Summary
On October 3, 2011, at approximately 12:17 JST, a Eurocopter AS350 B3, registered JA508A, operated by Toho Air Service Co., Ltd., crashed into the Choja-yashiki Campground in Kiyokawa Village, Kanagawa Prefecture. The helicopter was destroyed and a fire broke out. Two people were onboard: the pilot and an onboard mechanic. The pilot was killed and the mechanic was seriously injured.
Flight History
The helicopter departed from the Karasawa temporary helipad at around 09:30 to transport cargo for mountain trail construction. After multiple cargo transport flights, the pilot replaced the 27-meter sling cable with a seven-meter cable when approaching unloading site No. 6. Following unloading, the pilot turned back toward the Karasawa helipad.
Accident Sequence
During the return flight, the onboard mechanic heard a loud bang from the rear. The pilot remarked, "I think something hit the back." The mechanic observed that the tail rotor had stopped rotating, the upper blade was broken, and the tail lamp was hanging. Despite the damage, the pilot did not land immediately but searched for a suitable landing site. While circling, the helicopter began yawing left, then pitched down and crashed into trees at the campground. The mechanic escaped and called for help.
Probable Causes
The Japan Transport Safety Board determined that the accident occurred because the sling cable contacted the tail rotor, damaging the tail section and resulting in a loss of tail rotor thrust. The pilot did not make an emergency landing as soon as possible as stipulated in the Japanese Flight Manual, but instead continued to search for a suitable emergency landing site, during which time the damage to the tail section was exacerbated, leading to its rupture and loss of control.
The investigation also noted possible factors: the pilot increased airspeed excessively without monitoring the sling cable via the rearview mirror; the load factor decreased when adjusting the flight route downward, reducing distance between cable and tail; and the sling cable became unstable due to insufficient ballast weight. The pilot had not selected appropriate emergency landing sites before flight and did not anticipate that damage could worsen.