Actual Rope Break

I didn't have and actual rope break but had a PTT in 1997. I had about 110 hrs in gliders and was flying an ASW 15B. It was my second flight in that particular glider. The release occured at about 500 ft AGL and I was able to make and abbreviated circuit and land back at the take off Runway 14 at Dansville, NY. Thanks to excellent trainig at the Finger Lkkes Soaring Club, it was not unexpected or very dramatic. The PTT was due to an improper tow ring that someone had put on the tow rope. It was not a Tost Ring Set. I learned to look carefully at the tow ring set before hook up. This resulted in a cleanup of all the improper tow rings in the club. Lets see, 17 years ago, am I due for another one soon? I'm ready!

Chuck Zabinski
N839CZ

On Sunday, June 1, 2014 5:20:14 PM UTC-6, Kevin Christner wrote:
I had my first "rope break" ever today. I was approximately 200-250ft above the ground. Emergency procedures were not a problem.



The Tost released for some reason. Its a nose hook so the back release mechanism could not have been the culprit. Further testing with the wheel break on the ground revealed no problems so I took another tow and had no problems. The ring may not have been engaged properly (doubtful) or the slack I got in the rope was just too much when it tightened back up (probable but the release still should have held).



In any case has anyone else ever experienced an actual emergency unplanned release? In 14 years of flying I have never heard of one. Good thing we do practice this.



2C

I think it's worth inserting in this thread that the Germans suffer essentially zero aero tow PT3 accidents. The US could do the same by strictly observing FAR 91.309, FAR 91.9 and OSHA CFR 1915.112 on rope working strength where life is at risk as well as the Tost specifications on ring pair wear limits. The US and German regulations are essentially the same.

It works like this. Start by using a weak link at the glider as required by the glider flight manual (FAR 91.9) and a weak link 25% stronger at the tug (FAR 91.309) then use a rope 8x as strong as the strongest weak link (CFR 1915.112).

AFAIK, the strongest aero tow weak link called out in a glider manual is 2,436 Lbs-F (LAK-20) so CFR 1915.112 would require a rope with at least 19,500 Lb-F breaking strength. 3/8" Amsteel Blue from Sampson Rope would just meet this requirement. http://www.samsonrope.com/Pages/Prod...?ProductID=872

This rope would be much more expensive than what is commonly used now but it would last much longer - and the lives it would save would be priceless. Rope breaks would be a thing of the past.

A possible argument against Amsteel Blue is its very light weight so rope slack might float above the glider risking entanglement. This could be overcome by using a urethane rope coating as is used on helicopter slings to make the rope heavier and far more abrasion resistant. With a urethane coating, the rope should last decades.

I know someone will point out that a Schweizer hook is limited to 1,200 Lbs-F so it's impossible to comply with 91.9 and 91.309 using this hook. Yep, that's what it means. It IS possible to comply using the Tost tow plane hook.

On Sunday, June 1, 2014 7:20:14 PM UTC-4, Kevin Christner wrote:
I had my first "rope break" ever today. I was approximately 200-250ft above the ground. Emergency procedures were not a problem.



The Tost released for some reason. Its a nose hook so the back release mechanism could not have been the culprit. Further testing with the wheel break on the ground revealed no problems so I took another tow and had no problems. The ring may not have been engaged properly (doubtful) or the slack I got in the rope was just too much when it tightened back up (probable but the release still should have held).



In any case has anyone else ever experienced an actual emergency unplanned release? In 14 years of flying I have never heard of one. Good thing we do practice this.



2C

Kevin,

I had my first (and hopefully only) one with full water ballast 200 ft, windy conditions at Perry a couple of years ago. I was the first glider on the grid, and managed to get it back down safely, without running into the other 61 gliders on the grid! The line crew immediately turned me around and launched again, even before I had a chance to clean out my underwear ;-).

Frank (TA)

On Monday, June 2, 2014 4:08:14 PM UTC-5, Bill D wrote:
"I know someone will point out that a Schweizer hook is limited to 1,200 Lbs-F so it's impossible to comply with 91.9 and 91.309 using this hook. Yep, that's what it means. It IS possible to comply using the Tost tow plane hook."

Only flaw in that logic, Bill, is that the HOOK may be approved for that load, but the AIRPLANE it is attached to may NOT be approved for that load. Sort of like how on that old TV show "The Six Million Dollar Man", he could pick up the multi-thousand pound rock with his bionic arm, but they ignored the fact that his still human spine couldn't take the load and he would have been crushed.

I agree with you 100% that there are better things out there than Schweizer tow hitches. But, you can't suddenly legally pull 2000 lbs on the back end of your Cessna or Piper just because your tow hitch is now good for that much load. You have to know where the weakest link is, and either not exceed its limit, or you must increase the strength of it.

As to reliability, I have never known a Schwiezer release to let go of the still intact ring because the ring was worn. I have known of worn Schweizer releases releasing without pilot command, just like a Tost can do if the ring is worn, possibly beyond its allowed limits. So, neither one is perfect. If you have a choice as to which you want to use, make your decision and be willing to listen to the opinions of others.

Interesting that this thread is called "Actual Rope Break" when the rope in question doesn't appear to have broken? :-) We now return to our regularly scheduled programming...

Steve Leonard

On Tuesday, June 3, 2014 8:01:50 AM UTC-6, Steve Leonard wrote:
On Monday, June 2, 2014 4:08:14 PM UTC-5, Bill D wrote:

"I know someone will point out that a Schweizer hook is limited to 1,200 Lbs-F so it's impossible to comply with 91.9 and 91.309 using this hook. Yep, that's what it means. It IS possible to comply using the Tost tow plane hook."



Only flaw in that logic, Bill, is that the HOOK may be approved for that load, but the AIRPLANE it is attached to may NOT be approved for that load. Sort of like how on that old TV show "The Six Million Dollar Man", he could pick up the multi-thousand pound rock with his bionic arm, but they ignored the fact that his still human spine couldn't take the load and he would have been crushed.



I agree with you 100% that there are better things out there than Schweizer tow hitches. But, you can't suddenly legally pull 2000 lbs on the back end of your Cessna or Piper just because your tow hitch is now good for that much load. You have to know where the weakest link is, and either not exceed its limit, or you must increase the strength of it.



As to reliability, I have never known a Schwiezer release to let go of the still intact ring because the ring was worn. I have known of worn Schweizer releases releasing without pilot command, just like a Tost can do if the ring is worn, possibly beyond its allowed limits. So, neither one is perfect. If you have a choice as to which you want to use, make your decision and be willing to listen to the opinions of others.



Interesting that this thread is called "Actual Rope Break" when the rope in question doesn't appear to have broken? :-) We now return to our regularly scheduled programming...



Steve Leonard

Steve, you have a point. My defense is that the installation of a Tost tow plane hook requires an STC which, one presumes, would take aircraft structure into consideration. The Tost Hook itself is approved for 2,570 Lbs-F. If the aircraft structure is weaker, the STC should state a lower limit.

Most composite 2-seater flight manuals (Grob 103, ASK-21 etc...) call out a 600 daN blue weak link for aero tow so the tow plane link would be a red 750 daN link (1,686 Lbs-F) That's more than a Schweizer hook could take but much less than the Tost.

On Tuesday, June 3, 2014 8:01:50 AM UTC-6, Steve Leonard wrote:
On Monday, June 2, 2014 4:08:14 PM UTC-5, Bill D wrote:

"I know someone will point out that a Schweizer hook is limited to 1,200 Lbs-F so it's impossible to comply with 91.9 and 91.309 using this hook. Yep, that's what it means. It IS possible to comply using the Tost tow plane hook."



Only flaw in that logic, Bill, is that the HOOK may be approved for that load, but the AIRPLANE it is attached to may NOT be approved for that load. Sort of like how on that old TV show "The Six Million Dollar Man", he could pick up the multi-thousand pound rock with his bionic arm, but they ignored the fact that his still human spine couldn't take the load and he would have been crushed.



I agree with you 100% that there are better things out there than Schweizer tow hitches. But, you can't suddenly legally pull 2000 lbs on the back end of your Cessna or Piper just because your tow hitch is now good for that much load. You have to know where the weakest link is, and either not exceed its limit, or you must increase the strength of it.



As to reliability, I have never known a Schwiezer release to let go of the still intact ring because the ring was worn. I have known of worn Schweizer releases releasing without pilot command, just like a Tost can do if the ring is worn, possibly beyond its allowed limits. So, neither one is perfect. If you have a choice as to which you want to use, make your decision and be willing to listen to the opinions of others.



Interesting that this thread is called "Actual Rope Break" when the rope in question doesn't appear to have broken? :-) We now return to our regularly scheduled programming...



Steve Leonard

Thinking about this a bit more, a bigger problem is tow planes which are only approved for use with the Schweizer hook. Ex: Cessna 182's. Technically, they are only legal when towing gliders with a 965 Lb-F (439 daN) or less weak link which would exclude composite 2-seaters. FAR 91.9 trumps FAR 91.309 if the glider's Approved flight Manual contains an aero tow weak link specification.

Hi Bruce

thank you for noting there is extra tension in the aerorow rope due
to the angle of climb. If the climb rate is 3kts (which is typical for a
2-seater at my club) at a towing speed of 60kts, the angle of climb
(theta) = 3/60 radians = 3 degrees, and the extra tension in the
rope is (weight of glider)*(sin theta) = typically 1000*(sin 3
degrees) = 50lbs. This would add to the tension in the rope due to
glider drag, as you say, to make a typical aerotow rope tension of
35+50 = 85lbs.

My estimate of the tension in the aerotow rope during initial
ground roll, assuming acceleration of a 1000lb glider to 60kts in
10secs, is about 315lbs.

Whilst the numbers can be juggled for different sailplanes and
towplanes, I agree with you that the greatest rope tension is likely
to be during initial acceleration i.e. during the ground roll. So
aerotow ropes are 'proof tested' on every ground roll, to a useful
degree. This does not assure that the rope meets the full rated
breaking strain however.

I read Bill's comment of June 2 about the rope perhaps taking
some longer time to actually break. This is a new idea to me, and I
don't know what too make of it. I'd like to hear the evidence for
this effect.

The more important discussion, is whether it is a good idea to train
or teach 200ft turn-backs to our students. Despite it being long
accepted practice, in the USA anyway, I doubt that it is a good idea
in terms of reducing serious accident rates. I note nobody suggests
we teach students to do final turns under 200ft. I wonder why.

I was interested in the suggestion from others that Germany has a
much lower PT3 accident rate, due to their stronger ropes and
weak links, and that this could perhaps be allowed by the FAA.


At 07:04 02 June 2014, Bruce Hoult wrote:
On Monday, June 2, 2014 4:50:10 PM UTC+12, Andrew wrote:
My experience seems logical when one considers that after
liftoff, the tension on the rope should be close to the drag on
the
glider, i.e. about weight divided by L/D, i.e. about 35lbs or less.

No, that's not the case unless you're not climbing.

With a tug flying at 65 knots and climbing at 6 knots (typical for
our
glass two seaters) somewhere around 9% of the weight of the
glider (up to
600 kg or 1300 lb) is being borne by the rope. That's about 120
lbs in
addition to the 35 lbs from drag.

With a 300 kg all up single seater (PW5, Libelle etc) flying a bit
slower
and climbing at over 1000 fpm there is actually even more strain
on the
rope.

I do agree that if it doesn't break on initial acceleration then it
probably won't.

I'm not going to go into the turn back or not question again other
than to
say if you can land safely more or less straight ahead then of
course do
so, but you should also be competent to turn back if that's best.

On Wednesday, June 4, 2014 2:17:46 PM UTC+12, Andrew wrote:
I note nobody suggests we teach students to do final turns
under 200ft. I wonder why.

I believe we train students to try to manage the circuit to aim to make the turn to final at about 300 ft, depending on how close in they are.

If they get it wrong (or hit sink) and find themselves at 200 ft instead, I don't think anyone is going to advise them not to make the turn!


If you were *planning* to make the final turn at 200 ft (which would be perfectly safe) but actually ended up the same 100 ft lower then that's getting very low indeed.


Finding yourself *actually* at 200 ft is not the same thing as planning to be at 200 ft at some point in the future with a chance that you might be higher or lower.

Reasonable questions, Andrew. I'll try to answer some of them to a degree below.

On Tuesday, June 3, 2014 8:17:46 PM UTC-6, Andrew wrote:
Hi Bruce



thank you for noting there is extra tension in the aerorow rope due

to the angle of climb. If the climb rate is 3kts (which is typical for a

2-seater at my club) at a towing speed of 60kts, the angle of climb

(theta) = 3/60 radians = 3 degrees, and the extra tension in the

rope is (weight of glider)*(sin theta) = typically 1000*(sin 3

degrees) = 50lbs. This would add to the tension in the rope due to

glider drag, as you say, to make a typical aerotow rope tension of

35+50 = 85lbs.

My estimate of the tension in the aerotow rope during initial

ground roll, assuming acceleration of a 1000lb glider to 60kts in

10secs, is about 315lbs.
-----------------------------
The European certification standards under CS-22 mentions the aero tow rope tension designers should expect. I seem to recall that number is 150daN. That would be close to your estimate.
-----------------------------

Whilst the numbers can be juggled for different sailplanes and

towplanes, I agree with you that the greatest rope tension is likely

to be during initial acceleration i.e. during the ground roll. So

aerotow ropes are 'proof tested' on every ground roll, to a useful

degree. This does not assure that the rope meets the full rated

breaking strain however.

-------------------
I suspect the loads encountered in slack recovery could be the greatest.
-------------------


I read Bill's comment of June 2 about the rope perhaps taking

some longer time to actually break. This is a new idea to me, and I

don't know what too make of it. I'd like to hear the evidence for

this effect.
------------------
There are lots of engineering papers on rope testing and failure modes available on the Internet. The basic idea is a rope is a large bundle of twisted fibers. Not all those individual fibers are equal in strength nor are they loaded equally. When it gets overloaded some of those fibers reach their breaking point before others and the rope starts to unravel.

The unraveling process is usually spotted when the rope is inspected but sometimes it progresses fast enough for the rope to fail before it gets inspected. In almost all cases there is a time interval between fiber breakage and catastrophic failure, otherwise rope inspections wouldn't work.

All but a couple of the rope breaks I know of happened after lift off and below about 1500 feet with normal tension on the rope.
------------------

The more important discussion, is whether it is a good idea to train

or teach 200ft turn-backs to our students. Despite it being long

accepted practice, in the USA anyway, I doubt that it is a good idea

in terms of reducing serious accident rates. I note nobody suggests

we teach students to do final turns under 200ft. I wonder why.

----------------
This one is easy. A turn to final is a normal operating procedure not an emergency like a rope break recovery. There's no justification for practicing low turns to final. A rope break is a true emergency so the rule book can be disregarded as far as necessary to deal with it safely.

I think there are enough stories, some related here, to say lives have been saved and injuries avoided when pilots returned to the runway after a rope break. I can't recall any training accidents practicing the return-to-runway maneuver but there may be a few. I've done hundreds and don't recall sweating out one of them.
----------------

I was interested in the suggestion from others that Germany has a

much lower PT3 accident rate, due to their stronger ropes and

weak links, and that this could perhaps be allowed by the FAA.

----------------------
This is the ultimate solution - just eliminate rope breaks altogether by using a rope so strong it's hard to imagine it breaking. However, weak links could still break so I would continue the training.

It's not so much the US government "allows" stronger weak links and rope - the Federal Air Regulations and Occupational Safety and Health Administration rules require it. It's just that we haven't been following those rules.

This one is easy. A turn to final is a normal operating procedure not an emergency like a rope break recovery. There's no justification for practicing low turns to final.

Here I think I would disagree with you. In a number of land out situations (small fields) turning final at lower than 200ft would be preferable as it would increase the chance of safely landing in the field.