What happened
On January 28, 2024, an Alexander Schleicher ASK-13 sailplane, registration OO-ZKS, was performing a routine winch-launch takeoff at Weelde Airfield (EBWE). The operation, conducted by the Koninklijke Kempische Aeroclub, began normally, with the aircraft entering a light controlled ascent. However, shortly after lifting off at an altitude of approximately 30 to 35 meters, the aircraft failed to accelerate as expected.
As the pilot sensed the loss of momentum, the winch operator attempted to increase engine power but found the throttle stuck. The engine could not exceed 1200 RPM, far below the 1700 RPM required for a successful launch. The operator immediately broadcast a "break, break" warning via radio. The pilot attempted to execute a recovery maneuver by pushing the stick forward to prevent a stall, but the aircraft's airspeed was insufficient to maintain flight. The sailplane struck the ground nose-first, bounced, and came to a stop. While the pilot sustained minor injuries, the passenger, seated above the main wheel, suffered one serious injury involving spinal trauma.
The investigation
The AAIU(Be) examined the winch mechanism and the aircraft's flight path. The investigation focused on why the winch failed to provide the necessary power during the critical takeoff phase. Investigators analyzed the mechanical setup of the Leopard 4 winch and the maintenance records of the aeroclub.
Findings
Technical analysis revealed that the primary cause of the power loss was a jammed throttle control push-pull cable within its synthetic sheath. This failure was attributed to the following factors:
- The push-pull cable had become crunched, preventing the throttle from reaching the necessary RPM.
- The specific installation of the throttle lever created a lateral force on the cable conduit, inducing internal wear over 15 years of operation.
- The winch's maintenance program, based on the manufacturer's instructions, did not include specific inspections or scheduled replacements for this particular push-pull cable.
- The winch was a unique model; unlike other Leopard winches that use electrical motors or rods, this unit utilized the problematic push-pull cable system.
Safety action
Following the investigation, the aeroclub has implemented a new yearly inspection of the push-pull throttle control cable into its official maintenance program to prevent similar mechanical failures.