I should have caught up on intervening posts before responding to the
previous one. Thanks very much, Doug for a clear and useful explanation.


Jack

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Doug Haluza wrote:

The information I received from my rope distributor is that the initial
breaking strength is reduced by 25% with use. We incorporated this in
the operating procedures at our club (LISA). We use 250' (230' when
doubled at the ends) of 5/16" hollow braided polypropeleyne rope with a
nominal breaking strength of 1500 lb.

When the rope is new, we intall a new 1/4" poly rope weak link with
nominal breaking strength of 1200 lb at the glider end. This allows us
to tow gliders between 600 and 1488 lb, so it covers a 1-26E through a
2-32. Since the main rope strength is not more than 25% greater than
the weak link, no separate weak link is required at the
towplane--because there is a weak point at the towplane end where the
rope is doubled (and another at the glider end).

Once the ropes have been used for about a day, we discard the 1/4" weak
link and just use the 5/16" rope, under the assumption that it's
breaking strength has been reduced by approx 25%. This allows us to
contunue to tow the entire fleet with the same rope. We have to discard
the weak link, because it is assumed to also be reduced in breaking
strength, but it only uses about $1 worth of rope.

Now, of course there is a lot of variation in inital and used breaking
strength of actual ropes, so this is not an exact process. But it does
represent a good faith effort to comply with the regs, and is a
workable operating procedure.

Note that the maximum weak link strength is 1200 lbs for Schweizer
hooks on the tow plane, so this is the upper limit for breaking
strength for most tow planes. Some gliders have lower maximum breaking
strength, depending on the tow hook type and mounting structure. So if
you have one of these gliders, you may need to bring your own weak
links.

Another important note--do not splice a broken aero-tow rope! The point
where the rope is doubled will wear out faster from abrasion. At the
tow plane end, this is not an issue, because it is held off the ground,
and can be checked at the beginning of each day. At the glider end, it
can be checked at each hook-up. But if it is in the middle of the rope,
it will not get checked, and will wear until it breaks in flight.
Besides, if the rope broke once already, it's time to replace it. Use
the broken half to replace that ratty rope on the tow car that's too
short anyway.