Incident Overview
During an emergency, the pilot of a helicopter reported that engine RPM did not decay. The only indications of a fault were the low rotor RPM horn and a low rotor RPM reading on the rotor tachometer. The pilot could not exclude activation of the clutch light during the emergency, but post-recovery inspections and ground run tests showed no indication of belt slippage. An internal inspection of the clutch assembly did not find evidence of clutch slippage.
Pilot Actions
When the pilot manoeuvred the helicopter prior to entering autorotation, they noticed that rotor RPM decayed whenever the collective was raised. When the collective was raised to cushion the landing, the helicopter responded in a power-off manner. The pilot had elected to delay recovering RPM until a safe landing site could be reached, as immediately lowering the collective would have resulted in ditching the helicopter in the ocean.
Investigation Findings
According to the investigation, if there had been no loss of power to the rotor, the helicopter could have been expected to climb as a result of raising the collective to cushion the landing. Therefore, the low rotor RPM was probably the result of a reduction of power input to the rotor from the engine. However, during post-recovery inspections and ground run tests, no fault was found which could explain this condition.
ATSB Comment
The ATSB notes that the pilot operating handbook directs the pilot to lower the collective immediately to maintain rotor RPM between 97 and 108 per cent following a power failure. In this case, the pilot faced a dilemma where immediate compliance would have led to ditching the helicopter in the ocean.