What happened
On October 8, 2014, a Boeing 747-400F cargo aircraft was on final approach to Frankfurt am Main, Germany, arriving from Moscow, Russia. While the crew was transitioning the flaps from 20° to 30° at an altitude of approximately 2,500 ft AMSL, they heard a noise similar to the landing gear extending. Immediately following this sound, the aircraft experienced a 7° left roll. The pilot disconnected the autopilot and manually maintained the ILS approach path. The aircraft landed without further incident, but upon reaching the parking position, the crew discovered that a section of the left inboard fore flap, measuring approximately 4.5 x 1 meters, was missing. There were no fatalities and no injuries to the four persons on board.
The investigation
The BFU examined the aircraft, the flight data recorder, and the cockpit voice recorder. Investigators recovered the detached flap component from a nearby forest. Laboratory analysis of the broken flap bracket, performed at the Technical University of Braunschweig, focused on the fracture surfaces. The investigation also reviewed historical data regarding similar incidents involving the 747-400 series, including a 2009 occurrence in Frankfurt where flap components struck the aircraft fuselage.
Findings
Technical analysis revealed that the failure was caused by corrosion fatigue. The fracture originated from a pit caused by corrosion on the inner surface of the bracket. The investigation found that the bracket, made of an EN AW 7075 aluminum alloy, exhibited clear fatigue striations under electron microscopy. The corrosion was particularly prominent in areas outside the lubrication groove of the bronze bushing. The fatigue crack propagated through the material until the remaining cross-section could no longer withstand the increased loads applied during flap extension, resulting in a final ductile failure. While the loss of the flap caused a loss of lift, the pilot was able to compensate manually during the landing.
Safety action
- The aircraft operator should inspect the flap brackets on all Boeing 747-400 aircraft in accordance with revised manufacturer Service Bulletins and Service Letters.
- The manufacturer should improve the corrosion resistance of the flap brackets, specifically in the area of the bearing seat.