The NTSB has said its investigation of the fuselage skin rupture on a Southwest Airlines 737-300 on 1st April 2011 has found alignment issues with rivets and rivet holes in the area of the fracture. This could point to problems during the manufacturing process.
The fuselage skin with the two foot hole and another section of skin located forward of the hole were removed from the 737-300 to NTSB’s Materials Laboratory in Washington. "Non-destructive eddy current inspections conducted around intact rivets on the removed skin section forward of the rupture revealed crack indications at nine rivet holes in the lower rivet row of the lap joint," NTSB said in an investigation update.
Further inspection "revealed gaps between the shank portions of several rivets and the corresponding rivet holes for many rivets. 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." In addition, it said "evidence of blue paint was … found inside the joint between the upper and lower skin and on several areas of the skin fracture surface," which suggests there could have been gaps or overlaps that allowed livery paint from the fuselage surface to seep through. The board said that 136 aircraft worldwide have been inspected since US FAA issued an April 5 emergency airworthiness directive requiring operators of "specific" 737-300/400/500 series aircraft to conduct initial and repetitive electromagnetic inspections for fatigue damage.Four of these aircraft were found to have crack indications at a single rivet and one aircraft was found to have crack indications at two rivets the NTSB stated.
NTSB noted that electrical conductivity measurements, hardness tests and X-ray energy dispersive spectroscopy elemental analysis of the skin in the area of the fuselage hole "revealed that the aluminum skin material was consistent with the specified material. The skin was the specified thickness." This moves the investigation once and for all away from metal fatigue.