About The Cover Image

Cover, 4th Edition

In this photo, a Tucon AZ rescuer rides a highline at the Phoenix Fire Department training academy. This was a training exercise in a 2006 AZ State instructor course.

Friends and colleagues in the technical rescue industry have recently questioned the effectiveness of the rope system illustrated on the cover of Edition 4 of The Essential Technical Rescue Field Operations Guide.

In question is the configuration of the tag lines shown on the carriage and what problems might result due to this configuration if a catastrophic track line failure occurred. In particular, what potential problems could result from having tag line ends cross as shown in figure 1.

The Kootenay highline system has long taught to use 8mm prusiks to bypass the knots and terminate the ends of the taglines at the same carabiner that the prusik for that tagline is connected to.

The Students at the time thought to backup the carabiners. The discussion was that the single carabiners were a single point of failure. I know that is overly redundant as the entire tagline system functions as a secondary, but that was the thought.

Fig. 1 - Tag lines crossing

While this differs from the conventional configuration, I had no concern for a safety issue then nor do I have one now. How the ends of the tagline terminate past the three-wrap 8mm prusik makes no difference whatsoever.

Looking at the system, in a catastrophic failure of the tracklines, both tag line prusiks would need to completely fail for a twisting force to occur. Like many of you, I have pulled lots of 8mm prusiks to major slippage on half inch rope over the past 15 years during pull testing class (figure 2).

Fig. 2- 8mm prusik after 12 inch slip

My experience has been that 8mm prusiks begin to creep at about 1200 pounds force. After a major slip of about 8 to 10 inches, the peak force read is on average 3500 pounds force. This is of course a fast slow pull (running haulers).

I have on two occasions cut single tracklines with a 100 kg load at the mid point of a 75 foot span to demonstrate tagline belay effectiveness. There was no evidence of slipping of prusiks in either case after a 10 foot drop.

Reed Thorne tells me that he cut the trackline on a 300 foot span with a 200 kg load and had no slippage or evidence of damage after a 24 foot drop.

My guess is that it will require in excess of 4000 pounds of peak force at the midpoint in order to get the prusiks to slip just to a point of equilibrium with the knots. At that point, there would still be no twisting of the carriage, as there would be equilibrium between the prusik and the knot.

There is just a lot of rope stretch in the tag lines to absorb energy. Peak forces necessary to slip or fail the prusiks would not develop.

I don’t know how to calculate the statistical probability of both prusiks failing but I think it’s probably nearly the same as one of the carabiners failing on the non-crossing method.

Fig. 3- Long tail bowline variation

I really think this comes down to style and what makes a nice photo. I do not believe that crossed tag line tails is less effective or less safe than uncrossed tails. I simply think it makes no difference whatsoever.

Fig. 4- Double Overhand Bend

Personally, I favor the double overhand bend method in figure 4. However, I want to be sure to address this cover issue in the best way. My thinking is that it is a style that is just not familiar but is no less safe than the other method.

As an author and publisher of an industry standard text, it is very important to me to publish the most accurate information that conveys best practice in the industry. That being said, it also must be stated that this is an industry that is still developing. In many cases there is more than one procedure or technique for a particular skill. What is considered best practice is often dependent on who is talking.

 

Another phenomenon concerning best practice is simply that we tend to take what has been taught in the past as gospel as long as it makes sense. As practitioners, we all benefit by communicating about alternatives and improvements to standard practices. I encourage you to post your comments or questions about this system, or any others in the Field Guide, here.

Thanks, Tom

During the same course that the cover image was taken, I shot a photo sequence showing catastrophic failure of the trackilne. The load was 90 kg and the span was 75 feet. You can clearly see that there is no slippage of the prusiks that attach the tag lines to the carriage. 

Track line failure 1

 

 

 

 

 

 

Trackline failure 2

 

 

 

 

 

 

Trackline Failure 3

 

 

 

 

 

 

Trackline failure 4

 

 

 

 

 

 

Trackline Failure 5

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7 Responses to About The Cover Image

  1. Nicholas Ota says:

    I am wondering if you have, or know where I can get information about types of systems. I am looking for data on number of times certain types of systems have been used in the United States. A breakdown of year by year would be preferable. Mainly English Reeve, Kootany High lines, and Guiding lines.

    • admin says:

      Nicholas,
      Thanks for your question.
      I was recently up at the International Technical Rescue Symposium in Fort Collins CO. I happened to speak with a park ranger from Zion National Park. He said they use a highline on a rescue once or twice a year on average. I think they favor the english reeve.

      Otherwise I have not heard of use stats. I think it is quite low on actual rescues due to the time and logistics penalty for setting them up.

      I think most people practice them because they are part of the NFPA standard (thats why I added highlines to my guide.) The next reason is that people think they are cool. Ultimately, I think most rope rescue practitioners find more practical and expedient ways to perform their rescue.

      I do not know of anyone who collects statistics on highline use.

      I believe offsets like guiding and tracking lines are gaining favor and are very useful. I think we will see use of offsets continue to increase.

      Hope that helps and please send any other questions.

      Tom

  2. Jacob Brock says:

    What is your thoughts on the use of 5/8 rope for the track line, as opposed to the 1/2 rope. I often teach (we use 1/2 rope) that slippage of the prusiks is an indication that we are starting to exceed our 10:1 safety ratio as these prusiks (9mm) tend to slip about 1,000 to 1,200 lbs. Have you done any test on slippage froces using standard 9mm rope prusiks on 5/8 rope?

    • admin says:

      Jason,

      I havent seen any 5/8 rope in at least 10 years (I know its out there though). 8mm prusiks start creeping at 1000 – 1200 lbs force but you dont see a slip in earnest until about 2500 to 3000 lbs force (this is where you see the 10 inch slip and a puff of smoke). I had much discussion on this topic with Reed Thorne and others and in general most agree that with 100 feet of rope on either side of the carriage, there is so much potential stretch that you would never see enough peak force generated to slip the two prusiks.

      I located a sequence of shots that I took that day showing us cutting the track line. There is only a 200 lb load but there is not even a hint of creep much less slippage. I will add them to the “about the cover page”.

      Now if you happen to have long lengths of 5/8 rope for track line, sure, why not use it. Its just really heavy and I dont think its really needed as there is plenty of experience showing good performance of 1/2 rope for the track line provided you don’t exceed recommended pre-tension.

      Great discussion though.

      Thanks for the post.

      Tom

  3. Donald R says:

    Tom:

    I recently came across an article on the web that talked about prusik configurations and the forces that they slip at. It mentioned that in a tandem prusik setup the slippage resistance added by the additional prusik was minimal (200 to 300lbs more). I had read and heard this elsewhere as well, how accurate is this?

    In that same article they mentioned that a test was ran with a loan sharing prusik configuration where two prusik hitches are tied on the same rope using the same prusik cord and are connected via a biner between the prusik (http://i1192.photobucket.com/albums/aa322/Tanglerwr/DSC_0272-640.jpg) it seems that the slippage resistance in this configuration was almost double! (roughly over 3100lbs) have you seen this configuration before?

    I wonder if this set up does offer that much resistance why is it not more widely adopted?

    • admin says:

      Donald,

      You have some good questions. I have pulled a lot of 8mm prusiks on 1/2 inch rope to failure over the years doing slow pull testing to show students what happens. As I mentioned in this post, on average you see a single 8mm prusik begin to creep at about 1200 lbs. force (+/- 100 lbs or so). The single prusik usually binds up after moving only an inch or two and if force continues to increase, there will be a sudden slip (4-10 inches) resulting with prusik melting and subsequent welding to the host rope. That big slip usually occurs between 3500 and 4000 Lbs Force.

      Throughout the 90s and early 2000s, we did a slow pull test in every Rescue technician class we taught (2 to 3) per year. I also did several at fire rescue conferences in las Vegas. I kept records on most of them. Very consistently, tandem prusiks do their big slip between 5000 and 6000 Lbs force. Almost always 1500 to 2000 pounds more than a single 8mm prusik.

      This type of backyard test has been done countless times by rescue teams over the past 30 years with similar results. I am fairly certain that if the configuration shown in the photo (dsc_0272) worked better that a simple set (long, short) of 3 wrap 8mm prusiks, we would be doing it.

      Also remember, this is slow pull. Drop testing with 200kg and 280kg 1m fall on 3m of rope is much more severe. as researchers raised the fall height to 2m on 3m of rope single prusiks simply burned through. Tandem prusiks take longer to burn through (there is twice as much prusik material). It is a bit counter intuitive I guess but so much testing has been done to show that a long and short prusik performs consistently to catch extreem falls. This stuff you speak of really is reinventing past research that proved to be a dead end.

      I will do a full post on this as I have lots of photos.

      Thanks for the question, I hope my answer was not to round about.

      Tom

      • Donald R says:

        Tom Thanks for the reply. I appreciate you including some of those test results, that gives me a more accurate idea on what actually happens during the slip tests. It seems that the information I had gotten from other sources was way off.

        I had just recently found some information on the load sharing prusik configuration and was wondering what your thoughts were on it. Thanks for sharing!