Aerotow ropes: short or long, breakable or unbreakable?

[son_of_flubber]

With the dewpoint at 15d F. today, I'm ready to heat things up.

Regulations on aerotow ropes vary by region. What is the evidence to support the various approaches?

PTT (Premature Termination of Tow)(aka rope break) correlates with a number of fatal accidents. How often has a hard_to_break towrope led to a fatal accident?

Options to land after a PTT vary by airport and can be non-existent at certain altitudes. Why is an extra strong towrope not an option where the straight-ahead landing option is poor or non-existent?

In what year did the FAA set the FAR for towropes and what was the quality of towropes at that time? (obscure, but interesting question).

At the airport where I am towed, the same towrope satisfies the FAR for both the frequently towed heavier two-seater, and my less frequently towed lighter weight single place glider. Does this mean that the two-place is more likely to break the towrope?

[[email protected]]

On Tuesday, November 12, 2013 11:22:14 AM UTC-5, son_of_flubber wrote:
> With the dewpoint at 15d F. today, I'm ready to heat things up. Regulations on aerotow ropes vary by region. What is the evidence to support the various approaches? PTT (Premature Termination of Tow)(aka rope break) correlates with a number of fatal accidents. How often has a hard_to_break towrope led to a fatal accident? Options to land after a PTT vary by airport and can be non-existent at certain altitudes. Why is an extra strong towrope not an option where the straight-ahead landing option is poor or non-existent? In what year did the FAA set the FAR for towropes and what was the quality of towropes at that time? (obscure, but interesting question). At the airport where I am towed, the same towrope satisfies the FAR for both the frequently towed heavier two-seater, and my less frequently towed lighter weight single place glider. Does this mean that the two-place is more likely to break the towrope?

There are no regulations that vary by region, but practices may vary in different operations.
I personally have never heard of an accident caused by a rope that was too strong. Over the years there have been a few accidents caused by rope breaks at low altitude and at least one fatality that I recall out west. Whether this was due to selected rope strength or wear, or both, I don't know.
All failures of ropes at our operation are at the time of initial acceleration when a worn rope that has not been inspected fails at the ring. Stronger ropes tend to reduce the liklihood of this happening in my experience.
Avoiding the use of short ropes is a good way to help ensure long tow pilot life. Errors in glider positioning are magnified by short ropes.
Long, strong, inspect.
FWIW
UH

[[email protected]]

On Wednesday, November 13, 2013 5:22:14 AM UTC+13, son_of_flubber wrote:

> PTT (Premature Termination of Tow)(aka rope break) correlates with a number of fatal accidents. How often has a hard_to_break towrope led to a fatal accident?

In the case of aerotow upset, if the rope doesn't break the towplane and glider will both hit the ground. That's a good reason to have a weaklink (or weak rope). I know of four tow upsets where I fly, fortunately all high enough to not hit the ground. Three weak links let go, one towpilot released manually. The towpilot who released had his first upset a week before the second, so he was primed and ready on the release.


--
Philip Plane

[son_of_flubber]

On Tuesday, November 12, 2013 1:21:34 PM UTC-5, wrote:

> There are no regulations that vary by region, but practices may vary in different operations.

I thought that the concept of a breakable towrope was considered misguided in South Africa.

[SF]

I have has some interesting experiences with students. Normally the rope breaks during the initial acceleration on the ground, which is no big deal. I had one student that was low and to the left on tow at altitude and briskly put the plane into a 45 Degree left bank before I could catch it. The rope broke, and at that point I think that the tow pilot and I really wanted it to break because the glider and the tow plane were not friends at that point. Long ropes are easier on everyone involved. long tows on short ropes are pretty nerve wracking. Why make something potentially dangerous harder to do? After my experience with the student I think that you should use a rope that will break at the appropriate load or use the weak link in the POH for winching. The student that broke the rope at altitude never came back and that was probably a very good thing. He seemed to have no concept of wings level, which was almost surprising since he was a high time power pilot that had recently decided not to renew his medical. can you say "Danger Will Robinson, Danger"

[WAVEGURU]

Tow ropes don't always break during the initial acceleration. I had one go on me at about 150ft, and it didn't go bang, it was just sort of a soft fffffftt. I almost couldn't believe it as I watched the frayed end run away from me.

Boggs

[son_of_flubber]

On Tuesday, November 12, 2013 1:21:34 PM UTC-5, wrote:

> Avoiding the use of short ropes is a good way to help ensure long tow pilot life. Errors in glider positioning are magnified by short ropes.

This rational makes perfect sense, so why do the Europeans use shorter tow ropes that the North Americans? Is it just "common sense and tradition" or do the accident statistics prove one approach better than the other?

[Cookie]

I have a number of questions about tow ropes on my Pre solo written test which I give to my students.

After explaining the FAR's , I then ask them, "thinking as a glider pilot, would you rather have a tow rope which is on the stronger side, or on the weaker side?" They almost always answer "stronger", and I agree. But I caution, stronger within reason. In my way of thinking...I have the option to pull the release if things get too gnarly...but at least that release is on my terms, not the rope's.

My rough calculations show that the "normal" tension on a tow rope in smooth tow is very low...It is basically the drag on the glider, plus a tiny bit extra due to the climb vector...so maybe 60 lbs tension...

Almost all ropes I've seen at glider ports are in the 2000 lb breaking strength area....so rope breaks "should" be unlikely. But obviously they do break...so I attribute this to wear of damage....

I think the 80% ~ 200% is a pretty good "rule"...but again I want close to the 200%.

In 8000 flights, I have had 5 (if I remember correctly) tow rope "incidents".

Only two were classic rope breaks...and these happened on the ground roll..where the rope tension is quite high.

Another was a failure at the attachment of the rope to a weak link, not the rope actually breaking...(long story here)

another was the failure of the tow ring...but this was not an "approved" tow ring...it was somebody's idea of a "new and improved" ring...which of course failed...luckily at 400' and we were certainly expecting it. Funny thing about this one...it was in 2-33....at 400' I heard "tink"....which I figured was the tow ring breaking. We returned to the airport to find that the ring was in perfect condition. We looked at the tow hook on the Schweizer and it was still closed and in perfect condition. The guy in front was flying at the time...he got a bit of slack in the rope, when the slack come out, the rope was disconnected. What I think happened was the slack allowed the ring to twist around and back release the schweizer hook. They have a habit of bouncing back closed so it looked like it never released and the ring separated as if by magic.

Lastly, while doing a scenic ride, I noticed something first near the tow plane, which then seemed to be headed toward me...a black thing....turns out this was part of the tail spring, to which the tow rope was attached. It came straight and level back toward me for quite a while, but then finally arced and fell under the glider. So again, not a real rope break.

As for rope length..some operators will cut out a short section of worn rope at the end, and splice it back on the ring...so over time the rope gets shorter and shorter. I has a tow just last week with a student...and a really short rope. First of all, that Pawnee looked really "big"...but the student had a bit more trouble on tow than usual.

I've done long distance tows, where we attached two 200' ropes together to give 400' tow rope. This is luxury! The interaction between glider and tow plane is cut down dramatically!


Cookie








On Tuesday, November 12, 2013 11:22:14 AM UTC-5, son_of_flubber wrote:
> With the dewpoint at 15d F. today, I'm ready to heat things up.
>
>
>
> Regulations on aerotow ropes vary by region. What is the evidence to support the various approaches?
>
>
>
> PTT (Premature Termination of Tow)(aka rope break) correlates with a number of fatal accidents. How often has a hard_to_break towrope led to a fatal accident?
>
>
>
> Options to land after a PTT vary by airport and can be non-existent at certain altitudes. Why is an extra strong towrope not an option where the straight-ahead landing option is poor or non-existent?
>
>
>
> In what year did the FAA set the FAR for towropes and what was the quality of towropes at that time? (obscure, but interesting question).
>
>
>
> At the airport where I am towed, the same towrope satisfies the FAR for both the frequently towed heavier two-seater, and my less frequently towed lighter weight single place glider. Does this mean that the two-place is more likely to break the towrope?

[GM]

On Tuesday, November 12, 2013 11:22:14 AM UTC-5, son_of_flubber wrote:
> With the dewpoint at 15d F. today, I'm ready to heat things up.
>
>
>
> Regulations on aerotow ropes vary by region. What is the evidence to support the various approaches?
>
>
>
> PTT (Premature Termination of Tow)(aka rope break) correlates with a number of fatal accidents. How often has a hard_to_break towrope led to a fatal accident?
>
>
>
> Options to land after a PTT vary by airport and can be non-existent at certain altitudes. Why is an extra strong towrope not an option where the straight-ahead landing option is poor or non-existent?
>
>
>
> In what year did the FAA set the FAR for towropes and what was the quality of towropes at that time? (obscure, but interesting question).
>
>
>
> At the airport where I am towed, the same towrope satisfies the FAR for both the frequently towed heavier two-seater, and my less frequently towed lighter weight single place glider. Does this mean that the two-place is more likely to break the towrope?

The 80-200% rule is in the FARs but it DOES NOT APPLY to ANY glider which - at the time of US-certification (standard or experimental) - had a manufacturer's issued and approved POH, which spelled out the breaking strength of a weak-link for the CG and aero-tow hook! In some cases, these may be the same; in most cases, the aero-tow weak-link strength is lower.
In Europe, typically very strong tow ropes (mooring lines, as one US visitor to Europe called them) are used but there is a proper weak-link attached on the glider end.
A short while ago, I did launch a quick survey on the German forum to find out how many tow rope failure anyone had. The response: zero tow rope failures!

[Evan Ludeman[_4_]]

It's rare, but I have twice seen ropes break when they snagged something solid while the tug was on final. Something to consider before going to an "unbreakable" rope with a weak link only at glider end.

T8

[Bob Cook[_2_]]

I've seen the tow rope snag the fence across the street form the airport
several times...sometimes the rope breaks, sometimes the fence
breaks....once I actually saw the rope wind up once around the top pole of
the fence , then unwind without breaking anything.

Another time the tow rope wound up around the outside rear view mirror of a
passing car. It neatly removed the mirror from the car. The driver
arrived a the glider shack, mirror in hand. He was not too mad...he had
just taken a glider ride...and enjoyed it...He said it did scare him a bit
though...insurance payed for the car repair.


BTW...If you read the FAR...two weak links are required. The FAR is
worded funny...they use the singular twice, instead of the plural. "a week
link at the glider" further down, "a weak link at the tow plane"

Students almost always get my question about this wrong...they stop at "a
week link"..

Cookie




At 11:50 13 November 2013, Evan Ludeman wrote:
>It's rare, but I have twice seen ropes break when they snagged something
>solid while the tug was on final. Something to consider before going to
an
>"unbreakable" rope with a weak link only at glider end.
>
>T8
>

[Tom[_12_]]

The reality is there have indeed been fatalities due to over-strong tow ropes, and one of the preventative measures is to use a rope designed to break in case of over-stress.

The load on the rope is relatively light and can be figured using the L/D of the glider.

The "L" is the lift required to make the glider fly, which equals the weight of the glider.

Dividing the glide ratio into the weight equals the drag, or load on the tow rope.

A gllder with an L/D of 30, weighing 900 pounds would have a load on the tow rope of only 30 pounds.

Since the glider is being towed uphill, the load is slightly more. (Towed by the space shuttle, straight up, the load would equal the weight of the glider, disregarding acceleration forces.)

Anyway, the load on the tow rope is small. This load can be simulated by pulling the glider by the rope by hand on a smooth surface.

Typically, we use a tow rope with a breaking strength of 1,200 pounds, (regulated by the rules - weight of the loaded glider) or a much stronger, more durable rope with a much higher breaking strength with complying weak links on both ends of the tow rope.

The weak links can be the mechanical types made by Tost. These are expensive and weigh enough to cause the tow rope to fly very low behind the descending towplane.

The weak links can also be made from smaller tow ropes that comply with the strength requirements. The rope manufacturer must be trusted to conform to the requirments.

Bottom line is weak links are an important, regulated safety requirement. They rarely are needed, but there are occasions when they can save lives.

Finally, pilots and line crews need to be vigilant about the condition of launch equipment, glider and towplane.

Tom Knauff

[[email protected]]

I know of at least one Ka-8 breaking up in mid-air after an aerotow upset at an altitude sufficient for the tug to pull out of the dive. The weak link at the tug side was the black one (900kg), put there to protect the tug in case of catching an obstacle with the rope while coming in to land. The weak link broke too late to save the Ka-8. So yes, too strong towing ropes do kill people.

And I personnaly experienced three real rope breaks, 2 at height during instruction flights (broken weak link when taking out the slack of the rope after a slightly botched exercise), one at 150 ft resulting in an outlanding (that rope was heavily frayed inside the retracting system and broke without real cause).

So my advice is: strong rope, correct strength of weak link at sailplane end, and check frequently.

[James Metcalfe]

Previous posters have written
- "...plus a tiny bit extra due to the climb vector", and
- "Since the glider is being towed uphill, the load is slightly more."

Actually the load due to being pulled uphill is typically 10% of the
weight of the glider (thinking of a typical climb of 6kts at 60kts
airspeed).
This is significantly larger than the drag component, which is in the
range 2.5% to 2% for a decent performance glider (say L/D of 40
to 50), and maybe 3.5% for an old 2-seater (L/D of 28).

Pedantry? ;)

[Bob Cook[_2_]]

I guess I used kinda vague terms like "tiny bit", and "slightly".

You did the math and put some actual numbers on it. ( Large compared to the
drag...small compared to the rope strength)

So if we say the tension on the rope is smooth tow is 12% ~ 15% of the
glider's weight, and the rope strength is 80% to 200% of the glider's
weight, we have a pretty good safety factor there.

Yet a rope that should break if things get "hairy"..

Cookie




At 15:24 13 November 2013, James Metcalfe wrote:
>Previous posters have written
> - "...plus a tiny bit extra due to the climb vector", and
> - "Since the glider is being towed uphill, the load is slightly more."
>
>Actually the load due to being pulled uphill is typically 10% of the
>weight of the glider (thinking of a typical climb of 6kts at 60kts
>airspeed).
>This is significantly larger than the drag component, which is in the
>range 2.5% to 2% for a decent performance glider (say L/D of 40
>to 50), and maybe 3.5% for an old 2-seater (L/D of 28).
>
>Pedantry? ;)
>
>

[Bob Cook[_2_]]

I would like to think that in such a situation, I would have pulled the
release before the situation got that bad. Maybe not always as easy as it
seems?

So I agree with the notion of a rope on the strong side, but within specs
for breaking before something more important breaks.

I guess what really worries me, is not so much the actual strength of a
"new" rope, but how quickly that degrades due to use, wear, exposure, and
actual damage.


Cookie






At 14:23 13 November 2013, wrote:
>I know of at least one Ka-8 breaking up in mid-air after an aerotow upset
>a=
>t an altitude sufficient for the tug to pull out of the dive. The weak
>link=
> at the tug side was the black one (900kg), put there to protect the tug
>in=
> case of catching an obstacle with the rope while coming in to land. The
>we=
>ak link broke too late to save the Ka-8. So yes, too strong towing ropes
>do=
> kill people.=20
>
>And I personnaly experienced three real rope breaks, 2 at height during
>ins=
>truction flights (broken weak link when taking out the slack of the rope
>af=
>ter a slightly botched exercise), one at 150 ft resulting in an
outlanding
>=
>(that rope was heavily frayed inside the retracting system and broke
>withou=
>t real cause).
>
>So my advice is: strong rope, correct strength of weak link at sailplane
>en=
>d, and check frequently.
>

[Martin Gregorie[_5_]]

On Tue, 12 Nov 2013 18:43:41 -0800, Cookie wrote:

> My rough calculations show that the "normal" tension on a tow rope in
> smooth tow is very low...It is basically the drag on the glider, plus a
> tiny bit extra due to the climb vector...so maybe 60 lbs tension...
>
I think its a bit more than that, but not much. Some time ago I did a
fairly detailed spreadsheet calculation for my Libelle being towed at 60
kts in a 600 fpm climb and got a total rope tension of 37.62 kg, which
was more or less what I'd guestimated from a mental round number
calculation.

The makeup of the rope tension did surprise me though: like you I thought
drag would be the major part of it, but the spread sheet gave the
glider's drag as 9.97 kg and the force needed to haul my 280 kg Libelle
up a 5.67 degree slope at 60kts as 27.65 kg.

Yeah, I know, I should have only kept 1 decimal point: 2 decimals is
spurious accuracy.

Of course, the other interesting number would be what fraction of the
total drag of a climbing tug+glider combination was due to the glider. I
suspect the glider is contributing less than Wg/Wt where Wg is the glider
weight and Wt is the tow planes's weight. I'd expect the straight ratio
to over-estimate the glider's contribution because it is likely to fly
rather more efficiently than the tug.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[Bob Cook[_2_]]

yes, I underestimated....

Say a 800 lb glider at 40:1 L/D so the drag is about 20 lbs...but the
tension component in the rope due to the climb angle (say 10:1) is going
to be 80lbs....so you've got 100 lb tension on the rope.

But a rope "in spec" would offer a safety factor of 6x up to 16x.

Some tow planes can climb better than that...so more tension...

Cookie







At 23:46 13 November 2013, Martin Gregorie wrote:
>On Tue, 12 Nov 2013 18:43:41 -0800, Cookie wrote:
>
>> My rough calculations show that the "normal" tension on a tow rope in
>> smooth tow is very low...It is basically the drag on the glider, plus a
>> tiny bit extra due to the climb vector...so maybe 60 lbs tension...
>>
>I think its a bit more than that, but not much. Some time ago I did a
>fairly detailed spreadsheet calculation for my Libelle being towed at 60
>kts in a 600 fpm climb and got a total rope tension of 37.62 kg, which
>was more or less what I'd guestimated from a mental round number
>calculation.
>
>The makeup of the rope tension did surprise me though: like you I thought

>drag would be the major part of it, but the spread sheet gave the
>glider's drag as 9.97 kg and the force needed to haul my 280 kg Libelle
>up a 5.67 degree slope at 60kts as 27.65 kg.
>
>Yeah, I know, I should have only kept 1 decimal point: 2 decimals is
>spurious accuracy.
>
>Of course, the other interesting number would be what fraction of the
>total drag of a climbing tug+glider combination was due to the glider. I
>suspect the glider is contributing less than Wg/Wt where Wg is the glider

>weight and Wt is the tow planes's weight. I'd expect the straight ratio
>to over-estimate the glider's contribution because it is likely to fly
>rather more efficiently than the tug.
>
>
>--
>martin@ | Martin Gregorie
>gregorie. | Essex, UK
>org |
>

[Tom Claffey]

IF the glider is flown perfectly balanced in the perfect position behind
the tow
plane and NEVER gets out of station or forms slack in the rope.
;)

Tom



At 00:22 14 November 2013, Bob Cook wrote:
>yes, I underestimated....
>
>Say a 800 lb glider at 40:1 L/D so the drag is about 20 lbs...but
the
>tension component in the rope due to the climb angle (say 10:1) is going
>to be 80lbs....so you've got 100 lb tension on the rope.
>
>But a rope "in spec" would offer a safety factor of 6x up to 16x.
>
>Some tow planes can climb better than that...so more tension...
>
>Cookie
>
>
>
>
>
>
>
>At 23:46 13 November 2013, Martin Gregorie wrote:
>>On Tue, 12 Nov 2013 18:43:41 -0800, Cookie wrote:
>>
>>> My rough calculations show that the "normal" tension on a tow rope in
>>> smooth tow is very low...It is basically the drag on the glider, plus
a
>>> tiny bit extra due to the climb vector...so maybe 60 lbs tension...
>>>
>>I think its a bit more than that, but not much. Some time ago I did a
>>fairly detailed spreadsheet calculation for my Libelle being towed at 60

>>kts in a 600 fpm climb and got a total rope tension of 37.62 kg, which
>>was more or less what I'd guestimated from a mental round number
>>calculation.
>>
>>The makeup of the rope tension did surprise me though: like you I
thought
>
>>drag would be the major part of it, but the spread sheet gave the
>>glider's drag as 9.97 kg and the force needed to haul my 280 kg Libelle
>>up a 5.67 degree slope at 60kts as 27.65 kg.
>>
>>Yeah, I know, I should have only kept 1 decimal point: 2 decimals is
>>spurious accuracy.
>>
>>Of course, the other interesting number would be what fraction of the
>>total drag of a climbing tug+glider combination was due to the glider. I

>>suspect the glider is contributing less than Wg/Wt where Wg is the
glider
>
>>weight and Wt is the tow planes's weight. I'd expect the straight ratio
>>to over-estimate the glider's contribution because it is likely to fly
>>rather more efficiently than the tug.
>>
>>
>>--
>>martin@ | Martin Gregorie
>>gregorie. | Essex, UK
>>org |
>>
>
>

[Bob Cook[_2_]]

Of course!


Dynamic forces could be many times the static forces...in fact all the way
up to the limit of the rope!

Again good reason for an upper limit on the rope.

Cookie

At 01:34 14 November 2013, Tom Claffey wrote:
>IF the glider is flown perfectly balanced in the perfect position behin
>the tow
>plane and NEVER gets out of station or forms slack in the rope.
>;)
>
>Tom
>
>
>
>At 00:22 14 November 2013, Bob Cook wrote:
>>yes, I underestimated....
>>
>>Say a 800 lb glider at 40:1 L/D so the drag is about 20 lbs...bu
>the
>>tension component in the rope due to the climb angle (say 10:1) is
going
>>to be 80lbs....so you've got 100 lb tension on the rope.
>>
>>But a rope "in spec" would offer a safety factor of 6x up to 16x.
>>
>>Some tow planes can climb better than that...so more tension...
>>
>>Cookie
>>
>>
>>
>>
>>
>>
>>
>>At 23:46 13 November 2013, Martin Gregorie wrote:
>>>On Tue, 12 Nov 2013 18:43:41 -0800, Cookie wrote:
>>>
>>>> My rough calculations show that the "normal" tension on a tow rope in
>>>> smooth tow is very low...It is basically the drag on the glider, plu
>a
>>>> tiny bit extra due to the climb vector...so maybe 60 lbs tension...
>>>>
>>>I think its a bit more than that, but not much. Some time ago I did a
>>>fairly detailed spreadsheet calculation for my Libelle being towed at 6
>
>>>kts in a 600 fpm climb and got a total rope tension of 37.62 kg, which
>>>was more or less what I'd guestimated from a mental round number
>>>calculation.
>>>
>>>The makeup of the rope tension did surprise me though: like you I
>thought
>>
>>>drag would be the major part of it, but the spread sheet gave the
>>>glider's drag as 9.97 kg and the force needed to haul my 280 kg Libelle

>>>up a 5.67 degree slope at 60kts as 27.65 kg.
>>>
>>>Yeah, I know, I should have only kept 1 decimal point: 2 decimals is
>>>spurious accuracy.
>>>
>>>Of course, the other interesting number would be what fraction of the
>>>total drag of a climbing tug+glider combination was due to the glider.
>
>>>suspect the glider is contributing less than Wg/Wt where Wg is th
>glider
>>
>>>weight and Wt is the tow planes's weight. I'd expect the straight ratio

>>>to over-estimate the glider's contribution because it is likely to fly
>>>rather more efficiently than the tug.
>>>
>>>
>>>--
>>>martin@ | Martin Gregorie
>>>gregorie. | Essex, UK
>>>org |
>>>
>>
>>
>
>

[son_of_flubber]

On Wednesday, November 13, 2013 8:17:22 AM UTC-5, Tom wrote:
> The reality is there have indeed been fatalities due to over-strong tow ropes, and one of the preventative measures is to use a rope designed to break in case of over-stress.

The problem is that rope degradation is not recognized and defective ropes are used until they fail prematurely.

Is it a problem that we use tow ropes of a type that is subject to rapid degradation? The structural part of the rope is exposed to abrasion and UV. The open weave of the rope allows grit to penetrate and destroy the rope from the inside out. We use ropes that are dirty and show signs of "acceptable" wear. We test our ropes by towing the next glider.

Rock climbers approach this problem differently. Their ropes are designed to minimize the penetration of grit. The function of the outermost wrapping of the rope is to protect the structural core from grit. When the outermost protective wrapping is worn, the rope is discarded. In the old days, rock climbers would wash their ropes in the washing machine to remove micro grit.

Three questions:
1)Would a simple visual inspection find 99.9% of bad tow ropes before they break (Prematurely)? Do "good" towropes ever break?
2)Are we using the right type of rope?
3)Is there a way to test a tow rope on the ground without destroying it?

If you applied a known load to a tow rope (on the ground) and measured the elongation, would this not give an objective estimation of the condition of the rope? (As fibers break or weaken, the rope will elongate more under a known load). If the elongation is outside the limits, the rope is retired.

Would a magnifying glass improve the visual inspection process?

[Evan Ludeman[_4_]]

On Thursday, November 14, 2013 9:28:52 AM UTC-5, son_of_flubber wrote:

> The problem is that rope degradation is not recognized and defective ropes are used until they fail prematurely.

Not at my club.

The rope requires a preflight inspection, just like our gliders and airplanes. If you aren't doing this, you are doing it wrong.

T8

[son_of_flubber]

On Thursday, November 14, 2013 9:55:42 AM UTC-5, Evan Ludeman wrote:
> On Thursday, November 14, 2013 9:28:52 AM UTC-5, son_of_flubber wrote:
>
>
>
> > The problem is that rope degradation is not recognized and defective ropes are used until they fail prematurely.
>
>
> The rope requires a preflight inspection, just like our gliders and airplanes. If you aren't doing this, you are doing it wrong.
>

I thought it was obvious that "used until they fail" refers to the specific ropes that failed prematurely on tow. Are you saying that visual preflight inspection will detect 99.99% defective ropes before they fail in the air?

One problem is that ropes are not retired when they show "normal wear", and visual inspection is very subjective. I was surprised to see what is considered "normal wear" in AZ where the tow rope is regularly dragged through grit. The effects of grit sawing away at the fibers inside the weave of the rope cannot be seen from the outside.

[Dan Marotta]

Are we gonna fly today or will the eight hour test and evaluation session
use up all available daylight?


"son_of_flubber" > wrote in message
...
On Wednesday, November 13, 2013 8:17:22 AM UTC-5, Tom wrote:
> The reality is there have indeed been fatalities due to over-strong tow
> ropes, and one of the preventative measures is to use a rope designed to
> break in case of over-stress.

The problem is that rope degradation is not recognized and defective ropes
are used until they fail prematurely.

Is it a problem that we use tow ropes of a type that is subject to rapid
degradation? The structural part of the rope is exposed to abrasion and UV.
The open weave of the rope allows grit to penetrate and destroy the rope
from the inside out. We use ropes that are dirty and show signs of
"acceptable" wear. We test our ropes by towing the next glider.

Rock climbers approach this problem differently. Their ropes are designed
to minimize the penetration of grit. The function of the outermost wrapping
of the rope is to protect the structural core from grit. When the outermost
protective wrapping is worn, the rope is discarded. In the old days, rock
climbers would wash their ropes in the washing machine to remove micro grit.

Three questions:
1)Would a simple visual inspection find 99.9% of bad tow ropes before they
break (Prematurely)? Do "good" towropes ever break?
2)Are we using the right type of rope?
3)Is there a way to test a tow rope on the ground without destroying it?

If you applied a known load to a tow rope (on the ground) and measured the
elongation, would this not give an objective estimation of the condition of
the rope? (As fibers break or weaken, the rope will elongate more under a
known load). If the elongation is outside the limits, the rope is retired.

Would a magnifying glass improve the visual inspection process?

[son_of_flubber]

On Thursday, November 14, 2013 10:50:46 AM UTC-5, Dan Marotta wrote:
> Are we gonna fly today or will the eight hour test and evaluation session
>
> use up all available daylight?

Good point. Time is limited.

If an elongation test would detect weakening ropes, you would only need to do it once a week or so because many forms of degradation is gradual. If you had a permanent jig for testing elongation, it would take 10 minutes. Or you could test the ropes at the beginning of wave season when you know they are gonna get stressed (or even better, you could replace your ropes at the beginning of wave season).

Ever have a rope break at Moriarty? I know that you have a lot of grit.

[Evan Ludeman[_4_]]

On Thursday, November 14, 2013 10:18:20 AM UTC-5, son_of_flubber wrote:
> On Thursday, November 14, 2013 9:55:42 AM UTC-5, Evan Ludeman wrote:
>
> > On Thursday, November 14, 2013 9:28:52 AM UTC-5, son_of_flubber wrote:
>
> >
>
> >
>
> >
>
> > > The problem is that rope degradation is not recognized and defective ropes are used until they fail prematurely.
>
> >
>
> >
>
> > The rope requires a preflight inspection, just like our gliders and airplanes. If you aren't doing this, you are doing it wrong.
>
> >
>
>
>
> I thought it was obvious that "used until they fail" refers to the specific ropes that failed prematurely on tow. Are you saying that visual preflight inspection will detect 99.99% defective ropes before they fail in the air?
>
>
>
> One problem is that ropes are not retired when they show "normal wear", and visual inspection is very subjective. I was surprised to see what is considered "normal wear" in AZ where the tow rope is regularly dragged through grit. The effects of grit sawing away at the fibers inside the weave of the rope cannot be seen from the outside.

So... is this operation experiencing rope failure without overload? In 27 years in the sport, I've never seen this. If I did see it I'd change my procedures.

At some of my favorite places to fly, a PTT at 50 - 300' has some significant risks, so we take launch prep fairly seriously, the tow plane pre-launch checkout includes a warmup flight, etc. We don't break ropes, we inspect them visually and service them before they get ugly. We have no need for an ASTM approved test program....

[son_of_flubber]

On Thursday, November 14, 2013 11:35:41 AM UTC-5, Evan Ludeman wrote:

> So... is this operation experiencing rope failure without overload?
That does not tell us that their current procedure has a reasonable safety margin.
> In 27 years in the sport, I've never seen this.
> If I did see it I'd change my procedures.

Wait for a failure before evaluating procedures??

> At some of my favorite places to fly, a PTT at 50 - 300' has some significant risks, so we take launch prep fairly seriously, the tow plane pre-launch checkout includes a warmup flight, etc. We don't break ropes, we inspect them visually and service them before they get ugly. We have no need for an ASTM approved test program....

The collective wisdom and experience of many people have developed the status quo, and I'm sure that it makes sense on balance, but does anyone know how close to failure ropes get before they are retired?

And there is the matter of how our uncertainty about the rope affects training and flight reviews. I watched a glider enter a spin after a "simulated rope break" during a biennial flight review (glider totaled, no injuries, pilot retired). Maybe "that should not have happened", but I'm left with the impression that PTT simulation is risky.

That the probability of a PTT is currently high enough to justify the risk of PTT simulation is part of my motivation. Can the probability of a PTT be lowered to the point that the risk of PTT simulation is no longer justified? How far can better rope material choice and better (practical) inspection go towards eliminating the possibility of PTT?

[kirk.stant]

On Thursday, November 14, 2013 11:36:01 AM UTC-6, son_of_flubber wrote:
>
> That the probability of a PTT is currently high enough to justify the risk of PTT simulation is part of my motivation. Can the probability of a PTT be lowered to the point that the risk of PTT simulation is no longer justified? How far can better rope material choice and better (practical) inspection go towards eliminating the possibility of PTT?

In over 2500 hours of glider flying, and maybe 700 towing, I've never had a rope break.

As a tow pilot, I inspect the rope at the start of the day, and when there is a break in the action (as part of laying out the rope behind the towplane to get it out of the way).

I've had two (2) PTTs in that time - neither caused by a rope breaking. One was an incorrectly hooked up Schweizer to hook (muddy) that gave me a PTT at about 200' in a loaded 2-32 (quick 180 and rolled back to the doofus who had hooked me up and made him do it again, correctly) and once when the tow pilot ran out of gas and released his end without telling me (but the funny thing was that it happened at exactly 3000' agl and I released at the same time - we never did find that rope!).

And all this at glider fields all over the US.

So I really think our procedures are pretty sound. Can ropes break? Of course. So can tow releases, weak links, etc...

As far as PTT training being dangerous - you can tow with a steel cable and still get a PTT. So yes, it is very important. And really, it's not a Chuck Yeager bit of airmanship, after all...spinning off a PTT is remarkably bad flying - and probably the sign of someone who was scared of stalls or steep banks at low altitude and didn't really know how to fly to the limits of his glider. You want to worry about something, worry about the currency and proficiency of your fellow glider pilots!

Kirk
66

[John Carlyle]

Tow ropes do break, but in my experience it’s pretty rare. I’ve been soaring for only 8 years, primarily off of grass fields, and out of about 20,000 tows that have occurred during that time at the two sites I’ve flown from, I know of only 6 rope breaks. Four of those breaks occurred above 2,000 feet AGL, during slack line removal practice. One break occurred above 1,000 feet AGL, when a glider got high and upset the tow plane. One break occurred at about 10 feet AGL, when the tow rope came apart in its middle via unraveling.

The first four breaks don’t really bother me at all. The fifth break bothered a whole bunch of us, but not from a tow rope standpoint! The sixth break was initially disturbing, but we quickly discovered that a poorly manufactured batch of rope had been imported into the country (the substandard nature of the rope was pretty obvious simply by running the line through your hand).

Most of our tow rope wear occurs where the line is wrapped twice through the round part of the Tost ring. Pilots and line men see this area “growing hair” at the inspection done just before hook-up, and a worn tow rope gets rejected quickly. Our line men use a short stick with a metal hook in its end to guide the rope as the tow plane taxis into take off position. The feel of the rope going through this hook is a virtual mini-inspection, and has resulted in our finding knots as well as rope damage caused by trees and rocks.

-John, Q3

[son_of_flubber]

On Thursday, November 14, 2013 1:27:22 PM UTC-5, kirk.stant wrote:
>after all...spinning off a PTT is remarkably bad flying - and probably the sign of someone who was scared of stalls or steep banks at low altitude and didn't really know how to fly to the limits of his glider.

I agree that it "should not have happened", but the pilot in the front seat had thousands of hours over decades of flying in multiple gliders. He had been a top glider pilot at one time. He put off his retirement from flying for too long.

I'm confident flying after a PTT if there is a place to land, but as Evan notes there are fields where the options after a PTT at 50-300 AGL are limited. I fully subscribe to Evan's emphasis on pre-flight inspections.

[[email protected]]

On Thursday, November 14, 2013 12:36:01 PM UTC-5, son_of_flubber wrote:
> On Thursday, November 14, 2013 11:35:41 AM UTC-5, Evan Ludeman wrote: > So... is this operation experiencing rope failure without overload? That does not tell us that their current procedure has a reasonable safety margin. > In 27 years in the sport, I've never seen this. > If I did see it I'd change my procedures. Wait for a failure before evaluating procedures?? > At some of my favorite places to fly, a PTT at 50 - 300' has some significant risks, so we take launch prep fairly seriously, the tow plane pre-launch checkout includes a warmup flight, etc. We don't break ropes, we inspect them visually and service them before they get ugly. We have no need for an ASTM approved test program.... The collective wisdom and experience of many people have developed the status quo, and I'm sure that it makes sense on balance, but does anyone know how close to failure ropes get before they are retired? And there is the matter of how our uncertainty about the rope affects training and flight reviews. I watched a glider enter a spin after a "simulated rope break" during a biennial flight review (glider totaled, no injuries, pilot retired). Maybe "that should not have happened", but I'm left with the impression that PTT simulation is risky. That the probability of a PTT is currently high enough to justify the risk of PTT simulation is part of my motivation. Can the probability of a PTT be lowered to the point that the risk of PTT simulation is no longer justified? How far can better rope material choice and better (practical) inspection go towards eliminating the possibility of PTT?

12000 glider flights, about 7500 tows given, and the only rope breaks have been during initial acceleration. Maybe 1/2 dozen or so of those. And a couple on the tug end from hanging the rope in a tree.
I've done hundreds of PTT and do not see why they should create an increased risk that would justify not doing that training. I want to KNOW(as well as possible) that the person I'm teaching or reviewing will respond correctly in the event that a failure happens. That failure could be many reasons, broken rope being one.
Real rope breaks are, thankfully, pretty rare. From this I conclude most operations are taking suitable care.
FWIW
UH

[Dave Springford]

Well... if you had a winch, that 100-300 ft rope break would occur while you still have 2000 ft of runway ahead of you - I like that option a whole lot better than the trees off the end of the runway whilst on aerotow.

Sorry, it's November and I just couldn't resist throwing the winch wrench into the discussion :)

[Evan Ludeman[_4_]]

On Thursday, November 14, 2013 6:54:40 PM UTC-5, Dave Springford wrote:
> Well... if you had a winch, that 100-300 ft rope break would occur while you still have 2000 ft of runway ahead of you - I like that option a whole lot better than the trees off the end of the runway whilst on aerotow.
>
>
>
> Sorry, it's November and I just couldn't resist throwing the winch wrench into the discussion :)

I wish we had the space for winch launch! The undeniable advantage of a good tow plane is being able to operate out of a small airport.

T8

[Dave Springford]

What is small? We have 3000 ft fence to fence and use about 2500 for the launches. Typical heights 800-900 AGL in the summer, 1100-1200 in the winter. Best launch was 1600 AGL.

satellite image of the airport:
http://goo.gl/maps/ngz44

[Dan Marotta]

Our ropes are 200 ft long so the jig would either have to be very large or
the rope would need to mounted and dismounted numerous times.

I've had three rope breaks in the past three years while flying the tug at
Moriarty - never before in the previous 20+ years. Two were the fault of
the glider pilot horribly mishandling the glider and one of those resulted
in the rope being wrapped around the wing of the HP-14. The wing was cut
back to the spar before the rope broke. The third rope break had one of
our instructors flying his Libelle and the rope broke at about 300' AGL on
tow. He and I were both surprised and he handled the emergency perfectly.
The rope broke about 10 feet in front of the glider, probably in an area of
high wear.

We inspect our ropes daily in the morning and during operations throughout
the day. If sufficient wear is noted, the rope is replaced. More often
it's the weak link which gets replaced due to abrasion with the pavement.
We use the top half of drinking water bottles, slipped over the weak link,
and wrapped heavily with duct tape. These work well, but, if the tug lands
such that the protector hits the end of the runway, it's ripped off. We
then replace the weak link.

(Whew!)


"son_of_flubber" > wrote in message
...
On Thursday, November 14, 2013 10:50:46 AM UTC-5, Dan Marotta wrote:
> Are we gonna fly today or will the eight hour test and evaluation session
>
> use up all available daylight?

Good point. Time is limited.

If an elongation test would detect weakening ropes, you would only need to
do it once a week or so because many forms of degradation is gradual. If
you had a permanent jig for testing elongation, it would take 10 minutes.
Or you could test the ropes at the beginning of wave season when you know
they are gonna get stressed (or even better, you could replace your ropes at
the beginning of wave season).

Ever have a rope break at Moriarty? I know that you have a lot of grit.

[Evan Ludeman[_4_]]

On Thursday, November 14, 2013 7:44:29 PM UTC-5, Dave Springford wrote:
> What is small? We have 3000 ft fence to fence and use about 2500 for the launches. Typical heights 800-900 AGL in the summer, 1100-1200 in the winter. Best launch was 1600 AGL.

http://www.airnav.com/airport/2B9

Evan Ludeman / T8

[Dave Springford]

That is small!

[Peter Higgs]

Is there a difference between a Winch launch Weak Link, and an Aerotow Weak
Link?

I would think the Winch launch requires a much stronger link because of the
higher angle of attack.


At 00:46 15 November 2013, Dan Marotta wrote:
>Our ropes are 200 ft long so the jig would either have to be very large or

>the rope would need to mounted and dismounted numerous times.
>
>I've had three rope breaks in the past three years while flying the tug at

>Moriarty - never before in the previous 20+ years. Two were the fault of

>the glider pilot horribly mishandling the glider and one of those resulted

>in the rope being wrapped around the wing of the HP-14. The wing was cut

>back to the spar before the rope broke. The third rope break had one of

>our instructors flying his Libelle and the rope broke at about 300' AGL on

>tow. He and I were both surprised and he handled the emergency perfectly.

>The rope broke about 10 feet in front of the glider, probably in an area
of
>
>high wear.
>
>We inspect our ropes daily in the morning and during operations throughout

>the day. If sufficient wear is noted, the rope is replaced. More often
>it's the weak link which gets replaced due to abrasion with the pavement.

>We use the top half of drinking water bottles, slipped over the weak link,

>and wrapped heavily with duct tape. These work well, but, if the tug
lands
>
>such that the protector hits the end of the runway, it's ripped off. We
>then replace the weak link.
>
>(Whew!)
>
>

[GM]

On Friday, November 15, 2013 7:04:22 AM UTC-5, Peter Higgs wrote:
> Is there a difference between a Winch launch Weak Link, and an Aerotow Weak
>
> Link?
>
>
>
> I would think the Winch launch requires a much stronger link because of the
>
> higher angle of attack.
>


Peter - you seem to confuse angle of attack with deck angle. The angle of attack of the glider during a winch launch is no different than if the glider was in free flight at the same speed! During a winch launch, the glider is flying up a trajectory with a deck angle of up to 45degr.
And yes, the weak link strength spelled out in the plane's POH typically call for a stronger one for the CG hook vs. the nose hook.
With our winch, we measured the line forces and the results show an interesting force distribution - not at all what everybody expected.

Uli Neumann
'GM'

[James Metcalfe]

At 12:39 15 November 2013, GM wrote:
>... The angle of attack of the glider during a winch launch is no
different than if the glider was in free flight at the same speed!

Actually, it is significantly different: the wing has to deal with the
tension in the cable (including the cable self-weight) as well as
supporting the weight of the glider. This could easily double the
AoA required in steady flight & smoth conditions towards the end of
a winch launch.

[Steve Leonard[_2_]]

On Friday, November 15, 2013 7:41:22 AM UTC-6, James Metcalfe wrote:
> At 12:39 15 November 2013, GM wrote: >... The angle of attack of the glider during a winch launch is no different than if the glider was in free flight at the same speed! Actually, it is significantly different: the wing has to deal with the tension in the cable (including the cable self-weight) as well as supporting the weight of the glider. This could easily double the AoA required in steady flight & smoth conditions towards the end of a winch launch.

Well, James, now you have gone and confused AoA with lift coefficient. You do not necessarily double AOA to double lift coefficient. Lift coefficient generally goes up about. 1 per degree of AoA while you are below separation of flow on the wing surface. At some point on the curve, doubling AoA doubles Cl, but only at that one point. And I would guess that point is no where near where you operate on a winch launch.

Steve

[StaPo]

as different to many above statements,
we in central Europe are mostly connecting weak link of tow rope to towing plane release end, not to the glider end.
Two good reasons:
- there is usually practised towing plane landings with tow rope still connected due to economical and practical reasons. In this case, the rope end with weak link is not get dirty by contact with soft ground and not subjected by direct hit to possibly hard surface, then damage probability of weak link assy is lowered. And yes, we are mostly using mounting climber type braided nylon ropes for our aerotows (dia 7-12mm), on which the damage usually starts by weared rope braid on glider end (mostly from hit to ground during landings), which indicates that there should be some corrective action taken (rope shortening during saison, replacing after saison).
- when towing plane hits some obstruction via towing rope during landing, there is safety weak link very useful to avoid damage to towing plane and pilot.
- exception: tow planes with retractable rope device by electric drum winch (still not so common, but slowly grows) are equipped with weak link on glider end. But they do not land with rope extracted, then above two points do not apply to them.

[son_of_flubber]

On Friday, November 15, 2013 2:37:32 PM UTC-5, StaPo wrote:

> we in central Europe

How long are your tow ropes?

[James Metcalfe]

At 16:02 15 November 2013, Steve Leonard wrote:
>Well, James, now you have gone and confused AoA with lift
coefficient. You do not necessarily double AOA to double lift
coefficient. Lift coefficient generally goes up about. 1 per degree of
AoA while you are below separation of flow on the wing surface. At
some point on the curve, doubling AoA doubles Cl, but only at that
one point. And I would guess that point is no where near where you
operate on a winch launch.

No confusion in my mind ... when I mentioned doubling I was just
pointing out that the difference was substantial. I could have been
less conservative and said 'tripled' or 'quadrupled'!

If you look at a typical graph of lift coefficient vs AoA you will see
that to double the lift you can expect to much more than double the
AoA. (Although the curve is substantially linear for most of the AoA
range, it doesn't pass through the origin). Not that I said anything
at all about doubling the lift!
J.

[StaPo]

Dne pátek, 15. listopadu 2013 23:00:04 UTC+1 son_of_flubber napsal(a):
> On Friday, November 15, 2013 2:37:32 PM UTC-5, StaPo wrote:
>
>
>
> > we in central Europe
>
>
>
> How long are your tow ropes?

40m standard length, 15m special version for off-field aerotows (quite rare nowadays). I remember that 31 years ago (when I was learning to fly) there was rule for 30m ropes without weak link, but I do not remember when it was changed to current 40m. Many years ago...

[son_of_flubber]

On Friday, November 15, 2013 6:53:35 PM UTC-5, StaPo wrote:

> > How long are your tow ropes?
>
> 40m standard length, 15m special version for off-field aerotows (quite rare nowadays). I remember that 31 years ago (when I was learning to fly) there was rule for 30m ropes without weak link, but I do not remember when it was changed to current 40m. Many years ago...

For our USA friends... 40m = 131 feet. The usual North American tow rope is 200 feet (say 61 meters). That seems like a big difference (some of the physics is not linear).

It seems odd that the experts inside a country can agree on tow rope length, while the experts in different countries see things so differently. I had (naively) thought that tow rope length would be a matter decided by physics and engineering, but it appears to be decided by cultural perspective and tradition.

What other operational parameters and procedures in soaring are matters of culture and tradition?

Would my Polish SZD glider fly better behind a 131 foot tow rope?

[Piotr Szafranski]

Regular use of shorter tow ropes probably prepares pilots better for tows in turbulent air. This is how I understand this tradition here in C. Europe. Also, I think it would be harder to tow on a long rope while tightly (perhaps up to 45 deg bank) circling in a thermal, which is practiced by some tow pilots here. Sure such tows are not boring.

One case is not a proof of anything, but this summer I had seen whole bunch of fairly early (about 20-30h) solo students dealing ok with tight circling tows in strong (turbulence) thermals, while a MUCH more experienced foreign pilot had a rope break. I would guess being (not)used to following a bit shorter rope was a factor, but it is just my guess. I do not question this pilot skills, just the issue of being familiar with this setup.

So I would guess the SZDs do like to fly on a shorter rope :-)

On Saturday, November 16, 2013 4:27:57 AM UTC+1, son_of_flubber wrote:
> On Friday, November 15, 2013 6:53:35 PM UTC-5, StaPo wrote:
>
>
>
> > > How long are your tow ropes?
>
> >
>
> > 40m standard length, 15m special version for off-field aerotows (quite rare nowadays). I remember that 31 years ago (when I was learning to fly) there was rule for 30m ropes without weak link, but I do not remember when it was changed to current 40m. Many years ago...
>
>
>
> For our USA friends... 40m = 131 feet. The usual North American tow rope is 200 feet (say 61 meters). That seems like a big difference (some of the physics is not linear).
>
>
>
> It seems odd that the experts inside a country can agree on tow rope length, while the experts in different countries see things so differently. I had (naively) thought that tow rope length would be a matter decided by physics and engineering, but it appears to be decided by cultural perspective and tradition.
>
>
>
> What other operational parameters and procedures in soaring are matters of culture and tradition?
>
>
>
> Would my Polish SZD glider fly better behind a 131 foot tow rope?

[GM]

On Saturday, November 16, 2013 12:42:24 AM UTC+1, James Metcalfe wrote:
> At 16:02 15 November 2013, Steve Leonard wrote:
>
> >Well, James, now you have gone and confused AoA with lift
>
> coefficient. You do not necessarily double AOA to double lift
>
> coefficient. Lift coefficient generally goes up about. 1 per degree of
>
> AoA while you are below separation of flow on the wing surface. At
>
> some point on the curve, doubling AoA doubles Cl, but only at that
>
> one point. And I would guess that point is no where near where you
>
> operate on a winch launch.
>
>
>
> No confusion in my mind ... when I mentioned doubling I was just
>
> pointing out that the difference was substantial. I could have been
>
> less conservative and said 'tripled' or 'quadrupled'!
>
>
>
> If you look at a typical graph of lift coefficient vs AoA you will see
>
> that to double the lift you can expect to much more than double the
>
> AoA. (Although the curve is substantially linear for most of the AoA
>
> range, it doesn't pass through the origin). Not that I said anything
>
> at all about doubling the lift!
>
> J.

James, Steve

not wanting to hi-jack this thread but let me re-state what I meant to say more precisely: the AoA during the winch launch is no where near the the stall AoA. We did fly several winch launches with a side-string attached on the outside of the canopy. Since the line pull forces during the steepest part of the climb are nowhere near the forces shortly before the release and since we are using Spectra, which weighs a few pounds for the 4,500ft we can lay out, the loading on the glider is well below 2g. So the AoA should be that of a glider loaded at 1.5 - 2g.

Uli

[Bill D]

On Tuesday, November 12, 2013 9:16:19 PM UTC-7, GM wrote:
> On Tuesday, November 12, 2013 11:22:14 AM UTC-5, son_of_flubber wrote:
>
> > With the dewpoint at 15d F. today, I'm ready to heat things up.
>
> >
>
> >
>
> >
>
> > Regulations on aerotow ropes vary by region. What is the evidence to support the various approaches?
>
> >
>
> >
>
> >
>
> > PTT (Premature Termination of Tow)(aka rope break) correlates with a number of fatal accidents. How often has a hard_to_break towrope led to a fatal accident?
>
> >
>
> >
>
> >
>
> > Options to land after a PTT vary by airport and can be non-existent at certain altitudes. Why is an extra strong towrope not an option where the straight-ahead landing option is poor or non-existent?
>
> >
>
> >
>
> >
>
> > In what year did the FAA set the FAR for towropes and what was the quality of towropes at that time? (obscure, but interesting question).
>
> >
>
> >
>
> >
>
> > At the airport where I am towed, the same towrope satisfies the FAR for both the frequently towed heavier two-seater, and my less frequently towed lighter weight single place glider. Does this mean that the two-place is more likely to break the towrope?
>
>
>
> The 80-200% rule is in the FARs but it DOES NOT APPLY to ANY glider which - at the time of US-certification (standard or experimental) - had a manufacturer's issued and approved POH, which spelled out the breaking strength of a weak-link for the CG and aero-tow hook! In some cases, these may be the same; in most cases, the aero-tow weak-link strength is lower.
>
> In Europe, typically very strong tow ropes (mooring lines, as one US visitor to Europe called them) are used but there is a proper weak-link attached on the glider end.
>
> A short while ago, I did launch a quick survey on the German forum to find out how many tow rope failure anyone had. The response: zero tow rope failures!

Precisely!

FAR 91.309 (the 80 - 200% rule) is for gliders whose POH didn't specify aero tow weak links (Think Schweizer). FAR 91.9 is for those which do. (Think any glider certificated under JAR-22/CS-22) If your manual specifies an aero tow weak link, 91.9 requires you to use it. 91.9 trumps 91.309 - that's the law.

The combination of 91.309 and 91.9 pretty much requires a weak links on each end of the rope with the one on the tow plane end 25% stronger than the one on the glider end. With two weak-links why would any sane person want to use a weak towrope?

We should use massively strong ropes and let the weak-links handle overloads. That way there should never, ever be a rope break. It's not what we do - and we pay the price.