Incident
On 6 July 2014 at 16:16 local time, a Robin DR 400/180 R (registration HB-EXU) was towing a Duo Discus (HB-3416) from Schänis airfield (LSZX) for a glider tow flight. Shortly after takeoff from runway 34, the glider's canopy opened for unknown reasons. The glider pilot speculated that the passenger might have held onto the canopy latch due to turbulence, inadvertently opening it.
Glider Pilot's Account
While attempting to close the canopy, the glider pilot lost sight of the towplane. Judging that the altitude was sufficient for a landing on runway 16, he released the tow rope and performed an uneventful landing.
Tow Pilot's Account
The tow pilot noticed a nose-down attitude change and countered with elevator input to maintain climb speed. Shortly after, the rope was either released by the glider pilot or broke. The tow pilot continued the climb, reeling in the rope with the electric winch. However, due to knots in the rope, it could not be fully retracted. He executed a steep approach to runway 34 to avoid obstacles with the rope, and landed without incident.
Outcome
The tow rope sustained damage. No damage occurred to the glider. There were no serious injuries; the glider pilot and passenger sustained minor or no injuries.