inspection found 10 instances of cracking in the frame, which is part of the fuselage, and another 11 instances of cracked stringer clips, which help hold the plane's skin on, according to an AP review of FAA records of maintenance problems.

The records for the plane show the cracks were either repaired or the damaged parts replaced. Cracking accounted for a majority of the 28 problem reports filed as a result of that inspection.

It's common for fuselage cracks to be found during inspections of aging planes, especially during scheduled heavy-maintenance checks in which planes are taken apart so that inspectors can see into areas not normally visible.

The Arizona jetliner had gone through about 39,000 cycles of pressurizing, generally a count of takeoffs and landings. Cracks can develop from the constant cycle of pressurizing for flight, then releasing the pressure.

Southwest officials said the plane was given a routine inspection Tuesday and underwent its last so-called heavy check, a more costly and extensive overhaul, in March 2010.

The decompression happened about 18 minutes after takeoff from Phoenix Sky Harbor International Airport.

At cruising altitude of some 34,000 feet, the pilots declared an emergency and briefly considered returning to Phoenix before the cabin crew told them of the extent of the damage, Sumwalt said. The plane's voice and data recorders were being examined in Washington.

Southwest operates about 170 of the 737-300s in its fleet of 548 planes, but it replaced the aluminum skin on many of the 300s in recent years, a spokeswoman said. The planes that were grounded over the weekend have not had their skin replaced.

US Airways operates 18 of the older-model 737-300s, but the company said Monday it has been told by Boeing that its jets aren't affected.

Lufthansa has a fleet of 63 Boeing 737s, including 33 of the 300 series. But just three of those are from the same series as the Southwest jet.

"As a precautionary measure, all three aircraft underwent additional checks over the weekend, which didn't yield any further findings. Accordingly, the aircraft are operating as scheduled," said airline spokesman Martin Riecken. "We are in close contact with Boeing regarding this issue and are awaiting further advice from the manufacturer."

In July 2009, a football-sized hole opened up in-flight in the fuselage of another of Southwest's Boeing 737s, depressurizing the cabin. Sumwalt said the two incidents appeared to be unrelated.

A fuselage failure, although extremely rare, can have deadly consequences. In 1988, cracks caused part of the roof of an Aloha Airlines Boeing 737 to peel open while the jet flew from Hilo to Honolulu. The flight attendant died, and dozens of passengers were injured.

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