What happened
On May 5, 2016, a privately owned Glasflügel 304CZ-17 glider, registration JA21BB, departed from Kakuda Glider Field in Miyagi Prefecture for a training flight. The aircraft was being aero-towed for a planned long-distance mission. During the flight, the pilot climbed to significant altitudes, reaching approximately 25,000 feet.
At around 14:00 JST, communication with the Tokyo Area Control Center became degraded. The pilot reported an altitude that was actually 6,000 feet lower than his true position. Shortly after, a sound resembling a moan was recorded on the radio, followed by silence. At approximately 14:05, the glider crashed into a mountain forest in Miharu Town, Fukushima Prefecture. A witness near the site reported seeing the aircraft pass overhead appearing to have no wings. The pilot sustained fatal injuries, and the aircraft was destroyed upon impact.
The investigation
The Japan Transport Safety Board (JTSB) examined the wreckage and analyzed flight track data from a smartphone and ATC records. The investigation focused on the structural failure of the aircraft and the physiological state of the pilot. Investigators analyzed the damage to the main wings, which had separated from the fuselage, and examined the oxygen supply system. The investigation also considered meteorological conditions, specifically the presence of mountain waves and turbulence in the area.
Findings
- The investigation established that the pilot forgot to open the oxygen valve prior to takeoff, meaning the flight began without any oxygen supply.
- As the pilot climbed through 18,000 feet and up to 25,000 feet, the lack of oxygen led to a state of hypoxia, rendering the pilot semi-conscious.
- Due to this impaired state, the pilot entered a steep turn that resulted in an aerodynamic stall and a pronounced nose-down attitude, including a spin.
- The structural integrity of the glider failed because the aircraft was subjected to aerodynamic loads exceeding its ultimate maneuvering limit, exacerbated by passing through turbulent air.
- The pilot was likely unable to perform recovery maneuvers or utilize the onboard parachute due to his incapacitated state.