X-ray inspections have exposed problems with rivets near the spot where a portion of a Boeing 737-300 fuselage panel ripped open during flight, forcing Southwest Airlines flight 812 to make an emergency landing on 1 April, according to the US NTSB.
The findings from the ongoing investigation will put more scrutiny on Boeing's past manufacturing techniques after US FAA Administrator Randy Babbitt first questioned the company's workmanship nearly two weeks ago.
"People have leaped to the conclusion that it was fatigue," Babbitt told the MRO Americas conference on 14 April. "The airplane didn't have that many cycles on it so we're looking at other things, [such as] the manufacturing techniques. Boeing is very interested too. This is not good for anybody's business."
The NTSB followed-up Babbitt's remarks with initial findings from X-ray inspections of the surviving portion of the fuselage panel from N632SW, the airplane involved in the depressurisation earlier this month.
"This inspection revealed gaps between the shank portions of several rivets and the corresponding rivet holes for many rivets associated with [the fuselage panel]," the NTSB report says.
"Upon removing selected rivets, the holes in the upper and lower skin were found to be slightly offset relative to each other and many of the holes on the lower skin were out of round," the report adds.
The NTSB's Materials Laboratory is now conducting further inspections of the rivets from the damaged panel and other areas of the fuselage, including analyses of skin fractures and fatigue cracks.
The Southwest aircraft had accumulated more than 39,000 takeoff and landing cycles, according to the NTSB.
Fleet inspections since the NTSB issued a service bulletin on 5 April have checked 136 aircraft, with four tail-numbers reporting crack indications at a single rivet and one tail-number discovered with cracking at two rivets.