What happened
Following a normal touchdown and rollout, a Boeing 747 cargo airplane exited the runway and entered a high-speed taxiway. During the taxi phase, the captain observed that the aircraft was dragging to the right. The crew determined that an excessive amount of power was required to maintain taxi speed, prompting them to stop the aircraft and apply the parking brake.
The investigation
An on-site examination of the aircraft revealed that the right outboard main landing gear trennion had completely fractured and separated several inches inboard of the aft trunnion bearing. A visual inspection of the fracture site showed evidence of corrosion and a pre-existing crack on the upper portion of the outer cylinder of the trunnion. Subsequent metallurgical examination confirmed the presence of stress corrosion cracking (SCC).
Records indicated that while Airworthiness Directive 90-06-18 R1 required visual or eddy current inspections for cracks and corrosion, a more detailed borescope inspection mandated by AD 2001-17-25 had not yet been performed. The operator's records showed this specific inspection was not due until April 3, 2002. Investigation into Service Difficulty Reports revealed 19 previous occurrences of stress corrosion cracking associated with corrosion in the aft trunnion outer cylinder areas.