Background
On October 23, 2008, an Aerospatiale SA315B Alouette III, registered JA6117, operated by Toho Air Service Co., Ltd., was conducting cargo transport for the construction of a TV broadcasting relay antenna. The helicopter departed from Naruko 1 temporary helibase at approximately 11:26 JST with a pilot in command, a mechanic, and a passenger on board. The flight plan specified visual flight rules and an estimated total elapsed time of 1 hour 30 minutes.
Flight History
Before descending to Tazawako-Obonai temporary helibase (Obonai helibase), the pilot conducted a reconnaissance flight over a mountain top where afternoon work was planned. At about 12:15, during the approach and touchdown to the helipad, blue polyester tarpaulin sheets placed near the fuel storage area were blown into the air by the helicopter's downwash. One sheet was sucked into the main rotor disc, causing the accident.
According to the pilot, the approach was normal; he observed folded blue sheets with weights placed on them from about 10 to 15 meters above the helipad. The helicopter hovered at 2 meters and began a slow descent. Just before touchdown, sudden short-cycle vibrations and a yaw occurred. After landing on steel plates, the skids trembled, and the helicopter yawed left. The pilot shut down the engine and applied the rotor brake. An eyewitness reported seeing a blue sheet blow up and enter the rotor disc.
Damage
The helicopter sustained substantial damage. Specific component damage included: a bent tailboom and tail rotor drive shaft with destroyed support tubes, a broken and snapped left shock strut, damage to one of three main rotor blades, and detachment of two of three spacing cables.
Personnel and Aircraft Information
The pilot held a commercial pilot certificate with rotorcraft and instrument ratings, a class 1 medical certificate valid until May 26, 2009, and had a total flight time of 4,472 hours 55 minutes, including 424 hours 34 minutes on the accident aircraft type. The helicopter was an Aerospatiale SA315B Alouette III, serial number 2020-13, manufactured in 1968, with a certificate of airworthiness valid until August 19, 2009. Total time in service was 3,808 hours 03 minutes, and time since the last periodical check (October 4, 2008) was 33 hours 04 minutes. Estimated weight and balance at the time of the accident were within allowable limits.
Meteorological Conditions
The pilot reported cloudy weather, wind from east-southeast to southeast at 5–7 m/s, visibility more than 10 km, and no cloud cover over the helipad.
Accident Site
The accident occurred at Obonai helibase, located about 3.4 km east of Lake Tazawako in Semboku City, Akita Prefecture. The helipad was constructed of steel plates.