PHOENIX >> Flight attendants had just begun to take drink orders when the explosion rocked the cabin.

Aboard Southwest Flight 812, Shawna Malvini Redden covered her ears, then felt a brisk wind rush by. Oxygen masks fell, the cabin lost pressure, and Redden, now suddenly lightheaded, fumbled to maneuver the mask in place.

Then she prayed. And, instinctively, reached out to the stranger seated next to her in Row 8 as the pilot of the damaged aircraft began a rapid descent from about 34,400 feet in the sky.

"I don’t know this dude, but I was like, ‘I’m going to just hold your hand,’" Redden, a 28-year-old doctoral student at Arizona State University, recalled Saturday, a day after her Phoenix-to-Sacramento flight was forced into an emergency landing at a military base in Yuma, Ariz., with a hole a few feet long in the roof of the passenger cabin.

No serious injuries were reported among the 118 people aboard, according to Southwest officials.

What caused part of the fuselage to rupture on the 15-year-old Boeing 737-300 was a mystery, and investigators from the National Transportation Safety Board arrived Saturday in Yuma to begin an inquiry.

NTSB board member Robert Sumwalt said investigators were going to cut a piece out of the fuselage, which would be studied for fracture patterns. He said they would also examine the plane’s black box and flight recorders, which arrived Saturday at NTSB headquarters in Washington, D.C.

Southwest, meanwhile, grounded about 80 similar planes so that they could be inspected, and said that as a result some 300 flights were being canceled Saturday. Airline spokeswoman Linda Rutherford said it was too soon to estimate the cost of grounding a portion of its fleet.

Southwest operates about 170 of the 737-300s in its fleet of about 540 planes, but it replaced the aluminum skin on many of the 300s in recent years, Rutherford said. The planes that were grounded Saturday have not had their skin replaced, she said.

"Obviously we’re dealing with a skin issue, and we believe that these 80 airplanes are covered by a set of (federal safety rules) that make them candidates to do this additional inspection that Boeing is devising for us," Rutherford said.

Julie O’Donnell, an aviation safety spokeswoman for Seattle-based Boeing Commercial Airplanes, confirmed "a hole in the fuselage and a depressurization event" in the latest incident but declined to speculate on what caused it.

A total of 288 Boeing 737-300s currently operate in the U.S. fleet, and 931 operate worldwide, according to the Federal Aviation Administration. "The FAA is working closely with the NTSB, Southwest Airlines and Boeing to determine what actions may be necessary," the FAA said Saturday.

Southwest officials said the Arizona plane had undergone all inspections required by the FAA. They 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.

An Associated Press review of FAA records of maintenance problems for the plane show that in March 2010 at least eight instances were found of cracking in the aircraft frame, which is part of the fuselage. Those cracks were repaired, the records indicated.

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

The 737-300 is the oldest plane in Southwest’s fleet, and the Dallas-based company is retiring 300s as it takes deliveries of new models. But the process of replacing all the 300s could take years.

Seated one row from the mid-cabin rupture, Don Nelson said it took about four noisy minutes for the plane to dip to less than 10,000 feet. "You could tell there was an oxygen deficiency," he said.

"People were dropping," said Christine Ziegler, a 44-year-old project manager from Sacramento who watched as a flight attendant and a passenger nearby fainted. Nelson and Ziegler spoke Friday after a substitute flight took them on to Sacramento.

Brenda Reese described the hole as "at the top of the plane, right up above where you store your luggage."

At an altitude above 34,000 feet, the Southwest pilots would have had only 10 to 20 seconds of "useful consciousness" to get their oxygen masks on or pass out, said John Gadzinski, an airline pilot and aviation safety consultant.

"The higher you are the less useful consciousness time you have," said Gadzinski, president of Four Winds Consulting in Virginia Beach, Va. "It’s a credit to the pilots that they responded so quickly."

A loss of cabin pressure just after takeoff knocked out the pilots of a Helios Airways Boeing 737 in August 2005. The plane flew into a hillside north of Athens in Greece, killing all 121 people aboard. In that case, an investigation found the pilots had failed to heed a warning that the pressurization system wasn’t working correctly.

In this case, the hole and subsequent depressurization wouldn’t have affected the pilots’ ability to control the plane as long as they had their oxygen masks on, Gadzinski said.

"The fact that you have a breach hole doesn’t affect the aerodynamics of the plane. The plane still flies exactly the same," he said.

A similar incident happened in July 2009 when a football-sized hole opened up in-flight in the fuselage of another of Southwest’s Boeing 737s, depressurizing the cabin. The plane made an emergency landing in Charleston, W.Va. It was later determined that the hole was caused by metal fatigue.

In response to that incident, Southwest changed its maintenance plan to include additional inspections, which FAA reviewed and accepted, said John Goglia, a former NTSB member and an expert on airline maintenance. The details of the plan are considered proprietary and aren’t made public, he said.

The latest incident "certainly makes me think there is something wrong with the maintenance system at Southwest, and it makes me think there is something wrong with the (FAA) principal maintenance inspector down there that after that big event they weren’t watching this more closely," Goglia said.

There was "never any danger that the plane would fall out of the sky," Goglia said. "However, anybody on that airplane with any sort of respiratory problems certainly was at risk."

Four months before that emergency landing, Southwest had agreed to pay $7.5 million to settle charges that it operated planes that had missed required safety inspections for cracks in the fuselage. The airline inspected nearly 200 of its planes back then, found no cracks and put them back in the sky.

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. A flight attendant was sucked out of the plane and plunged to her death, and dozens of passengers were injured.

After that incident, Boeing made changes to its designs "to prevent the airplane from ever coming apart in that way again, by using different materials and manufacturing methods," Goglia said.

Three years ago, an exploding oxygen cylinder ripped a hole the fuselage of a Qantas Boeing 747-438 carrying 365 people. The plane descended thousands of feet with the loss of cabin pressure and made a successful emergency landing.

As for Friday’s flight, there was obvious relief when it touched down safely. When the pilot emerged after the landing, passengers "clapped and cheered," Redden said.

"If overhead bins weren’t in the way, I’m pretty sure we would’ve given him a standing ovation," she said.

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Associated Press writers Lien Hoang, Don Thompson and Adam Weintraub contributed to this report from Sacramento, Calif.; David Koenig contributed from Dallas; Joan Lowy from Washington, D.C.; and Mark Evans and Robert Seavey from Phoenix.