Nimbus 4DT accident 31 July 2000 in Spain.

On Tue, 5 Jul 2005 16:57:36 UTC, "F.L. Whiteley"
wrote:

NDT would work if you jigged up an accurate test bed. I would be more
concerned with rope weak links used in aerotowing than TOST weak links if I
wanted to test something. From many years of observation I'm pretty
confident that the TOST links perform as expected.

Me too. Tost aerotow weak links are pretty good as well - I thought
just about everyone had gone over to them in place of rope. Mind you,
I knew one club which put a good stout length of rope in parallel with
the Tost link "because they are so expensive if they break and we lose
half the connector."

Ho hum.

Ian


--

"Ian Johnston" wrote in message
news:dzZo7CxomoOm-pn2-1IU6gvmrdIOd@localhost...
On Tue, 5 Jul 2005 15:59:24 UTC, M B
wrote:

Is there any commonly known way to test a weak
link non-destructively (other than launching a glider)?

The Tost system uses two in parallel, one slightly longer than the
other. Whoever hooks up the glider should always check that both links
are intact and that only one is taking the load.

Ian

To expand on what Ian wrote, the parallel weak-link system places only the
primary weak-link under load with a unloaded spare in place to take the load
should the first one fail below its rated strength as a result of fatigue
from repeated load cycles. If the failure is due to an overload, the spare
will fail a millisecond after the primary and the glider will be protected
from excessive cable force.

If the person hooking up the glider to the winch line shows these weak links
to the pilot, a crack in the primary will be obvious and can be quickly
replaced with a new one of the correct strength for the glider.

The Tost weak-link system is very well thought out and correct weak-link
color is universally referenced in German glider handbooks. It should be
used in every winch operation. I find it very comforting to know that the
glider will be protected from excessive loads while the weak-link itself is
backed up by a full strength spare.

Using either over or understrength weak-links is very hazardous.

Bill Daniels

On 5 Jul 2005 17:11:09 GMT, "Ian Johnston"
wrote:

Secondly, the point is not to get the weak link breaking at all. As long as
you stay within the limits of max winch tow speed, the overload is zero.

Except you can stay within that limit with 250dN load more on the
winch hook and the wing attachments than the designers of the aircraft
thought safe.

As Bert already stated: As long as you stay within the safe speed
range, there is no way to exceed the stress limits of the glider -
simply there isn't enough lift available.

Besides: I've never seen an ASW-20 break a weak link on my home
airfield. We were using the blue ones for the 20.

Would you fly with 250kg of unofficial extra ballast in the fuselage?
That's not the point since this "unofficial ballast" never shows up if
the speed is kept in the safe range.

One example of how a designer got it wrong is the SF-34:
Officially the only allowed weak link is the blue one. Unfortunately
with this weak link it is nearly impossible to complete a winch launch
- the weak link fails in the moment the glider starts to accelerate.
Solution: a stronger weak link, and careful speed control.



Bye
Andreas

Ian Johnston wrote:

On Tue, 5 Jul 2005 16:57:36 UTC, "F.L. Whiteley"
wrote:

NDT would work if you jigged up an accurate test bed. I would be more
concerned with rope weak links used in aerotowing than TOST weak links if
I
wanted to test something. From many years of observation I'm pretty
confident that the TOST links perform as expected.

Me too. Tost aerotow weak links are pretty good as well - I thought
just about everyone had gone over to them in place of rope. Mind you,
I knew one club which put a good stout length of rope in parallel with
the Tost link "because they are so expensive if they break and we lose
half the connector."

Ho hum.

Ian


--
I don't recall having seen a TOST aerotow weak link in use in the US.
Perhaps some are using them.

Frank

Ian Johnston wrote:
On Tue, 5 Jul 2005 10:54:57 UTC, Bill Gribble
wrote:


As far as character is concerned, I'm pretty certain the last time I
launched an ASH25 (as signaller, at least) it was on a black link. Or it
would have been if that was what the pilot had asked for.


Would you launch a K8 on a black link if the pilot requested it? In
the old days, would you have signalled "all out" to the winch driver
if the pilot requested it and despite open brakes? Would you launch a
glider with a faulty back-release if the pilot said "Oh, that's OK,
just launch me."

I don't think /anyone/ on an airfield is entitled to overlook a clear
safety risk of this sort.

Ian

Experience at two clubs - one uses weak links, other not.

Tost weak links are expensive ,and difficult to come by in our backwater. But
both clubs have pretty much the same cost on weak links.

We launch our entire fleet on the Red link (although the book says some of our
ships are OK for Black)

In the last three yeast we have not had a single weak link failure. In both
cases the wire used is 1930Mpa class C, put a knot in it (unavoidable as it has
to connect to the parachute) and you have an automatic reduction in strength to
under the strength of a black link.

At the other club an engineer member with an enquiring mind put the various tost
links specified for the club and private fleet on a tensiometer attached to a
piece of wire with the standard four turn knot in it. Even the blue link
survived the test. Conversely, there were a number of dangerous launch failures
with the Twin Astir breaking weak links.
Subsequently said club has steadfastly refused to use weak links, for many
thousands of launches - with a powerful winch.

Bottom line is , if you keep the speed in the correct range, and have a sane
cable strength, you are unlikely to need a weak link. By the time the weak link
/ cable breaks your structure has already transmitted the load.

Personally I prefer having a weak link in place, but I don't really believe that
it helps much.

--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.

"F.L. Whiteley" wrote in message
...
Ian Johnston wrote:

On Tue, 5 Jul 2005 16:57:36 UTC, "F.L. Whiteley"
wrote:

NDT would work if you jigged up an accurate test bed. I would be more
concerned with rope weak links used in aerotowing than TOST weak links
if
I
wanted to test something. From many years of observation I'm pretty
confident that the TOST links perform as expected.

Me too. Tost aerotow weak links are pretty good as well - I thought
just about everyone had gone over to them in place of rope. Mind you,
I knew one club which put a good stout length of rope in parallel with
the Tost link "because they are so expensive if they break and we lose
half the connector."

Ho hum.

Ian


--
I don't recall having seen a TOST aerotow weak link in use in the US.
Perhaps some are using them.

Frank

There's no (legal) reason not to use the Tost weak-links for air tow in the
USA as long as the strength satisfies the FAR 91.309 80%-200% rule. (Note
that 200% of the MTOW allowed by that FAR is generally much stronger than
the manufacturer's mandated winch weak-link strength.)

The POH for my Nimbus 2C states that the weak-link for air tow is the same
(92% of MTOW) as for winch launch. It's interesting to note that S-H
specifies the weak-link strength for air tow just as tightly as for winch
which may imply that the designers concern was not the wing but the tow hook
mounting structure.

Bill Daniels

On Tue, 5 Jul 2005 18:05:20 UTC, "Bill Daniels"
wrote:

If the person hooking up the glider to the winch line shows these weak links
to the pilot, a crack in the primary will be obvious and can be quickly
replaced with a new one of the correct strength for the glider.

It's also easy to check that under light tension one link is taking
the load and one is free to rattle around, which is an easy way to
make sure that the primary link hasn't stretched, or got elongated
holes. The downside is that the metal holder for the links does tend
to get rammed full of mud on grass airfields.

Ian


--

On Tue, 5 Jul 2005 18:24:21 UTC, (Andreas Maurer)
wrote:

As Bert already stated: As long as you stay within the safe speed
range, there is no way to exceed the stress limits of the glider -
simply there isn't enough lift available.

Um, must check, but am pretty sure angle of attack influences lift as
well...

Besides: I've never seen an ASW-20 break a weak link on my home
airfield. We were using the blue ones for the 20.

So why would anyone fly with a 33% over strength link?

Would you fly with 250kg of unofficial extra ballast in the fuselage?
That's not the point since this "unofficial ballast" never shows up if
the speed is kept in the safe range.

Unless the glider hits a gust, or the pilot pulls back too hard, or
the throttle of the winch suddenly shoots open or ...

One example of how a designer got it wrong is the SF-34:
Officially the only allowed weak link is the blue one. Unfortunately
with this weak link it is nearly impossible to complete a winch launch
- the weak link fails in the moment the glider starts to accelerate.
Solution: a stronger weak link, and careful speed control.

It's been ages since I did a winch launch in an SF34 and I really
can't remember much about it, but surely in that case the manufacturer
should be approached about changing the link officially, and issuing a
strengthening modification if required?

I wonder if Centrair changed this on the Alliance 34? I must check the
handbook - we have one at my current (all aerotow!) club.

Ian
--

If you fly at max allowed speed at Ca(max), even a gust cannot produce more
lift.

Just to get your numbers straightened out: The wing attachement is designed
for a maximum load of 5.3g, that equivalents in the case of an ASW20 beyond
1000kg of non lift producing parts (which are around 200 kg of mass).

Ad it's pretty clear that if you don't use a weak link, you need to watch
your speed _and_ your angle of attack.


--
Bert Willing

ASW20 "TW"


"Ian Johnston" a écrit dans le message de news:
dzZo7CxomoOm-pn2-sdKTFDfjQBvY@localhost...
On Tue, 5 Jul 2005 18:24:21 UTC, (Andreas Maurer)
wrote:

As Bert already stated: As long as you stay within the safe speed
range, there is no way to exceed the stress limits of the glider -
simply there isn't enough lift available.

Um, must check, but am pretty sure angle of attack influences lift as
well...

Ian Johnston writes
Would you launch a K8 on a black link if the pilot requested it? In the
old days, would you have signalled "all out" to the winch driver if the
pilot requested it and despite open brakes? Would you launch a glider
with a faulty back-release if the pilot said "Oh, that's OK, just
launch me."

No, absolutely not. And I agree with you, to a point; I would consider
it a moral obligation to make my views known and then not participate in
a launch (or any other aspect of the flying operation) that I considered
unsafe.

I frequently fly a Ka8, so know that it takes a blue link. If somebody
asked for a black, I'd laugh at them and, on realising that they were
serious, refuse.

If I can see the airbrakes are unlocked, of course I'm going to stop the
launch, at it happens, irrespective of my role (or otherwise) at the
launch point.

Faulty back release? Again, being aware of it, no.

ASH25 on a black link? I wouldn't know any better, if that's what the
pilot asked for. Knowing now that it should be a brown link I'll likely
mention it to the pilot concerned if I come across such a situation in
the future, but if the pilot insists on black, I suspect I'd defer to
his judgement in such an instance and use black.

A black link instead of brown is a whole different magnitude to black
instead of blue, or open brakes or an obvious and known fault with the
back release.

I don't think /anyone/ on an airfield is entitled to overlook a clear
safety risk of this sort.

Obviously safety on the airfield is paramount and the responsibility of
all. The main point of my original answer to your "character test" was
to observe that, in our operation at least, the winch driver wouldn't be
aware of which link was being used at the other end of the field.
Moreover, the launch marshal would, in many instances, be reliant upon
the pilot to identify the correct link required.


--
Bill Gribble
http://www.scapegoatsanon.demon.co.uk
- Learn from the mistakes of others.
- You won't live long enough to make all of them yourself.