Introduction
On 29 April 2003, a McDonnell Douglas DC-8-63F cargo aircraft, registered 9G-MKO, experienced a fracture of its right main landing gear while performing a 180° taxi turn at RAF Lyneham, Wiltshire. The aircraft was prepared for a cargo flight to the Middle East with a takeoff weight of 297,996 lb. The four crew members on board were uninjured, and no passengers were carried.
History of the Flight
After taxiing to runway 24, the aircraft backtracked to the turning area and began a 180° right turn to line up for takeoff. With about 20° of the turn remaining, the crew heard a loud noise and the aircraft stopped. Increasing engine power did not move the aircraft, and ground engineers discovered that parts of the right main landing gear had fractured, with the truck rotated approximately 45° left relative to the aircraft heading. The aircraft remained on the runway for about 36 hours while the landing gear was replaced.
Aircraft and Landing Gear Description
The McDonnell Douglas DC-8-63 is a four-engined freighter with a tricycle landing gear. The main landing gear leg consists of a cylinder-piston oleo shock strut. The piston is manufactured from 4340 steel, heat-treated to an ultimate tensile strength of 260-280 ksi, and protected by cadmium plating. The scissor linkage prevents piston rotation relative to the cylinder. The piston is approximately 44 inches long with a 9.5-inch diameter and 0.63-inch wall thickness.
Maintenance Background
At the time of the accident, the aircraft had accumulated 48,076 flight hours and 16,533 cycles. The right main landing gear (serial number HA002) had been overhauled in the USA and fitted during a C Check in December 1999, operating 2,379 flight hours since overhaul—about 10% of its specified overhaul life of 23,000 hours.
Wreckage Examination
Examination revealed extensive fracturing of the shock-strut piston into four main pieces and fragments. A generally axial fracture passed through the lower torsion link clevis lugs and the aft limb of the inboard truck beam clevis lug. Two fractures extended up the left side-wall, with one becoming circumferential about 9 inches from the base, separating the lower part. One piece remained jammed in the cylinder, supporting the aircraft. The lower torsion link fractured at the apex pin, and the forward limb of the left truck beam clevis lug twisted about 40°.
Materials Assessment
Metallographic examination showed the piston material met specifications. The fracture origin was at the edge of the aft lug of the lower torsion link clevis, at a blend radius on the chromed bearing face. A stained area about 5 mm across and 2 mm deep exhibited intergranular morphology characteristic of stress corrosion cracking, while the surrounding fracture surface showed ductile dimples from overload. Three distinct discoloration bands suggested intermittent crack growth. Cadmium plating was absent near the crack origin, and a step in the blend radius with local machining damage was present. No grinding burns were found, and shot peening evidence was inconclusive.
Probable Cause
The fracture originated from a pre-existing stress corrosion crack in the shock-strut piston, initiated by abnormally high local stresses associated with a step in a blend radius and surface scratches where cadmium plating was absent, leading to rapid extensive fracturing under normal operating loads.