What happened
On April 9, 2023, a privately owned PZL-Bielsko SZD-55-1 glider, registration JA2502, crashed into the bank slope of Yamba-Agulated Lake in Naganohara Town, Gunma Prefecture. The flight began as a winch-launched training session from the Nagano City gliding field. During the flight, the pilot successfully reached altitudes of approximately 2,600 meters near Mt. Neko, but after crossing the prefectural boundary into Gunma, the aircraft's trajectory changed.
As the glider moved toward the lee side of the ridge, it encountered significant downdrafts. The pilot attempted to find a suitable area for an off-field landing near a clearing. However, while performing a low-altitude turn to avoid trees, the aircraft entered a stall and subsequent spin. The glider struck the terrain in an inverted position, resulting in one fatality and the total destruction of the aircraft.
The investigation
The Japan Transport Safety Board (JTSB) examined GPS data from the pilot's smartphone, surveillance footage from Yamba-Agatsuma Lake, and meteorological charts. The investigation focused on the aircraft's flight path relative to the ridge line and the aerodynamic conditions present during the final maneuvers. Investigators also reviewed the pilot's flight history and the wind conditions at the time of the accident, noting that strong northwest winds were present, which were known to create powerful updrafts and downdrafts near the mountain ridges.
Findings
- The pilot drifted into a downdraft on the lee side of the ridge, which caused a loss of altitude and prevented the aircraft from returning to the windward side.
- The pilot likely misjudged the complex topography of the ridge at the prefectural boundary, leading to the aircraft entering a no-fly zone on the Gunma side.
- While attempting an off-field landing, the pilot encountered obstacles in the form of trees.
- The aircraft stalled and entered a spin due to excessive control corrections—either excessive bank angle or uncoordinated rudder usage—made while trying to avoid these trees at a very low altitude.
- The pilot was unable to identify a suitable landing site early enough to execute a stabilized approach.