Thinking about ... back protectors
But then the flying time comes to an end, after a record-breaking cross-country or a simple local flight. The pilot prepares his approach to the field, begins his final, and lands gently, as if to bring this magnificent day to a close.
But as we all know, things don't always go according to plan! Wrong landing, obstacles, strong breeze, wind gradient, turbulence, downwind final, etc. In these cases, the approach to the field can become more complicated, and the landing less gentle. The legs remain the best shock absorber for a smooth landing, but sometimes this is not possible, and the pilot ends up with his buttocks on the ground.
Manufacturers have long sought to equip their harnesses so-called "passive" protection, designed to absorb (instead of the pilot) any impact upon contact with the ground and disperse this energy to limit potential injuries. The concept of "passive" safety is important because it does not require any direct action, unlike "active" safety, which directly involves the pilot through their choices, movements, adaptation to the situation, etc.
There are different types of protection, each with its own advantages and disadvantages (as always!).
Foam bag protection consists of one or more blocks of foam, wrapped in a fabric cover. Thicknesses, shapes and densities can vary. The foam contained in the fabric cover is made up of cells and traps air. In the event of an impact, the foam compresses, expelling the air contained in the cells. This air will attempt to pass through the fabric of the cover (by porosity) and the seams. It's the air's difficulty in escaping that provides the shock-absorbing effect, combined with the foam's intrinsic characteristics (density) that also have a shock-absorbing effect. Due to the almost constant volume of foam, thicknesses are generally just sufficient to provide correct cushioning. The greater the foam thickness, the more effective the protection is likely to be. It seems established that for a "classic" foam bag, a thickness of less than 15-17cm no longer constitutes protection worthy of the name.
The quality of foam bag protection therefore comes from :
- Foam quality
- Foam thickness
- Partitioning of foam blocks
- The quality of the fabric used for the covers
- Leakage control (flow and speed)
After a shock, the foam bag must be able to regain its initial characteristics (shape, condition of the cover), otherwise it will certainly not be able to guarantee an acceptable level of absorption during a subsequent shock.
The airbag principle is the same: a volume of trapped air which, following an impact, seeks to escape. In principle, airbags are inflated by the relative wind generated during flight, which penetrates the chamber via a system of scoops (different solutions exist: side, front) and valves to retain the trapped air. The damping principle remains the same as for the mousse bag: gradually evacuate the air contained in the chamber to obtain a shock-absorbing effect. In the event of an impact, the chamber takes a certain amount of time to "pressurize" (air being compressible) and begin to act as a shock absorber, expelling air through the various leaks (fabric porosity, seams, valve system).
Airbags are generally larger than mousse bags, as they take up less space when folded. This ability to inflate larger volumes gives airbags better cushioning characteristics than mousse bags.
As with the foam bag, the intrinsic quality of an airbag comes from :
- Its thickness
- Its shape, allowing more or less rapid pressurization
- Leakage control (flow and speed)
- Scoop performance (placement and shape)
- The quality of the envelope fabric
DESTRUCTIVE PROTECTION :
destructive protection, unlike traditional foam bags, does not work on the principle of a trapped volume of air that absorbs energy by compressing.
This type of protection is made up of "harder" elements that will deform under the effect of impact. We're talking here about mechanical deformation, just as in modern vehicle bumpers, which absorb energy by gradually bending.
This technical solution makes it possible to reduce the thickness of the protection, with good results in the standard tests. In tests we carried out on a prototype, the results were better than with a 45% thicker foam bag.
On the other hand, tests carried out for low-level falls were significantly less good than for airbags or foam bags. This type of protection requires a minimum amount of energy before it starts to deform, and will therefore not be very effective on low impacts. Be careful in this respect.
The quality of a "mechanical deformation" type of protection depends on:
- the type of deformable mechanical elements
- their shape and material
their number and arrangement
After an impact, this type of protection is damaged and cannot be considered operational in the event of a new impact. It is therefore imperative to check them regularly for any degradation that could prevent the protection from functioning properly.
To avoid making protections totally "disposable", manufacturers generally integrate this aspect into their design and ensure that only damaged modules can be replaced.
INFLATABLE PROTECTION :
Unlike an airbag, which compresses the trapped air and lets the pressure escape through controlled leaks, inflatable protection is airtight. When pressure increases on impact, the entire structure of the bag deforms elastically to provide sufficient volume for the compressed air, without allowing it to escape.
Its behavior is similar to that of a foam bag, although it is much more effective in terms of the normative test (165cm).
This type of protection is more effective than a foam bag of equivalent thickness, and like it, has no tendency to shift to the side on off-axis impacts. From the airbag, it recovers the compactness of storage once emptied.
The quality of an inflatable protection comes from:
- its thickness
- the elasticity and watertightness of the pouch
Inflatable protection is designed to be reusable, but it's important to check that it hasn't been punctured, as this would render it unfit for future impacts.
To be able to test the protections and determine their effectiveness, it was necessary to determine a repeatable protocol that would, if possible, be representative of a real-life situation. It was therefore decided at the time that the test should correspond to landing under reserve parachute at 5.5m/s (approx. 20km/h).
harnesses certification standards therefore harnesses this test for back protectors, allowing them to determine whether the level of shock absorption can be considered acceptable in terms of what the human body can withstand without causing significant injury.
Le test effectué consiste en une chute verticale à une hauteur donnée (1m65), protection en place sur un mannequin de 50kg en position assise. Le dispositif enregistre l’énergie absorbée (en g) par la protection, ce qui correspond à l’impact encaissée par le pilote. Cet impact doit se situer au-dessous d’une valeur limite (50 g), et doit également avoir une durée limitée lorsqu’il dépasse certains seuils (<7ms si >38g / <25ms si >20g).
In theory, this value makes it possible to compare different back protectors in terms of "pure" impact absorption.
NOTE: There has also been mandatory CE certification in France since 2017 to market back protectors (level 2 PPE), whether removable or not.
ATTENTION: this certification test represents a single case of landing under your reserve parachute! It should not be considered as a guarantee that the protection in question will be effective in all situations. The result of this test represents only a small aspect of what your passive protection may have to endure: sideways impacts, slipping, on your back, etc.
In real-life situations, the pilot rarely falls perfectly vertically onto the protection. He can arrive from higher up, sideways, with horizontal speed, etc. The possibilities are numerous.
An airbag is often more effective than a foam bag in the event of a vertical fall, but may "shift" in the event of an off-center impact or slide. A foam bag is heavier and bulkier, but often stays in place better during this type of impact. An airbag will also be more sensitive to repeated impacts since it is directly exposed to friction and other damage. A foam bag is placed inside the harness and is therefore better protected from these kinds of problems. It therefore has a longer lifespan and is easy to replace, unlike an airbag, which is often an integral part of the harness. It is therefore important to be aware of the advantages and disadvantages of each type of protection when choosing a harness.
It's also important to consider the protection's coverage zone. As the test is only carried out for a seated position in a perfectly vertical fall, what happens if the pilot is in a prone position, or if he moves backwards on impact (in the event of a stall close to the ground, for example)?
Let's take the theoretical test situation: a perfectly vertical fall from a seated position. In reality, the pilot is slightly inclined (even in the seated position). On impact, with the center of gravity shifted backwards in relation to the axis of the fall, the pilot will move backwards and impact the ground with his back. It is not possible to simulate this during the test, as the dummy is mounted on rails which keep it upright whatever happens.
The manufacturer is therefore free to design the protection of his choice, as long as it is sufficiently effective to pass the test.
As a result, you'll see a number of advertisements touting the benefits of protection:
- Optimum passive safety
- High-performance protection in its class
- Best LTF homologation test results
- All-round protection, from thighs to upper back
- Full back protection
- ...
But be careful to analyze what's really behind it...
- "Extremely high performance protection": The results of certification tests may be within the limit, yes, but by how much? If we find ourselves in the 40-50G range, we've gone over the limit!
- "Integral protection": since the back has not been tested, does the protection in this area seem satisfactory? what thickness? what system (airbag or foam bag)? what volume? Some manufacturers claim to offer full back protection, but with only 4-5 cm of foam in the back area, this is doubtful...
- "Back protection":In the event of a fall onto my back, is only the protection between the ground and my back? Certain positions of the reserve parachute at lumbar level can raise questions in the event of impact ...
- ...
CONCLUSION:
As you can see, the result of the homologation test is a factor to be taken into account in your choice, but it's not enough.
As a pilot, it's up to you to carefully compare the solutions available to you on the market, based on :
- Your flying expectations (touring, competition, cross-country, addictive, etc.)
- Your level and experience. A beginner may need it a little more often.
If you have any questions about the back protection of a particular harness, contact the manufacturer for details!
It's often said that "the best reserve parachute is the one you don't have to use", and so it is with back protection. But just like the parachute, it's important that the back protector is effective if you ever need to use it.
HAPPY FLIGHTS!