What happened
During low-level mustering operations, a Robinson R4CO helicopter experienced a sudden loss of rotor drive. The pilot, operating at approximately 60 feet above the ground, observed smoke emanating from the rear of VH-YMU shortly before the aircraft impacted the terrain. Due to the low altitude and the nature of the mechanical failure, there was insufficient time to establish an autorotation to cushion the landing. However, the pilot managed to maneuver the aircraft into a clear area, landing in an upright position, which helped mitigate the severity of the injuries sustained.
The investigation
The investigation focused on the failure of the rotor drive system, specifically examining the fanwheel and drive belts. Investigators identified pre-existing fatigue cracking in the welded regions of the fanwheel vanes and the outer support ring. While the exact cause of the initial breakup was difficult to determine due to limited cracking evidence, the fragmentation of the fanwheel created a significant mass imbalance.
This imbalance likely caused the forward drive belt to dislodge from its sheaves and become entangled in the rotating engine and rotor components. Evidence of heat and abrasion was found on the forward belt, which appeared to be a relatively new component. Following the impact, the rear drive belt likely failed due to the tensile stress caused by the engine's downward motion during the collision.
Findings
- Fatigue cracks in the fanwheel's outer support ring and vane welds compromised the component's structural integrity.
- The fragmentation of the fanwheel caused a drivetrain imbalance that led to the dislodgement of the forward drive belt.
- The low-level nature of the mustering flight left the pilot with minimal time to react to the loss of rotor drive.
- A delay in confirming the accident occurred because the emergency locator transmitter (ELT) registration had not been updated following a change in aircraft ownership.
- There appears to be a lack of formal defect reporting regarding fanwheel cracking within the CASA Defect Reporting Service, despite known risks among operators.
Safety message
Operators must ensure that all in-service defects, including identified cracks in components, are reported to CASA via the online defect reporting service. Furthermore, it is critical that ELT registration details are updated immediately following a change in aircraft ownership to ensure rapid search and rescue response.