Exterior blue paint found inside a joint on Southwest Airlines Flight 812 may hold critical information for federal investigators probing the April 1 rupture of the jet’s fuselage while in flight.
The National Transportation Safety Board released an update Monday (April 25) into its investigation of the mishap, in which the Boeing 737-300 experienced rapid depressurization from a large hole in the fuselage while the flight was at 34,000 feet.
The crew made an emergency descent and landed at Yuma International Airport, with no serious injuries.
Focus on Metal Fatigue
The NTSB has been focusing on metal fatigue as a cause of the accident. Fatigue is a common result of the cycles of pressurization and depressurization that occur with takeoffs and landings, along with repeated heating and cooling of the aircraft’s skin, but Boeing said the problem normally does not happen so early in a plane’s lifespan.
The Southwest aircraft had been in service since June 13, 1996, and had undergone 39,781 cycles (a cycle is a takeoff and landing).
After the mishap, the Federal Aviation Administration ordered inspections of certain Boeing 737-300, 400, and 500 aircraft.
Of 136 inspected worldwide, four planes showed crack indications at a single rivet and one showed crack indications at two rivets in the same area as the rupture on Flight 812.Those planes all had 40,000 to 45,000 cycles. Those sections of the affected planes have been removed and are now part of NTSB’s investigation.
On Flight 812, the rectangular hole—nine inches wide and 59 inches long—occurred in the aluminum fuselage skin at a "lap joint," where the crown skin overlaps the lower skin. The two skins are connected at the joint by an upper, middle and lower row of rivets. The fracture was through the lower skin and connected 58 consecutive rivet holes in the lower row of lap joint rivets.
| A photograph shows the interior and exterior surfaces of the ruptured fuselage. |
The exterior surface of the skin in that area is painted blue. And NTSB found this week that “evidence of blue paint was also found inside the joint between the upper and lower skin and on several areas of the skin fracture surface,” indicating that the paint had room to seep between the layers.
The report does not say when the plane was painted or repainted, information that could indicate when a problem first appeared.
The aluminum skin material was consistent with the specified material and was of the specified thickness, NTSB reported.
However, microscopic examination of the ruptured skin revealed fatigue cracks emanating from at least 42 of the 58 rivet holes connected by the fracture, NTSB said.
Cracks were also found at nine rivet holes forward of the rupture, and X-ray inspections of that area revealed gaps between the shank portions of several rivets and the corresponding rivet. Upon removing selected rivets, the NTSB found holes in the upper and lower skin to be slightly offset relative to each other and many of the holes on the lower skin out of round.
|A close-up of the torn interior fuselage shows traces of blue exterior paint along the fracture line.|
NTSB said its Materials Laboratory was continuing to conduct additional inspections and examinations in the probe. The agency has not said whether the problems it has uncovered thus far are isolated or systemic to the aircraft models.