Originally, the C-141 was scheduled to depart Memphis on 20 December 2001. During preflight inspection a fuel leak was discovered behind engine number two. Maintenance personnel evaluated the fuel leak and determined that repairs would be required prior to departure. The plane was defueled and towed to the fuel cell hangar for repairs. Maintenance personnel were unable to locate or repair the fuel leak, so they inserted fuel tank vent plugs and pressurized the fuel tank to locate the leak. The leak was found and repaired. Once the repair was completed, the fuel systems technician exited the fuel tank but failed to document or request an in-progress inspection (IPI). to ensure all tools and materials are removed from the fuel tank; and fuel tank plugs (if used) are extracted from the fuel tank vents. Thus a fuel vent plug was not removed. As the aircraft was being refueled, upon reaching approximately 120.000 pounds of jet fuel, the interior left wing fuel tank pressure exceeded wing structural tolerances, as pressure was unable to vent due to the forgotten fuel vent plug. Over-pressurization resulted in catastrophic failure of the left wing structure at the wing root. PROBABLE CAUSE: “The AIB President found by clear and convincing evidence that the primary cause of the mishap was over-pressurization of the left wing fuel tanks. The over-pressurization was the direct result of the fuel systems technician’s failure to remove a fuel tank vent plug. The forgotten fuel tank vent plug, in turn, caused the fuel tank overpressurization leading to its rupture.”