What happened
On November 26, 2010, a Boeing 747-40 and operated by a German airline was taxiing toward runway 18 at Frankfurt Airport, destined for Buenos Aires, Argentina. After 257 passengers and 17 crew members had boarded, the aircraft pushed back from Terminal 1 at 22:55 local time.
While taxiing via routes N5 and N, the crew received clearance to proceed behind a departing Airbus A319. As the aircraft slowed near taxiway hold position N at approximately 23:10, the crew experienced a violent, loud impact on the right side, which they described as being similar to driving through a deep pothole.
Following the impact, the crew moved the aircraft to taxiway L to avoid blocking the runway and to allow for an inspection. The aircraft was eventually parked outside the runway safety area, where passengers were evacuated via fire service stairs and transferred to buses. There were no fatalities or injuries during the event, though the aircraft sustained heavy damage.
The investigation
An investigation by the BFU, supported by the NTSB and the aircraft manufacturer, focused on the structural failure of the right wing landing gear. Initial inspections revealed that the right wing landing gear had broken at the trunnion, specifically the rear mounting of the landing gear cylinder. This failure caused secondary damage, including breaches in the landing gear bay and gear doors, as well as hydraulic fluid leakage. The impact was severe enough to puncture the right wing and cause internal damage to the fuselage skin above the right wing.
Investigators examined the outer cylinder, which was a modified component originally designed for older B747-200/300 models. Because the B747-400 does not use the leveling system found in earlier models, the filling port for that system had been plugged with a stopper and secured with a bolt to allow the part to be used in the newer airframe. The investigation involved metallurgical analysis of the fractured material, identified as AISI 4340M (300M).
Findings
- The primary cause of the failure was stress corrosion cracking.
- The fracture originated at the inner diameter of the filling hole, specifically below the nickel plating.
- The crack initiation site showed evidence of minor corrosion.
- The fracture path consisted of a transgranular component followed by a ductile, high-energy failure.
- The component in question was a modified part that had undergone approximately 27,750 cycles over its lifetime, with 4,500 cycles since its last overhaul.
- The failure of the trunnion caused the stopper to be forced out of the filling port, leading to the structural breaches in the wing and fuselage.