What happened
On 13 June 2022, during a routine post-flight maintenance inspection at Norwich Airport, engineers performing a walkaround on a Leonardo AW139, registration G-CIMU, discovered significant wear damage. The inspection revealed a hole worn through the cover protecting the tail rotor duplex bearing and the end of the tail rotor control actuator. Upon removing the cover, the maintenance team found a large accumulation of carbon dust inside the component. Further examination showed that the actuator was protruding beyond its normal position and the bearing race was heavily damaged, with the ball bearings in the outboard race becoming visible.
During the attempt to remove the bearing for further study, the component was so degraded that it disintegrated into its constituent parts. The aircraft had previously completed a passenger flight to North Sea oil rigs earlier that day before being moved into the hangar for the daily check.
The investigation
The AAIB launched an investigation due to the potential relevance of the failure to ongoing investigations involving similar bearings on AW169 and AW1 and AW189 helicopter models. Investigators examined the damaged bearing and actuator, which were sent for independent forensic analysis. The investigation also reviewed the technical design of the yaw control system, specifically the hydraulic actuator and the hybrid duplex bearing, which utilizes silicon nitride ceramic ball bearings.
While the investigation sought to determine the exact cause of the failure, the physical condition of the bearing was too poor to allow for a definitive forensic conclusion. The investigation also looked into the maintenance history and the existing airworthiness processes regarding the inspection of these critical components.
Findings
- The failure of the tail rotor duplex bearing was identified via visible wear damage to the actuator cover.
- The extent of the damage to the bearing was such that a loss of tail rotor control would likely have occurred if the helicopter had continued to fly.
- The bearing is classified as a critical part, as its failure could have catastrophic consequences for the aircraft.
- The investigation found that the maintenance process used by the operator to refit bearings at their facility did not comply with the Aircraft Maintenance Manual (AMM) procedures, specifically regarding the prohibited reuse of certain components.