From section 3.4 of the 1999 HGFA Towing Manual:

Recommended breaking load of a weak link is 1g. - i.e. the combined weight of pilot, harness and glider (dependant on pilot weight - usually approximately 90 to 100 kg for solo operations; or approximately. 175 kg for tandem operations).

Each pilot should have his/her own weak link of appropriate strength.

It is recommended that a new weak link is used for every launch; or a fabric sheath is used to cover the weak link to protect it as it is dragged along the ground.

Testing weak links tied from "No 8" builders string line has shown that the type of knot used does not greatly affect the breaking strain of the weak link.

I have chosen only the relevant part of the graphic in the manual. Notice that it uses No. 8 bricklayers twine as the weaklink material. Here is the requirement from the 2007 Worlds local rules (which I wrote) for weaklinks:

Appropriate aerotow bridles

Competitors must use appropriate aerotow bridles as determined by the Meet Director and Safety Director and their designated officials. Bridles must include secondary releases (as determined by the Safety Director). Bridles must be able to be connected to the tow line within two seconds. The only appropriate bridles can be found here: http://www.ozreport.com/9.039#0 and http://ozreport.com/9.041#2.

Pilots who have not already had their bridles inspected during the practice days must bring their bridles to the mandatory pilot safety briefing and have them reviewed. Pilots with inappropriate bridles may purchase appropriate bridles from the meet organizer.

Weaklinks

Pilots must use weaklinks provided by the meet organizers and in a manner approved by the meet organizers. All weaklinks will be checked and use of inappropriate weaklinks will require the pilot to go to the end of the launch line to change the weaklink.

Weaklinks will consist of a single loop of Cortland 130 lb Greenspot braided Dacron Tolling line http://www.cortlandline.com/catalog/braid.html and should be placed at one end of a shoulder bridle. The tow forces on the weaklink will be roughly divided in half by this placement. Pilots will be shown how to tie the weaklink so that it more likely breaks at its rating breaking strength.

For many years a number of us (US pilots) have felt that #8 bricklayers nylon line was not an appropriate material to use for weaklinks as it is not as consistent in its breaking strength (as far as we are concerned) as the Greenspot line used in the US. At the 2008 Forbes Flatlands Greenspot for the first time was used as the standard weaklink material (thanks in large part to the efforts of Bobby Bailey). We applaud these efforts to improve the safety of aerotowing by using a better weaklink material.

If the weaklink is at one end of the bridle then there is little to no reason to replace the weaklink after each flight. The weaklink should be replaced if it shows any signs of wear as its strength may be reduced. The weaklink is constructed using "fisherman's knots."

A single loop of weaklink is used at the end of the V-bridle or the end of the pro tow bridle.

Pilots at the 2007 Worlds were not actually required to use the bridles pictures above unless their bridle couldn't be hooked up quickly to the carabineer at the end of the tow rope. They could have a weaklink (four strands) that connected their bridle to the carabineer. They just had to have the loops of the weaklink available to be hooked to by the ground crew. This would obviously require a carabineer at the end of the tow rope, and would leave the weaklink attached to the carabineer after the pilot releases, requiring that it be taken off by the ground crew before hooking up to the next pilot. Of course, a locking carabineer could be used.

It was required that pilots be able to be connected to the tow line quickly both in order to be fair to all pilots and get them in the air in time to compete with each other, as well as to promote safety. It is safer to have a simple uniform release/bridle system that the ground crew is familiar with and can determine if there is a problem. The simpler and more uniform the safer, system wide.

Getting pilots into the air quickly is also safer as it reduces the stress that pilots feel on the ground and keeps them focused on their job which is to launch safely and without hassling the ground crew or themselves. When we look at safety we have to look at the whole system, not just one component of that system. One pilot may feel that one component is unsafe from his point of view and desire a different approach, but accommodating one pilot can reduce the overall safety of the system.

I look forward to any response from the HGFA or other interested persons. Again, I have a direct personal and minor financial interest in the issues raised by this discussion, but no financial interest when it comes to weaklink material.