Twisted Riser Syndrome
Cutting Away: Why It’s Difficult Sometimes
We read in a recent APF News-Sheet of yet another hard cutaway experienced by an Australian skydiver. It prompted me to dig out a still very relevant article from the archives.
Several years ago, there occurred a rash of hard/impossible pulls on cutaway systems, prompting John Sherman to conduct a series of experiments in May 2003, to accurately define the problem. John has written several definitive papers on riser construction and 3-Ring technology (they can be found on www.plabsinc.com under technical Articles).
In one of the experiments a jumper was hung in a suspended harness with two other jumpers hanging on to each Main Lift Web to simulate a “3-G” load. This is often referred to as the “3 bloke test”, as something similar to it was first described by British skydivers. The harness was outfitted with a standard set of Type 17 (1 inch), mini 3-ring risers. In one case the cutaway cable passed through the Type 2a locking loop, which holds the assembly together, but was not inserted into the channel that normally houses it, on the back of the rear risers. Severe twists were put in the risers suspending the “3 bloke” load. The jumpers (in one case a male, in another case a female), were unable to pull the cutaway handle. In another test, the cutaway cable passed through the Type 2a locking loop, and was inserted into the channel that normally houses it, and once again the jumpers were unable to cutaway.
Continuing on with the experiments, the jumpers were suspended with their “3 bloke” load, and this time the cutaway cable bypassed the Type 2a locking loop, and a temporary cable (kind of like a temp pin, but made out of cutaway cable material), was inserted in the loop to hold the assembly together. The excess cutaway cable attached to the handle was stowed in the channel on the back of the risers. The load was twisted to simulate a highly loaded 3-G malfunction. While there was resistance, in all cases the jumpers were able to cutaway from this setup.
Two contributing factors were found to interfere with the ability to cut away: one major, one minor. The first and most significant factor was the side loading or twisting of the rings themselves. This caused the ring to load excessively and transferred that load to the locking loop. This problem has no proposed solution other than avoidance.
The second contributing factor was the "Finger Trapping" of the release cable in the riser channel. The amount of lubrication or the type of cable coating (Teflon vs. Lolon), made a big difference on this factor. The "Finger Trapping" phenomenon is caused by the Type 3, ¾-inch channel being sewn flat to the rear riser web. When this web is loaded to above 1 G it also loads the Type 3 channel creating a very tight relationship between them. Any cable between the channel and riser web is then "Finger Trapped".
Several organizations have attempted to solve this "Finger Trap" problem by providing a hard housing or tubing inside the channel to prevent the channel from collapsing around the cable and trapping it. This solution will lessen the severity of the problem if it is executed properly, and maintained so the tube doesn't slip down into the locking loop preventing cutaway. The drawback is that it is bulky and inflexible.
Our experiments have shown that all that is necessary to achieve similar results is to sew the sides of the channel closer together so there is a looser, roomier “tunnel” for the cable, even when the riser and channel are loaded. This additional space in the tunnel eliminates the "Finger Trapping" potential of the tunnel around the cable.
The primary reason Parachute Labs has not thoroughly embraced the hard-housing-inserted-into-the- riser solution, is that it has minimal effect in solving the problem of twisted “riser syndrome.” The root cause of the difficult cut away is the side-loading of the rings exerting excessive load on the loop. So what’s the solution? We propose a multi-pronged attack strategy! Firstly, jump a canopy that does not exceed your own ability to control it, through opening. Highly loaded, high-performance canopies can have tendency to spin up. Continue to fly your body through the opening until the canopy is completely open and on heading. Second, make sure your rig is equipped with non-compressible, hard housings for the cutaway cables. If you have soft housings – get rid of them! They can cost you your life. Third, ensure that your cutaway cables are either Teflon (they are red or orange in color, and do not require oiling), or if they are Lolon (yellow), oil them at least once per month.
AFF & Tandem I/E
President, Parachute Labs, Inc.