Southwest Airlines 737-300 rivet/rivet hole alignment issues discovered by NTSB

US National Transportation Safety Board said its investigation of the April 1 midair fuselage skin rupture on a Southwest Airlines 737-300 has found alignment issues with rivets and rivet holes in the area of the fracture, raising the prospect that manufacturing problems could have played a role in the incident in addition to, or instead of, aircraft fatigue.


A portion of the fuselage skin that contained the 9-inch wide, 59-inch long hole and another section of skin located forward of the hole were excised from the 737-300 and moved 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.


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."


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 airplanes were found to have crack indications at a single rivet and one airplane was found to have crack indications at two rivets," NTSB stated.


Boeing said in a statement that it would be "premature and speculative" to reach any conclusions from NTSB's investigation.

(Aaron Karp - ATWOnline News)