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's no similar problem on the 737-Next Generation planes it's now building.
Paul Richter, Boeing’s chief project engineer for the 737-300/400/500 models, said the 1-by-5-foot hole opened up because of fatigue cracks in the metal emanating from the fastener holes at the so-called lap joints, where two panels overlap and are spliced together.
Richter said that the specific design of this lap joint was installed on all classic 737s models built between 1993 and 2000, a total of 570 airplanes.
Boeing has issued a service bulletin for airlines recommending immediate inspections and the Federal Aviation Administration is set to release an airworthiness directive Tuesday, mandating that airlines inspect all 737s with this lap joint design that have flown more than 30,000 take-off and landing cycles.
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Richter said the lap joint that failed was an improved design, developed after Boeing determined that an earlier version of the lap joint could result in the cracking along the inside row of fastener holes.
“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,” Richter said.
“Obviously, the cracking in this airplane is significantly before that,” he added. The plane that ripped open Friday had flown only about 40,000 cycles.
As a result, Richter said, “We are having to adjust our plan.”
The service bulletin issued by Boeing 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 the costs of the inspections and of any repairs that are required.
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.
The inspections should take about eight hours, Richter said, and if repairs are required that may take another 8 to 16 hours.
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 be subject to the same fatigue problem at that low number of cycles, according to Boeing’s testing.
“We remain completely confident that there is no (lap joint inner panel) cracking issue on the NG design,” said Richter.