Incident Overview
On 8 January 2023, the pilot of a Robinson R44 helicopter, registered VH-ZUJ, observed a persistent clutch warning light while on approach to Hamilton Island Airport in Queensland. Following the emergency procedure for the clutch warning light, the helicopter landed safely. Ground crew subsequently discovered that the clutch actuator electric drive motor had detached from the gearmotor assembly and had become lodged between the drive belts and the right-hand fan shroud.
Investigation Findings
The Australian Transport Safety Bureau (ATSB) investigated the incident and determined that during the assembly of the gearmotor, the required thread adhesive was either not applied or was applied in a manner that failed to prevent the loosening of the electric motor retaining nut. Over time, normal helicopter vibrations caused the retaining nut to loosen, which allowed the electric motor to separate from the gearmotor assembly during flight.
Examination of the threaded sections of the incident gearmotor under ultraviolet light revealed that the thread adhesive was not visible in an area that would reliably secure the motor. The absence of thread locking adhesive on the mating surfaces likely reduced the intended design clamping force between the gear assembly and the electric motor. However, the ATSB could not determine the reason for this absence, nor could they chemically identify residue found in the gear assembly threads to assess its impact on adhesion or lubrication. The extent to which these factors influenced the failure was not determined, though it was noted that a reduced clamping load may have contributed.
Corrective Actions
In response to the incident, Robinson Helicopter Company stated that they are actively working with the component manufacturer to address identified quality issues related to the gearmotor assembly. They are also considering updates to the inspection procedures for the clutch actuator assembly.
Safety Recommendations
The ATSB emphasized the importance of physically checking the security of the R44 clutch gearmotor assembly on a regular basis. Pilots and maintenance engineers are encouraged to hand-check the motor for security with electrical power off, or to conduct a visual inspection for motor rotation during operation. These methods may help detect a loosening motor before complete separation occurs. If a defect is identified, the gearmotor assembly should be replaced, and the defect should be reported to the Civil Aviation Safety Authority (CASA) and the manufacturer.
The ATSB also noted that the integrity and effectiveness of thread adhesives are difficult to determine by visual inspection alone, and typically require specialized tooling or procedures. Therefore, it is likely that the inspecting licensed aircraft maintenance engineers would have been unable to detect the gearmotor defect by visual inspection had it existed at the time of the last scheduled inspection.