Duo Discus canopy opens on aerotow; pilot releases and lands on runway

A Duo Discus on aerotow from Schänis (Switzerland) had its canopy open shortly after takeoff from RWY 34; no occupants were injured and both aircraft landed safely. Despite a pre-takeoff check the cause was unknown, with the possibility that the passenger had gripped the latch in turbulence. While closing the canopy the glider pilot lost sight of the Robin DR 400 tug and, judging the altitude sufficient, released and landed on RWY 16. The tug pilot felt a nose-down change, corrected with elevator, then noticed the rope was gone; knots prevented full retraction. He flew a steep approach on RWY 34 to keep the rope clear and landed normally.

1. Aerotow start r34 ~16:16 LT: At 16:16 LT on 6 July 2014 a Duo Discus with a pilot and one passenger started an aerotow behind a Robin DR 400/180 R from runway 34 at Flugfeld Schänis (LSZX). A pre-takeoff check had been completed.
2. Canopy opens shortly after takeoff: Shortly after takeoff the canopy over the glider cockpit opened for unknown reasons, despite the pre-takeoff check.
3. Possible passenger latch grip: The SUST notes it as possible that the passenger, reacting to turbulence, held onto the canopy latch and inadvertently unlocked it. This remains a hypothesis.
4. Glider releases at safe altitude: The glider pilot focused on closing the canopy and lost sight of the tug. Judging that the altitude was sufficient for a landing on runway 16, he released from tow.
5. Tug feels nose-down; rope gone: The tug pilot felt a nose-down attitude change and counteracted with elevator to maintain the planned climb speed. The rope was then either released by the glider pilot or torn. He continued the climb normally and used the electric winch to retract the rope, but knots in the rope prevented full retraction.
6. Glider lands r16; tug lands r34: The glider landed without incident on runway 16. The tug pilot flew a steep approach on runway 34 to keep the trailing rope clear of obstacles, and landed normally. No injuries. Neither aircraft was damaged; the tow rope was torn.