Investigation Summary
The structural failure occurred at the S-4L lap joint, a longitudinal seam where fuselage skin panels are riveted together. This specific area was part of a split manufacturing process: the fuselage sections were built by Boeing in Wichita, Kansas, but the final drilling and riveting of the crown skin lap joints were completed at the Renton, Washington facility. Investigators found that the rivet holes had been improperly aligned during assembly, leading to the use of undersized rivets and high local stresses. These manufacturing discrepancies caused multiple site damage (MSD) fatigue cracking that was not detectable through the visual inspection protocols in place at the time. Notably, the accident aircraft (N632SW) had cracks found and repaired in this exact lap joint location in March 2010, just one year prior to the failure. The cracks eventually coalesced, causing a 5-foot-long section of the skin to flap open under the pressure differential of high-altitude flight. The incident mirrored a 2009 failure on another Southwest 737-300 (Flight 2294), prompting a massive re-evaluation of inspection intervals for high-cycle Boeing 737 Classic airframes.
Final Conclusions
The NTSB determined the probable cause was the improper installation of the fuselage crown skin panel at the S-4L lap joint during manufacturing. This error resulted in multiple site damage fatigue cracking and the eventual failure of the lower skin panel. A contributing factor to the injuries was a flight attendant's incorrect assessment of his time of useful consciousness, which led to a delay in donning his oxygen mask and subsequent loss of consciousness. The investigation concluded that existing visual inspection requirements were insufficient to detect the subsurface fatigue cracking caused by the manufacturing defects.
Video Analysis
The Plane That Was Ripped Apart | Southwest Airlines 812 — A detailed look at the structural failure of N632SW, exploring how a manufacturing defect at a Boeing factory led to a mid-air rupture and the subsequent emergency landing in Yuma.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Systems & Failure Modes
Fuselage Lap Joint
The longitudinal seam (S-4L) where upper and lower skin panels overlap and are riveted together; the site of the fatigue failure.
Passenger Oxygen System
Automatically deployed masks in the cabin following the rapid decompression, though two occupants lost consciousness before they could use them.
Pressurization System
Maintains cabin altitude at a breathable level; the system failed when the structural integrity of the pressure vessel was breached.
Interesting Facts
- 01The fuselage rupture was measured post-accident at approximately 60 inches long by 8 inches wide; cabin crew initially estimated it as a two-foot hole.
- 02The aircraft had completed 39,786 cycles and 48,748 hours at the time of the accident.
- 03Official NTSB statistics record one minor injury (a flight attendant with a broken nose), though an off-duty employee also sustained a minor head injury.
- 04Cracks had been previously detected and repaired in the same lap joint location on N632SW in March 2010.
- 05Post-accident inspections of the Southwest fleet found similar cracks in five other aircraft.
- 06The failure occurred at a lap joint where manufacturing work was split between Boeing's Wichita and Renton facilities.
Safety Actions & Advisories
FAA Emergency Airworthiness Directive 2011-08-51
Required immediate electromagnetic inspections of lap joints on Boeing 737-300, -400, and -500 series aircraft with high flight cycles (over 30,000).
Revised Inspection Intervals
Boeing issued new service bulletins mandating eddy-current inspections instead of just visual checks for lap joint fatigue.
TUC Training Revision
The NTSB recommended that the FAA revise its 'Time of Useful Consciousness' (TUC) training and tables, noting they were overly optimistic and contributed to the flight attendant's delay in donning oxygen.