On Sunday, June 1, 2014 5:20:14 PM UTC-6, Kevin Christner wrote:
I had my first "rope break" ever today. I was approximately 200-250ft above the ground. Emergency procedures were not a problem.



The Tost released for some reason. Its a nose hook so the back release mechanism could not have been the culprit. Further testing with the wheel break on the ground revealed no problems so I took another tow and had no problems. The ring may not have been engaged properly (doubtful) or the slack I got in the rope was just too much when it tightened back up (probable but the release still should have held).



In any case has anyone else ever experienced an actual emergency unplanned release? In 14 years of flying I have never heard of one. Good thing we do practice this.



2C

I've had two uncommanded Tost releases myself and observed two more. (No drama ensued.) In another case, the hook wouldn't initially release until the pilot worked it loose by maneuvering the glider on tow. In all cases the release itself was found to be within its service life and in excellent condition with springs intact.

The one common thing with all these incidents was a badly worn Tost ring pair. An abrasive runway had noticeably removed metal from the small ring leaving it with a rough, pitted surface. The point of the hooks "beak" could capture the rough ring against the cage with friction even though it wasn't fully inserted into the release. Even when tested by pulling hard on the rope, it sometimes wouldn't come loose but it could still work loose during the tow. The "beak" can't capture a new, smooth ring pair.

We speculate the fail-to-release incident was the result on a worn, undersized ring getting cocked sideways inside the cage.

If the tow operator can't be convinced to replace the ring pair, have the wing runner shake the rope to make sure it rattles in the hook after it is attached. If it's captured by friction, it won't rattle.