Two-Tension Rope Systems

By Tom Pendley
Published Tuesday, July 1, 2014 | From the July 2014 Issue of FireRescue

With new awareness and research in rope rescue safety, experts are now questioning the effectiveness of the non-tensioned belay. This article will examine pros and cons of this system, as well as alternatives that use a tensioned belay, or two-tensioned rope system (TTRS).

The Evolution of Belay

Two tension systems are the wave of the future. They can be safer and more efficient for rope rescue operations.

Two tension systems are the wave of the future. They can be safer and more efficient for rope rescue operations.

Rope rescue has changed a lot in the past 40 years. We went from gold line and single rope technique in the 1970s to high-tech specialty gear and double rope technique (DRT) today. The transition to DRT was promoted in the 1980s in the interest of safety. In the DRT system, one of the two ropes is traditionally non-tensioned, or “not loaded,” and referred to as the belay line. The belay is a secondary line that provides redundancy during any operation with exposure to falling. The tensioned rope is simply referred to as the mainline or working line.

Plenty of evidence supports the benefits of a two rope system. While it’s nice to have a backup if a mistake is made, there is growing concern that a non-tensioned line may not be the safest configuration for a belay in a rescue system. Because non-tensioned belay lines are widespread, the idea of an alternative is a bit controversial.

The 2013 edition of NFPA 1006: Standard for Technical Rescuer Professional Qualifications specifically states that the belay is “not loaded unless actuated.” This standard was upheld even though many rescue organizations outside of the fire service use tensioned belay lines or “mirrored” rope systems. Still, the merit of a two-tensioned rope system is worth a closer look—no longer can “that’s they way we have always done it” justify ignoring evidence.

Rude Awakening

In 2010, the Phoenix region conducted informal belay testing with all of our rescue technicians as part of regular training (see Belay Test Yields Surprising Results). We created more than 400 main-line failures in a structured setting to allow rescuers the experience of arresting a falling 500-lb. load. Using a tandem prusik non-tensioned rescue belay, the load was arrested with very little downward movement in more than 50% of the simulated main-line failures. But in 10% of the tests, the belay system allowed a disturbing amount of travel (eight to 10 feet) before the load was arrested. The 10 feet of movement was significant, as we began each test cycle with only about 12 feet of rope in play.

This was a rude awakening for many rescuers, as they might be hanging below the basket as the litter attendant in such a scenario. The idea of an immediate stop was replaced with visions of being mowed over by the basket and pounded into the terrain.

Of greater concern is that low-stretch rescue ropes are actually quite stretchy when there is more than 100 feet in use. For example, if using 100 feet of ½” nylon rescue rope for a 600-lb. load (even if the belayer keeps an absolute minimum of slack in the belay), the litter would drop three to five feet just on rope stretch alone. In a real-life case, the litter would likely fall much farther due to the inherent slack of a non-tensioned system. This greatly exposes the rescuer and victim to impact with terrain, especially in a non-vertical scenario. The actual fall potential with a tandem prusik belay is significant and would probably be considered unacceptable to most rescuers if shown the effects of a main-line system failure.

The following video demonstrates the drop tests and the results:

A Controversial Alternative

The TTRS, also called a mirrored system, is growing popular because the potential for big movement is greatly reduced should one side of the system fail. A TTRS shares the load between two ropes, both under tension. Some of the positive aspects of a TTRS are:

  • Force is distributed between two systems, reducing force concentration on any one component
  • Rope stretch potential in the system is greatly reduced in the event that one side of the system fails
  • Rigging is the same for both sides of the system, which lowers complexity in operations and training
  • Rock fall is reduced by limiting loose rope, which has a tendency to wrap around and dislodge rock
  • Both ropes are active, which reduces the tendency of belayer complacency (moving rope in or out without an event or action)
The key to effectively managing a two tension rope system ifs for the two operators to work together toward the goal of maintaining 50-50 tension.

The key to effectively managing a two tension rope system ifs for the two operators to work together toward the goal of maintaining 50-50 tension.

TTRS is not a new concept—it’s been around in various forms for more than 30 years.2 Historically, TTRS was rigged on two descent control devices (DCDs), such as the brake bar rack or two rescue eights. Each DCD was operated by someone maintaining enough friction to control half the system load. The problem with TTRS has always been that in the event of a catastrophic failure of one side, the other brake operator would suddenly be in a struggle to control the entire load when they only had enough friction set for half. This was not a reliable system.

The greatest change for practical use of the TTRS has been the introduction of the CMC Rescue Multi-Purpose Device (MPD). The MPD allows for lowering as well as raising, and it provides progress capture, which means it allows the system to pass the “whistle test”—if someone blows a whistle at any time and says “Hands off the system,” there would be no catastrophic failure. Another DCD that also works in this system is the Petzl ID, yet while the ID is very similar in the lowering function, it creates more friction when it comes to raising.

Two-TensionED Rope System

Two-tensioned rope systems are really quite simple, and once a team is proficient in their use, safety improves tremendously. The required personnel is just like that of a standard system: litter attendant, edge person, safety, mainline, belay line and team leader. (It’s easier to still refer to ropes as mainline and belay line, even if they are essentially identical—both under tension and operated the same way.)

Both ropes require a DCD so they bear part of the load yet retain the ability to hold the entire load if needed. Although it may be necessary to augment the configuration due to terrain, basically two mainline configurations are assembled in a mirrored system.

To begin, the team leader will identify the place where rope will go over the edge (the fall line). Assignments are made and rigging is set up. The team leader confers with the main and belay riggers to come up with the best rigging plan and choice of anchors.

The edge person then establishes a personal travel restriction so that they can operate near the edge. They are also responsible for setting up edge protection or an artificial high directional to protect the rope from friction.

The belay and main operators each give the end of their rope to the litter attendant so that they can rig the basket. Once anchors have been rigged, the ropes are placed in the DCDs in preparation for a system pretension and safety check. If a high directional will be used, it is set up and tethered in position.

Once all rigging is complete, the team leader calls for a safety check and the system is pre-tensioned within the safe area to confirm that all is ready.

Edge Transition


High directional anchors greatly improve the transition over the edge. When using a high directional like an Arizona Vortex, the belay line can be supported by a small pulley system like the AZTEK set of fours, which allows for easy height adjustment. Once the litter is below the edge, lower the belay to a point high enough to keep it off the rock. The belay can be placed under tension immediately when using a high directional. If you are using a low directional like a roller or a pad, the belay line is managed without tension (only hand tight) for the edge transition.

Once the attendant is over the edge and in control, the second rope gets about half of the tension and each rope operates identically to the other.

Hauling with TTRS

When the time comes to haul with a mirrored system, simply construct a 3:1 or a 5:1 MA system on each rope and haul simultaneously. Though it can be tricky to coordinate the dual haul, the advantage is that force is distributed over two anchors and both lines are tensioned, minimizing potential stretch.

The goal with TTRS is to keep tension roughly at 50% on each line. In practice, however, that will be less than consistent. The tension can go from 80:20 to 50:50 to 20:80 and everything in between. It’s okay to have this variance in tension—just shooting for 50:50 still minimizes stretch in both lines.


Belay systems are a complex topic that cannot be completely covered in one article. The intent of this article is to start thoughtful discussion about the pros and cons of techniques available to rescuers. Based upon my personal tests on fall potential from rope stretch, I’ve found that TTRS is a safer method to use when the operational distance is greater than 100 feet. A non-tensioned belay rope system may be better suited for short operations, where slack in the belay can be more easily managed.

The best advice I was given as a young rescuer was to ask questions and conduct my own testing. Following this advice, I have been able to gain a better understanding of systems and make changes accordingly. I would give that same advice to any rescuer: Don’t take my word for it—prove it to yourself.

See the video below for Tom Pendley’s overview of TTRS:


1. Mauthner, Kirk. Mirrored Rope Rescue Systems. Parks Canada—Terrestrial Commission, IKAR 2011. Retrieved from

2. Thorne, Reed. A Young Person’s Guide to Two-Tensioned Rope Systems. Technical Rescue, Issue 63, pg. 28

Sidebar – Origins of the Non-Tensioned Belay in Rescue
After our surprising belay test results, I began to wonder about the origin of the non-tensioned belay requirement in NFPA 1006. In fact, when the 2013 edition was in its public comment period, I monitored the outside input on the revision. One comment by a rescuer proposed additional language to allow a tensioned belay with NFPA-compliant devices. The request was denied and the belay language in Section 5 was kept as it was.

I reached out to industry experts and former NFPA committee members for more insight, yet even veteran rescue leaders couldn’t think of a concrete reason that influenced the “system is not loaded unless actuated” language in the NFPA standard.

A common reason for a non-tensioned belay line is the belief that a slack line is less likely to be cut by rock fall. Of course tensioned nylon rope can be cut easier than non-tensioned nylon, but the case could be made that tensioned rope would make a much harder target for a rock to hit. In addition, non-tensioned belay lines are known to cause rock fall.

This concept may have originated from the climbing community, where climbers are belayed with a non-tensioned rope to enable solo climbing. After all, most of the early rope rescuers in the fire service were rock climbers or mountaineers and borrowed extensively from their climbing experience.

Rescue loads and their potential forces are hugely different from those in recreational climbing. You can catch a 170-lb. falling rock climber with a stretchy dynamic belay—that lead climber manages the risk and fall consequence as they climb. A 600-lb. rescue load is a completely different scenario. A stretchy, non-tensioned belay is a frightening prospect given a 600-lb. load in free fall. Try setting this scenario up with 100 feet of non-tensioned belay rope in play (even with minimal slack). You would not want to be on that basket.


This entry was posted in Training. Bookmark the permalink.

Comments are closed.