Weak links
Hans Trautenberg t2466
: What is the rules about weak links in your country?
Within Germany you have to use the weak link that is specified in the LBA
approved flight manual of the glider. For aero tow to also have to use a
weak link in accordence to the LBA approved flight manual of the tow plane.
SOmetimes you need different weak links at the both ends, and even this
regulation is usefull. The weak linke to avaiod damge of the tow plane when
the rope catches some trees might have a much higher brake load than the
weak link at the glider, hat should ensure tha the structure of the glider
is not dammaged due to sudden movements of the tow plane.
: Do people stick to the rules?
People in general stick to this rule. Sometimes a weak link of the class
below or very seldom one class above is used.
: How do one decide on what strength weak link to use (winch/aerotow)?
The engineer at the glider manufacturer determins the weak link to be used.
=============================================================================
Dipl. Phys. Hans L. Trautenberg Universitaet Regensburg
Institut fuer Experimentelle und Angewandte Physik
phone (49) 941 943 2466 Polymerphysik
fax (49) 941 943 3196 D-93040 Regensburg
e-mail hans.tra...@physik.uni-regensburg.de Germany
privat
phone (49) 941 949211 Flurstr. 14
fax (49) 941 930792 D-92348 Berg
Germany
=============================================================================
Axel Morgenstjerne
pok...@puknet.puk.ac.za (Jonker, MP) writes:
> change the weak link for every glider? Surely if you use the same weak
> link for the heavy two seater full of water and the Ka-8 you could just
> as well use no weak link at all."
>
> What is the rules about weak links in your country?
> Do people stick to the rules?
> How do one decide on what strength weak link to use (winch/aerotow)?
In Denmark we use weak links as discribed by the instruction manual for the
gliders, with the following color codes (which I suppose are international):
Airtows:
Green: 400 kg
Winch launch:
White: 500 kg (Astir, ...)
Blue: 600 kg (K-8, Discus, ...)
Red: 750 kg (K-13, Cirrus VTC, ...)
Brown: 900 kg
Black:1000 kg (ASK-21, ...)
These are from memory, so please check your manual!
Anyway, in danish clubs we change to the right weak links prior to each
start, acording to the plane. The standard in my club is, that you can
choose between white, blue and red while we don't have brown and black.
The change of weak link is a very simple action, which takes only 1 second
for the system we use. At the same time the weak link can be inspected, so
you don't start with a damaged weak link.
In general it is my impression, that the rules are obeyed very carefully,
except most pilots prefears the blue to the white weak link for the Astir,
as the white link breaks too often and the Astir is belived (though it is
against the manual) to be strong enough for the blue - we have lost an Astir
due to broken white links (of course, it was just the initiation of the
accident), but never experienced any damage due to overload in the winch
launch using a blue weak link.
Axel Morgenstjerne, AEF, bygn.322, DTU, DK-2800 Lyngby, Danmark
tel:+45 4525 3450, fax:+45 4588 7133, privat:+45 4593 5510
Phil Irvine
pok...@puknet.puk.ac.za (Jonker, MP) wrote:
>I would like some comment on the following statement that one sometimes hear
>on glider fields and more often at the bar after a couple of beers:
>
>"Weak links is just a nuisance. They usually break at the most embarassing
>times. Perhaps you could use them for winch launching if you realy wishes,
>but on aerotowing the forces are so light there is no need for them. In any
>case how would you know how strong these things should be. Must we change
>the heavy two seater full of water and the Ka-8 you could just as well use
>no weak link at all."
>
>What is the rules about weak links in your country?
>Do people stick to the rules?
>How do one decide on what strength weak link to use (winch/aerotow)?
>
At my club, in Australia, we use aerotow launch. 500 Kg. (Yellow) TOST weak
links are used at the tug end of the rope. There are no weak links at the
glider end.
We do not have problems with breakage unless the load is really high, such as
the rope catching on the fence when the tug is landing (makes the tuggie
unpopular!) or a severe jerk on tow as a result of slack rope. Just the times
it should break for safety!.
Other clubs I have seen use sections of smaller rope as a weak link but I am
not convinced of the reliability of this. The TOST links are very dependable
but hard to get at times.
Phil Irvine
Phil Irvine
Geoff Butler
pok...@puknet.puk.ac.za "Jonker, MP" writes:
> I would like some comment on the following statement that one sometimes hear
> on glider fields and more often at the bar after a couple of beers:
>
> "Weak links is just a nuisance. They usually break at the most embarassing
> times. Perhaps you could use them for winch launching if you realy wishes,
> but on aerotowing the forces are so light there is no need for them. In any
> case how would you know how strong these things should be. Must we change
> the weak link for every glider? Surely if you use the same weak link for
> the heavy two seater full of water and the Ka-8 you could just as well use
> no weak link at all."
On a winch, the correct weak link is essential, because the forces
involved are high enough to overstress the airframe. Our club has a
list, which I assume was published by the BGA, of the correct links
to use for most aircraft flying in the UK, and we follow the rules
very carefully. We normally use three different weak link ratings.
Checking the link is part of the launch ritual ("cable attached and
secure, brown link").
Changing the link takes about five seconds. We mount each link in
an aluminium channel, and we have three different links permanently
attached to a shackle at the end of the shock rope. The strop is
attached to the required link by a piece of kit made by Tost,
consisting of an elongated ring with a flat on the long side and
a hook that slips over the flat.
Geoff Butler
Doug Haluza
>In article <pokm...@puknet.puk.ac.za>
>pok...@puknet.puk.ac.za "Jonker, MP" writes:
>
>> I would like some comment on the following statement that one sometimes
hear
>> on glider fields and more often at the bar after a couple of beers:
>>
>> "Weak links is just a nuisance. They usually break at the most
embarassing
>> times. Perhaps you could use them for winch launching if you realy
wishes,
>> but on aerotowing the forces are so light there is no need for them. In
any
>> case how would you know how strong these things should be. Must we
change
>> the weak link for every glider? Surely if you use the same weak link
for
>> the heavy two seater full of water and the Ka-8 you could just as well
use
>> no weak link at all."
When we revised the Long Island Soaring Association Handbook, we wrestled
with this problem too. The most importaant constraint for the weak link
used on aerotow is not overstressing the glider, but overstressing the
towplane. The Schweizer type hookup used on most towplanes in the US is
placarded for a max breaking strength of 1200 pounds. The procedure we came
up with is listed below:
3.15. Tow Rope Procedures
1. Tow ropes used for Club operations must comply with FAR 91.309(3):
· Minimum breaking strength is 80% of the maximum gross weight of the
glider.
· Maximum breaking strength is 200% of the maximum gross weight of the
glider.
· Maximum breaking strength is limited to 1200 pounds by the release
mechanism on the tow plane
2. The maximum gross weight range for Club gliders is:
· SGS 1-26E: 700 pounds--Maximum breaking strength: 1400 pounds.
· SGS 2-32: 1430 pounds--Minimum breaking strength 1144 pounds.
3. Tow ropes are rated on initial breaking strength. After the rope is
stressed, the breaking strength is reduced by as much as 25%. Therefore,
the following procedure will be used for Club operations:
· Tow ropes shall be made from 5/16" polypropylene hollow braided rope
having an initial breaking strength of approximately 1400 pounds. This rope
must be obtained from a source that advertises and regularly sells this
rope specifically for the purpose of towing gliders. An initial length of
250' of rope is to be cut from the reel. This will allow enough length to
terminate the ends with enough rope remaining to re-terminate the ends if
necessary.
· On initial use, the rope shall have a brand new weak link added to the
glider end to reduce the breaking strength. The weak link shall be made of
1/4" polypropylene hollow braided rope having an initial breaking strength
of approximately 1200 pounds.
· No additional weak link is required on the towplane end. The area where
the doubled rope tapers down to a single rope is the towplane end weak
link.
· The weak link shall be used for the first day’s flying, or approximately
ten tows. After this break-in time, the breaking strength of each rope has
been reduced.
4. After the break-in is complete, the weak link shall be removed and
destroyed (worn weak links have had their breaking strength reduced also
and no longer meet minimum strength requirements). After break-in, no weak
link is required for towing Club gliders.
5. A weak link may be required for towing other gliders under 700 lbs.
gross weight, such as a 1-26A. Before towing light weight gliders, check
with the private owner for weak link requirements.
6. The neck of a plastic bottle should be used to protect the glider end of
the tow rope and the tow ring. Worn tow ropes can be reused by
re-terminating the end of the rope to remove the worn portion. Ropes must
be discarded when less than 200 feet of usable length remains.
Doug Haluza
Mike Cohler
>I would like some comment on the following statement that one sometimes hear
>on glider fields and more often at the bar after a couple of beers:
>
>"Weak links is just a nuisance. They usually break at the most embarassing
>times. Perhaps you could use them for winch launching if you realy wishes,
>but on aerotowing the forces are so light there is no need for them. In any
On a normal aerotow if your are carrying a glider with a glide ratio of
about 30:1 with a glider weight of around 1000 lbs, then you can see that
for level flight the rope tension will be about 1000/30 = 35 lbs or so.
However if the glider were to make an excursion from normal flight and
let the rope go slack, but then the tug accelerates there can be a large
and sudden pull on the rope when the rope finally goes tight again. For
this reason there is normally a weak link of around 1000 lbs in an
aerotow line in the UK. It protects the situation where the glider
might pull the tug into an awkward flight attitude and protects the
glider from excessive structural forces. In all cases the weak link is
not required in normal smooth flight. If there is wave then there is a
greater chance of getting bows in the rope and sudden re-tensioning due
to the turbulence in the rotor, and generally at the lower levels.
>case how would you know how strong these things should be. Must we change
>the weak link for every glider? Surely if you use the same weak link for
>the heavy two seater full of water and the Ka-8 you could just as well use
>no weak link at all."
Each glider in the UK has a recommended strength of weak link for winch
operations, but in general the weak links are only usually of 3 different
strengths, from 1000 lbs to 2000 lbs to cope with the difference from the
light single seater to the heavy two-seaters. They are usually fairly
simple to change, especially if the system of using specially made rapid
attachment links are used (such as those by Tost).
The links are also there for safety. IN a winch launch there is an
increasing bending moment on the wing roots as you climb up the launch,
since the cable tension is increasingly becoming more perpendicular to
the flight path. ie the wings are required to provide more and more
lift to support not only the glider's weight but also the tension of the
wire in the direction opposite to the lift. So the effective "g"
loading on the aircraft becomes higher as you climb, and the wings are
pulling up, whilst the cable and weight are pulling down on the fuselage.
The points at which the bending moments are acting at their worst is the
wing root area. Hence it would be possible to pull back on the stick
sufficiently for the bending moments to exceed the structural limit of
the aircraft and a catastrophic failure of the main spars half way up a
winch launch would lead to almost inevitable death of the pilot. Hence
the weak link is absolutely vital for winch launching, and it MUST be the
correct breaking tension. In addition be aware that the stall speed
increases with the "g" load, so that near the top of a winch launch you
are flying extremely close to the stall. If you were to continue to add
"g" (which you do NOT feel because the pull of the wings is being
balanced largely by the cable tension) could alow the aircraft to stall
whilst under a large cable tension.... and a spin from an accelerated
stall on the wire is not a thought to relish.... so the weak link will
prevent this happening also, because by relieving the tension the stall
speed will decrease again before you reach the stall condition.
So make sure you fly the winch with the correct weak links in place....
it may save your life one day.
>
>What is the rules about weak links in your country?
>Do people stick to the rules?
In the UK we stick totally rigidly to the rules... because lives depend
on it.
>How do one decide on what strength weak link to use (winch/aerotow)?
See above. If you ask about a specific glider, someone on the net will
reply and tell you.
Mike
I Johnston
: In the UK we stick totally rigidly to the rules... because lives depend
: on it.
With a few exceptions: more than one club flies PZL Juniors off weak
links one stronger than the "official" one (blue instead of white, or
maybe the other way round) because the official ones break so frequently
that the CFI's in question have decided that the risk of repeated cable
breaks is greater than the risk of stronger weak links...
: >How do one decide on what strength weak link to use (winch/aerotow)?
: See above. If you ask about a specific glider, someone on the net will
: reply and tell you.
There is a Tost list which gives the links for a hundred or so types. If
the glider in question is not on the list then as I recall the Tost
recommendation is for 1.4 times the maximum AUW of the glider.
(presumably because the maximum wing loading is 2.4g, which still leaves
a margin for gusts before reaching the maximum).
Ian
PS I have flown at a club which uses a weak link on the aerotow rope ...
with a rope in parallel so that even if the weak link goes the rope
won't break. An interesting idea.
Robert Hawley
Jonker, MP (pok...@puknet.puk.ac.za) writes:
>
> What is the rules about weak links in your country?
> Do people stick to the rules?
Where I fly, we use a knot in the rope at the glider end. This takes
about 1/3 away from the strength of the rope at the knot. If the rope
breaks, it _should_ break without alot hanging onto the glider.
--
http://www.inasec.ca/hawley/rob.htm _____ / |
__(_o__) \______/--|-
o--------------{Rob Hawley}------------{________------------
Rich Carr
: Where I fly, we use a knot in the rope at the glider end. This takes
: about 1/3 away from the strength of the rope at the knot. If the rope
: breaks, it _should_ break without alot hanging onto the glider.
In spite of the fact that, like most US clubs, mine uses rope weak
links, I've always thought it a poor method due to the
rapidly-changing strength of the rope with abrasion, exposure to UV,
and general degradation with load cycles. Using a knot in a rope
where it becomes the focus of abrasion seems singularly bad to me.
- Rich Carr
Rich Carr
: There is a Tost list which gives the links for a hundred or so types.
Before relying on the following, consider how many typographical
errors I made in typing it in. Consult your owner's manual for The
Truth.
I have also made this list available at
http://www.csn.net/~rjc/tostlink.html.
- Rich Carr
-------------
B.G.A. WINCH/AUTO TOW WEAK LINKS
Revised April 1991 From TOST DATA SHEET 2/4/90
With Amendment As Authorized by B.G.A.
TOST COLOR CODING
Black No. 1 1000 daN 2200 lbs
Brown No. 2 850 1870
Red No. 3 750 1650
Blue No. 4 600 1320
White No. 5 500 1100
If in doubt, apply a factor of 1.3 x max all-up
weight of glider to determine weak link
strength for winch/auto tow.
Astir Single 500 No. 5
Astir Twin 845 No. 3
ASH 25 900 No. 2
ASK 14 830 No. 3
ASW 15 500 No. 5
ASW 17 600 No. 4
ASW 19 600 No. 4
ASW 20 600 No. 4
ASK 21 1000 No. 1
ASK 22 900 No. 2
ASK 23 680 No. 4
ASK 24 600 No. 4
AV.36 600 No. 4
Austria Std 670 No. 4
BergFalke 2 970 No. 2
BergFalke 3 1070 No. 1
BergFalke 4 750 No. 3
Bijave (WA30) 600 No. 4
Blanik 630 No. 4
Bocians 1000 No. 1
Bruguet 905 600 No. 4
BG. 135 600 No. 4
Cadet Mk1 & 2 500 No. 5
Cadet Mk3 (T31) 500 No.5
Caproni A21 600 No. 4
Capstan 600 No. 4
Carman JP15 600 No. 5
Centrair 101 600 No. 4
Cirrus 860 No. 2
Cirrus Std 500 No. 5
Cumulus 540 No. 5
Cobra 600 No. 4
Condor 1000 No. 1
Dart 15/17 500 No. 5
Delphin 700 No. 4
Diamant 16.5/18 935 No. 2
Discus 650 No. 4
DG100/200 500 No. 5
DG 400 500 No. 5
DG 300/600 680 No. 4
Doppleraab 800 No. 3
Eagle 600 No. 4
EON Primary 500 No. 5
EON Baby 600 No. 4
ELF S2 540 No. 5
Falcon 500 No. 5
Fauvel 500 No. 5
Fauvette 905 500 No. 5
FOKA 3/4/5 720 No. 4
Geier II 765 No. 3
Glasflugel 604 850 No. 2
Goevier III 1030 No. 1
Grunau /5 540 No. 4
Gull 1/3/4 500 No. 5
Harbinger 500 No. 5
Hornet 500 No. 5
Hutter 17 500 No. 5
Iris (D77) 500 No. 5
IS 28B2 600 No. 4
IS 29/30/32 500 No. 5
Jantar Std 530 No. 5
Jantar 2 600 No. 4
Jantar 3 600 No. 4
Janus B 600 No. 4
Janus C 750 No. 3
Jaskolka 500 No. 5
Javelot 500 No. 5
Junior 500 No. 5
JP 36A 500 No. 5
Ka 1/3 450 No. 6
Ka 2 600 No. 4
Ka 4 900 No. 2
Ka 6 650 No. 4
Ka 7 1080 No. 1
Ka 8 668 No. 4
Ka 13 1080 No. 1
Kestrel 17/19 630 No. 4
Kite 1.2B 500 No. 5
Kranich II/III 960 No. 2
Kranjanek 500 No. 5
LAK 12 600 No. 4
Libelle 201 500 No. 5
Libelle H301 670 No. 4
LS 1 500 Nol 5
LS 3 600 No. 4
LS 4 600 No. 4
LS 6 600 No. 4
LS 7 600 No. 4
LO-100 650 No. 4
M 100 500 No. 5
M 200 600 No. 4
Meise 670 No. 4
MG 19A 950 No. 2
Mosquito 650 No. 4
Moswey 650 No. 4
Minimoa 500 No. 5
Mucha Std 820 No. 3
MU 13 535 No. 5
Nimbus 2 600 No. 2
Nimbus 3 750 No. 3
Nimbus 3/24 1040 No. 1
Nimbus 3D 1040 No. 1
Nimbus-Mini 600 No. 4
Olympia 1/2 500 No. 5
Olympia 460/463 500 No. 5
Olympia 419 600 No. 4
Peak 100 600 No. 4
Ptrel 500 No. 5
Phoebus (all) 1000 No. 1
PIK 20E 600 No. 4
PIK 16/20 530 No. 5
Pilatus B4 500 No. 5
Pirat 600 No. 4
Prefect 500 No. 5
Puchacz 750 No. 3
Rheinland 500 No. 5
Rhonlander 2 500 No. 5
Rhonlerche 2 900 No. 2
Rhonsperber 500 No. 5
Sagitta 600 No. 4
SB 5 600 No. 4
SF 26 650 No. 4
SF 27A 750 No. 3
SF 34 600 No. 4
SG 38 300 No. 7
SHK 700 No. 4
SIE 3 700 No. 4
Silene (E 78) 600 No. 4
Sky 500 No. 5
Skylark 1/2/3/4 500 No. 5
Spatz 520 No. 5
Sperber 1030 No. 1
Suid III 500 No. 5
Swallow 500 No. 5
Swift 500 No. 5
T21 500 No. 5
T31 500 No. 5
T53/YS53 750 No. 3
Torva 500 No. 5
Tutor 500 No. 5
Vega 600 No. 4
Ventus 650 No. 4
Viking (VGC) 500 No. 5
WassamerWA26 500 No. 5
Weihe 670 No. 4
Zugvogel 1/2 720 No. 4
Zugvogel 3 742 No. 4
Zugvogel 4 690 No. 4
John Giddy
> PS I have flown at a club which uses a weak link on the aerotow rope ...
> with a rope in parallel so that even if the weak link goes the rope
> won't break. An interesting idea.
>
John G.
I Johnston
Search me. I think the argument goes something like...
"The parallel rope (which is much thinner than the tow rope) is the real
weak link. However, to stop it failing from abrasion we use a tost weak
link set to tighten first. That is prone to fatigue failure, so if
it goes for no good reason the rope will take over. In the event of a
real violent bang both will break anyway."
Personally I think it's full of holes and I stress that I am not putting
this side of the argument myself, merely repeating what I heard.
Ian
Jeffrey M. Matthews
Phil Irvine <p.ir...@syd.dwt.csiro.au> wrote:
>
>Other clubs I have seen use sections of smaller rope as a weak link but I am
>not convinced of the reliability of this. The TOST links are very dependable
>but hard to get at times.
>
I've used both smaller rope weak links and knots. We broke many of the
former, and a couple of the latter, without damaging either the glider or
the towplane. It's quite imprecise, to be sure. But it gets the job done.
We did a lot of vigorous slack rope where I started flying gliders down in
Georgia, using 1/4 inch water-ski rope. We broke many of them doing slack
rope exercises, but never _ever_ had one break when we weren't deliberately
abusing it. Even when they got pretty ratty looking, they were completely
dependable. We towed everything from 1-26C's to 2-32's with three aboard
with the same rope. Perhaps a new rope was a bit too strong for the 1-26,
or a ratty one a bit too weak for the Tank, when taken strictly by the book,
but within practical limits, it worked well.
I suspect that using accurately calibrated weak links may give a level of
precision which isn't needed. But then, Schweizers were built pretty
tough (same people built Ag-Cats, and you know what their reputation is),
and some of the "other" ships might not be as tolerant of abuse.
Another poster alluded to protecting the towplane from unusual attitudes
caused by out-of-position gliders. I seriously doubt that the weak link
ever has that effect. It takes far less force to overpower the towplane's
controls than to break even the weakest rope. The only way the towplane
can remain controllable is if the glider pilot stays in a reasonable pos-
ition--doesn't have to be precise (though that's nice), but there are
limits beyond which the tow pilot absolutely can't keep the tug under con-
trol.
The glider has a huge leverage advantage over the towplane, since the re-
lease on the glider is much closer to the aerodynamic center of the wing
than the tail assembly. On the tug, it's 1:1 because the release and the
tail are pretty much in the same place. Even on the Schweizers with the
release in the nose, there's about a 4:1 mechanical advantage. On some
ships with cg releases, it's probably 10:1 or higher. On every takeoff
the glider pilot has the tow pilot's life in his hands.
Jeff Matthews
Doug Haluza
'jmma...@apgea.army.mil (Jeffrey M. Matthews)' wrote:
>Another poster alluded to protecting the towplane from unusual attitudes
>caused by out-of-position gliders. I seriously doubt that the weak link
>ever has that effect. It takes far less force to overpower the towplane's
>controls than to break even the weakest rope. The only way the towplane
>can remain controllable is if the glider pilot stays in a reasonable pos-
>ition--doesn't have to be precise (though that's nice), but there are
>limits beyond which the tow pilot absolutely can't keep the tug under con-
>trol.
--
I don't recall seeing a posting referring to upset (could be my news server
dropped it). If you were referring to my earlier posting, let me clarify
it. The Schweizer style tow hook installed on the towplane is placarded for
a maximum breaking strength of 1200 pounds. You cannot legally exceed this.
This is not to protect the towplane against upset. A hundred pounds or so
of up force on the tail would exceed the elevator authority of the
towplane. This cannot be avoided with a practical weak link.
The real problem is overstressing the tail structure of the towplane. The
structure was designed to handle aerodynamic and landing loads only. Since
the landing loads can be quite high, the vertical strength of the tail
structure is good. But a heavy side pull on the towplane could damage the
airframe. Note how much taller than wide the tail structure is on a Super
Cub or similar towplane. We found a broken longeron near the tail of our
Super Cub and this was probably caused by towing loads (you might want to
pay particular attention to the tail structure on your towplane
inspections).
Doug Haluza
Ken Kochanski
>
> On every takeoff the glider pilot has the tow pilot's life in his hands.
>
In January 1989, I and other club members watched one of our pilots die
in an accident towing a 1-26. The mid-winter weekend day was wonderfully
warm (mid 60's) and the airport was packed with tourists and club
members. The 1-26 was a little wild as it lifted off the ground, but the
pilot regained enough control to start the climb out. At 200- 300 feet
it was apparent to the observers that the glider pilot was having
problems. The tow pilot was apparently also aware of the problem,
because he turned back over the field ... we assume to get the glider
over the airport so it could release and land safely. However, the
glider hung on and the L-19 again traveled the length of the airport and
began a turn with glider in tow. As we watched, the L-19 just
snap-rolled and accelerated into the ground. We'll never really know
what happened, but many observers think the 1-26 was high and off to the
side ... and if so, then the tow pilot probably had the stick back in his
lap, and was probably standing on the rudder. This of course is the
perfect setup for a snap-roll, and that's exactly what happened when the
rope released. Nobody actually knows who released ... or saw the rope
release ... it landed close to the back of the L-19, but was not actually
attached. I read parts of the NTSB report and it put blame on both
pilots. Blame won't bring our friends back ... or prevent this from
happening again.
This is not happy stuff :-( and I was inclined to cancel this post, but I
knew the tow pilot well, I talk to his mom and dad now and then, and I
hope this account just highlights (again) the life and death power that
the glider pilot has over the tow pilot.
KK