Incident Sequence
While approaching the outer marker, a loud bang was heard from the aircraft. The flight crew needed to apply left aileron input to maintain heading. The landing was completed without further issue until the flaps were raised during taxi. At that point, the flight engineer reported a failure of the No. 4 hydraulic system. The captain subsequently lost normal brakes and had to use the standby brakes. A split flap indication was also observed.
Inspection Findings
Post-incident inspection revealed that the flap track had fractured approximately 6 inches aft of the forward attach point. The fracture originated on the outboard flange at the No. 1 bolt hole wall due to pitting corrosion. A portion of the fore flap and a small wing panel had separated from the aircraft.
Sequence of Failures
During flap retraction after landing, the transmission ball nut jammed. This caused the drive torque tube to fracture. The broken end of the rotating torque tube then damaged hydraulic lines, resulting in a loss of hydraulic fluid.
Aftermath
Following this incident, Boeing issued an alert service bulletin. United Airlines initiated a fleet-wide inspection to check for similar issues.