Are Weak Links really Necessary for Aero Tow?

A very good friend of mine died several years ago in
a tug upset.
He was aero-towing I believe a Ka6 out of a wave site
when turbulence caused the glider to loose sight of
the tug at about 300ft. The glider pilot did not release
and the tug pilot could not as his inertia reel harness
had locked and he was unable to reach the release (conclusion
of the AAIB). The uspet was so bad that the tug was
hanging vertically from the glider before the rope
broke. The resulting dive was irrecoverable and the
pilot died when the tug hit the ground, the glider
pilot of course survived.

I recall fitting a new rear end to a Tiger Moth that
had been used for glider towing, 3 of the four rear
longerons were almost pulled completely apart. None
of the other Tiger Moths, which were not tugs, serviced
by the same organisation had the same problem. Co-incidentaly
the main pilot of the Tiger Moth was the pilot mentioned
above but he was not flying the Tiger in the incident.

The answer to the original poster is, yes they bl00dy
well are, and you would do well to remember it especially
if you are a tuggie.
Glider pilots always seem to survive tug upsets it's
the tuggie that gets it.

At 21:06 17 September 2006, Doug Haluza wrote:

KM wrote:
OK now focus here Doug, the math is not in dispute.
The question is
whether a glider could exert this force while on tow.

If both aircraft are in a steep dive from a high altitude
upset, and
the glider pilot panics and pulls the sitck, it certainly
can. But it
really doesn't matter--using a dockline as a tow rope
means it won't
break before one of the aircraft does.

But what makes you think the dive would be 'Unrecoverable'
just because
the tow plane is past its manurering speed?

The dive after an upset will be unrecoverable as long
as the glider
stays attached to the towplane.

This topic is worthy of discussion, so it would be nice to avoid having
it degenerate into the typical RAS morass. I've flown at commercial
operations that use large diameter "tugboat" rope. The explanation
for this is usually something like, "Oh well, we put a knot at the
end that cuts the breaking strength in half", or "We can't afford
to keep changing those little skinny ropes...".

Let's start at the top of the list: Number one - In the US, FAR
Part 91.309(3) seems crystal clear in terms of requirements - A
towline (or weaklink) is required to be not less than 80% of max glider
weight and not more than twice the max operating weight. I am not
aware of any options in the regulations to do something different. In
fact, if you are doing something different I suspect the FAA, your
insurance company, or maybe some plaintiff's attorney might be very
interested should an accident occur.

Doug and Papa3 are right in their assessment and experience that a
glider can damage a towplane. I've seen an overstress failure of a
tow hitch which broke and bent parts in the attachment assembly.
Notwithstanding excess structural loads on the empennage, the very real
danger is the tow plane running out of pitch control in the event of a
kiting incident.

I witnessed a kiting accident caused by a glider elevator misconnect.
The glider climb resembled a winch launch with the tow plane at 50-100
feet off the ground. The tow rope broke at about the same time the
towpilot had the stick on the back stop and was pitching down. This
happened so fast, had the rope not broken as it was designed to it is
very doubtful the towplane could have either released or recovered.
There would have most likely been two fatalities that day if the tow
rope had not been the correct breaking strength.

It would take a foolhardy towpilot to believe: 1) An over-strength rope
would be legal 2) An over-strength rope could not cause structural
damage to the towplane 3) The tow pilot could react quickly enough to
release in a low level kiting incident.

The tow "system" works with multiple layers of protection if the
rope is the proper specification.

Bob

KM wrote:

Are you telling us that the tail can only handle its own weight?You are
using apples and oranges here because you used the FLIGHT loads of a
sailplane and the actual weight of the tow plane.In other words, if a
towplane can sustain 4.4Gs (In the utility category) shouldnt the tail
of said towplane ALSO sustain 4.4Gs.


Happy Landings
KMU

KM,

I think what you're missing here is the difference between the limiting
load factor on the towplane as a system (ie. the 4.4Gs) vs. the
limiting load factor on a component. Typically, the primary
load-bearing components in flight are the wing spars and the
carry-through structure; this is carrying the majority of the load.
The tail section will never sustain anything near those sorts of loads
in normal flight.

Think of this way. Suppose your towplane weights 1,500 lbs (just an
illustration). It's pretty easy to imagine it supporting 6,000 lbs of
sandbags spread along the wings (just think of those impressive ads
showing proof-testing of spars). Now, imagine piling those same
6,000 lbs of sandbags on the empennage (assuming you could find the
space to stack them :-))

Anyway, I can probably come up with some guesstimates on what the tail
section of a typical welded steel cluster could withstand, but why
bother?

P3

Bob's post below is clear and correct.

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.

Actually, reading the POH for several German gliders, I note the weak link
for aerotow is specified as as exact figure. For example, the weak link for
both aero tow and winch for my Nimbus 2C is specified as 600 Kg (1323 Lbs)
or a blue Tost weak link. The tolerance is + or -10%. The US Airworthiness
Certificate specifies that the Nimbus 2C is to be flown as specified in the
Pilots Handbook (POH). Considering the possible flying weights, this ranges
between 95% - 160% which is a narrower range than specified in the FAR's.

Make me wonder if we should be using Tost weak links instead of old bits of
rope.

Bill Daniels

"RL" wrote in message
ups.com...
This topic is worthy of discussion, so it would be nice to avoid having
it degenerate into the typical RAS morass. I've flown at commercial
operations that use large diameter "tugboat" rope. The explanation
for this is usually something like, "Oh well, we put a knot at the
end that cuts the breaking strength in half", or "We can't afford
to keep changing those little skinny ropes...".

Let's start at the top of the list: Number one - In the US, FAR
Part 91.309(3) seems crystal clear in terms of requirements - A
towline (or weaklink) is required to be not less than 80% of max glider
weight and not more than twice the max operating weight. I am not
aware of any options in the regulations to do something different. In
fact, if you are doing something different I suspect the FAA, your
insurance company, or maybe some plaintiff's attorney might be very
interested should an accident occur.

Doug and Papa3 are right in their assessment and experience that a
glider can damage a towplane. I've seen an overstress failure of a
tow hitch which broke and bent parts in the attachment assembly.
Notwithstanding excess structural loads on the empennage, the very real
danger is the tow plane running out of pitch control in the event of a
kiting incident.

I witnessed a kiting accident caused by a glider elevator misconnect.
The glider climb resembled a winch launch with the tow plane at 50-100
feet off the ground. The tow rope broke at about the same time the
towpilot had the stick on the back stop and was pitching down. This
happened so fast, had the rope not broken as it was designed to it is
very doubtful the towplane could have either released or recovered.
There would have most likely been two fatalities that day if the tow
rope had not been the correct breaking strength.

It would take a foolhardy towpilot to believe: 1) An over-strength rope
would be legal 2) An over-strength rope could not cause structural
damage to the towplane 3) The tow pilot could react quickly enough to
release in a low level kiting incident.

The tow "system" works with multiple layers of protection if the
rope is the proper specification.

Bob

The tail loads imposed by a glider during a normal
aero tow are really quite small; probably less than
50lbs. The weak links are only really necessary to
cope with upset situations, or if the rope snags a
tree or power lines on the approach to landing. Obviously
they must break before the tail is damaged, or the
aircraft is slowed down enough to stall in.

Derek Copeland

At 13:36 18 September 2006, Bill Daniels wrote:
Bob's post below is clear and correct.

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.

Actually, reading the POH for several German gliders,
I note the weak link
for aerotow is specified as as exact figure. For example,
the weak link for
both aero tow and winch for my Nimbus 2C is specified
as 600 Kg (1323 Lbs)
or a blue Tost weak link. The tolerance is + or -10%.
The US Airworthiness
Certificate specifies that the Nimbus 2C is to be flown
as specified in the
Pilots Handbook (POH). Considering the possible flying
weights, this ranges
between 95% - 160% which is a narrower range than specified
in the FAR's.

Make me wonder if we should be using Tost weak links
instead of old bits of
rope.

Bill Daniels

'RL' wrote in message
oups.com...
This topic is worthy of discussion, so it would be
nice to avoid having
it degenerate into the typical RAS morass. I've flown
at commercial
operations that use large diameter 'tugboat' rope.
The explanation
for this is usually something like, 'Oh well, we put
a knot at the
end that cuts the breaking strength in half', or 'We
can't afford
to keep changing those little skinny ropes...'.

Let's start at the top of the list: Number one - In
the US, FAR
Part 91.309(3) seems crystal clear in terms of requirements
- A
towline (or weaklink) is required to be not less than
80% of max glider
weight and not more than twice the max operating weight.
I am not
aware of any options in the regulations to do something
different. In
fact, if you are doing something different I suspect
the FAA, your
insurance company, or maybe some plaintiff's attorney
might be very
interested should an accident occur.

Doug and Papa3 are right in their assessment and experience
that a
glider can damage a towplane. I've seen an overstress
failure of a
tow hitch which broke and bent parts in the attachment
assembly.
Notwithstanding excess structural loads on the empennage,
the very real
danger is the tow plane running out of pitch control
in the event of a
kiting incident.

I witnessed a kiting accident caused by a glider elevator
misconnect.
The glider climb resembled a winch launch with the
tow plane at 50-100
feet off the ground. The tow rope broke at about the
same time the
towpilot had the stick on the back stop and was pitching
down. This
happened so fast, had the rope not broken as it was
designed to it is
very doubtful the towplane could have either released
or recovered.
There would have most likely been two fatalities that
day if the tow
rope had not been the correct breaking strength.

It would take a foolhardy towpilot to believe: 1)
An over-strength rope
would be legal 2) An over-strength rope could not
cause structural
damage to the towplane 3) The tow pilot could react
quickly enough to
release in a low level kiting incident.

The tow 'system' works with multiple layers of protection
if the
rope is the proper specification.

Bob

Bill, thank you for the addendum - You are right in pointing out that
within the FAA 80%-200% limitation there may be a more restrictive
specification in a glider's POH. That's why, in many cases, private
glider owners carry their own weaklink to be sure the rope doesn't
exceed the manufacturer's requirement.

I also agree that a PT3 caused by a towline failure has a history of
bad endings.... better to be toward the higher side within the limits.

Bob

RL wrote:
Bill, thank you for the addendum - You are right in pointing out that
within the FAA 80%-200% limitation there may be a more restrictive
specification in a glider's POH. That's why, in many cases, private
glider owners carry their own weaklink to be sure the rope doesn't
exceed the manufacturer's requirement.

I also agree that a PT3 caused by a towline failure has a history of
bad endings.... better to be toward the higher side within the limits.

Bob

Unfortunately, here's a recent PT3.

http://tinyurl.com/gsp6s

Frank

"RL" wrote in message
oups.com...

Bill, thank you for the addendum - You are right in pointing out that
within the FAA 80%-200% limitation there may be a more restrictive
specification in a glider's POH. That's why, in many cases, private
glider owners carry their own weaklink to be sure the rope doesn't
exceed the manufacturer's requirement.

I also agree that a PT3 caused by a towline failure has a history of
bad endings.... better to be toward the higher side within the limits.

Bob


Bob, as a tow operator, maybe you can answer this.

In consideration of the rule that the tug end weak link must be 25% stronger
than the glider end weak link and that the glider being towed may be
anything from a Sparrowhawk to a fully ballasted ASH 25, how do you insure
that the rope and/or tug end weak link is within the rules?

A partial answer to my own question is that towing at a FAI class contest
the glider weights may be within a narrow range but what about sports class
or just a commercial tow operation?

Bill Daniels

"RL" wrote in message
oups.com...

Bill, thank you for the addendum - You are right in pointing out that
within the FAA 80%-200% limitation there may be a more restrictive
specification in a glider's POH. That's why, in many cases, private
glider owners carry their own weaklink to be sure the rope doesn't
exceed the manufacturer's requirement.

I also agree that a PT3 caused by a towline failure has a history of
bad endings.... better to be toward the higher side within the limits.

Bob

Uh, "PT3" ?

I know what a PT6 is.

Al G

Premature Termination of The Tow.

Tom Knauff has many examples of causes and preventive measures he
http://www.eglider.org/newsarticles/...mergencies.htm

Some
"Al" wrote in message
...



I know what a PT6 is.

Al G