Authorities say it was nearly four dozen older planes, Boeing 737's, that Southwest failed to inspect. The mandatory inspections are designed to uncover any cracks in the body of a plane.
Last March, Southwest notified the FAA it had mistakenly missed the inspections. Some were nine months overdue.
"[The inspections are critical] to make sure that the health of the airplane structurally is maintained throughout the life of the airplane," Former NTSB board member John Golgia said.
The planes should have been grounded immediately. They were not. An FAA supervisor told the airline it could continue to fly the jets until they were inspected, but that took another nine days.
"If the FAA knew about the problem, didn't correct the problem, continued to allow Southwest to fly, basically it's nothing more than a wink and a nod," said Michael Barr, of the USC Aviation Safety and Security Program.
The government says Southwest flew more than 60,000 flights with these planes without their required inspections.
When the airline finally did inspect the 46 planes, it found cracks in six of them. They were cracks that, by aviation standards, were significant.
"What you want to do is find these cracks when they're very, very small, so they can be repaired," Golgia said. "When the cracks get to be large, it indicates, it could mean in impending failure."
It was after an older Aloha jet peeled apart at 24,000 feet in 1988 that the FAA realized that older planes could suffer metal fatigue and needed frequent inspections.
The U.S. fleet is growing older as many financially-strapped carriers have not invested in new planes. The average age of the fleet is now at least 12 years old.
