Weak links

[Tad Eareckson]

http://ozreport.com/forum/viewtopic.php?t=15716
weak links

Martin Henry - 2009/04/26 16:23:56 UTC

Tad,

Its astonishing how manage to take a simple short little piece of string and and stretch it into an 11 page (and still going) thread

Like George says, your posts are getting better and there is some actual "content" , still, for some strange reason I can't help but click on your posts once in a while, like putting on some Abba tunes and never admitting I liked the music...

The reason I thought I would post is, well your very caught up in your end of the towline you seemed to be in a bit of a battle with the guy (or gal?) on the other end of the line. I have absolutely no challenge to your 1.4 mass weak link (Mainly because there is only so much time I can dedicate to string...) but... If the guy on the front end of the line does not like it, then I think you should respect their side of the argument?

Trike Tug pilots (most), Bailey Tugs with the smaller two strokes or anything that is simply underpowered have a pretty small margin of safety when it comes to the towing envelope. It's a complicated balance. I've heard the expression, "it's like flying two aircraft at once". Getting your tail hauled up, down, left and right buy less then cooperative customer can be down right nerve wracking. If your tug pilot wants you on a "weak" link, you should respect his wishes. If you don't like it, find a tug pilot that can handle the challenge. If he's a lousy tug pilot, he will sooner or later be out of business, If he's good, pilots will line up to be towed. My point there are two aircraft involved. If your being towed up by a big 914T your tug pilot might be able to muscle his way out of a problem, but if he's driving a wheeze two stroke he simply may not have the power to put up with much trouble caused by a strong weak link.

For a sad example of what can happen to a tug when things go wrong...

In Washington State a few years back a Sailplane operation had Tug go down. During the roll, the Spoilers "popped" open on the two seater sailplane (with student at the controls). The Tug pilot sensed a problem as the tow progressed... things where simply not happening as they should. He glanced back and saw the spoilers open. He should have given the glider the rope. He didn't, he powered hard into the tow and got on the radio and screamed at the student to get his spoilers closed. There was plenty of energy building and the student realized his error and slammed the spoilers shut. The glider, now with speed, ballooned rapidly and the student did not react fast enough to compensate, shooting way too high. The high tow position, Tug at full power (trying to power his way out of the problem) the glider was pitching the tug down. The rope/weak link failed. The Tug still low to the ground at full power, close to stall, nose down, drove straight into the rough ground off the end of the runway and the Tug pilot was killed.

Flying a Tug with a heavy customer at the end of the towline is a ballet dance with a fat lady, you have to do it right or somebody could get killed.

Now its back to my "Abba's Greatest Hits"...

Cheers

Martin

Tad Eareckson - 2009/04/26 20:05:29 UTC

Martin,

That short little piece of string (which - in my opinion - should not even be a short little piece of string) can/has/will kill people when it breaks at the wrong time, causes total havoc in flight operations, and is virtually NEVER required for the safety of a flight. (Notice the total dearth of posts from people with evidence to the contrary?)

I have absolutely no challenge to your 1.4 mass weak link...

It's not mine. This is pretty much an aviation standard outside of the inbred little world of hang gliding.

If the guy on the front end of the line does not like it, then I think you should respect their side of the argument?

I generally find that virtually no one in the sport on EITHER end of the line has enough of clue about the purpose of a weak link to BEGIN to present a hint of a side of a rational argument. The vast majority don't even have a clue as to the breaking strengths of what they're using or how these strengths translate to tow line tension.

If your tug pilot wants you on a "weak" link, you should respect his wishes.

Sorry but - BULLSHIT. USHPA defines a safe upper limit of two Gs but was too stupid to define a lower end at which things become dangerous. I SAY anything much south of 1.4 is DANGEROUS to me and it's his job to make sure I don't get stuck with the rope in the event that things get really far outta hand - which ain't never gonna happen anyway.

If you assume that the tug's release is rusted shut the back end bozos who can't get to their releases are the ones who present the biggest threat to both planes - but we never hear much about them 'cause majority tends to trump common sense.

In Washington State...and the Tug pilot was killed.

And this is pertinent to the discussion HOW?

In Washington State the planes are REQUIRED under federal law to be connected with a line and/or weak links which hold to a MINIMUM of 0.8 Gs. The part of your paragraph giving any indication that this situation either progressed above that tension or needed to to achieve the end result is completely absent.

The tug was killed, not as a consequence of anything above what needed to be and probably was only minimal tension, but as a consequence of how far out of position the sailplane was and how quickly it got there. If the rope was strong enough to get the sailplane aloft at all - it was also strong enough to let the glider kill the tug.

Now to this part of the paragraph...

He should have given the glider the rope.

Once again with emphasis...

HE SHOULD HAVE GIVEN THE FREAKIN' GLIDER THE FREAKIN' ROPE.

but...

He didn't...

http://www.dynamicflight.com.au/WeakLinks.html

In all aviation the pilot is usually the weakest link...

http://groups.yahoo.com/group/skysailingtowing/message/4633
Weaklinks and aerotowing (ONLY)

Steve Kroop - 2005/02/10 04:50:59 UTC

Weak links are there to protect the equipment not the glider pilot. Anyone who believes otherwise is setting them selves up for disaster. The pilot activating his or her releases is their way to save themselves.

In sailplaning it's ALWAYS the tug that gets killed and NEVER the glider.

In hang gliding it's just the opposite 'cause the Dragonfly's control authority blows the glider's away and the latter can't get out of position fast enough to do any real damage.

Flying a Tug with a heavy customer at the end of the towline...

I'm NOT a heavy customer - I'm two hundred pounds under some of their tandems.

Take a chance on me...

http://ozreport.com/forum/viewtopic.php?t=24846
Is this a joke ?

Zack C - 2011/08/31 02:45:17 UTC

Also...isn't it is not so much the magnitude of the tow force that endangers the tug (and glider) pilot but rather its direction? As a tug pilot, would you rather have 500 lbs pulling straight aft or 240 pulling straight up? Could not even a glider using a 130 lb Greenspot loop put the tug pilot in danger?

http://ozreport.com/forum/viewtopic.php?t=24846
Is this a joke ?

John Fritsche - 2011/08/26 06:07:24 UTC

I intentionally broke a weak link by severely and intentionally locking out on aerotow at about 2,000 feet. Why I did this is a long story. But the weak link served its purpose. As I was locking out, I was forcing the tail of the tug up and sideways in a big way, and the tug pilot said he was about to give me the rope when the weak link (at my end) snapped.

And when I ask the question of Martin and Zack asks it of Rooney the response we get is EXACTLY the same.

...

[deltaman]

sailplanes kill tugs
tugs kill hanggliders
and for a single surface microlight with the power of a Rotax 512, who endanger who ?

[Tad Eareckson]

- A plane:
-- that's going up is always safer than one going down
-- with a working engine and gas always has the ability to go up
-- without an engine below two hundred feet is always going down

- With extremely rare exceptions, below two hundred feet a glider tied to a plane with an engine going up has much better prospects for survival than a glider below two hundred feet not tied to a plane with an engine.

- It's NEVER safer for a powered plane to have ANYTHING - especially something heavy - tied to its tail. (It's also never safer for a passenger plane to be carrying passengers - higher stall speeds, longer runway requirements...)

- The situation for a tug ALWAYS and IMMEDIATELY gets dramatically better when it loses its glider.

- With extremely rare exceptions, the situation for a glider temporarily gets worse - sometimes lethally - when it loses its tug. It will always suffer an increase in angle of attack and occasionally stall severely if it was slow to begin with or rolling into a lockout.

- Someone who flies a hang glider frequently can expect to lock out - often violently but virtually always at safe altitude - once in a while.

- When was the last time you heard about a tug:

-- locking out?

http://www.youtube.com/watch?v=F_n5B3-MIC4


-- suffering from a dangerous loss of control by even a lethally locking out hang glider?

11-1814


http://ozreport.com/forum/viewtopic.php?t=24846
Is this a joke ?

John Fritsche - 2011/08/26 06:07:24 UTC

I intentionally broke a weak link by severely and intentionally locking out on aerotow at about 2,000 feet. As I was locking out, I was forcing the tail of the tug up and sideways in a big way, and the tug pilot said he was about to give me the rope when the weak link (at my end) snapped.

The tug's getting pretty screwed up by a deliberate and severe lockout even with this loop of 130 pound Greenspot that's supposed to keep it from getting pretty screwed up but he's still not so compromised that he needs to squeeze the lever.

1984 - A trike tug, Dave Garrison, and the glider both screwed up, allowed a lot of slack to develop, got a snag on and tore the trike's left wing. The trike went down in flames but Dave got away with a broken wrist.

1985/07/17 - A trike tug, Chris Bulger, was tumbled and killed (no parachute onboard) because with a deliberate and unnecessary hard turn he induced a control compromised situation and had a piece o' shit release which couldn't handle a reasonable weak link.

1996/07/25 - A trike tug, Dave Farkas, put himself in a dangerous control compromised situation by inadvertently but incompetently, severely, and fatally stalling a heavy tandem glider and ALSO having a piece o' shit two-string release release which couldn't handle a reasonable weak link. He lucked out when the front end weak link blew.

1992/05/17 - A Dragonfly, Dick Reynolds, lost a engine when he was already flying slow - out of too much consideration for the glider, delayed pulling the lever - also out of too much consideration for the glider, and attempted to pull out of the stall too soon. He pancaked and was badly injured.

2011/07/30 - A Dragonfly, Peter, stalled on takeoff and neither tug nor pilot left in very good shape. Zilch in the way of details but seems like it was all pilot error.

That's about all I know.

So to answer your question...

A hang glider can't ever endanger any tug as long the tug keeps the towline tight or dumps it if he can't, has a good release, which I'm pretty sure they all do nowadays, and...

Towing Aloft - 1998/01

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!

...a properly configured weak link, which NO Dragonfly does.

He can get screwed up if he loses an engine and/or doesn't fly competently - but that really doesn't have anything much to do with the glider.

[Zack C]

In a recent aerotow weak link discussion someone pointed out that the AirBorne Edge series of trikes have the following on their Hang Glider Aerotow Limitations Placard:

Maximum takeoff weight of hang glider (kg): 250
Maximum weak link strength (kg): 150
Maximum hang glider towing speed (KTS IAS): 45
Aircraft to be flown solo only for aerotow operations

The Pegasus XL-R has a maximum weak link specified of 100 kg.

I replied that:

Tandems in the US typically use a double loop of 130 Greenspot as their weak link, which allows a tow force of about 158 kg. That's in excess of the maximum stated for the Edge even without a margin above that for the trike end. And the Edge interestingly permits a glider weight of 250 kg, for which their stated weak link maximum is only 0.6 Gs.

Be that as it may, it's hard to argue with manufacturer stated operating limitations...

Zack

[Steve Davy]

http://www.hanggliding.org/viewtopic.php?t=24033
Got my AT rating at Quest! (Now with write-up)

Matt Christensen - 2011/11/24 00:39:40 UTC

This day I did nine tows with four weak link breaks. The weak link breaks were all early on in the tow and all but one were in basically level flight. Either the weak link material that I was using was bad or the knots were compromising the strength, either way I got some good practice in dealing with breaks!

[Steve Davy]

All up weight has nothing to do with weak links. The only thing that's important is the MAXIMUM CERTIFIED FLYING WEIGHT of your glider.

I'm having trouble getting my head around this. Please define "Maximum certified flying weight".

Edit : Found it
http://www.energykitesystems.net/Lift/hgh/TadEareckson/Tostweaklinktable.pdf

[Tad Eareckson]

Damn. I didn't catch that you no longer had the question before I answered it - undoubtedly with WAY more than you needed or wanted. But since the work is done and in case anyone else has an issue on the subject...
---

I'm having trouble getting my head around this.

Yeah, so did I for a long time - thanks to a couple of decades worth of hang gliding brainwashing.

Hang gliding - largely thanks to Donnell Hewett - has pretty solidly convinced itself that the purpose of the weak link is "to break before you can get into too much trouble." Thus the "thinking" is that you base the weak link on the actual flying weight of the glider (glider plus everything hanging from it and clamped to the control frame) on the "theory" that if you keep the weak link at a value - at an ABSOLUTE MAXIMUM - equal to that figure but preferably just an ounce or two over normal smooth air tow tension then the severity of a lockout will be limited to something that can easily be recovered from.

This, of course, is total bullshit. When a low glider gets hit by something nasty there's no possible way to predict whether or not the situation will be survivable whether or not it's on tow.

In REAL aviation...

Tost Flugzeuggerätebau

Weak links protect your aircraft against overloading.

...or the microscopic fraction of hang gliding that's got it right...

http://www.dynamicflight.com.au/WeakLinks.html

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 weak link serves ONLY to blow before the glider gets broken or bent under positive loading. And there's no guarantee whatsoever that you'll still be alive three seconds after that happens. And if you ever let yourself get into a situation below a couple of hundred feet in which a weak link blow is necessary there's a really excellent chance that you won't be.

Here's what the FAA says with respect to sailplanes:

A safety link is installed at the point of attachment of the towline to the glider with a breaking strength not less than 80 percent of the maximum certificated operating weight of the glider and not greater than twice this operating weight.

That lower figure is dangerous for sailplanes...

http://www.aviationbanter.com/archive/index.php/t-40965.html
Are Weak Links really Necessary for Aero Tow?

Bill Daniels - 2006/09/18 14:30

I would like to add, however, that at least my reading of accident reports suggest that a fatal glider accident is more likely when the towline fails prematurely. For that reason, I like to stay near the stronger end of the FAR 80-200% range.

...insanely dangerous for hang gliders...

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

...and unbelievably insanely dangerous for paragliders...

http://www.paraglidingforum.com/viewtopic.php?t=28641
Weak links in towing paragliders with pay out winch

Steve Beach - 2010/01/10 16:24:45 UTC

My father reports a weak link failure at thirty feet that had him surge into the ground from exactly the most dangerous altitude possible for his glider's surge tendencies. Only his timely brake input and perfectly executed forward tumble prevented serious injury. His tow was proceeding perfectly until the weak link failure nearly caused significant injury. I am convinced stronger - not weaker weak links are best.

...because the:

- poorer your lift to drag ratio the greater the percentage of your flying weight you need coming through the towline to get and keep you airborne

- narrower your speed range the closer to stall you'll be flying

But (to get back on topic) the FAA got the upper end and "maximum certificated operating weight" parts right.

- Planes not designed for aerobatics - like ours - are not really expected or intended to be operated over three Gs when fully loaded, even though they're built to take a lot more.

- But the goal is to keep the plane from being stressed beyond three Gs and that's plenty reasonable enough for me.

- Let's throw out the additional G loading you experience from coordinated turns 'cause you have no good reason to be doing high G turns on tow and if you're really determined to break the glider no weak link can stop you anyway.

- So you normally fly at one G and a two G weak link guarantees that you won't stress the glider to (much over) three on tow.

- If you're flying your sailplane at below its maximum certificated operating weight or hang glider at below its maximum hook-in weight you get to pull more radical aerobatics and use heavier weak links to put the same stress on the plane that it would get if it were topped off.

Elaborating...

The hook-in range for a 68 pound Wills Wing U2 160 is 160 to 260 pounds. If you hooked in at the top of the range you could use a 656 pound weak link to keep the glider from going over its intended stress level. If you hooked in at the bottom you could go to 756 without permitting more total strain.

[Tad Eareckson]

http://ozreport.com/forum/viewtopic.php?t=23414
Step Towing

Bart Doets - 2011/12/18 09:43:51 UTC

In the recent Delta Magazine (Dutch HG mag) someone describes the experience of a weaklink break caused by a drum snag on payout.

Seems like a relative minor event provided the weaklink is properly chosen (the club where this happened uses 150 and 175 kilos, for lighter and heavier pilots).

Because a drum snag is a very sudden thing (compared to all other loads while towing) a proper chosen weaklink will break at a drum snag yet function as long as things go right.

Dutch pilots have many years experience step towing; our country is made up of relatively small fields devided by fences and roads, and for many years aerotow was illegal so we had to!

I don't consider myself a specialist but I've done my share of step towing as well.

In the recent Delta Magazine (Dutch HG mag) someone describes the experience of a weaklink break caused by a drum snag on payout.

Weak link breaks aren't caused by snags on payout. Weak link breaks are caused by line tension which exceeds the strength of the weak link. That may or may not happen as a result of a snag.

Seems like a relative minor event...

Yeah. EVERYTHING seems like a relatively minor event...

http://www.hanggliding.org/viewtopic.php?t=21033
barrels release without any tension except weight of rope..

Bart Weghorst - 2011/02/25 19:06:26 UTC

No stress because I was high.

...IF you're high enough.

...provided the weaklink is properly chosen...

Properly chosen for WHAT, Bart? To make a line snag on a downwind run from a winch SEEM like a relatively minor event? Yeah, it'll probably do that. Kinda like all those successful shuttle re-entries prior to 2003/02/01 made all those foam insulation strikes on and damage to the heat shielding SEEM like relatively minor events. Kinda like tens of thousands of ramp launches in which no one is killed after failing to perform a hook-in checks just prior to launch make a failure to perform a hook-in check just prior to launch SEEM like a relatively minor event.

...(the club where this happened uses 150 and 175 kilos...

For us English system Neanderthals, that translates to 331 and 386 pounds. For a Wills Wing T2 154 loaded to the max - 360 pounds - we're talking under 1.1 Gs on the heavy weak link. A 200 pound glider on the light weak link would be over 1.6. So it's a no brainer that the vast majority of these gliders are flying well in excess of one G.

...for lighter and heavier pilots).

Right. You "properly choose" weak links for lighter and heavier pilots. This is hang gliding after all. This isn't like sailplaning in which you properly choose weak links for lighter and heavier capacity gliders.

Because a drum snag is a very sudden thing (compared to all other loads while towing)...

Yes. The load of snag runs from about 0.3 to 1.2 seconds while all other loads while towing run from about one to three hundred pounds. And after we do the conversion from seconds to pounds we find that this statement makes no sense whatsoever.

...a proper chosen weaklink will break at a drum snag yet function as long as things go right.

- Can you give us an optimal G figure we can properly choose which will break at a drum snag yet function as long as things go right? Should we go with one G 'cause that's a good rule of thumb?

- What are the laws of physics that make that a given under real world conditions?

http://www.hanggliding.org/viewtopic.php?t=20529
Shane Smith - RIP

Mark Knight - 2011/01/17 23:59:32 UTC

The rope was cut by the operator.

The rope went slack and he made a 90 degree turn away from the turnaround pulley and the rope caught something else on the ground causing lockout.

- How come Shane Smith died when his line snagged? Was his his weak link as improperly chosen as his bridle/release configuration?

- Precisely how will it function as long as things go right?

Dave Broyles - 1990/11

I talked to a lot of pilots at Hobbs, and the consensus was that in the course of Eric Aasletten's accident, had a weak link break occurred instead of the manual or auto release that apparently did occur, the outcome would have been the same.

http://ozreport.com/forum/viewtopic.php?t=865
Tandem pilot and passenger death

Mike Van Kuiken - 2005/10/13 19:47:26 UTC

The weak link broke from the tow plane side. The towline was found underneath the wreck, and attached to the glider by the weaklink. The glider basically fell on the towline.

- How will it function when things AREN'T going right?

Ya know, Bart...

- Even if the probability of a weak link blowing in response to a snag on payout is high it's a REALLY BAD idea to fly with the expectation that it WILL.

- And it's an even worse idea to be telling everybody that it will.

- If you're running downwind away from the winch low enough to be dependent upon a weak link blow to keep you alive in the event of a snag you're out of your mind.

- The component of the tow system whose function it is to RELEASE the glider from tow in an emergency requiring RELEASE from tow is called a RELEASE.

- You guys are UNDOUBTEDLY using the Koch two stage release which requires the pilot to take a hand off the basetube and slap a paddle on his chest. While this is the best of the need-to-take-a-hand-off release options, you're saying - in effect - that you can't get to it when you really need to.

- If you're running downwind away from a winch low enough to be dependent upon being able to blow your release within two seconds of a snag to keep you alive you're still out of your mind.

- In a snag situation in which one of your "properly chosen" weak link is gonna blow the tension will be building up so fast that the extra time it takes for an "improperly chosen" 1.5 or 2.0 G weak link to blow will be immeasurable. An extra two hundred pounds won't make enough difference to be worth talking about.

- If you're running downwind at a sane altitude and you get a snag, even if the Koch release is less than ideal, you're gonna be able to use it to blow tow and recover just fine.

- Do try to learn the difference between a release and a weak link and how to properly choose one of the latter.

Tost Flugzeuggerätebau

Weak links protect your aircraft against overloading.

And EVERY TIME one of these damned things HAPPENS to blow at the right time to do the job the pilot and his release were supposed to, this crap:

7. INFALLIBLE WEAK LINK

The system must include a weak link which will infallibly and automatically release the glider from tow whenever the tow line tension exceeds the limit for safe operation.

just keeps getting more deeply entrenched in the culture.

[Tad Eareckson]

http://www.youtube.com/watch?v=sKVsHAxRAcE
Drum Tangle

Mark Dowsett - 2011/04/03
Instinct Windsports

http://www.youtube.com/watch?v=sKVsHAxRAcE


Drum Tangle

...while on downwind. This is why we turn before getting down to 500'!

Our first day of towing for the year. I had a drum tangle at about 700'-ish. I was looking down at the rope since I was getting low (almost ready to turn back upwind) as I knew I should be ready to release if something happened...good thing! No biggie (if you react fast).

I released before the weaklink broke. It was the first flight of the year and as you can see from me looking down, I was watching for a tangle. But this is why you don't step tow PGers and also why we stay over 500' at all times. But you probably tell, I only lost about 20' of altitude...no big deal!

Bart Doets - 2011/12/18

I doubt the weaklink would have broken if you had not released. The peak of the snag was over before you could reach for the release, it would have had no more reason to break.

In Holland they do also step tow paragliders, and we have steptowed HG's for almost as long as we tow. It's hard enough to find a towing field, and when you have one it's never long enough to get a reasonable height in one run. I read the weaklinks used will break at a drum snag.

- He's almost at the end of his downwind run.

- Thus he has about as much line out and is about as low as he's gonna get.

- He's around seven hundred feet (and maintaining a floor of five) so he's in pretty good shape no matter what.

- He probably has a couple of thousand feet of spectra out so there will be a lot of sag and this will dampen any jolt resulting from a snag well enough to keep the weak link from blowing.

- But - BFD. He has plenty of air under him, he's level and not far off trim so he just slaps his chest and blows the second stage of his Koch release.

No biggie (if you react fast).

At that altitude it's no biggie even if you don't react fast.

I read the weaklinks used will break at a drum snag.

Yeah, I read that at Wallaby:

If you fail to maintain the correct tow position (centered, with the wheels of the tug on the horizon), the weak link will break before you can get into too much trouble.

Those guys are full o' shit too. Use the goddam weak link to protect the glider from overload and use the goddam release to keep from crashing.

[Tad Eareckson]

http://ozreport.com/forum/viewtopic.php?t=23414
Step Towing

Andrew Stakhov (Macsux) - 2011/12/22 17:44:32 UTC

I flew a few times at Mark's field. It's a stationary winch, with a looooong rope, I think I got like 3000' stop towing, and you can go like 2 fields away from winch before you have to turn back. I actually really enjoy launching with stationary winch towing, especially the "rocket launch" effect you can get going up at 45 degrees all the while telling yourself to relax and not shit your pants Only downside is when rope breaks and the transition from 45 degree nose up to 45 degree nose down 30 feet off the ground - that one is unnerving.

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 actually think winch towing is in a way less involved for the pilot then aerotowing because mistakes can be corrected on the ground by lowing tow pressure. I fly at soga now and found that I have to make corrections much faster and aggressively.

FAA Glider Flying Handbook

The hook-up should be done deliberately and correctly, and the release mechanism should be checked for proper operation. The launch crewmember applies tension to the towrope and signals the glider pilot to activate the release.

Gawd it must be nice to be fly in a sport which tows using tension instead of pressure.
(Line above composed, edited, proof read, spell checked on a Mac.)