What happened
On May 4, 2008, a Schleicher ASK 23 B glider, registration PH-929, was performing a winch launch from runway 05 at Marinevliegkamp Valkenburg. Shortly after the aircraft left the ground at an altitude of approximately 20 to 30 meters, a mechanical failure occurred within the winch mechanism, causing the towing tension to vanish. As the aircraft continued to move forward, it overtook the slackening cable, triggering an automatic release.
The pilot, a 16-year-old student, initially interpreted the loss of tension as insufficient winch speed and lowered the nose to build momentum. However, the loss of power was total. Unable to regain sufficient airspeed at such a low altitude, the pilot was forced to perform an immediate landing on the airfield. The aircraft approached the ground with high vertical velocity. Despite an attempt to pull the nose up just before impact, the Schleering ASK 23 B struck the ground heavily, resulting in the tail breaking off and the wing skin tearing. The pilot sustained serious injuries to the back, though no other passengers were on board.
The investigation
The investigation examined the winch mechanism, the aircraft's condition, and the pilot's actions. Investigators found that the winch failure could not be reproduced during subsequent testing, and no specific mechanical fault was identified in the machinery itself. However, evidence suggested that the winch had a known history of intermittent failures.
The investigation also reviewed the pilot's flight training and experience. At the time of the accident, the pilot was a solo student with 24 total flight hours. The investigation focused on the sequence of events between the loss of tension and the moment the pilot realized the launch had failed, specifically regarding the execution of the BOKS procedure (a standard protocol for managing interrupted launches).
Findings
- The primary cause of the accident was the delayed recognition by the pilot that the winch launch had failed, which prevented an earlier, safer abort.
- The loss of tension was caused by an undetermined technical malfunction in the winch.
- The winch was known within the flying club to experience frequent, unexpected operational failures.
- The pilot's attempt to build speed by lowering the nose was initially a standard response to low winch speed, but because the power was already gone, it contributed to a loss of airspeed and potential aerodynamic stalling.
- The pilot's limited flight experience may have contributed to the difficulty in managing the emergency at such low altitude.