What happened
On May 30, 2015, a privately owned Schempp-Hirth Duo Discus, registration JA07KD, was undergoing a winch launch at Kirigamine Gliding Field in Nagano Prefecture, Japan. The flight was intended as a familiarization mission with the pilot in the front seat and a passenger in the rear.
During the climb, while the aircraft was at a pitch angle of approximately 20° to 30°, the towline snapped. The pilot immediately released the remaining towline and attempted to stabilize the glider by pitching the nose down to gain airspeed. Because the aircraft was at a very low altitude—estimated between 30 and 50 meters—the pilot determined that a straight landing would result in a collision with the winch. To avoid this, the pilot initiated a 180-degree right turn toward a grassy area.
During this maneuver, the glider experienced a sudden and significant loss of altitude. The pilot's attempt to manage the turn at low altitude resulted in a loss of control balance, leading to a side slip. The aircraft's descent was further exacerbated by a downward draft common to the airfield's geography during north winds. The glider eventually struck the ground with a wingtip, resulting in the destruction of the fuselage and wings. The pilot and passenger sustained two serious injuries.
The investigation
The Japan Transport Safety Board (JTSB) examined the mechanical state of the towline components and the meteorological conditions at the time of the accident. Investigators found that the fuses installed in the towline were of a lower tensile strength than required. Specifically, blue fuses, which have lower intensity, had been used instead of the standard black fuses.
The investigation also reviewed the ground operations, noting that the person responsible for attaching the towline had misidentified the fuse color. This error was attributed to the faded appearance of the fuse cases and the fact that the ground crew member was wearing sunglasses. Furthermore, the JTSB analyzed the flight path and the aerodynamic challenges of performing a circling landing at such low altitude under the influence of localized downdrafts.
Findings
- The primary cause of the towline breakage was the incorrect installation of low-intensity fuses on the towline.
- The crash was caused by a significant descent in altitude during the emergency turn, resulting from an operational imbalance during the circling maneuver.
- A localized down draft contributed to the rapid loss of altitude.
- The misidentification of the fuse color was made possible by the lack of maintenance on the faded fuse cases and the use of sunglasses by the ground personnel.