Accident Overview
On Saturday, March 18, 2017, a privately owned Robinson R44 helicopter, registration JA7907, sustained damage when it rolled over after contacting a snowy slope during landing at the Temporary Airfield in Odaira, Itoigawa City, Niigata Prefecture. The accident occurred at around 15:13 Japan Standard Time. The pilot in command (PIC) and three passengers were on board; no injuries were reported.
Flight History
According to statements from the PIC and passengers, along with video footage and photographs, the flight was a leisure flight with the Temporary Airfield as its hub. At approximately 15:10, the helicopter took off from Eboshidakeshita Temporary Airfield. After takeoff, the helicopter flew at a low altitude along a valley, changing course multiple times. Approaching the airfield from the south-southeast, the helicopter passed over the helipad, then performed a steep left turn with the nose down to avoid trees. During this maneuver, the rotor sound changed and the low rotor RPM warning horn activated. Immediately after turning northeast with a shallow bank, the helicopter's left side contacted the snowy slope southwest of the helipad. The main rotor touched the snow, causing the helicopter to roll over and the engine to stop. The helicopter came to rest on its right side, facing south. The bubble window was broken, and one rotor blade tip was broken and protruded from the snow; the other blade and the rotor head and mast were buried. The left-side ski rack detached and was found about three meters southwest along the slope.
Damage and Injuries
The helicopter was destroyed. Specific damage included a broken bubble window and door frame, broken main rotor blades, a broken rotor head and mast, and a detached ski rack. Subsequent handling by a snow compaction vehicle may have expanded the damage. The PIC and three passengers evacuated through the broken bubble window without reported injuries.
Investigation Findings
The Japan Transport Safety Board (JTSB) investigated the accident. The PIC held a commercial pilot certificate with a rotorcraft rating and had 1,779 hours and 48 minutes total flight time, including 222 hours and 42 minutes on the same type. The helicopter was a Robinson R44 (serial number 0050) manufactured in 1994, with a valid certificate of airworthiness. Meteorological conditions at the airfield were reported as fine with northwest wind under 5 knots. The accident site was a snowy slope about 20 meters southeast of the helipad, approximately two meters above it. The report noted that the helipad's boundary line was not defined at the time, but the PIC considered takeoff and landing safe. The featureless terrain illusion, caused by snow-covered ground, was referenced as a known phenomenon. The low rotor RPM warning horn is designed to activate when main rotor speed drops below 96-97% of specified values. The rotorcraft's safety notice emphasizes immediate throttle increase and collective lowering in low RPM situations. The PIC did not report the accident to the authorities until 13 days later. The investigation determined that pilot involvement contributed to the accident, but no specific probable cause was stated.