What happened
On July 6, 2014, at 16:16 local time, a winch-assisted glider launch was underway at Schänis airfield (LSZX). A Robin DR 400/180 R, registration HB-EXU, was being used to tow a Duo Discus glider, registration HB-3416, in a private VFR operation.
Shortly after departing from runway 34, the canopy of the towing aircraft unexpectedly opened while the plane was in its climb phase. During the attempt to close the canopy, the pilot lost visual contact with the glider. Consequently, the pilot decided to release the tow rope. The glider pilot noted a sudden change in aircraft attitude, specifically a nose-down movement, which required corrective elevator input to maintain the intended climb speed. Following the release of the rope, the tow aircraft continued its climb and eventually retracted the cable using the electric winch. Due to knots in the cable, the winch could not fully retract the line, necessitating a steep approach to runway 34 to avoid ground obstacles. Both aircraft completed their landings without further incident.
The investigation
The investigation focused on the sudden opening of the canopy on the Robin DR 400/180 R. Although the canopy had been inspected prior to takeoff, the cause of the failure remained undetermined. Investigators considered the possibility that the passenger may have inadvertently manipulated the locking mechanism while bracing against turbulence.
Findings
- The cockpit canopy of the HB-EXU opened during the climb phase for unknown reasons.
- The pilot of the towing aircraft lost sight of the HB-3416 during the process of attempting to reseal the canopy.
- The unintended opening of the canopy led to the loss of visual contact and the subsequent release of the tow rope.