What happened
During flight, the pilot of the helicopter experienced a significant drop in rotor RPM, accompanied by the activation of the low rotor RPM warning horn. As the pilot attempted to manoeuvre the aircraft before initiating an autorotation, they observed that raising the collective caused the rotor RPM to decay. When the pilot subsequently raised the collective to cushion the landing, the aircraft responded as if in a power-off state. To avoid ditching the aircraft in the ocean, the pilot elected to delay lowering the collective, instead focusing on reaching a safe landing site.
The investigation
Investigators examined the aircraft following the incident to identify the source of the power loss. The pilot noted that while the engine RPM remained stable, the rotor tachometer indicated low RPM. During post-recovery inspections and ground run tests, investigators found no evidence of belt slippage. Furthermore, a detailed internal inspection of the clutch assembly revealed no signs of clutch slippage occurring under high-power settings. Despite these efforts, the technical team could not find a mechanical fault that would explain the observed reduction in power input to the rotor.
Findings
- The low rotor RPM was likely caused by a reduction of power input to the rotor from the engine.
- Post-recovery testing and ground runs failed to reproduce the fault.
- There was no evidence of slippage within the clutch assembly during the inspection.
- The pilot's decision to delay lowering the collective was driven by the need to avoid an immediate ditching in the ocean.