What happened
On May 1, 2015, at approximately 14:38 JST, a Grob G109B motor glider, registration JA2569, was engaged in a leisure flight departing from Hida Airpark in Takayama City, Gifu Prefecture. While the original flight plan intended to remain within a nine-kilometer radius of the airpark, the crew altered their route toward Norikuradake Mountain to take advantage of favorable weather conditions.
As the aircraft was climbing along the western slope of the ridge, it encountered a sudden downdraft. The pilot attempted to increase the rate of climb by applying full throttle, but the aircraft was unable to gain sufficient altitude to overcome the steep gradient of the terrain. In an attempt to avoid the slope, the pilot sought to turn the aircraft around; however, the altitude was already too low to permit a banked turn without the wingtips striking the trees. Consequently, the aircraft struck the mountain slope while maintaining a climbing attitude. The impact resulted in the destruction of the aircraft, including broken propeller blades and structural failure of the wings and fuselage. The pilot and passenger successfully egressed from the wreckage and, after moving to higher ground, were rescued by a Gifu Police helicopter.
The investigation
The Japan Transport Safety Board (JTSB) examined the meteorological conditions, the flight path, and the mechanical state of the aircraft. Investigators analyzed wind data from a nearby automated station, which showed west-southwest winds ranging from 3.9 to 5.3 m/s with gusts up to 9.2 m/s. The investigation also reviewed the pilot's flight maneuvers and the aircraft's performance capabilities, specifically the propeller pitch settings and engine functionality. Additionally, the JTSB looked into the regulatory aspects of the flight plan, noting that because the crew had deviated from their original route to fly beyond the nine-kilometer radius, a flight plan update had not been submitted to the authorities.
Findings
- The aircraft encountered a downdraft likely caused by the concentration of wind over the ridge due to local geographic features.
- The pilot approached the mountain slope at an altitude insufficient to recover from the sudden loss of altitude caused by the descending air current.
- The aircraft's altitude became too low to execute a turning maneuver without risking wingtip contact with the terrain.
- The aircraft's engine and control systems were functioning normally at the time of the accident.
- The flight plan had not been updated via radio to reflect the change in the intended flight area.