Aerotow ropes: short or long, breakable or unbreakable?

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

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?

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?

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?

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

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.

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....

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?

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

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