Safety problem - Convertible rucksack harness
Every second paraglider pilot now chooses to fly a convertible rucksack-harness. How do these harnesses compare on the safety side? The DHV has taken a closer look at five of the most popular convertibles. For the tests, we chose convertible harnesses which have a comparatively normal set of features: e.g. integrated reserve containers and seat boards. All the tested harnesses had two chamber airbags: the lower part of the airbag is separated from the upper by an internal baffle and valve system which allows air to pass to the top.
Airbag energy absorption
Firstly, we tested the new harnesses according to the LTF test specifications. For this, the harnesses were mounted on the DHV Protector test rig (an identical set-up to that of the EAPR, where the harnesses were originally tested) and the airbags were then filled from a fan blowing at 7m/s. After switching off the fan and waiting for exactly 5 seconds, the harness then falls from a height of 165cm (bottom of dummy) and the deceleration is measured. The following maximum values must not be exceeded: 20G for longer than 25milliseconds (ms), 37G for longer than 7ms and 50G as the absolute maximum . All three criteria must be fulfilled. The LTF does not exactly specify if a reserve parachute should be mounted in the harness when testing the protector or not, so to determine the effect of the reserve, we tested all harnesses both with and without mounted reserve systems, when without then with completely opened outer containers.
Protector test set-up
Two of the harnesses did not offer sufficient energy absorption when tested without installed reserve parachutes. The Sup Air Hybrid was particularly poor, with peak values of more than 60G. The Charly Globe had by far the best test results, with peak values under 25G.
The first interesting result of our investigations – the worst-case-sceniaro for the airbags is always when no reserve parachutes are installed and the outer containers are open. This is a very real possibility for landings on reserves – it is often not possible to land standing due to swinging motions etc. Reserve parachutes mounted beneath seat boards offer significant extra protection. Products from Airwave and Sup Air only managed to achieve the LTF maximum of 50G when reserve parachutes were installed in the harnesses. The test winner was the Charly Globe, with a maximum value of just over 20G.
Airbag protectors depend upon a constant air current from the front in order to remain filled. If for some reason the air current is not from the front (e.g. when descending on a reserve or flying a deep parachutal stall), then designs which do not allow the airbag to be filled from this direction will consequently no longer be completely filled.
To investigate this, we performed some additional tests (which are not included in the LTF specifications) with angled air currents. For these tests, a lage fan was placed vertically under the harness airbag, and regulated to produce a current of 7m/s (+/- 20% as the larger fan cannot be as easily calibrated as the smaller one). After filling the airbag normally, the harness was then exposed to the air currents from below for a period of 20 seconds. As expected, only designs which allowed air to enter the bag from below managed to fulfil the LTF test limits.
Practical protector tests
Investigations on the protector test machine have shown that airbags usually only achieve reasonable energy absorption values when they are fully inflated. Partially inflated areas, dents or folds in an airbag often result in a failed test – deceleration values of over 50G. In these cases the air in the airbag first flows to the partially inflated area before it begins to compress and absorb energy. This allows the dummy to hit the baseplate, causing deceleration values of 80-100G to be recorded by the accelerometer. During the practical tests we particularly observed the time taken for airbags to fully inflate – the results give cause for concern – no harness achieved an acceptable result.
Charly Globe
It is most important to check that the airbag plate on this harness is not deformed, before it is put on. Check that the shape of the bottom plate is curved outwards by pulling left and right of it. When this is the case, then the lower part of the airbag fills quickly after take-off, but the top part needs longer to fill properly. If the bottom plate remains deformed, then filling takes much longer. It is not possible to fill the airbag completely within 10 seconds after take-off Looking at the airbag, folds can still be seen when it is completely filled – these were also present on the protector test machine, so we can expect that the good energy absorption values achieved there will be realistic in everyday use.
Swing Connect Reverse
The airbag on this harness remained badly filled during take-off Once in the air, as soon as the pilot sat back in the harness it filled relatively quickly and looked free of folds. The airbag looked better filled in flight than on the protector tests machine, realistic energy absorption should be very good. Swing have reacted quickly to our criticism, and now supply a modification to the harness to help filling during take-off Repeated testing on the protector test machine indicated that the modified airbag can now just manage to fulfil the LTF specifications when merely shaken out and not completely filled (from 4 tests two were positive and two were negative).
Airwave GT-Light
The lightest harness in test took its time to fill properly. As seen in the pictures the airbag is only slightly filled after the start run, and even after the pilot sits back in the harness, the airbag still needs about 20 seconds to fill. Folds and dents in the airbag can easily be recognised in flight, but as these were also present during tests on the protector test machine it is to be expected that the tested values are realistic.
Skyline Piz
The Skyline harness performed similarly. From the near-empty airbag at the end of the start run, nothing much had changed after 10 seconds. Even after 30 seconds, the airbag had still not taken on a completely filled appearance. During tests on the protector test machine the airbag appeared better filled, therefore it can be expected that realistic energy absorption values will lie somewhat lower than the test results.
Sup Air Hybrid
The Sup Air airbag fills badly while the pilot is in an upright position. The fill rate is good once the pilot sits back in the harness, but large folds and dents can be observed in the airbag even after longer periods of flight. Due to this, it is expected that the airbag will perform worse than the already marginal energy absorption values achieved on the protector test machine.
Practical test results conclusions
None of the tested harnesses offered anything like the back protection required in the LTF standards during the start run and take-off phase. It is to be noted that good back protection while close to the ground during launching is particularly important – accidents frequently occur here due to collapses, knots in lines or pilot errors in applying too much brake and resultant crashes with back injuries.
Note: a few points which help make airbag filling better:
✔ Shaking out the airbag or pre-filling it (whirl it through the air – although the filling usually doesn't hold for too long) before putting it on makes for faster filling during the start run.
✔ Consciously pre-fill the airbag by facing into the wind at the start (stronger winds help more).
✔ Plates or elements built into the airbag to help it keep its form should be handled carefully. Bends or deformations in these elements can prolong or prevent a complete filling of the airbag. Always pack the harness to prevent unnecessary bending or deformation of such elements. Check in the harness handbook to see that everything is correctly assembled when unpacked.
✔ Once you have put on your harness, do not sit on the airbag – doing so can deform the above mentioned plates or stiffeners, and also completely empties the airbag.
✔ Mechanical stress and ultraviolet radiation are significant factors on the lifespan of the airbag, especially when frequently used as a rucksack. Frequent packing / unpacking, damage during transport and general wear and tear can degrade the airbag. For maximum lifespan, these harnesses should only be used as rucksacks when necessary (e.g. Walk&Fly tours). For general lift-flying it is better to pack equipment in a separate rucksack.
Shaking-out harness Do not sit on the airbag
The results of these tests indicate that there is a lot of room for improvement in the current LTF protector tests specifications, particularly with regard to protection during the take-off phase. The DHV will propose additional requirements for airbag design in the next revision of the LTF specifications for testing, for example protection which is not dependant on air currents coming only from one direction. In addition, protector tests are often strongly influenced by the presence of a reserve parachute. Here the DHV will continue to test without reserves and with opened outer containers as this configuration is always harder to fulfil, and proposes that this requirement also be included in the next LTF revision.
