What happened
On May 16, 1965, during a public flight display celebrating the inauguration of the Alpine Glider School Schänis AG, a collision occurred involving two Ka-6 gliders and a Dewoitine D-26 tug aircraft. The operation was a double tow, with the glider HB-683 positioned in front of HB-703.
As the aircraft began their takeoff roll on runway 34 at Schänis airfield, a ground assistant signaled the pilot of the Dewoitine D-26, HB-RE (identified in records as HB-RAE), to apply full power. At the moment of acceleration, the section of the tow rope connecting the tug to the trailing glider, HB-703, was not yet fully tensioned. The sudden surge in power caused the rope to snap under the abrupt tension.
Upon noticing the rope failure, the pilot of the lead glider, HB-683, immediately released the tow line and banked the aircraft to the left to prevent a ground loop. During this maneuver, the tail skid of HB-683 struck the left wingtip of HB-703, which was already lowered toward the ground by its wingman. The impact resulted in a broken main spar on the trailing aircraft.
The investigation
The investigation examined the flight procedures, the coordination between the pilots and ground crew, and the mechanical failure of the tow rope. Investigators reviewed the credentials of all involved pilots, noting that all were properly licensed for the specific flight types and possessed sufficient experience for the maneuver. The investigation also looked into the communication chain between the wingmen, the ground assistant, and the tug pilot.
Findings
- The primary cause of the accident was the abrupt tensioning of the tow rope following an premature start signal.
- The takeoff signal was given to the tug pilot before the tow rope for the lead aircraft had been fully straightened and tensioned.
- There was a lack of specific instruction or coordination provided to the ground assistance crew regarding the precise sequence of signals required for a double tow operation.
- There were no injuries to any personnel during the incident.