Misshapen and misaligned rivet holes in parts of the fuselage removed from a Southwest Airlines 737-300 has lent more credence to theories that a manufacturing flaw led to the eventual failure of the lap joint during an April 1 accident in which a five-foot-long tear developed in the roof of the airplane while en route from Phoenix to Sacramento. A rapid depressurization occurred at 34,000 feet, forcing the crew to divert to Yuma, Ariz.
Non-destructive eddy current inspections conducted around intact rivets on the removed skin section forward of the actual tear revealed cracks at nine rivet holes in the lower rivet row of the lap joint. An X-ray inspection of the skin located forward of the rupture location revealed gaps between the shank portions of several rivets and the corresponding rivet holes for many rivets associated with the suspect portion of fuselage. Upon removing selected rivets, inspectors found a slight offset in the holes in the upper and lower skin and “many” out-of-round holes on the lower skin.
In the tear itself, microscope examination of the fracture faces revealed fatigue cracks coming from at least 42 of the 58 rivet holes connected by the fracture. However, electrical conductivity measurements, hardness tests and X-ray energy dispersive spectroscopy elemental analysis of the skin in the area of the fracture revealed no discrepancies from the specifications required of the aluminum skin.
Contacted by AIN for a reaction to the findings, a Boeing spokesman said the company would wait until the NTSB finishes its investigation before addressing the issue directly. “We will not speculate on what the NTSB’s initial findings may suggest about the root cause of the April 1 737-300 decompression event, which the investigation will determine in due course,” Boeing said in a statement.
However, during an April 27 earnings call, Boeing CEO Jim McNerney acknowledged that the early findings suggest a problem with “workmanship” rather than design.
Southwest has returned to service all five of the 737s on which it found fatigue cracking following the April 1 “depressurization event.”
Boeing issued an alert service bulletin on April 4 instructing operators of certain Boeing 737-300s, 400s, and 500s delivered between 1994 and 1996 to inspect the lower row of fasteners along a length of the fuselage for cracking in the lower skin of the lap joint. A day later the FAA issued an emergency AD mandating the inspections in the service bulletin. The NTSB reported last week that 136 airplanes out of some 190 airplanes covered in the AD have undergone inspection, including all U.S.-registered airplanes covered by the directive. Over the course of the inspections, technicians found four airplanes that showed cracking at a single rivet and one airplane with crack indications at two rivets. Those airplanes had accumulated between 40,000 and 45,000 total cycles.