What happened
On August 8, 2016, an AERO ASAHI Corporation Kawasaki BK117C-2, registered JA6917, was conducting an emergency medical flight to transport a sick and wounded individual. The mission involved landing at a temporary helipad in Hadano City, Kanagawa Prefecture. The aircraft was carrying five occupants, including the pilot, a maintenance engineer, and three medical staff.
During the approach, the pilot deviated from the planned southern approach path because the landing site was not yet cleared by ground personnel. Instead, the pilot transitioned to an approach from the west, flying over a 27-meter-high steel tower. To compensate for the steep approach angle and high descent rate, the pilot attempted to pull up the collective pitch (CP) near the ground to transition into a hover. However, the aircraft failed to stabilize and instead entered a rapid descent accompanied by a right-hand spin. The helicopter struck the ground with significant force, resulting in the destruction of the tail boom and tail fin, as well as damage to the main rotor, tail rotor, and skids.
The investigation
The JTSB investigation examined flight data from the GPS and Cockpit Voice Recorder, as well as video footage from a standby fire engine. Investigators analyzed the aircraft's flight path, the descent rate across four distinct phases of the approach, and the aerodynamic conditions at the time of the accident. The investigation specifically looked into whether the aircraft entered a Vortex Ring State (VRS), a condition where a helicopter's descent rate matches its downwash, causing a loss of lift.
Findings
- The pilot approached the helipad using an unauthorized path that lacked a cleared approach surface, flying over a tall obstacle.
- The approach was characterized by a high descent rate and a steep angle, which exceeded the safety margins recommended in the flight manual.
- As the pilot reduced forward airspeed to transition to a hover, the main rotor entered Vortex Ring State (Vulnerable to VRS), meaning that increasing the collective pitch failed to generate the necessary lift to arrest the descent.
- The pilot's focus on the landing target rather than the vertical speed indicator likely prevented the timely detection of the increasing descent rate.
- The sudden application of collective pitch likely contributed to the loss of directional control, leading to the right-hand spin.