As part of its continuing investigation of the April 1, 2011 accident involving Southwest Airlines flight 812 that experienced a rupture in the fuselage while in flight, the National Transportation Safety Board releases the following update.
On April 1, 2011, Southwest Airlines flight 812, a Boeing 737-300 registration N632SW, experienced a rapid depressurization caused by a rupture in the fuselage. The flight was at 34,000 feet when the depressurization occurred. The flight crew conducted an emergency descent and diverted the flight to Yuma International Airport, Yuma, AZ. At the time of the accident, the aircraft had accumulated 48,740 hours of service and 39,781 cycles (a cycle is a takeoff and landing). The accident aircraft was delivered to Southwest Airlines on June 13, 1996.
On-scene inspection by NTSB investigators revealed an approximately 9-inch wide by 59-inch long rectangular-shaped hole in the fuselage crown on the left side of the airplane, aft of the over-wing exit. The 59-inch longitudinal fracture occurred in the aluminum fuselage skin along the lap joint at stringer-4 left (S-4L) between body station (BS) 666 and BS 725. At S-4L, the crown skin overlaps the lower skin forming a lap joint. The two skins are connected at the lap joint by three rows of rivets (referred to as lower, middle, and upper row of rivets.) The fracture was through the lower skin and connected 58 consecutive rivet holes in the lower row of lap joint rivets. The exterior surface of the skin in the area of S-4L is painted blue. 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.
Following an on-scene examination of the accident aircraft, a portion of the fuselage skin that contained the hole and another portion of the skin located forward of the hole (total size 116 inches by 19 inches) were excised from the accident aircraft and transported to the NTSB Materials Laboratory in Washington, DC. The airplane was then released back to the operator.
At the NTSB Materials Laboratory, microscope examination of the fracture faces of the ruptured skin revealed fatigue cracks emanating from at least 42 of the 58 rivet holes connected by the fracture. Electrical conductivity measurements, hardness tests, and X-ray energy dispersive spectroscopy elemental analysis of the skin in the area of the fracture revealed that the aluminum skin material was consistent with the specified material. The skin was the specified thickness.
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. To assess the condition of the intact rivets and the skin rivet holes, X-ray inspections were performed on the skin located forward of the rupture location. This inspection revealed gaps between the shank portions of several rivets and the corresponding rivet holes for many rivets associated with S-4L. 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 this ongoing investigation, the NTSB Materials Laboratory work is actively conducting additional inspections and examinations in the following areas:
- Removal of rivets and examination of rivet hole dimensions, rivet dimensions, and rivet hole alignment between upper and lower skins.
- Detailed fractographic analysis of the skin fractures emanating from the rivet holes using optical and scanning electron microscopes.
- Fatigue striation analysis using a scanning electron microscope of specific skin fractures to determine the rate of crack propagation.
- Additional portions of the lap joints from the accident aircraft.
Following the depressurization accident and on-scene examination of the accident aircraft, Boeing issued Alert Service Bulletin SB 737 53A1319-00 on April 4 instructing operators of certain Boeing 737-300, 400, and 500 aircraft to inspect the lower row of fasteners at stringer S-4R and S-4L, from BS 360 to BS 908 for cracking in the lower skin of the lap joint on airplanes.
The Federal Aviation Administration issued Emergency Airworthiness Directive AD 2011-08-51 on April 5 mandating the inspections in the Boeing Service Bulletin. To date, the NTSB has been informed that 136 airplanes have been inspected worldwide in accordance with the Service Bulletin and Airworthiness Directive including all U.S. registered airplanes covered by the Directive. As stated in a previous release, 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. These airplanes had accumulated between 40,000 and 45,000 total cycles. The lap joints from these areas of the subject airplanes have been removed and will be fully documented as part of the NTSB investigation.
Photograph showing the interior and exterior surfaces of the ruptured section of the fuselage:
Click for full-sized image