What happened
On September 16, 2018, a Boeing 747-428F cargo aircraft landed at Frankfurt am Main Airport following a flight from Dammam, Saudi Arabia. During the landing roll on runway 07R, a portion of the right wing's inboard fore flap detached from the aircraft. The detached components, measuring approximately 8 x 1 meters, struck the aircraft's fuselage and vertical stabilizer before coming to rest on the runway near taxiways R17 and M2लपुर M27.
The damage included punctures to the right side of the fuselage and a dent in the leading edge of the vertical stabilizer. Because the aircraft was landing in darkness and the flaps were retracted after the aircraft came to a stop, the crew did not immediately realize the component had been lost. The damage was discovered during a subsequent inspection at the parking stand when technicians noticed holes in the fuselage and damage to the vertical stabilizer.
The investigation
The BFU investigation focused on the structural failure of the flap attachment hardware. Examination of the broken outer flap holder revealed two distinct fracture surfaces. Microscopic analysis performed in collaboration with the Technical University of Braunschweig identified that the failure originated from within the surface of the holder.
Investigators found evidence of corrosion at the crack initiation sites. Specifically, the internal surface of the holder showed significant oxidative damage and a distinct "track" where mechanical stress was concentrated. The analysis confirmed that the crack propagated through the material via fatigue, eventually leading to a ductile failure of the remaining cross-section, likely triggered by the impact of the landing.
Findings
- The primary cause of the failure was corrosion fatigue within the flap holder.
- Corrosion at the crack initiation sites acted as a stress concentrator, allowing fatigue cracks to develop under operational loads.
- The material used, a copper- and zinc-containing aluminum alloy, is inherently susceptible to such corrosion.
- The internal area of the bearing seat, where the corrosion occurred, is not visible during standard external inspections, making the defect extremely difficult to detect through routine maintenance.
- While the operator had been performing required inspections according to existing Airworthiness Directations (AD), these measures were insufficient to identify the hidden internal corrosion.