Introduction
On the morning of 27 June 2023, two Robinson Helicopter Company R22 helicopters were conducting mustering operations near Limbunya Station, Northern Territory. One helicopter, registered VH-PSC and operated by Top End Mustering, was piloted by a single pilot. The second helicopter, registered VH-RCS, was also operating in the area. After a period without communication from the pilot of VH-PSC, the pilot of VH-RCS attempted contact and received no response. The ground mustering crew also reported no recent communication. The pilot of VH-RCS began a search and located the wreckage of VH-PSC approximately 15 to 20 minutes later. The helicopter was destroyed, and the pilot had sustained fatal injuries.
Findings
The Australian Transport Safety Bureau (ATSB) examined the accident. For reasons that could not be determined, VH-PSC collided with terrain in a nose and right-side down orientation. The wreckage examination revealed signatures consistent with low rotor energy and low-to-nil engine power. There was no evidence of any flight control or mechanical system abnormality that would have prevented the helicopter from operating normally. In the absence of an identified problem with the helicopter, the reason for the loss of control could not be determined.
Engine Power and Maintenance Issues
The ATSB considered potential reasons for the reduction in engine power, including engine or system defects, fuel contamination or starvation, carburettor ice, and pilot-induced throttle reduction. Examination of the engine and fuel system did not identify any fault or condition that would have prevented normal operation. Fuel quantity and quality were also unremarkable. While an intermittent interruption could not be completely discounted, no evidence supported that scenario.
Review of meteorological conditions from the nearest Bureau of Meteorology weather station, 144 km away, indicated conditions on the edge of the serious icing envelope for carburettor ice. However, the pilot of the other R22 operating in the same area reported that local conditions did not require carburettor heat use and no signs of carburettor ice were observed.
The ATSB considered the possibility that the pilot intentionally reduced throttle to perform a quick descent. An inadvertent engine stoppage could result from the throttle being reduced too quickly, potentially at a height too low for a successful autorotation. The collision orientation was also consistent with a quick descent maneuver from which recovery did not occur. Alternatively, if the engine was at low power when the main rotor blades struck a tree and the ground, the impact could have stopped the engine.
Loss of Control
No evidence of a collision with a bird or object such as a wirestrike was found. Witness observations indicated that weather conditions did not affect control. There was no indication of mast bumping or control system issues. The possibility of a loose object interfering with flight controls could not be confirmed due to cabin damage. However, any such interference would not explain the low rotor energy state.
Distraction was considered, but the other R22 was not operating in the immediate area. An engine power reduction, if not intentional, could have been a distraction requiring autorotation. The ATSB's research indicates distraction events more often result in incidents than accidents. Effective management of main rotor RPM above 97% is required for a successful autorotation during a power reduction.
Maintenance Release Deficiencies
The maintenance release for VH-PSC lacked endorsements for daily inspection certification, hours flown, total time in service, and engine oil uplift. The release had been issued 13 days prior, and the helicopter was operated daily since. A scheduled 25-hour engine oil and filter change had reportedly been conducted but was not certified. The absence of recorded operating hours increased the risk of the helicopter being operated beyond scheduled maintenance requirements.
Conclusion
The ATSB determined that the helicopter collided with terrain in a nose and right-side down orientation for reasons that could not be identified. The wreckage exhibited low rotor energy and low engine power at impact. No mechanical or flight control anomalies were found. The maintenance release was not properly completed, but this did not directly contribute to the accident. The investigation could not establish a definitive cause for the loss of control.