What happened
On March 16, 2013, a privately owned Robinson R22 Beta, registration JA23TN, departed from a temporary site in Fukuyama City, Hiroshima Prefecture, for a leisure flight toward Matsuyama Airport. During the cruise phase at approximately 1,000 feet and 80 knots, the pilot detected an abnormal engine sound and observed that the manifold pressure indicator was abnormally high.
Believing the engine and rotor RPM were over-speeding and potentially out of control, the pilot attempted to adjust the governor and throttle. However, the abnormal vibrations and engine sounds persisted. Fearing imminent engine failure and attempting to avoid an approaching mountain, the pilot executed a forced landing into a bamboo grove near Asanamihara, Matsuna City, Ehime Prefecture. The impact with the trees caused the aircraft to overturn and resulted in the destruction of the airframe. The pilot sustained a minor injury, while the passenger was uninjured.
The investigation
The Japan Transport Safety Board (JTSB) examined the aircraft's mechanical state, the flight history, and the pilot's decision-making process. Investigators analyzed the engine/rotor RPM behavior and the functionality of the RPM governor system. The investigation also included a review of the aircraft's electrical components, specifically the alternator and master battery. Additionally, the quality of the fuel used in the flight was tested, and the suitability of the installed master battery was evaluated.