What happened
On 29 April 2003, a McDonnell Douglas DC-8-63F, registration 9G-MKO, was preparing for a cargo flight from RAF Lyneham to the Middle East. While performing a 180-degree right-hand turn on the runway to line up for takeoff, the crew heard a loud noise, and the aircraft abruptly stopped moving.
Upon inspection by ground engineers, it was discovered that the right main landing gear had suffered extensive fracturing of its shock-strut piston. This failure caused the landing gear truck to deviate approximately 45 degrees from the aircraft's heading. Although the gear was severely damaged, a portion of the broken piston remained lodged in the cylinder, providing enough support to prevent a total collapse. There were no injuries to the four crew members on board.
The investigation
Investigators examined the fractured components of the right main landing gear. The piston, manufactured from high-strength 4340 steel, had been separated into several pieces. Specialist metallurgical analysis revealed that the primary cause of the failure was stress corrosion cracking.
Detailed examination of the fracture origin showed that the crack had started in an area where cadmium plating was missing. The investigation identified a step in the blend radius and surface scratches at the site of the crack origin. These surface irregularities likely acted as stress raisers. While the piston had only completed about 10% of its specified overhaul life, the investigation concluded that these machining-related defects and the lack of protective plating should have been detectable during the most recent overhaul in the United States.
Findings
- The right main landing gear shock-strut piston suffered catastrophic fracturing due to a pre-existing stress corrosion crack.
- The crack originated at a location featuring a step in the blend radius and surface scratches, likely introduced during a previous overhaul.
- The absence of cadmium plating in the crack origin area left the high-strength steel vulnerable to corrosion in the damp environment.
- The high notch sensitivity of the 4340 steel alloy allowed a relatively small crack to lead to rapid, extensive failure under normal operating loads.