What happened
On 7 December 2002, a Boeing 747-367, registration AP-BFV, was taxiing for departure at London Heathrow Airport. While the aircraft was turning at Block 104, the crew heard a loud bang and felt the aircraft nose pitch downward. Shortly after, a 'STAB TRIM' warning appeared on the master warning panel.
Although the aircraft's steering continued to function normally, the commander decided to cancel the departure to Lahore, Pakistan, and return to the stand. During the taxi, debris was discovered on the taxiway by airfield personnel. The largest piece of debris, a 2 kg section of the nose landing gear outer cylinder, had been ejected from the component. The aircraft eventually returned to its stand, where the 457 passengers and 15 crew were disembarked without injury.
The investigation
The AAIB investigation focused on the structural integrity of the nose landing gear outer cylinder. Metallurgical analysis revealed that the cylinder had suffered a circumferential fracture. Investigators found a square-shaped groove machined into approximately 70% of the internal circumference of the cylinder. This groove, which appeared to have been created by a rotating cutting wheel, was not part of any approved design or maintenance specification.
Further examination showed that the fracture originated from fatigue at the base of this groove. The investigation also identified that the cylinder wall had been thinned due to improper rework and that certain areas had not undergone the required shot peening process, which is essential for preventing fatigue cracks. Additionally, the investigation noted that the repair station's records allowed for the identification of the component, though the specific unit had escaped a previously established inspection programme due to record-keeping irregularities.
Findings
- The primary cause of the failure was fatigue crack propagation initiated at the base of a non-specification groove in the outer cylinder.
- The groove was created during the removal of nickel plating 'trees' using a hand-held cutting wheel.
- The cylinder wall thickness was reduced below minimum requirements due to improper blending and rework.
- A lack of required shot peening left the material vulnerable to fatigue.
- The component had escaped a mandatory inspection programme following a similar failure in 1998 due to irregularities in record-keeping.