The U.S. government will order emergency checks of some Boeing Co (NYSE:BA) 737s for the kind of fatigue cracks that prompted Southwest Airlines Co (NYSE:LUV) to ground dozens of planes and cancel hundreds of flights after a hole opened in one of its jets.

The Federal Aviation Administration directive on 737-300, 400, and 500 fuselage inspections, expected on Tuesday, will apply to the most frequently flown models. It involves a time consuming and repetitive electromagnetic check not previously required for the area of the plane in question.

"This action is designed to detect cracking in a specific part of the aircraft that cannot be spotted with visual inspection," FAA Administrator Randy Babbitt said in a statement on Monday.
Boeing promised its own bulletin similar to the FAA order and will detail steps for inspecting joints in the fuselage that bond layers of aircraft skin.

The National Transportation Safety Board NTSB.L, which is investigating Friday's incident in which a 5-foot (1.52 meter) hole opened on Southwest Flight 812 at 36,000 feet, will look to see if there is a wider problem.

But safety board member Robert Sumwalt said investigators believe the FAA and Boeing actions should take care of it.

"We have no reason to believe there are deficiencies in the fleet," Sumwalt told reporters at a news conference in Yuma, Arizona, near the military base where the stricken Southwest jet heading from Phoenix to Sacramento landed safely.

The FAA said most of the 80 jets needing new checks in the United States are flown by Southwest that have accumulated high numbers of takeoffs and landings. Overseas regulators are expected to adopt the FAA order for 95 additional planes.

Southwest grounded planes and launched its own inspections of 79 jets over the weekend. The company said in a statement late on Monday that it had completed most of the inspections and returned 64 jets to service. Subsurface fuselage cracks were found on three other jets that will stay grounded until they are repaired.

Southwest said it believes the steps it has taken so far will meet the requirement of the pending FAA order.

"We expect to complete the inspections and be able to launch a full operation on Tuesday," the airline said.

Southwest cancelled 600 flights over the weekend and another 70 on Monday.

Southwest has had fatigue cracks on older aircraft in its all-737 fleet, which safety experts and other insiders attribute to the aggressive way it uses those planes.

"We all know that Southwest has rapid turnarounds and short hops. It's a different type of use, so you have different types of maintenance and oversight," said Jim Hall, a former NTSB chairman.

Friday was the second time in two years that a Southwest plane experienced a fuselage rupture. The other plane also landed safely.

Additionally, Southwest paid a $7.5 million FAA fine for operating 737s without required fuselage inspections in the 2006-2007 period.

Southwest said the location of cracks on Flight 812 was a problem it had not seen before. That plane entered service 15 years ago and had almost 40,000 takeoffs and landings.

The NTSB also will take a fresh look at Southwest's maintenance practices, FAA oversight of the carrier, and Boeing's service recommendations, safety experts said.

"They are going to be pushed very hard on this," said one safety expert familiar with the thinking of transportation investigators.

Sumwalt said investigators had reviewed maintenance records of the Flight 812 aircraft. He said the aircraft was fully compliant with maintenance and inspection requirements.

James Higgins, an analyst with Soleil Securities, said the flight cancellations would cause some revenue loss for Southwest, but he does not expect lasting fallout.

"I don't think there is anything systematic or persistent here," Higgins said. "This is more noteworthy for its headline generation than for its likely damage to the company."

Shares of Southwest, which only operates Boeing 737s, closed Monday down 1.7 percent at $12.46, while Boeing lost 6 cents to end at $73.95.