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Detailed inspections of a Southwest Airlines Co jet that experienced a mid-flight fuselage rupture revealed possible manufacturing flaws and further evidence of fatigue cracks.
A National Transportation Safety Board preliminary report Monday on the April 1 incident raised new questions about the process for bonding aluminum fuselage skin and how wear and tear may affect certain older model Boeing Co 737-300 series aircraft.
Those planes have been a workhorse for decades in the global commercial fleet. Fatigue wear and cracks have long been a concern with older aircraft, including the so-called “classic” 737s made before the late 1990s.
Southwest Flight 812 made an emergency landing in Yuma, Arizona, with a five-foot roof tear over the left wing. The damaged area peeled back like the lid of a can. No one was hurt.
An examination by safety board investigators of the plane and a cutaway of the damaged skin showed microscopic cracks extending from at least 42 of 58 rivet holes connected to the rupture. Cracks also extended in an area forward of the hole.
A separate inspection of intact rivets showed imperfections in the location and size of several rivet holes, but the NTSB does not know if the problem is related to wear, manufacturing, or another cause.
Additionally, evidence of Southwest’s blue livery paint was found inside a joint where the upper and lower fuselage skin meet and where microscopic cracks had been painted over.
Boeing said in a statement that it could not speculate on what the NTSB’s preliminary findings might suggest about the root cause of the incident involving the 15-year-old jetliner.
Southwest said in a statement that the initial findings “were another step in this ongoing investigation” and pledged cooperation with investigators and regulators “in an effort to determine the cause of events.”
There were nearly six hundred 737-300, 400, 500 series planes made between 1993-2000 with fuselage assembly at the company’s Wichita, Kansas, facility. Boeing changed its 737 skin bonding techniques early on based on analysis and other factors including a similar incident in 1988 involving an Aloha Airlines 737-200. Boeing said it is unclear whether the change contributed to the Southwest incident.
Boeing said it was working closely with the NTSB and any attempt to draw conclusions about the Southwest incident “would be premature and speculative.”
The Federal Aviation Administration, in consultation with Boeing after the Southwest rupture, ordered airlines to inspect 190 of those 737-300,400, 500s worldwide. Those planes, like the damaged Southwest jet, had a high number of takeoffs and landings. Expansion and contraction of the fuselage during flight can cause cracks.
Southwest grounded 79 of its jets before the FAA order that included its jets.
Boeing said inspections were completed worldwide on nearly 80 percent of the planes affected with cracks showing up on four — all from Southwest. Southwest, which has an all-737 fleet, said it has since repaired those planes and put them back in service.
The jet with the ruptured skin remains sidelined, the company said.
FAA records show a series of directives over the years aimed at preventing and detecting cracks in the 737-300 family.
Copyright © 2011 Reuters