What happened
During a flight, a Robinson R44 experienced a significant reduction in engine power that prevented the pilot from maintaining altitude. The engine began misfiring due to issues within the right magneto, which also caused the engine governor to provide inaccurate throttle inputs. These fluctuations led to an overspeed condition where both engine and rotor RPM exceeded manufacturer limits.
In response to the power loss, the pilot transitioned the aircraft into an autorotation and closed the throttle to stabilize engine RPM. During the descent, the pilot deployed the emergency pop-out floats. The aircraft performed a controlled ditching on the water, remaining upright and buoyant. The pilot remained safely inside the cabin until emergency responders arrived. There were no fatalities or injuries.
The investigation
Investigators examined the right magneto and found that a loose electrode in the distributor gear had caused the gear to jam, resulting in damaged gear teeth. This mechanical failure disrupted the electrical timing to the spark plugs, leading to engine misfires. While the exact cause of the electrode loosening could not be determined, the manufacturer noted that fatigue, vibration, or heat could be contributing factors.
Further inspection of the engine revealed damage to the exhaust valve sealing face. The investigation determined that the altered ignition timing caused by the faulty magneto led to combustion during the exhaust stroke. This overheating caused the valve to burn, which compromised compression in the cylinder and further reduced available power. The aircraft's history of operating in the corrosive salt air of the Whitsunday Islands and Great Barrier Reef was also noted as a factor in valve erosion.
Findings
- A loose electrode in the magneto distributor gear caused the gear to jam and the gear teeth to fail.
- The resulting timing issues caused the engine to misfire and led to the damage of the number one cylinder exhaust valve.
- The damaged valve caused a loss of compression, contributing to the loss of engine power.
- Erroneous signals from the damaged magneto caused the engine governor to provide inaccurate throttle inputs, resulting in an engine and rotor RPM overspeed.