NTSB: Rivet holes too big on 737 that ripped open

WASHINGTON — Investigators say they found rivet holes that were too big and appear to be misshapen from wear in a Southwest Airlines jet that peeled open in flight earlier this month.

The National Transportation Safety Board said Monday that its examination of a section of the Boeing 737's fuselage skin adjacent to where a 5-foot hole tore open revealed rivet holes that were too wide for the rivets. The rivets hold overlapping sections of the plane's skin together.

The board also said in a statement that some rivet holes were worn into irregular shapes instead of being round. The board didn't offer any conclusions based on the evidence. Its investigation is continuing.

The NTSB statement followed earlier reports that investigators are focusing on the manufacturing process at Boeing.

Government and industry officials say investigators noticed that the stricken jet and five other Southwest planes that had cracks in their metal skins were all built at about the same time in the same Boeing plant.

The officials cautioned Monday that no final determination has been made about why the planes developed cracks in an area of the fuselage many years before Boeing expected to see problems.

The fuselages of 737s are all made in Wichita at a plant that was owned by Boeing until 2005, when it was sold and renamed Spirit Aerosystems. The planes are assembled in Boeing's Renton plant.

The 5-foot hole tore open in the roof of the Southwest Boeing 737 as Flight 812 climbed to 34,000 feet above Arizona on April 1. The pilot guided the plane to a safe emergency landing, and there were no serious injuries.

Metal fatigue was initially suspected to have caused tiny subsurface cracks in the aluminum skin, which gave way during flight. Now investigators think the seeds of the near-disaster might have been planted when the plane was built.

Officials said investigators are looking at several possible mistakes during assembly, including the size and the way rivets and sealants were used to hold aluminum panels together on the plane's roof.

After the Southwest incident, Boeing told airlines that owned about 190 other 737s built in the 1990s to immediately conduct electromagnetic inspections of an area of the roof called the lap joint, where overlapping panels of skin are riveted together.

Boeing said Monday that inspections have been completed on about three-fourths of those planes, and only the five at Southwest were found to have cracks. Boeing said it was analyzing portions from panels of those planes "to validate the initial inspection findings," but added that no final conclusions have been drawn.

The Southwest plane had made about 39,000 flights. A senior Boeing engineer said this month that the company didn't expect airlines would need to inspect the lap joints for metal fatigue until about 60,000 flights.