US National Transportation Safety Board (NTSB) member Robert Sumwalt states the lap joint on the Southwest Boeing 737-300 that suffered a decompression event on 1 April was not believed to be an area that would fail on the aircraft.
The aircraft, N632SW, made an emergency landing in Yuma, Arizona after a hole opened in the top of the fuselage.
Southwest initiated inspections of 79 Boeing 737-300s and subsequently the FAA said it was readying an emergency airworthiness directive covering roughly 175 737-300/400/500 models to conduct initial and repetitive electromagnetic testing for fatigue damage.
The NTSB investigating the incident onsite in Yuma concluded their was clear evidence the skin on the aircraft separated at the lower rivet line, and preliminary examinations showed pre-existing fatigue along the entire fracture surface.
During a briefing in Yuma, Sumwalt stated the FAA, the manufacturer and the industry "have not believed that this particular lap joint on this model aircraft was one that warranted attention on an aircraft that had this amount of takeoff and landings".
There were additional cracks found on the incident aircraft, says Sumwalt, who states the cracks are about a quarter of an inch long.
Boeing is expected to release a service bulletin outlining dual frequency Eddy-current inspections, which entail a small probe moving across the aircraft skin, says Sumwalt.
He explains he has no specifics of the Boeing service bulletin, but anticipates it would require inspection of both lap joints running the entire length of the fuselage.
Sumwalt reiterates that previously it was unknown that the lap joints on an aircraft "of this number of cycles" should be inspected. The assumption, he says, was that 737-300s at 40,000 cycles did not need to undergo non-destructive testing.
Flightlglobal's ACAS database shows that N632SW had logged roughly 29,980 cycles.
Southwest states it has completed inspections on all 79 aircraft 737-300s and five remain out of service after minor subsurface cracking was found. The aircraft will remain grounded until "Boeing recommends appropriate repairs and those repairs have been completed".