What happened
Two separate incidents involved Robinson R22 helicopters experiencing sudden drive train failures. In the first event involving VH-8H8, the pilot detected a burning rubber smell followed shortly by a loud bang. This was immediately accompanied by a significant drop in main rotor RPM and a spike in engine RPM. The flight had lasted only 10 to 15 minutes prior to the failure.
In the second occurrence involving VH-HFQ, the pilot also reported a rapid decay in main rotor RPM, consistent with drive belt separation observed on the ground. This flight had been longer, lasting approximately 30 minutes, and no burning rubber smell was reported beforehand. In both cases, the pilots recognized the emergency as an engine or drive system failure and immediately lowered the collective to enter autorotation.
The investigation
The ATSB examined the physical evidence and pilot reports to determine the causal chain. For VH-8H8, uneven belt wear indicated that the forward belt was not correctly seated in the drive sheave grooves. This misalignment likely occurred during startup, leading to abnormal wear and eventual failure of the forward belt. The failed forward belt then interfered with the rear belt, causing it to fracture and resulting in a complete disconnect of the drive train.
For VH-HFQ, the mechanism of failure could not be determined because the belts were unavailable for inspection. The longer flight time and lack of pre-failure odor suggested a different failure mode than the first incident. In both cases, the helicopters had recently been inspected, the belts were relatively new, and pre-flight checks showed no issues. Notably, two pilots assessed the belt tension on VH-8H8 as acceptable, making excessive slack an unlikely cause.
Findings
The investigation highlighted that power loss events can occur with limited prior warning. In both instances, the initial indications of failure coincided almost exactly with the rotor RPM decay, leaving no time for a controlled power-on landing. The pilot of VH-8H8 noted a few seconds' delay in diagnosing the condition before lowering the collective, during which rotor speed dropped to minimum allowable levels, increasing risk at low altitude.
Records from maintainers indicated a recent increase in low-time drive belt replacements across the fleet. Several affected helicopters, including those in these occurrences, used belts from the same lot number. This suggested a possible manufacturing anomaly predisposing certain batches to stretching, although this could not be confirmed. No widespread evidence of a broader manufacturing defect was found.
Safety message
Pilots must remain intimately familiar with emergency procedures and be prepared to act immediately upon any abnormal in-flight indications. Given the potential for sudden power loss with minimal warning, rapid recognition and execution of autorotation are critical to mitigating risk.