crack on upper fuselage of Southwest – Boeing – B737-3H4 (N-632NW) flight WN812

Southwest flight WN812 : on April 1, 2011, about 1558 mountain standard time (MST),1 a Boeing 737-3H4, N632SW, operating as Southwest Airlines flight 812 experienced a rapid decompression while climbing through flight level 340.

The flight crew conducted an emergency descent and diverted to Yuma International Airport (NYL), Yuma, Arizona.

Of the 5 crewmembers and 117 passengers on board, one crewmember and one nonrevenue off-duty airline employee passenger sustained minor injuries. The airplane sustained substantial damage; postaccident inspection revealed that a section of fuselage skin about 60 inches long by 8 inches wide had fractured and flapped open on the upper left side above the wing.

The flight was conducted under the provisions of 14 Code of Federal Regulations (CFR) Part 121 as a regularly scheduled domestic passenger flight from Phoenix Sky Harbor International Airport, Phoenix, Arizona, to Sacramento International Airport, Sacramento, California.
According to the flight crew and recorded data, the takeoff and initial climb were normal.

Southwest flight WN812 descended from cruising level to 11,000 feet within 5 minutes

At 1558:05, an unidentified sound was recorded on the cockpit area microphone. About 2 seconds later, the captain announced that the airplane had lost cabin pressurization and called for oxygen masks on; sounds consistent with increased wind noise were heard on the cockpit voice recording.

The captain declared an emergency with air traffic control and requested a lower altitude. The air traffic controller provided lower altitude clearances, and the flight crew descended the airplane to 11,000 feet within 5 minutes.

Cabin oxygen masks deployed, and about 1605, the cabin crew began relaying condition reports to the flight crew describing a 2-foot hole in the fuselage and one broken-nose injury of a cabin crewmember.

The airplane was cleared for further descent to 9,000 feet, and the captain requested radar vectors to the nearest suitable airport (NYL). The airplane landed about 1629 on runway 21R at NYL without further incident. The passengers deplaned via airstairs.

The National Transportation Safety Board determines that the probable cause of this accident was the improper installation of the fuselage crown skin panel at the S-4L lap joint during the manufacturing process, which resulted in multiple site damage fatigue cracking and eventual failure of the lower skin panel.

Contributing to the injuries was flight attendant A’s incorrect assessment of his time of useful consciousness, which led to his failure to follow procedures requiring immediate donning of an oxygen mask when cabin pressure is lost.