On 5/20/2015 9:59 AM, Bob Pasker wrote:
well shucks. corrected:

On Wednesday, May 20, 2015 at 11:58:12 AM UTC-4, Bob Pasker wrote:
The other question is who is responsible for compliance with the tow
regs?

The TOW pilot (who probably never sees the rope)?
In my club, a daily responsibility of the tow pilot prior to the day's first
tow is visually inspecting the entire length of the towrope. Decision to use
as-is, shorten, or discard (meaning: cut into "obviously too-short-for-towing"
lengths) is entirely the towpilot's responsibility.

The glider pilot (who only gets to see one end if the ground crew happens
to show it to him)?
Every tow, SOP is for the wing runner to show (and hand-to, if requested by
the glider pilot) the glider-end of the towrope to the glider pilot for his or
her inspection, after which the glider type can choose to launch, replace the
weak link or (very rarely) request the line be shortened or replaced prior to
the tow.

The ground crewmember (who has no regulatory responsibility)?
See above...

A host of obvious questions likely springs to each reader's mind, all centered
on the "burning question": How can anyone KNOW any of this does any good/meets
regulatory compliance/etc.?

FWIW, strictly my own personal conclusions as a self-interested glider
sort/engineer, and based on having done my own digging over time into these
sorts of questions, here's my take on things:

a) "Actual line safety," "legally-binding regulatory safety," and "known
regulatory compliance" are pretty much different things, with VERY fuzzy,
arguably smallish-to-largish overlapping subsets. Some of the fuzziness is due
to unavoidable, practical, realities...e.g. testing/methodology,
continuing-use vs. degradation testing, correlating visual degradation to
measurable strength, etc. Some is due to the inherent difficulties in trying
to define/describe/place hard/legal bounds on engineering problems. Some is, I
would bet, almost certainly due to bureaucratic hand-washing/disinterest/etc.

b) At least one (engineer) member of my club built a pull-to-failure test rig
(resides in the towplane hangar for any curious club member to "play with")
and (more than once) performed extensive parametric testing over the years, of
new and used tow ropes and weak link methodologies (e.g. separate links of
smaller-than-towrope diameters, knots in the rope, etc.). In large part the
club's daily procedures described above are derived from this testing, said
testing coupling at some level with regulatory guidelines.

c) A former (late) on-field glider FBO (CFIG, FAA designated examiner, A&P [w.
IA?], "engineer-head") had done his own independent pull-to-failure testing
along the lines of B) above...and interestingly came to essentially the same
conclusions as my (operationally independent) club, regarding "the best
method" to safely "meet the intent of regulations."

Strictly from personal engineering curiosity, I picked the brains of both men
once learning of their test efforts. My interest was in doing what I could to
reassure myself the club's towrope guidelines conveyed to every member,
weren't simply picked out of the air or otherwise "of dubious provenance." I
had zero interest in bringing "Philadelphia lawyer-ism" to the regulatory
question of "What's safe?"

Bottom line is in over 30 years of operations from the field - over which time
I've seen towrope/weaklink methods evolve and "reinvent a wheel" once or twice
- zero "entirely unforeseeable" towrope/weaklink issues come to mind, while I
*can* remember some unknown quantity of self-inflicted rope/link failures (aka
footshots from slack line training or PIC inattention). My conclusion?
Regardless of the fun-to-discuss, of-legal-implication issues surrounding how
"to best define & regulate" towrope/weaklink strengths, is that "for all
practical purposes" our gliderpilot derived, local towrope procedures, pretty
much "work acceptably" and have the benefit of simplicity.

In my mind, "If it ain't broke, don't fix it," applies on this front, though I
realize "Better is the enemy of good enough," might also be brought to the
discussion. Meanwhile, I'll bet Real Money that anyone "needing absolute
assurance" of towline/weaklink breaking strength on every given tow, is doomed
to the same unquantifiable disappointment as those arguing how many angels can
fit on the head of a pin. And, yes, I'm aware of Tost's metallic weaklinks,
and readily acknowledge their superiority for winch launching, while being
prepared to debate their "necessity" for aerotow.

YMMV,
Bob W.