2025-11-15: Robinson Helicopter Co R22 Beta (VH-8H8) — Georgina Pastoral Company Pty Ltd — Near Argadarga Aerodrome, NT, and Chillagoe Aerodrome, Qld, Northern Territory

Accident narrative

Drive belt failures The VH-8H8 pilot report of a burning rubber smell, followed shortly afterwards by the ‘bang’, the significant reduction in main rotor RPM and sudden increase in engine RPM, was all consistent with failure of the helicopter’s drive belts. The short (10 to 15 minute) duration of flight, rubber smell, and uneven belt wear, was consistent with the forward belt not being correctly seated in the drive sheave grooves. This belt misalignment most likely occurred during startup, as the flight duration was consistent with RHC’s experience of belt longevity when operated in this configuration. The abnormal wear indicated that the forward belt most likely failed first and, noting the very brief time interval between the 'bang' and the main rotor RPM decay, the forward belt probably interfered with the rear belt, which then fractured, resulting in the complete drive train disconnect.The VH-HFQ pilot report similarly indicated a rapid decay in main rotor RPM, consistent with the drive belt separation subsequently observed by the pilot on the ground. However, the mechanism of failure in this case was not determined. The drive belts were not available to the investigation, and the recounted circumstances of the occurrence were slightly different. Specifically, the significantly longer (30-minute) flight time and the absence of any reported burning rubber smell preceding the event. This may be indicative of a different failure mechanism, although the presence of any rubber smell would be dependent on local conditions and not necessarily detected.In each of the occurrences the helicopters had recently been inspected, the drive belts were relatively new, and pilots reported conducting the required pre-flight checks without observing any issues. It is worth noting that, although the pilot of VH-8H8 sought a second opinion on the belt tension, ultimately it was assessed as acceptable by 2 pilots and therefore unlikely to have been excessively slack. Otherwise, there were no obvious operational factors reported or identified that might have led to the drive belts becoming unseated from the sheaves or that would otherwise contribute to a belt failure. Pilot responses In both occurrences, the initial indications of drive train failure were reported as being almost coincident with the main rotor RPM decay. There was no prolonged or abnormal clutch light illumination, or any other timely indications preceding the failure, and therefore no opportunity to conduct any kind of power-on landing. As it was, both pilots had to assess the condition as an engine or drive system failure (noting the emergency procedure for both is the same), and immediately lower the collective to enter autorotation. Both pilots did this after observing the rapid main rotor RPM decay. The pilot of VH-8H8 later reflected on the few seconds delay in diagnosing the condition before lowering the collective and entering autorotation. In those few seconds the rotor speed decayed to the minimum allowable level, beyond which increased the risk of an adverse outcome, particularly at low level where time and altitude may be insufficient to recover low rotor RPM.Aside from rotor RPM, there are a number of variables that can influence the success of an autorotation landing (including, airspeed, weight, density altitude, manoeuvring, and the timing and magnitude of pilot control inputs), and because of this, the specific factors that contributed to the hard landings in these events were not explored in detail. Nevertheless, these occurrences serve as a reminder that power loss events may occur with limited prior warning. They reinforce that pilots should be intimately familiar with the POH emergency procedures and be prepared to act immediately in the event of any abnormal in-flight indications. Drive belt failure rates Reports and records provided to the ATSB by both of the helicopter maintainers indicated a relatively recent increase in low-time drive belt replacements. The reported issues were similar to those captured in the CASA defect reporting scheme database around 2020–21 which was also reflected in the occurrence involving VH-SUX (AO‑2021‑007).The occurrence belt sets, as well as the August low-time replacement on VH-HFQ and 2 other low-time replacements recorded by the maintainer of VH-8H8 were from the same lot number. This suggested the possibility that there was a manufacturing anomaly with a particular batch of belts that pre-disposed them to stretching. This would infer that the occurrence belts also had stretching issues, which was possible but could not be confirmed. There were also no reports to indicate that other belts, out of the 150 sets from the same lot number, had been replaced for a similar reason. Moreover, the increase in belt replacements had been occurring over the previous few years, which might suggest a more significant manufacturing anomaly than a single batch. Despite this, there was no evidence of a more widespread,