What happened
On October 3, 2011, a Eurocopter AS350 B3, registered JA508A, was performing cargo transport operations near Kiyokawa Village, Kanagawa Prefecture. The aircraft, operated by Toho Air Service Co., Ltd., was moving construction materials using a sling cable system. During the mission, the pilot was navigating between various unloading sites along a mountain ridge.
While the aircraft was in flight, the onboard mechanic heard a loud impact and observed that the tail rotor had stopped rotating and was visibly damaged. The tail lamp was also detached and hanging from an electric wire. Despite the critical failure, the pilot did not immediately attempt an emergency landing at the nearest suitable location. Instead, the pilot continued to fly while searching for a wider landing area, passing over the Arai helipad and attempting to locate a disaster prevention heliport.
As the aircraft maneuvered, the damage to the tail section and vertical stabilizer worsened, eventually leading to a structural rupture. The helicopter lost controllability, entered a yawing motion, and crashed into the trees at the Choja-yashiki Campground. The impact resulted in one fatality (the pilot) and one serious injury (the mechanic). A fire broke out upon impact, destroying the airframe.
The investigation
The Japan Transport Safety Board (JTSB) examined the wreckage, interviewed the surviving mechanic, ground operators, and witnesses, and reviewed flight manuals and maintenance records. The investigation focused on why the sling cable made contact with the tail rotor and why the pilot's emergency response failed to prevent the crash.
Findings
- The primary cause of the accident was the sling cable contacting the tail rotor, which damaged the tail section and caused a loss of tail rotor thrust.
- This contact likely occurred because the pilot increased airspeed excessively without adequately monitoring the cable's movement via the rearview mirror, and because the load factor decreased during a downward flight path adjustment, bringing the cable closer to the tail.
- The instability of the cable was exacerbated by insufficient ballast weight on the sling.
- The pilot failed to execute an emergency landing as soon as possible, as required by the Japanese Flight Manual, because appropriate landing sites had not been pre-selected and the pilot was unprepared for the specific emergency.
- The pilot continued flying in an attempt to find a better site, failing to anticipate that the structural damage to the tail section would worsen and render the aircraft uncontrollable.