What happened
On October 27, 2007, at approximately 15:05 JST, a Robinson R22 Beta helicopter, registration JA102D, crashed onto the Nankai Koya Line railway tracks in Sakai City, Osaka Prefecture. The aircraft, operated by Osaka Aviation Inc., had departed from Yao Airport for a scheduled 15-minute experience flight.
During the flight, the aircraft's flight path became irregular, with radar records showing fluctuating altitudes and ground speeds. Eyewitnesses reported seeing the helicopter flying at an unusually low altitude, appearing to wobble or lose balance. Witnesses also noted that the main rotor blades appeared to stop rotating or were not spinning normally before the impact. The aircraft struck the railway tracks between Asakayama and Abikomae Stations, causing the destruction of the airframe and a fire. The impact also severed six overhead high-voltage wires and damaged a power pole. The two fatalities included the captain and the passenger.
The investigation
The investigation examined the aircraft's flight history, radar data, and eyewitness accounts. Investigators analyzed the wreckage, which showed a broken fuselage, a detached tail cone, and broken rotor blades. Specifically, a large dent was found on the tail cone where a main rotor blade had struck it. Radar analysis confirmed the aircraft's erratic movement prior to disappearing from the screen. The investigation also looked into the weather conditions, noting strong, gusty winds and the approach of a typhoon on the day of the accident. Additionally, the medical history of the captain was reviewed, though no direct link between his health and the accident could be established.
Findings
Investigators determined that the primary cause of the accident was mast bumping, which occurred during flight. This mechanical failure led to a main rotor blade striking the tail cone, causing a rapid drop in rotor speed and rendering the helicopter uncontrollable.
A significant contributing factor was the presence of a passenger in the right seat who was not a qualified pilot. It is believed that when the aircraft encountered strong tail gusts, the passenger performed abrupt maneuvers that made it difficult for the captain to maintain control, ultimately triggering the mast bumping event.