What happened
On 29 July 2011, an MD900 helicopter, registration G-CEMS, was performing aerial work at Leeds Bradford Airport. After completing a takeoff and hover taxi, the aircraft landed at Hold Y. While stationary on the ground with engines at flight idle, a loud bang was heard, followed by the helicopter pitching nose-down and to the right. The crew immediately shut down both engines and exited the aircraft without injury. Upon inspection, the commander found that the forward landing gear cross tube had fractured, causing the fuselage to make contact with the broken component and resulting in delamination of the composite structure.
The investigation
AAIB investigators examined the fractured forward cross tube and identified a fatigue crack originating from a machining mark on the outer surface. The investigation revealed that the internal surface of the tube was significantly corroded, with evidence of exfoliation. This corrosion was linked to moisture accumulation inside the tube.
Further examination of the side stop clamps showed that the environmental sealant had not been renewed since the aircraft was manufactured in 2001. The investigation also scrutinized the Rotorcraft Maintenance Manual (RMM). While the RMM required the removal of side stop clamps to inspect dowel pin holes, it did not explicitly mandate inspecting the area beneath the clamps or the mating surfaces of the tube. Additionally, the internal surface of the cross tube had not been coated with epoxy primer, which was a deviation from production standards.
Findings
- The primary cause of the fracture was a fatigue crack originating from a surface machining mark.
- Significant corrosion on the internal surface of the cross tube was caused by moisture ingress due to inadequate sealant maintenance.
- The maintenance instructions in the RMM were ambiguous, failing to explicitly require the inspection of the cross tube area beneath the side stop clamps.
- The side stop clamps had not been removed for inspection since the aircraft's manufacture ten years prior.
- A production non-conformance meant the internal surface of the cross tube lacked the required epoxy primer.