What happened
On April 30, 2006, a Schleicher ASK-21 glider, registration PH-686, was performing a local training flight at Terlet airfield. The flight was being conducted by an instructor and a student. During the initial phase of a winch launch from runway 30, the winch motor suddenly stalled after the aircraft had reached a speed of approximately 65 to 70 km/h.
As the winch power failed to increase, the aircraft maintained a climbing attitude. The instructor noted that the cable parachute deployed at an altitude of about 10 meters, indicating insufficient tension from the winch. As the airspeed dropped to roughly 55 km/h, the instructor leveled the aircraft and released the winch cable while at an altitude between 10 and 15 meters. Due to the high nose attitude maintained after release, the glider performed a hard landing, with the nose and main wheels striking the ground simultaneously. The student sustained a broken vertebra, while the instructor was uninjured. The aircraft suffered significant damage, including delamination to the fuselage and dents to the underside of the wings.
The investigation
The investigation focused on the mechanical and electrical performance of the Munster van Gelder winch used for the launch. The winch had recently undergone winter maintenance and a full engine overhaul in 2003. The investigation examined the electrical circuit of the engine's fuel shut-off mechanism, which relies on an electric solenoid to keep the fuel valve open.
It was determined that the engine's DAF 625 diesel motor was equipped with an electrically controlled shut-off device that closes the fuel supply if electrical power is lost. The investigation analyzed the stability of the electrical connections under the vibration levels produced during a high-load launch sequence.