What happened
On May 8, 2009, a Boeing 747-400F cargo aircraft, operated by a South Korean carrier, was on final approach to Frankfurt am Main Airport, Germany. The flight, arriving from Vienna, was being operated by a crew of three, including a relief pilot.
While flying at approximately 1,400 ft AMSL, the crew deployed the flaps to the 30° position. Immediately following this action, the crew experienced a loud impact noise and significant vibrations. The aircraft experienced a 1° left roll, which the autopilot corrected. The aircraft subsequently landed safely on runway 25L and taxied to its parking position.
During the incident, a section of the left inboard trailing edge fore flap, measuring approximately 4.5 x 1 meters, detached from the wing. The detached component was later recovered from a forest area east of the airport. The loss of the flap caused severe structural damage, including a 3.5-meter puncture in the aircraft's fuselage that breached the pressure hull, as well as damage to the vertical stabilizer.
The investigation
The BFU examined the aircraft components and analyzed flight data from the FDR and CVR. Investigators recovered the detached flap section and the broken flap bracket (part number 65B3025-1) for metallurgical analysis.
Laboratory testing at the Technical University of Braunschweig revealed that the bracket had failed due to a fatigue crack. Microscopic examination (SEM) identified characteristic fatigue striations and a ductile fracture zone. Crucially, investigators found evidence of significant corrosion, specifically pitting and holes, on the internal surface of the bearing seat where the crack originated.