Accident Overview
On Saturday, March 16, 2013, around 10:04 Japan Standard Time, a privately owned Robinson R22 Beta helicopter, registered JA23TN, sustained damage during a forced landing near Asanamihara in Matsuyama City, Ehime Prefecture. The aircraft had departed from a temporary operation site in Fukuyama City, Hiroshima Prefecture, at approximately 09:30 for a leisure flight to Matsuyama Airport. The captain and one passenger were on board. The captain suffered a minor injury, and the helicopter was destroyed, but there was no outbreak of fire.
Flight History
According to the captain's statement, the pre-flight check revealed no abnormalities, and the aircraft departed with the captain and one passenger. The navigation light switch was not turned on due to daytime flight. No anomalies were noticed during the first 30 minutes of flight. At around 10:04, while cruising at an altitude of approximately 1,000 feet and a speed of about 80 knots, the captain perceived an abnormality in the engine sound. The engine and main rotor were rotating at an abnormally high speed. The manifold pressure indicator showed an abnormally high reading. The captain lowered the collective pitch lever, but the manifold pressure did not respond.
Believing the governor system to be malfunctioning, the captain turned the governor off, closed the throttle slightly, and toggled the governor switch on and off, but observed no change in engine sound or manifold pressure. The caution light for clutch disengagement did not illuminate. The captain concluded the engine was over-speeding and out of control. The helicopter felt overly sensitive to control inputs, and a loud clang was heard from the engine. Deciding a forced landing was necessary, the captain turned the aircraft right toward a bamboo grove in sight. He raised the nose, reduced speed, made a normal approach, and flared at the end, resulting in a low-speed impact into the bamboo. The captain estimated about 10 seconds elapsed from the onset of the abnormality to the forced landing.
The passenger reported that the yellow caution light did not illuminate during flight. She noticed the helicopter did not climb near a mountain, observed a troubled expression on the captain's face, and heard a very loud engine sound. The aircraft turned right and descended into the bamboo grove, impacting three times with loud sounds. After the forced landing, no instrument panel lamps were lit. When the passenger later re-entered the overturned helicopter to retrieve a bag, her right elbow inadvertently contacted a switch, producing a click, but she could not identify which switch.
A witness working about 50 meters from the site reported that the aircraft made a loud sound while flying, then turned toward the bamboo grove and flew straight like a paper airplane, with no smoke.
Injuries and Damage
The captain sustained a minor injury. The aircraft was destroyed, with damage to the landing gear, fuselage, windshield, door, tail gear box (broken), tail cone (bent), drive system (partially broken), and main and tail rotors.
Probable Causes
The Japan Transport Safety Board determined that the probable cause of the accident was that when the engine/rotor RPM increased while cruising, the captain could not deal with the situation, leading him to aim for a bamboo grove for a forced landing, resulting in airframe damage. The captain's inability to manage the situation was likely because he concluded that the rotor over-speeding was due to an out-of-control engine over-speed without confirming the engine/rotor RPM on the tachometer. It is somewhat likely that the RPM increase involved the alternator power switch being off, causing battery power depletion, loss of governor operation, and governor suspension. However, when the alternator switch became off could not be identified, so the reason for the RPM increase could not be determined.