What happened
On June 29, 2012, at 12:54 JST, a Bell 412EP helicopter, registered JA6817, was attempting to land at a temporary helipad upstream of the Nagashima Dam in Shizuoka Prefecture, Japan. The aircraft, owned by the Chubu Regional Bureau and operated by Nakanihon Air Service, was conducting a mission to verify damage from natural disasters in the Oi River basin.
During the final approach, the pilot executed a steep descent path while experiencing a tailwind. As the helicopter's forward airspeed decreased, the aircraft entered a state of unstable descent. Despite the pilot increasing the collective pitch to generate more lift, the aircraft continued to sink rapidly, eventually striking the ground with significant force. The impact caused the helicopter to bounce once before coming to a stop. The pilot sustained serious injuries involving a bone fracture, and one passenger sustained a minor injury.
The investigation
The Japan Transport Safety Board (JTSB) examined data from the digital flight data recorder (DFDR), the cockpit voice recorder (CVR), and GPS information, alongside interviews with the pilot, mechanic, and passengers. The investigation focused on the flight dynamics during the descent and the environmental conditions at the temporary landing site. Investigators noted that while the pilot had successfully landed at the same location earlier that day, the second approach involved different wind conditions and a steeper descent angle. The investigation also looked into the lack of wind direction indicators, such as windsocks, at the temporary helipad.
Findings
- The primary cause of the hard landing was the occurrence of a vortex ring state (VRS).
- The aircraft entered VRS because the forward airspeed continued to drop during a high rate of descent, causing the rotor's downwash to create large vortices around the upper surface of the main rotor tips.
- The descent rate could not be controlled by increasing the collective pitch because the aircraft could no longer generate sufficient lift due to the aerodynamic state.
- The conditions for VRS were facilitated by a steep approach path conducted under tailwind conditions, which caused the forward airspeed to decrease below critical levels.
- The lack of windsocks at the temporary site contributed to the pilot's inaccurate assessment of the wind conditions.