What happened
On December 30, 2020, a privately owned Robinson R66, registration JA77AR, departed from Isewan Heliport in Tsu City, Mie Prefecture, on a ferry flight destined for Yokohama. The aircraft was operated by the captain, who was the sole occupant of the helicopter.
During the flight, the helicopter transitioned through the mountainous terrain of Shizuoka Prefecture. While flying near Shizuoka Airport, the aircraft's attitude became unstable. Eyewitnesses reported seeing the helicopter swinging its tail and tilting heavily to the right and left before descending at a steep angle. At approximately 15:33 JST, the helicopter struck a mountain forest near Ojiro, Shimada City. The impact destroyed the aircraft, though no fire broke out. The captain sustained fatal injuries.
The investigation
Investigators from the JTSB analyzed flight plan data, GPS records, and the Engine Monitoring Unit (EMU) to reconstruct the final moments of the flight. The investigation also examined meteorological data, which indicated the presence of a low-pressure system and a winter pressure pattern in the region.
Technical analysis focused on the phenomenon of mast bumping. The EMU records showed significant power fluctuations and torque changes just prior to the crash. Investigators also reviewed the aircraft's physical condition, noting damage to the main rotor hub, drive shaft, and tail rotor. The investigation considered the pilot's flight speed and the impact of thermal convection on the aircraft's stability.
Findings
- The helicopter encountered strong downdrafts caused by roll-shaped thermal convection, a phenomenon common in mountainous regions when sunlight heats the ground.
- The aircraft entered a low-G condition due to the turbulence.
- Instead of following the flight manual's instruction to apply gentle aft cyclic to reload the rotor, the pilot likely attempted to correct a sudden right roll by applying left cyclic.
- This improper control input during a low-G state likely triggered mast bumping, leading to a catastrophic loss of flight control.
- The pilot maintained a high cruise speed of approximately 110 KIAS, which may have prevented an effective response to the sudden turbulence and contributed to the onset of mast bumping.