On Monday, August 20, 2012 10:18:48 AM UTC-4, Bill D wrote:
For the first time in my life I heard a 1st person story of a release failure. The story follows: The glider, an ASK-21, was jerked forward by the tug so the front wheel rolled over the rope at the start of an aero tow. The slack was then taken out and the tow was launched without inspecting the hook and release. The working theory is that this somehow cocked the ring set in the hook so it failed to release even with pilot and passenger pulling on the release knobs. The pilot eventually got a successful release by yawing the glider strongly. AFAIK, the hook in question had not reached it's 2000 tow life limit nor is there any history of repair or maintenance in the hook area. There had been cases where a commercial operator had insisted on providing tows using a chain link instead of a Tost ring set. We think it possible this had damaged the hook in some way. Operationally, should a roll-over happen again, we will release the rope and re-attach before proceeding with a launch. The release will get special attention in the annual inspection later this month. No chain links will be allowed for any purpose.

Lets look at the mechanics of things when the glider gets bumped and rolls over the rope: The rope is pulled with an incredible force down- and backwards. The large ring which is held half way inside the rigid cage of the release is reacting against that cage with a huge force, so it is not unreasonable to assume that the entire housing gets deformed plastically. This could possibly allow the position of the small ring in relation to the moving beak to change and it doesn't take a whole lot to pinch in the ring and lock up the entire mechanism.
We had a release failure on a 2-33 after an new member was allowed to hook up the line using the large Tost ring. The line force was enough to make the ring slide up the wedge shaped release hook and lock itself onto it and not letting go even after the release was pulled. I know that these two mechanisms are completely different but it demonstrated how small deformations combined with wedge shaped bodies can result in very strong connections.
In other countries, the tow ropes are much thicker and stronger than the ones used in the US (following the 80-200% rule, which is total B.S. to begine with) but the manufacturer's stipulated weak-link (part of the POH) is right at the double ring. In the desribed scenario, the weak-link would have broken and the whole episode would have not happened.

GM