Boeing details metal fatigue that ripped hole in 737

Boeing provided new details Tuesday about the rupture that tore a hole in the roof of a Southwest Airlines 737-300 last Friday, and said it's confident there is no similar problem on the 737-Next Generation planes it's now building.

However, airlines operating 570 older 737-300/400/500 models built between 1993 and 2000 now face an expensive regime of repeated inspections until more is known about the cause of the incident.

Paul Richter, Boeing's chief project engineer for those so-called Classic models, said the 1- by 5-foot hole opened up due to fatigue cracks in the metal emanating from the fastener holes at the lap joints, where two panels overlap and are spliced together.

Boeing issued a service bulletin to airlines Monday recommending immediate inspections, and the Federal Aviation Administration (FAA) followed Tuesday with an emergency airworthiness directive mandating that airlines inspect all 737s with this lap-joint design that have flown at least 30,000 takeoff and landing cycles.

The inspections are an interim measure until the root cause is identified. Richter said in an interview that it's possible some design modification could be needed later.

"If we determine through this investigation that an inspection regime only won't maintain the safety of the fleet, that would indicate a mandatory modification down the road," he said. "It's much too early in the investigation to know if that is what is eventually going to happen."

Richter said the lap joint that failed was an improved design, developed after Boeing determined an earlier version could result in the cracking along the lower row of fastener holes.

Boeing had required more than 2,200 planes with that earlier design be retrofitted with a modification that replaced the skin panels before the planes reached 50,000 cycles. The redesigned lap joint was then installed on new jets from 1993.

"This was the Boeing company's response to those issues," Richter said. "Analysis and testing ... led us to believe this (improved) lap joint's fatigue performance would allow us to delay any sort of inspection requirements until the fleet approached roughly 60,000 cycles.

"Obviously, the cracking in this airplane is significantly before that," he added.

The plane that ripped open Friday had flown just under 40,000 cycles.

As a result, Richter said, "We are having to seriously rethink that."

Tuesday's FAA airworthiness directive says aircraft that have clocked more than 35,000 cycles will have to be inspected within five days. Those with between 30,000 and 35,000 cycles have a 20-day grace period in which to perform the inspections.

Richter said the FAA will require repeat inspections every 500 cycles. Since operators such as Southwest fly the workhorse 737 for six or more cycles every day, that means a re-inspection roughly every three or four months — a demanding frequency for airlines that will have to bear all costs of the inspections and of any required repairs.

Richter acknowledged that the short re-inspection period is "a rare interval to impose," but said that until the root cause is understood "a very conservative repeat threshold" is appropriate.

He said the electromagnetic devices used to inspect the metal can find even very short cracks hidden under the surface.

"When done properly, these inspections will find cracks long before there is any risk of a decompression," Richter said.

The inspections should take about eight hours, he said; repairs might take eight to 16 more hours.

Southwest's inspections since Friday found five other 737s with fatigue cracks. Boeing has provided guidance repairs, which entail replacing about 18 inches of the affected lap joint.

Richter said that the latest model of the 737, the 737 Next Generation (NG), has a "significantly different and much improved" lap-joint design that will not have the same fatigue problem at that low number of cycles, according to Boeing's testing.

"We remain completely confident that there is no lower-row cracking issue with the lap joints on the NG design," said Richter, referring to the place on those joints that failed Friday on the Southwest 737.

In addition, Richter said a later modification to the NG's lap-joint design precludes another type of structural failure highlighted by a 2009 incident. In that one, a hole in the roof of another Southwest 737 ripped open because of cracking not at the fasteners but at the edge of the overlap of the lap-joint splice.

Invisible fatigue cracks in the skins of aging airplanes only became a concern after a terrifying 1988 incident when the top of an Aloha Airlines 737 ripped open and a flight attendant was sucked out. That was after the classic model 737-300/400/500s had been certified.

On the later 737-NG, introduced in 1996, Richter said, Boeing has done fatigue testing to three times the design life of 75,000 cycles — and found no cracking.

Dominic Gates: 206-464-2963 or dgates@seattletimes.com