The rivet holes on the Southwest Airlines jet that had developed a hole in its roof didn't appear to be lined up properly, and the rivet holes were "out of round," the National Transportation ...

The rivets hold overlapping sections of the plane's skin together. The board also said in a statement that some rivet hole were worn into irregular shapes instead of being round.

Southwest Flight 812 was about 34,000 feet over Arizona with 117 passengers on board on April 1 when a 5-foot-long hole opened in zipper-like fashion along a row of rivets in the roof.