What Happened
On 16 August 2020, a Robinson R22 Beta II helicopter, registered VH-YMU, was mustering cattle on a property about 44 km south of McArthur River Mine Airport, Northern Territory. The helicopter was operated by ENJAY Services, and the pilot was the sole occupant. While hovering at a height of about 60 ft, the helicopter experienced a sudden loss of rotor drive. The helicopter rapidly lost height and collided with terrain. The pilot sustained serious injuries and the helicopter was substantially damaged.
What the ATSB Found
The Australian Transport Safety Bureau (ATSB) found evidence of fatigue cracking in the fanwheel outer support ring and in the welded region of two vanes of the fanwheel assembly. These fatigue cracks probably weakened the fanwheel structure sufficiently to result in in-flight break‑up of the fanwheel. During this investigation, it was also found that there had been other instances where cracking had been identified in the welded regions of fanwheels. However, a search of the CASA Defect Report Service for R22 and R44 models identified only a single reported occurrence in 2013 for vane weld cracking.
The imbalance caused by the fanwheel break-up likely led to the forward drive belt migrating from the drive sheaves. The remaining rear drive belt likely failed under the load exerted on it when the helicopter impacted the ground, and the downward motion of the engine overloaded the drive belt to failure. Due to the helicopter being in a 60 ft hover at the time of the drive belt failure, the pilot had limited options to respond to the emergency, resulting in a heavy landing and serious injuries to the pilot.
It was also identified that the details for the emergency locator transmitter (ELT) had not been updated after the helicopter had been recently purchased by the operator. This led to a delay in the Joint Rescue Coordination Centre (JRCC) being able to confirm the accident. Fortunately, prompt assistance was provided to the pilot by other personnel involved in the muster.
Rotor Drive System Failure
Fanwheel Failure
It could not be determined why the fanwheel assembly began to break up during the flight, mainly due to the limited amount of cracking evident within the fanwheel structure itself. The presence of fatigue cracking in the welded regions from the fanwheel vanes and on the outer support ring were considered a pre-existing defect. There was no metallurgical evidence of continuous/intermittent crack growth from that region of damage to indicate that a period of progressive crack growth had occurred to the extent that the structural integrity of the fanwheel should have been affected.
Drive Belt Failure
Examination of the drive belts and photographs provided to the ATSB by the operator established that the forward drive belt dislodged and moved forward, entangling in the rotating components of the engine and rotor drive. This was further supported by the witness account from the pilot of an R44, who observed smoke from the rear of VH-YMU moments before the accident. The forward drive belt showed signs of being heated and abraded by frictional contact with the upper sheave and clutch shaft. It is likely that the remaining rear drive belt failed in tensile overstress from the downward motion of the engine during the ground collision. This is supported by the short time from fanwheel failure to ground impact and the inability of the clutch actuator to have travelled to its full extension to cause the drive belt to fail through overstretching. As the circumference and cross-sectional profile of the forward belt were consistent with a new Revision‑Z belt, dislodgement due to a belt defect was considered unlikely. The examination of the upper and lower sheaves showed that the sheave grooves did not exhibit any abnormal wear or damage to the painted surfaces, which also supported positive belt engagement.
Low-Level Operations
Mustering operations involve manoeuvring at low-level and at varying airspeeds. Such operations increase the risk associated with a loss of engine power. In this accident, the operational requirement to operate at about 60 ft above ground level did not allow sufficient time for the pilot to react to the drive belt failure or provide sufficient aircraft energy for an autorotation to cushion the landing. The high hover provided limited opportunity to conduct a safe forced landing, however the pilot, with the remaining energy in the rotor system, was able to manoeuvre to a clear area and land the helicopter in an upright attitude, lessening the injuries sustained.
Emergency Locator Transmitter
Activation of the ELT on impact and transmission of the 406 MHz signal was detected by the JRCC, although incorrect contact details delayed confirmation of the accident. Fortunately, this accident was reacted to quickly due to the mustering being performed by two helicopters operating in close proximity and nearby ground crew. In a circumstance where an aircraft may be operating alone, any delayed search and rescue (SAR) response has the potential to be a critical factor in occupant survival. ELT registration is entered onto a database that is always accessible to SAR authorities. Up-to-date contact information is vital in deploying SAR resources and medical assistance effectively when an activation is detected.
Defect Reporting
There is reportedly an awareness within operators of Robinson helicopters that cracking can occur at the welded regions of the cooling fanwheel, however this is not reflected in the CASA Defect Reporting Service data. Reporting of in-service defects, whether identified during operation or maintenance, must be reported to CASA via the online defect reporting service. This ensures a database can be maintained to assist in identifying reliability of aircraft components and systems, which benefits the wider aviation community. Additionally, feedback provided to the manufacturer permits awareness of defects to be identified and investigated, and system improvements to occur should it be required. This ensures that aircraft component reliability and safety are maintained at optimum levels.