Weak links

[Steve Davy]

Two questions.
How many weak links are required/suggested for a two point aerotow?
If a weak link is installed at each end of the tow line (on the line side of the rings) then what is the function of any other weak links in the system?
Steve
P.S. I've never towed, and I'm not real smart so if you could kind of dumb it down for me I would appreciate it.

[Tad Eareckson]

How many weak links...

Depends heavily upon to whom you're talking in this science free, opinion based sport.

Legally for two (or one) point aero...

The United States Hang Gliding and Paragliding Association, Inc.
Standard Operating Procedure
12. Rating System
02. Pilot Proficiency System
10. Hang Gliding Aerotow Ratings
-B. Aero Vehicle Requirements

05. A weak link must be placed at both ends of the tow line. The weak link at the glider end must have a breaking strength that will break before the towline tension exceeds twice the weight of the hang glider pilot and glider combination. The weak link at the tow plane end of the towline should break with a towline tension approximately 100 pounds greater than the glider end.

Sanely...

Specify a 1.3 G minimum and change the word "should" to "must". Then you're operating the way sailplanes have since the beginning of time.

HOWEVER...

- A weak link on the back end of a towline gets dragged, so if you do it that way, the back end weak link should be a Tost assembly - which is immune to dragging but kinda bulky and heavy.

- You can pick up a lot of safety edge if instead of (or in addition to) having weak links on the ends of the towline you have them at both ends of two point bridles.

All Dragonflies - by definition - use two point bridles and all gliders really should (but I don't get too bent out of shape if the high performance jobs don't).

Under Murphy's Law one MUST assume that a two point bridle WILL wrap at the tow ring - because it CAN.

Towing Aloft

I witnessed a tug pilot descend low over trees. His towline hit the trees and caught. His weak link broke but the bridle whipped around the towline and held it fast. The pilot was saved by the fact that the towline broke!

And, as these total idiots so conveniently demonstrate and document, this, again true to the predictions of Murphy, WILL happen at the worst possible time in the worst possible circumstances. The higher the tension under which a bridle end separates the more likely it is to tie itself in a knot (big surprise) in circumstances in which you almost certainly most wish it wouldn't.

A glider's two point bridle must NEVER be released from the bottom first 'cause if that end wraps, depending upon how you have your bridle/release assembly trimming the glider, at best you're towing from the hang point and having (maybe manageable) control issues and at worst you're towing from forward on the keel and WILL be put into an immediate severe dive which the glider likely won't survive.

So...

If you put a weak link at the top end of your bridle that blows at 1.5 Gs (of towline tension) for YOUR GLIDER you have a weak link which blows at 1.5 Gs for YOUR GLIDER. That's nicer than using or having to swap whatever happens to be on the end of the towline for your flight (but really not that much of a BFD (and the Tost system also allows you to just select from an in-place array)).

And the really cool part...

Put a weak link at the BOTTOM end of the bridle which holds safely over what the top does but not much more. I recommend a twenty percent increase - so for 1.5 at the top, 1.8 at the bottom.

The top weak link blows (at 1.5 Gs), the bridle wraps, the bottom 1.8 G weak link is now - after you do the trig to take the two point bridle apex angle out of the equation - a 1.035 G weak link, it's feeling 1.5 even before the brutal shock load, it's toast, you're gone before you can even think about a secondary release.

Also...

Although this shouldn't be taken as a given, even if the bridle wraps in the course of a routine normal tow tension release, the double plus shock loading still blows the secondary weak link before you can think. It's amazing how fast the tug (with no input from the driver) accelerates away when suddenly relieved of its payload.

The Dragonfly releases from the bottom only and wouldn't be in significant trouble if it found itself towing from the top of the tow mast only.

And there's a weak link between the bridle apex (at the tow ring) and top of the tow mast. In a wrap this weak link would also feel near double loading and some considerable shock (but not as much as with the glider 'cause it doesn't have the equivalent of the glider pilot's mass immediately in front of it - rather something bearing some semblance to a fishing rod).

But the morons who fly these things REFUSE to put a weak link BELOW the tow ring (or at the end of the towline as per the regulations). If you did it just like the glider, heavy above, heavy times 1.2 below, when your tow ring clipped itself to a tree or fence the top link would blow and, if the bridle wrapped - as per the Towing Aloft moron - the towline would be gone with the driver feeling practically nothing.

But the way these assholes configure, one of them could suddenly find himself tied to something on the ground with nothing lighter than two thousand pound Spectra limiting load to the tug - and its release.

Murphy predicts that - with enough monkeys and typewriters - somebody WILL kill himself this way. And if/when that happens I one hundred percent guarantee you that the discussion will be all about not flying low over trees or fences and nothing about using a legal, safe, and sane weak link configuration.

Note that - while it had zero bearing on Keavy Nenninger's fatal crash - the NTSB hasn't commented on the obvious illegal weak link configuration (and won't).

I've never towed...

Really appreciate the question from someone who's never towed.

...I'm not real smart...

http://ozreport.com/forum/viewtopic.php?t=22660
What can be learned from this "scooter" towing accident?

Mitch Shipley - 2011/01/31 15:22:59 UTC

The tow line to release interface

Jim, as I am starting to play with Elektra Tow (ET) at Quest Air (the battery powered "scooter" tow system you and Adam got me jazzed up about up at Highland) I've watched this thread closely. We all can learn.

Enjoy your posts, as always, and find your comments solid, based on hundreds of hours / tows of experience and backed up by a keen intellect/knowledge of the issues when it comes to most things in general and hang gliding AT/Towing in particular. Wanted to go on record in case anyone reading wanted to know one persons comments they should give weight to.

Being as dumb as a ten year old kid is a real asset in this sport. Keeps you from perfecting aerotowing the way the geniuses at Quest and Ridgely have.

...so if you could kind of dumb it down for me...

Summary... One at each end of:
- the towline (total - two) is mostly OK.
- each two point bridle (total - four max):
-- virtually eliminates bridle wrap issues
-- reduces the hassle of ensuring the glider has an optimal weak link rating
-- keeps extra junk off the trailing end of the towline with which the tug has to descend and land

And when there's a better safer way to do something with zero downsides in the way of cost, time, weight, drag you go for it.
---
2022/05/26 12:00:00 UTC

Fuck that.

http://1.bp.blogspot.com/-2lk6hbjuJ-c/Ujh-ENPLQrI/AAAAAAAA3IQ/vh46chqQX4M/s1600/JRS_0542.JPG


I get EXTREMELY bent out of shape when the high performance jobs don't.

[Steve Davy]

A perfect explanation, thank you sir. I had not considered that the lower weak link would blow at anything other than its rating. That's a neat set up.

Question about the 4 string emergency release (terminology?) Doesn't this release the bottom of the primary bridle?

Edit: Is the emergency release covered in mouse traps? I can't find it. Edit: Found it.

[Tad Eareckson]

That's a neat set up.

Thanks. I suggested something along those lines in the magazine fourteen and a half years ago - but nobody was interested.

Doesn't this release the bottom of the primary bridle?

Only after a two pointer has had a bridle wrap and a secondary weak link hold. In two point you don't ever expect to use it but it's there and it costs nothing to have in place.

A one pointer, however, can expect it to come in handy once in a while - almost always at altitude where it doesn't matter a whole lot - but I personally know one local aerotower whom it would've saved from a devastating crash.

[Steve Davy]

05. A weak link must be placed at both ends of the tow line.

So how did this get interpreted to: A weak link must be placed somewhere near the end of the tow line. On the top, bottom or both ends of the bridle is just fine. ?

Note that - while it had zero bearing on Keavy Nenninger's fatal crash - the NTSB hasn't commented on the obvious illegal weak link configuration (and won't).

What is illegal about this configuration?

Only after a two pointer has had a bridle wrap and a secondary weak link hold.

Yeah, I didn't think that one through too well.

[Tad Eareckson]

So how did this get interpreted to...

- Hey! This is hang gliding, dude. It's all about FREEDOM.

- In hang gliding we ignore the standards and throw whatever we feel like into the air. If it doesn't kill anybody in the first three flights by failing at a low enough altitude we declare it the pinnacle of perfection. If it does, hell, it's an insurmountable engineering problem that we've done our best to address. (But this is why we fly with hook knives.)

- BOTH ends of the bridle is fine - better in fact. Here's what I wrote for the USHGA SOPs revision:

The towline and/or bridle connections must be configured so as to not unduly impede control of either vehicle. Where connections incorporate the use of one or more bridles these are considered to be extensions of the towline.

Weak links must be installed at both ends of the towline proper and/or its extensions.

which legalizes what some of the saner aerotowers are doing.

- But here's what Dr. Tracy...

Subj: Re: [Tow] AT SOPs - proposed revisions
Date: 2009/05/10 02:08:52 UTC
From: cloud9sa@aol.com
To: skysailingtowing@yahoogroups.com
cc: GreggLudwig@aol.com, lisa@lisatateglass.com

Hi Tad.

I'm Tracy Tillman, on the USHPA BOD, on the Tow Committe, and I am an Aviation Safety Counselor on the FAA Safety Team (FAAST) for the Detroit FSDO area. As a rep of both the USHPA and FAA, I would like to help you, USHPA, and the FAA improve safety in flying, towing, and hang gliding.

The FAA gets a lot of letters of complaint from a lot of yahoos.

...S. Tillman, PhD wrote on the issue:

http://groups.yahoo.com/group/skysailingtowing/message/549
Weaklinks/bridles; was: high wire act world news

Tracy Tillman - 2001/05/16 15:14:55 UTC

We (and many others) do not recommend using a second weaklink on the non-released end of the bridle. That weaklink is just as likely to break as the one on the other end. If the lower weak link breaks, the bridle could get caught in the towline ring, and pull the glider from the 'biner or keel, causing it to tuck. The other weaklink (on top) may not break prior to the tuck.

'cause PhD shitheads (and many others) are incapable of grasping the ten year old kid concept of using a weak link on the bottom incapable of blowing before the one on the top. In this game if it ain't a loop of 130 pound Greenspot it simply ain't a weak link.

- And, of course, PhD shitheads (and many others) are also incapable figuring out that a loop of 130 pound Greenspot on the end of the secondary bridle is only feeling half the load of the one at the top end of the primary.

- And, of course...

The weaklink should be on the released end of the bridle, and the bridle should release from the top. That way, if the bridle does get caught on the ring, it is pulling from the body, rather than from the glider, and the glider may still be controllable--in which case you can use your secondary release (or hook knife if that fails) to release.

...at this point, one needn't worry about the glider and bent pin secondary release being overloaded - this is why we fly with hook knives.

What is illegal about this configuration?

The current regulation states:

A weak link must be placed at both ends of the tow line.

and, on the Dragonfly end, there was only a weak link above the tow ring.

Yeah, I didn't think that one through too well.

Don't worry about it. In hang gliding, never thinking anything through at all has never been the least impediment to being hailed as a genius by one and all. Major asset, in fact - considering the constituency.

[Steve Davy]

5. And, of course, PhD shitheads (and many others) are also incapable figuring out that a loop of 130 pound Greenspot on the end of the secondary bridle is only feeling half the load of the one at the top end of the primary.

Half of the load of the one at the top? Wouldn't each weak link feel about half the tow line tension?

I read aerotowing guidelines and found no reference to a configuration that would have the lower weak link feeling half the load of the top weak link.

[Tad Eareckson]

A weak link at the bottom end of the PRIMARY bridle will feel exactly what the one at the top does - half the towline tension plus - assuming a sixty degree bridle apex angle - fifteen percent.

The bottom end of the primary bridle is engaged by the SECONDARY bridle which splits in half that half plus fifteen percent and distributes it to your shoulders - through weak links if present (the secondary apex angle is so acute that you can ignore its effect).

So if you have 400 pounds of towline tension the primary bridle is under a 230 pound strain and the secondary feeling 115.

If you had a 400 pound weak link at the end of the towline, 230s at the ends of the primary bridle, and 115s at the ends of the secondary somebody would give up the ghost first and the other four would be so stressed that you wouldn't be able to use them again.

If you use the same weak link on the ends of your secondary bridle that you do at the top of your primary it's physically impossible for the bottom end to go first and you're in pretty good shape if the top end wraps.

[Steve Davy]

Hang Glider Lock-Out!
http://www.youtube.com/watch?v=fnrh9-pOiq4
MorphFX - 2011/03/20
A hang glider pilot learning to aerotow starts a mild PIO that leads into a lock-out.
dead
02-00319
http://farm8.staticflickr.com/7523/15601000807_705ab49977_o.png


http://farm6.staticflickr.com/5606/15600335029_3c234e6828_o.png
14-03708
http://www.kitestrings.org/post6995.html#p6995

What's your take on this?

Also, if the function of the link is to save the glider then why not go all the way up to 2 G's?

I can't help but wonder if the term 'weak' link had never been coined and instead something like fusible link had, if folks would be more inclined to use something less 'weak'.

[Tad Eareckson]

What's your take on this?

- A one G weak link makes a really crappy lockout protector and actually DOESN'T break before you can get into too much trouble.

Dynamic Flight - 2005

The purpose of a weak link is solely to prevent the tow force from increasing to a point that the glider can be stressed close to or beyond its structural limits.

- The purpose of a weak link is solely to prevent the tow force from increasing to a point that the glider can be stressed close to or beyond its structural limits. This is NOT drivel or a load of shit.

- Any halfway competent pilot with a decent release can always beat a halfway substantial weak link in a lockout.

- The end of the lockout is just the beginning of your problems.

- You can't afford to have a low level lockout because most of the time you won't have enough air in which to recover.

- Assholes like Rooney and Ryan Voight who tell people to increase the roll and pitch up to effect an instant hands free release need to have all their ratings and appointments revoked and jagged broken downtubes driven through their hearts.

- Don't count on your ass being saved by a tug releasing you. (And never encourage him to do so.)

- Tugs are never endangered and seldom even affected by severely locking out gliders and would have plenty of leeway to blow them off tow even if they were.

Manned Kiting
The Basic Handbook of Tow Launched Hang Gliding
Daniel F. Poynter
1974

"Never take your hands off the bar." - Tom Peghiny

09. Never take your hands off the bar.

- Towing one point is a mediocre idea at best and way less than stellar until you've gotten a good feel for two.

Also, if the function of the link is to save the glider then why not go all the way up to 2 G's?

- You can - legally.

- You COULD also go up to three safely - if not legally.

But...

- You'd probably have to really beef up your releases and attachments to handle the worst case scenario.

- One and a half is high enough so that you're never gonna blow off anyway. (Sailplane manufacturer specified weak links are all around 1.3 to 1.4 and they never break them.)

I can't help but wonder if the term 'weak' link had never been coined and instead something like fusible link had, if folks would be more inclined to use something less 'weak'.

- Hang glider folk are inclined only to follow what they were told on Day One.

- Most of this "the only good weak link is a constantly on the edge of blowing weak link" crap that's making our lives miserable dates back thirty years to a lot of totally clueless bullshit written by Donnell Hewett and sanctified by the international cult of hang gliding.

- I was told by Chad at Ridgely - on its first weekend of operation when 130 pound Greenspot was going off like popcorn and I asked why we didn't just beef things up - that a loop blew at 260 pounds and that increasing things beyond that would put a problematic stress on the glider.

- I was stupid enough to accept this for about half a dozen years - even though I KNEW from Day One that they were going off under normal tow tension - until I finally got suspicious enough to question those bastards and test it.

- Even after finding that it was half of what they were saying it was, I was STILL too stupid to then question why we were using it. It took another couple of years until the lightbulb came on and I totally understood what Dynamic Flight was saying.

Manned Kiting
The Basic Handbook of Tow Launched Hang Gliding
Daniel F. Poynter
1974

"The greatest dangers are a rope break or a premature release." - Richard Johnson

- I think we'd have better luck calling them whipstallers and showing a few supporting videos before we let people up to begin flight training. Thanks for the idea.