tow hook limitation question

[Chris]

Our club is looking at new tow plane. The plane has a Schweizer tow
hook with "1200 lbs max" stamped on it.
Does anybody know what that relates to?
Is this the max gross weight of the glider or max tow tension or...?
I've tried Schweizer, but they couldn't help since they are no longer
making tow hooks.

Thanks for any insights.

Chris

[Jeremy Zawodny]

Chris wrote:
> Our club is looking at new tow plane. The plane has a Schweizer tow
> hook with "1200 lbs max" stamped on it.
> Does anybody know what that relates to?

Max weight of the two pilot? :-)

Jeremy

[Bill Daniels]

If you check glider manufacturers weak link requirements as listed in the
respective POH's, you will find almost all are stronger than the maximum
lond permitted on a Schweizer tug hook. Overload a Schweizer hook and it
jams. To me, that says that all Schweizer tug hooks are dangerously
obsolete and should be replaced with Tost hooks - preferably with the tow
rope retract winch.

Bill Daniels

"Chris" > wrote in message
oups.com...
> Our club is looking at new tow plane. The plane has a Schweizer tow
> hook with "1200 lbs max" stamped on it.
> Does anybody know what that relates to?
> Is this the max gross weight of the glider or max tow tension or...?
> I've tried Schweizer, but they couldn't help since they are no longer
> making tow hooks.
>
> Thanks for any insights.
>
> Chris
>

[BT]

It is not the max weight of the glider, but the max tension applied to the
hook.
That said, the "weak link" should not be stronger than 1200#.
It does not take 1200# of "pull" to tow a 1200# glider.

Cindy has the formula.. and I'm looking for my reference.

BT

"Chris" > wrote in message
oups.com...
> Our club is looking at new tow plane. The plane has a Schweizer tow
> hook with "1200 lbs max" stamped on it.
> Does anybody know what that relates to?
> Is this the max gross weight of the glider or max tow tension or...?
> I've tried Schweizer, but they couldn't help since they are no longer
> making tow hooks.
>
> Thanks for any insights.
>
> Chris
>

[Soarin Again]

Bill's data is a bit skewed. The Discus is probably
the most common single place glider in the U.S. I
have flight manuals for both the Discus a and b as
well as a Discus 2a and b in front of me. Per the
mfg specs both manuals list the minimum weak link as
1157 lbs.
His other claim that exceeding the 1200 lb limit will
cause the schweizer release to jam is highly suspect.
The vast majority of tows in the U.S. are done with
schweizer releases with little if any actual complaints
by tow pilots. Several years ago in soaring magazine
there was an article about a glider kiting and the
tow pilot was unable to release. However the cause
was not that the schweizer release jammed, it was the
result of a poorly installed and secured release cable
housing. The cable ties used to secure the release
cable housing to the airframe failed, which then resulted
in excessive slack in the release cable. This excessive
slack meant that the tow pilots release handle was
incapable of pulling on the schweizer release lever.

At 16:30 30 January 2007, Bill Daniels wrote:
>If you check glider manufacturers weak link requirements
>as listed in the
>respective POH's, you will find almost all are stronger
>than the maximum
>lond permitted on a Schweizer tug hook. Overload a
>Schweizer hook and it
>jams. To me, that says that all Schweizer tug hooks
>are dangerously
>obsolete and should be replaced with Tost hooks - preferably
>with the tow
>rope retract winch.
>
>Bill Daniels
>
>'Chris' wrote in message
oups.com...
>> Our club is looking at new tow plane. The plane has
>>a Schweizer tow
>> hook with '1200 lbs max' stamped on it.
>> Does anybody know what that relates to?
>> Is this the max gross weight of the glider or max
>>tow tension or...?
>> I've tried Schweizer, but they couldn't help since
>>they are no longer
>> making tow hooks.
>>
>> Thanks for any insights.
>>
>> Chris
>>
>
>
>

[Bill Daniels]

OK, I'll accept that a Discus single place glider's minimum weak link
strength is 43 pounds less than the maximum rated strength for a Schweizer
hook - but I won't take much comfort from it. A typical 2-seater is a Grob
103 with a reccomended weak link of 1650 pounds.

Reccomended weak link strengths range up to 1000 kg force or 2200 pounds
force. A common safety factor is 50% which brings it to 3300 pounds or more
than 2.5 times the rated strength of the Schweizer hook.

I think (but I don't know) the Schweizer hook wouldn't actually break at
these loads but I suspect that the manufacturer had a good reason to put a
1200 pound limit on it. You don't size a tow hook for the average
glider, you size it for the maximum loads it will see in normal service plus
a safety factor. Common sense and good practice says don't exceed it. I'll
stand by my advice - get a Tost hook.

Bill Daniels

"Soarin Again" > wrote in message
...
> Bill's data is a bit skewed. The Discus is probably
> the most common single place glider in the U.S. I
> have flight manuals for both the Discus a and b as
> well as a Discus 2a and b in front of me. Per the
> mfg specs both manuals list the minimum weak link as
> 1157 lbs.
> His other claim that exceeding the 1200 lb limit will
> cause the schweizer release to jam is highly suspect.
> The vast majority of tows in the U.S. are done with
> schweizer releases with little if any actual complaints
> by tow pilots. Several years ago in soaring magazine
> there was an article about a glider kiting and the
> tow pilot was unable to release. However the cause
> was not that the schweizer release jammed, it was the
> result of a poorly installed and secured release cable
> housing. The cable ties used to secure the release
> cable housing to the airframe failed, which then resulted
> in excessive slack in the release cable. This excessive
> slack meant that the tow pilots release handle was
> incapable of pulling on the schweizer release lever.
>
> At 16:30 30 January 2007, Bill Daniels wrote:
>>If you check glider manufacturers weak link requirements
>>as listed in the
>>respective POH's, you will find almost all are stronger
>>than the maximum
>>lond permitted on a Schweizer tug hook. Overload a
>>Schweizer hook and it
>>jams. To me, that says that all Schweizer tug hooks
>>are dangerously
>>obsolete and should be replaced with Tost hooks - preferably
>>with the tow
>>rope retract winch.
>>
>>Bill Daniels
>>
>>'Chris' wrote in message
oups.com...
>>> Our club is looking at new tow plane. The plane has
>>>a Schweizer tow
>>> hook with '1200 lbs max' stamped on it.
>>> Does anybody know what that relates to?
>>> Is this the max gross weight of the glider or max
>>>tow tension or...?
>>> I've tried Schweizer, but they couldn't help since
>>>they are no longer
>>> making tow hooks.
>>>
>>> Thanks for any insights.
>>>
>>> Chris
>>>
>>
>>
>>
>
>
>

[BT]

Bill, I missed where the Grob 103 "recommended" weak link is 1650 lbs.
Our Grob 103 POH does say "max" rating of 1662 lbs. at Max Gross weight of
1279 lbs, perhaps we have different models of the G-103?

Based on FARs, 80-200% factor for weak links, that makes it as low as 1032
lbs or as high as 2558 lbs, much higher than Grob recommends.

We run a weak link that is "about" 1200 lbs when brand new, after a few
flights it is lower from initial use. This allows us to tow most everything
on the same weak link material. Yes we have them built up for both Tost and
Schweizer rings.

Have you seen an approved STC or Form 337 to put a TOST tow assembly on a
Pawnee?
I would be interested, as our last Schweizer hook wears out.

BT

"Bill Daniels" <bildan@comcast-dot-net> wrote in message
. ..
> OK, I'll accept that a Discus single place glider's minimum weak link
> strength is 43 pounds less than the maximum rated strength for a Schweizer
> hook - but I won't take much comfort from it. A typical 2-seater is a
> Grob 103 with a reccomended weak link of 1650 pounds.
>
> Reccomended weak link strengths range up to 1000 kg force or 2200 pounds
> force. A common safety factor is 50% which brings it to 3300 pounds or
> more than 2.5 times the rated strength of the Schweizer hook.
>
> I think (but I don't know) the Schweizer hook wouldn't actually break at
> these loads but I suspect that the manufacturer had a good reason to put a
> 1200 pound limit on it. You don't size a tow hook for the average
> glider, you size it for the maximum loads it will see in normal service
> plus a safety factor. Common sense and good practice says don't exceed
> it. I'll stand by my advice - get a Tost hook.
>
> Bill Daniels
>
> "Soarin Again" > wrote in message
> ...
>> Bill's data is a bit skewed. The Discus is probably
>> the most common single place glider in the U.S. I
>> have flight manuals for both the Discus a and b as
>> well as a Discus 2a and b in front of me. Per the
>> mfg specs both manuals list the minimum weak link as
>> 1157 lbs.
>> His other claim that exceeding the 1200 lb limit will
>> cause the schweizer release to jam is highly suspect.
>> The vast majority of tows in the U.S. are done with
>> schweizer releases with little if any actual complaints
>> by tow pilots. Several years ago in soaring magazine
>> there was an article about a glider kiting and the
>> tow pilot was unable to release. However the cause
>> was not that the schweizer release jammed, it was the
>> result of a poorly installed and secured release cable
>> housing. The cable ties used to secure the release
>> cable housing to the airframe failed, which then resulted
>> in excessive slack in the release cable. This excessive
>> slack meant that the tow pilots release handle was
>> incapable of pulling on the schweizer release lever.
>>
>> At 16:30 30 January 2007, Bill Daniels wrote:
>>>If you check glider manufacturers weak link requirements
>>>as listed in the
>>>respective POH's, you will find almost all are stronger
>>>than the maximum
>>>lond permitted on a Schweizer tug hook. Overload a
>>>Schweizer hook and it
>>>jams. To me, that says that all Schweizer tug hooks
>>>are dangerously
>>>obsolete and should be replaced with Tost hooks - preferably
>>>with the tow
>>>rope retract winch.
>>>
>>>Bill Daniels
>>>
>>>'Chris' wrote in message
oups.com...
>>>> Our club is looking at new tow plane. The plane has
>>>>a Schweizer tow
>>>> hook with '1200 lbs max' stamped on it.
>>>> Does anybody know what that relates to?
>>>> Is this the max gross weight of the glider or max
>>>>tow tension or...?
>>>> I've tried Schweizer, but they couldn't help since
>>>>they are no longer
>>>> making tow hooks.
>>>>
>>>> Thanks for any insights.
>>>>
>>>> Chris
>>>>
>>>
>>>
>>>
>>
>>
>>
>
>

[Brian Glick]

Talk to Tom Knauff, he recently put a Tost on his Pawnee and Scout



"BT" > wrote in message
...
> Bill, I missed where the Grob 103 "recommended" weak link is 1650 lbs.
> Our Grob 103 POH does say "max" rating of 1662 lbs. at Max Gross weight of
> 1279 lbs, perhaps we have different models of the G-103?
>
> Based on FARs, 80-200% factor for weak links, that makes it as low as 1032
> lbs or as high as 2558 lbs, much higher than Grob recommends.
>
> We run a weak link that is "about" 1200 lbs when brand new, after a few
> flights it is lower from initial use. This allows us to tow most
> everything on the same weak link material. Yes we have them built up for
> both Tost and Schweizer rings.
>
> Have you seen an approved STC or Form 337 to put a TOST tow assembly on a
> Pawnee?
> I would be interested, as our last Schweizer hook wears out.
>
> BT
>
> "Bill Daniels" <bildan@comcast-dot-net> wrote in message
> . ..
>> OK, I'll accept that a Discus single place glider's minimum weak link
>> strength is 43 pounds less than the maximum rated strength for a
>> Schweizer hook - but I won't take much comfort from it. A typical
>> 2-seater is a Grob 103 with a reccomended weak link of 1650 pounds.
>>
>> Reccomended weak link strengths range up to 1000 kg force or 2200 pounds
>> force. A common safety factor is 50% which brings it to 3300 pounds or
>> more than 2.5 times the rated strength of the Schweizer hook.
>>
>> I think (but I don't know) the Schweizer hook wouldn't actually break at
>> these loads but I suspect that the manufacturer had a good reason to put
>> a 1200 pound limit on it. You don't size a tow hook for the average
>> glider, you size it for the maximum loads it will see in normal service
>> plus a safety factor. Common sense and good practice says don't exceed
>> it. I'll stand by my advice - get a Tost hook.
>>
>> Bill Daniels
>>
>> "Soarin Again" > wrote in message
>> ...
>>> Bill's data is a bit skewed. The Discus is probably
>>> the most common single place glider in the U.S. I
>>> have flight manuals for both the Discus a and b as
>>> well as a Discus 2a and b in front of me. Per the
>>> mfg specs both manuals list the minimum weak link as
>>> 1157 lbs.
>>> His other claim that exceeding the 1200 lb limit will
>>> cause the schweizer release to jam is highly suspect.
>>> The vast majority of tows in the U.S. are done with
>>> schweizer releases with little if any actual complaints
>>> by tow pilots. Several years ago in soaring magazine
>>> there was an article about a glider kiting and the
>>> tow pilot was unable to release. However the cause
>>> was not that the schweizer release jammed, it was the
>>> result of a poorly installed and secured release cable
>>> housing. The cable ties used to secure the release
>>> cable housing to the airframe failed, which then resulted
>>> in excessive slack in the release cable. This excessive
>>> slack meant that the tow pilots release handle was
>>> incapable of pulling on the schweizer release lever.
>>>
>>> At 16:30 30 January 2007, Bill Daniels wrote:
>>>>If you check glider manufacturers weak link requirements
>>>>as listed in the
>>>>respective POH's, you will find almost all are stronger
>>>>than the maximum
>>>>lond permitted on a Schweizer tug hook. Overload a
>>>>Schweizer hook and it
>>>>jams. To me, that says that all Schweizer tug hooks
>>>>are dangerously
>>>>obsolete and should be replaced with Tost hooks - preferably
>>>>with the tow
>>>>rope retract winch.
>>>>
>>>>Bill Daniels
>>>>
>>>>'Chris' wrote in message
oups.com...
>>>>> Our club is looking at new tow plane. The plane has
>>>>>a Schweizer tow
>>>>> hook with '1200 lbs max' stamped on it.
>>>>> Does anybody know what that relates to?
>>>>> Is this the max gross weight of the glider or max
>>>>>tow tension or...?
>>>>> I've tried Schweizer, but they couldn't help since
>>>>>they are no longer
>>>>> making tow hooks.
>>>>>
>>>>> Thanks for any insights.
>>>>>
>>>>> Chris
>>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>>
>
>

[[email protected]]

> Several years ago in soaring magazine
> there was an article about a glider kiting and the
> tow pilot was unable to release. However the cause
> was not that the schweizer release jammed, it was the
> result of a poorly installed and secured release cable
> housing. The cable ties used to secure the release
> cable housing to the airframe failed, which then resulted
> in excessive slack in the release cable. This excessive
> slack meant that the tow pilots release handle was
> incapable of pulling on the schweizer release lever.
>

Many years ago there were tests of Schwiezer releases that showed that
they did, indeed jam. These should all be replaced ASAP. Anyone with
a brain can look at a Schwiezer release and see why it will jam under
high loads.

2c

[Jim Vincent]

> wrote in message
oups.com...
> > Several years ago in soaring magazine
>> there was an article about a glider kiting and the
>> tow pilot was unable to release. However the cause
>> was not that the schweizer release jammed, it was the
>> result of a poorly installed and secured release cable
>> housing. The cable ties used to secure the release
>> cable housing to the airframe failed, which then resulted
>> in excessive slack in the release cable. This excessive
>> slack meant that the tow pilots release handle was
>> incapable of pulling on the schweizer release lever.
>>
>
> Many years ago there were tests of Schwiezer releases that showed that
> they did, indeed jam. These should all be replaced ASAP. Anyone with
> a brain can look at a Schwiezer release and see why it will jam under
> high loads.
>
> 2c
>

I'll bite....why will it jam under high loads?

[[email protected]]

On Feb 4, 10:54 am, "Jim Vincent" > wrote:
> > wrote in message
>
> oups.com...
>
>
>
> > > Several years ago in soaring magazine
> >> there was an article about a glider kiting and the
> >> tow pilot was unable to release. However the cause
> >> was not that the schweizer release jammed, it was the
> >> result of a poorly installed and secured release cable
> >> housing. The cable ties used to secure the release
> >> cable housing to the airframe failed, which then resulted
> >> in excessive slack in the release cable. This excessive
> >> slack meant that the tow pilots release handle was
> >> incapable of pulling on the schweizer release lever.
>
> > Many years ago there were tests of Schwiezer releases that showed that
> > they did, indeed jam. These should all be replaced ASAP. Anyone with
> > a brain can look at a Schwiezer release and see why it will jam under
> > high loads.
>
> > 2c
>
> I'll bite....why will it jam under high loads?

This is extremely difficult to describe without a diagram, but the way
the release is constructed causes pressure to be applied to the
mechanism in such a way that higher forces are required in the cockpit
as upward pressure on the mechanism is applied (towplane low, glider
high). Too much pressure can cause this force to be great enough that
the pilot will not be able to deliver such pressure in an emergency
situation.

2c

[Jim Vincent]

> wrote in message
ups.com...
> On Feb 4, 10:54 am, "Jim Vincent" > wrote:
>> > wrote in message
>>
>> oups.com...
>>
>>
>>
>> > > Several years ago in soaring magazine
>> >> there was an article about a glider kiting and the
>> >> tow pilot was unable to release. However the cause
>> >> was not that the schweizer release jammed, it was the
>> >> result of a poorly installed and secured release cable
>> >> housing. The cable ties used to secure the release
>> >> cable housing to the airframe failed, which then resulted
>> >> in excessive slack in the release cable. This excessive
>> >> slack meant that the tow pilots release handle was
>> >> incapable of pulling on the schweizer release lever.
>>
>> > Many years ago there were tests of Schwiezer releases that showed that
>> > they did, indeed jam. These should all be replaced ASAP. Anyone with
>> > a brain can look at a Schwiezer release and see why it will jam under
>> > high loads.
>>
>> > 2c
>>
>> I'll bite....why will it jam under high loads?
>
> This is extremely difficult to describe without a diagram, but the way
> the release is constructed causes pressure to be applied to the
> mechanism in such a way that higher forces are required in the cockpit
> as upward pressure on the mechanism is applied (towplane low, glider
> high). Too much pressure can cause this force to be great enough that
> the pilot will not be able to deliver such pressure in an emergency
> situation.
>
> 2c
>

Kevin, do you have any known examples of this happening? Flying off the
winch is very similar to a glider high scenario, yet I've never had a
problem releasing even under tension.

[Marc Ramsey]

Jim Vincent wrote:
> Kevin, do you have any known examples of this happening? Flying off the
> winch is very similar to a glider high scenario, yet I've never had a
> problem releasing even under tension.

There have been a number of accidents apparently caused by Schweizer
hooks jamming on the tow plane when the glider gets too high. However,
most everyone has applied an approved fix from years ago, turn the
release assembly upside down and the jamming problem goes away...

Marc

[Marc Ramsey]

Marc Ramsey wrote:
> Jim Vincent wrote:
>> Kevin, do you have any known examples of this happening? Flying off
>> the winch is very similar to a glider high scenario, yet I've never
>> had a problem releasing even under tension.
>
> There have been a number of accidents apparently caused by Schweizer
> hooks jamming on the tow plane when the glider gets too high. However,
> most everyone has applied an approved fix from years ago, turn the
> release assembly upside down and the jamming problem goes away...

Here's a 337 for a Pawnee:

http://home.att.net/~jdburch/Form337.htm

Marc

[Marc Ramsey]

Marc Ramsey wrote:
> Marc Ramsey wrote:
>> Jim Vincent wrote:
>>> Kevin, do you have any known examples of this happening? Flying off
>>> the winch is very similar to a glider high scenario, yet I've never
>>> had a problem releasing even under tension.
>>
>> There have been a number of accidents apparently caused by Schweizer
>> hooks jamming on the tow plane when the glider gets too high.
>> However, most everyone has applied an approved fix from years ago,
>> turn the release assembly upside down and the jamming problem goes
>> away...
>
> Here's a 337 for a Pawnee:
>
> http://home.att.net/~jdburch/Form337.htm

And an article:

http://home.att.net/~jdburch/towhook.htm

Marc

[Tony Verhulst]

> Kevin, do you have any known examples of this happening? Flying off the
> winch is very similar to a glider high scenario, yet I've never had a
> problem releasing even under tension.

Jim,

Several years ago, the PIC pulled the release on an L13 and started his
right turn. The problem was that he was still on tow behind the Bird
Dog. A second later, both tow plane and glider were pointed at the
ground - still attached. The tow pilot reported that he pulled his
release and nothing happened. Several, much more "assertive" attempts
ensued and finally the 2 aircraft parted company - having lost 1000ft.
Had this been a pattern tow, there would have been fatalities.
Interestingly, both aircraft had problems releasing. The Birddog has a
Schweizer release. As I was chief instructor at the time, I asked for a
written statement from the (rated) glider pilot. The names have been
removed.

.................................................. ................

Glider pilot report

Saturday November 6th, being towed by the birddog L-19 in the Blanik
L-13, I had a serious release incident.

On preflight I often noticed that the release mechanism is not as clean
as on the Schweizer models. I had never given any attention to that
point and that was already the first mistake.

Reaching 5000', I over emphasized on the climbing turn after the
release. I pulled the release and made my right turn. Being busy
clearing the right turn And getting to the right airspeed, I did not
notice the rope was still attached until the glider violently yawed to
the left and caused the birddog to start to go into a spin.

Over emphasizing on the turn without verifying that the rope actually
released after pulling the release is obviously a terrible mistake.
Unlike the 2-33 in which the release is very sensitive, the blanik
requires you to pull the handle all the way back and will not release
with a side load on the jaws!

Having pulled the handle twice and noticing that this was hopeless, I
saw that the towplane could not release either. Indeed, the birddog
release also locked when its the tail got pulled up by a glider. To make
matter worse, the tow plane with its tail pulled up was forced in to a
dive by the glider. It could not level off even with full aft elevators.
From there the only hope was a release or a rope break before hitting
the ground (another reason for not using a too strong rope).

Trying to get back on normal tow position I dove. I quickly noticed that
this would not be possible. Thanks to the streamlined shape of the
Blanik I could dive fast enough to remove some of the load on towplane
tail. It allowed the tow pilot to release the rope with a strong yank on
his release handle.

Fortunately, this incident did not aggravate into a tragic accident.
This incident showed me how quickly things can deteriorate if I don't
use the proper procedures. Everybody knows how important verifying the
correct effect of one's input is. I did not know it enough on that flight.