Magneto Damage
The magneto distributor gear on helicopter VH-8HR had accumulated 335.6 flight hours since its last inspection, with the next inspection due at 164.4 hours. The cause of a loose electrode on the distributor gear could not be determined. The magneto manufacturer indicated that the rotating electrode could become loose due to fatigue, vibration, incorrect installation, or heat damage. The loose electrode likely caused the distributor gear to jam, damaging the gear. This damage or the jamming affected the timing of the electrical supply to the spark plugs, resulting in engine misfire.
Exhaust Valve Damage
The helicopter had flown 1,116.8 total hours, primarily over the Whitsunday Islands and Great Barrier Reef, exposing it to a corrosive salt air environment. The maintainer reported erosion damage to the valve sealing face, likely from the corrosive environment, fuel, or exhaust deposits. Continental Aerospace Technologies (CAT), the magneto manufacturer, stated that the valve damage was consistent with late ignition timing from the damaged magneto. The late timing caused combustion during the exhaust stroke, overheating and burning the exhaust valve, increasing the likelihood of premature failure. The damaged exhaust valve prevented adequate compression during the combustion cycle, reducing engine power.
Forced Landing
As a result of the damaged magneto and exhaust valve, the available power was insufficient for the pilot to maintain altitude. The pilot closed the throttle and entered autorotation, which removed the threat of further engine RPM fluctuations. During autorotation, rotor RPM increased from approximately 92% to over 100%, providing additional inertia that likely assisted the pilot in performing a controlled ditching without further damage to the helicopter or emergency pop-out floats. The pilot activated the emergency pop-out floats during the autorotation, which kept the aircraft upright and buoyant on landing. The occupants took refuge in the helicopter until first responders arrived.
Erratic Governor Control
The Robinson R44's engine-right magneto provided an electrical signal to the cockpit engine tachometer. The engine governor applied throttle inputs to maintain constant engine RPM. The damaged right magneto likely caused the tachometer to incorrectly read engine RPM, leading to inaccurate throttle inputs by the governor and resulting in engine RPM fluctuations. These fluctuations included several RPM reductions and one significant increase that exceeded the manufacturer's limits for both engine and rotor RPM, increasing the likelihood of damage to the helicopter. Following the overspeed, the pilot switched off the governor control and flew using manual throttle. Engine monitoring unit (EMU) data showed a 10-second period of stable RPM at about 98%, consistent with manual control, followed by a further momentary RPM reduction. The governor switch position was not recorded by the EMU, so the ATSB could not determine if that reduction was caused by governor inputs.