WASHINGTON (AP) — The Federal Aviation Administration is reviewing its safety regulations aimed at spotting metal fatigue in aging aircraft in response to the 5-foot hole that ripped open in a Southwest Airlines plane last week, the agency's chief told a House panel on Wednesday.

FAA Administrator Randy Babbitt said he has ordered a review of regulations that became effective in January. Those regulations lay out actions that aircraft manufacturers and operators must take to ensure they catch metal fatigue in planes before it becomes a safety problem.

The new rules give aircraft manufacturers as many as five years to develop inspection plans for about 4,000 older airliners. Airlines would have as many as six years to implement the inspections.

It's unclear why a section of the Southwest Boeing 737's fuselage came apart in flight despite an aircraft testing and maintenance regime already in place that is designed to prevent such problems, Babbitt told the House Appropriations Committee's transportation subcommittee.

The sudden opening of the hole at 34,400 feet caused an explosive decompression of the 15-year-old plane with 95 people on board. The plane was heading from Phoenix to Sacramento, Calif., but pilots made an emergency landing at a military base in Yuma, Ariz.

On Tuesday, FAA issued an emergency order requiring inspections on similar older-model Boeing 737-300s, 737-400s and 747-500s that have had at least 30,000 pressurization cycles, basically takeoffs and landings. Cracks can develop from the constant cycle of pressurizing the cabin for flight, and releasing it. The planes were built between 1993 and 2000.

Boeing said 579 airplanes will eventually have to be checked, but just 175 have that many cycles and need immediate inspections. Boeing issued a service bulletin detailing the required inspections earlier this week.

Aircraft manufacturers like Boeing have special equipment they use to simulate the effects on tens of thousands of takeoffs and landings on an airplane, Babbitt said.

"That's on the test stand. Are we getting a different performance out in the field? We don't know," Babbitt said.

"We don't know what happened with this plane. We have some suspicions," he said.

Babbitt was also asked by lawmakers if, in light of Southwest's history, whether he is confident the airline has been inspecting planes as required by FAA.

"We have no suspicions that they have done anything wrong in that area," Babbitt said.

Last year, a football-size hole forced an emergency landing, in Charleston, W.Va., of another of Southwest's 737s. In 2008, FAA proposed fining Southwest $10 million for allowing planes to fly without mandatory inspections for cracks and other signs of metal fatigue. The airline ultimately settled the fine for $7.5 million.