Aerotow ropes: short or long, breakable or unbreakable?

On Thursday, November 14, 2013 12:36:01 PM UTC-5, son_of_flubber wrote:
On Thursday, November 14, 2013 11:35:41 AM UTC-5, Evan Ludeman wrote: So... is this operation experiencing rope failure without overload? That does not tell us that their current procedure has a reasonable safety margin. In 27 years in the sport, I've never seen this. If I did see it I'd change my procedures. Wait for a failure before evaluating procedures?? At some of my favorite places to fly, a PTT at 50 - 300' has some significant risks, so we take launch prep fairly seriously, the tow plane pre-launch checkout includes a warmup flight, etc. We don't break ropes, we inspect them visually and service them before they get ugly. We have no need for an ASTM approved test program.... The collective wisdom and experience of many people have developed the status quo, and I'm sure that it makes sense on balance, but does anyone know how close to failure ropes get before they are retired? And there is the matter of how our uncertainty about the rope affects training and flight reviews. I watched a glider enter a spin after a "simulated rope break" during a biennial flight review (glider totaled, no injuries, pilot retired). Maybe "that should not have happened", but I'm left with the impression that PTT simulation is risky. That the probability of a PTT is currently high enough to justify the risk of PTT simulation is part of my motivation. Can the probability of a PTT be lowered to the point that the risk of PTT simulation is no longer justified? How far can better rope material choice and better (practical) inspection go towards eliminating the possibility of PTT?

12000 glider flights, about 7500 tows given, and the only rope breaks have been during initial acceleration. Maybe 1/2 dozen or so of those. And a couple on the tug end from hanging the rope in a tree.
I've done hundreds of PTT and do not see why they should create an increased risk that would justify not doing that training. I want to KNOW(as well as possible) that the person I'm teaching or reviewing will respond correctly in the event that a failure happens. That failure could be many reasons, broken rope being one.
Real rope breaks are, thankfully, pretty rare. From this I conclude most operations are taking suitable care.
FWIW
UH