Reasonable questions, Andrew. I'll try to answer some of them to a degree below.

On Tuesday, June 3, 2014 8:17:46 PM UTC-6, Andrew wrote:
Hi Bruce



thank you for noting there is extra tension in the aerorow rope due

to the angle of climb. If the climb rate is 3kts (which is typical for a

2-seater at my club) at a towing speed of 60kts, the angle of climb

(theta) = 3/60 radians = 3 degrees, and the extra tension in the

rope is (weight of glider)*(sin theta) = typically 1000*(sin 3

degrees) = 50lbs. This would add to the tension in the rope due to

glider drag, as you say, to make a typical aerotow rope tension of

35+50 = 85lbs.

My estimate of the tension in the aerotow rope during initial

ground roll, assuming acceleration of a 1000lb glider to 60kts in

10secs, is about 315lbs.
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The European certification standards under CS-22 mentions the aero tow rope tension designers should expect. I seem to recall that number is 150daN. That would be close to your estimate.
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Whilst the numbers can be juggled for different sailplanes and

towplanes, I agree with you that the greatest rope tension is likely

to be during initial acceleration i.e. during the ground roll. So

aerotow ropes are 'proof tested' on every ground roll, to a useful

degree. This does not assure that the rope meets the full rated

breaking strain however.

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I suspect the loads encountered in slack recovery could be the greatest.
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I read Bill's comment of June 2 about the rope perhaps taking

some longer time to actually break. This is a new idea to me, and I

don't know what too make of it. I'd like to hear the evidence for

this effect.
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There are lots of engineering papers on rope testing and failure modes available on the Internet. The basic idea is a rope is a large bundle of twisted fibers. Not all those individual fibers are equal in strength nor are they loaded equally. When it gets overloaded some of those fibers reach their breaking point before others and the rope starts to unravel.

The unraveling process is usually spotted when the rope is inspected but sometimes it progresses fast enough for the rope to fail before it gets inspected. In almost all cases there is a time interval between fiber breakage and catastrophic failure, otherwise rope inspections wouldn't work.

All but a couple of the rope breaks I know of happened after lift off and below about 1500 feet with normal tension on the rope.
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The more important discussion, is whether it is a good idea to train

or teach 200ft turn-backs to our students. Despite it being long

accepted practice, in the USA anyway, I doubt that it is a good idea

in terms of reducing serious accident rates. I note nobody suggests

we teach students to do final turns under 200ft. I wonder why.

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This one is easy. A turn to final is a normal operating procedure not an emergency like a rope break recovery. There's no justification for practicing low turns to final. A rope break is a true emergency so the rule book can be disregarded as far as necessary to deal with it safely.

I think there are enough stories, some related here, to say lives have been saved and injuries avoided when pilots returned to the runway after a rope break. I can't recall any training accidents practicing the return-to-runway maneuver but there may be a few. I've done hundreds and don't recall sweating out one of them.
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I was interested in the suggestion from others that Germany has a

much lower PT3 accident rate, due to their stronger ropes and

weak links, and that this could perhaps be allowed by the FAA.

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This is the ultimate solution - just eliminate rope breaks altogether by using a rope so strong it's hard to imagine it breaking. However, weak links could still break so I would continue the training.

It's not so much the US government "allows" stronger weak links and rope - the Federal Air Regulations and Occupational Safety and Health Administration rules require it. It's just that we haven't been following those rules.