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.
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).
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.
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.
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.
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.