What happened
During the final approach, just inside the outer marker, a loud bang was heard by the crew. Immediate left aileron input was required to maintain control of the aircraft's heading. Despite this disturbance, the landing itself proceeded without incident. However, complications arose after touchdown while the aircraft was taxiing. The flight engineer reported a failure of the number four hydraulic system. This occurred as the flaps were being raised. Consequently, the captain lost normal brake functionality and had to switch to the standby braking system. At this point, a split flap indication was observed on the instrument panel.
The investigation
A thorough inspection of the aircraft revealed significant structural damage. The flap track had fractured approximately six inches aft of its forward attach point. Further analysis showed that the fracture initiated on the outboard flange number one bolthole wall due to pitting corrosion. As a result of this failure, a portion of the fore flap and a small wing panel separated from the aircraft structure.
During the process of retracting the flaps after landing, the transmission ball nut became jammed. This jamming caused the drive torque tube to fracture. The broken end of the rotating torque tube then damaged nearby hydraulic lines. This damage resulted in a complete loss of hydraulic fluid, which explained the earlier system failure reported by the crew.
Findings
The primary cause of the accident was pitting corrosion on the flap track's outboard flange. This corrosion weakened the structure until it fractured under operational loads. The subsequent mechanical failures, including the torque tube fracture and hydraulic line damage, were direct consequences of the initial structural failure. Following this incident, Boeing issued an alert service bulletin. United Airlines subsequently initiated a fleet-wide inspection to identify similar issues in other aircraft.