What happened
During flight operations, the electric motor within a gearmotor assembly separated from the larger gearmotor assembly. This separation was triggered by the standard vibrations experienced during normal helicopter operation. The failure resulted in the loss of the motor's connection to the gear assembly.
The investigation
Investigators examined the gearmotor assembly to determine why the motor had become unseated. Using ultraviolet light to inspect the threaded components, the ATSB found that the thread adhesive, which is intended to serve as a secondary locking mechanism, was not present in a location sufficient to secure the motor. While the adhesive was designed to prevent loosening caused by vibration, its absence in critical areas meant the motor could move freely.
The investigation also looked into the assembly process itself. It was noted that the lack of adhesive may have also reduced the intended clamping force between the gear assembly and the electric motor. While some residue was present in the threads, its chemical composition was not identified, making it impossible to determine if it acted as a lubricant that further compromised the connection. Furthermore, the investigation could not establish why the adhesive was missing from the mating surfaces during the initial assembly.
Findings
- The primary cause of the separation was the loosening of the electric motor caused by normal aircraft vibrations.
- Thread adhesive was either not applied to the threads or was applied in a manner that failed to provide an effective locking mechanism.
- The reduction in clamping force, potentially caused by the lack of adhesive or the presence of unidentified residue, may have contributed to the failure.
- It is unlikely that licensed aircraft maintenance engineers could have detected this specific defect through standard visual inspections, as verifying the integrity of thread adhesives typically requires specialized tools or procedures.