Avalanche Airbags: How do they work and are they effective? (2018)
Anti-avalanche airbag (a.k.a. balloon) backpacks have become increasingly popular over the last 20 years. They have almost become the fourth essential piece of off-piste safety equipment, alongside a transceiver, shovel and probe. Airbag backpacks are an expensive thing to buy, so perhaps we should find out how they are supposed to work and if they actually do the job.
Before we answer these questions, we need to make it clear that the best way to stay safe in avalanche terrain is not be in an avalanche in the first place. Most skiers are relatively cash rich and time poor. It takes time to gain the knowledge and experience that are best tools for avoiding avalanches. If you only get to ski for 6 or 7 days per winter, are you going to spend 3 of those days going on an avalanche safety course? One solution is to hire a guide or instructor who can provide the knowledge and take the decisions for the group. Another perceived solution is to buy technology to in order to stay "safe". However, equipment such as airbags and transceivers are only there to help even up the odds when things have gone badly wrong: they are certainly not a perfect solution.
If you are caught in un-survivable terrain, by definition you won’t survive no matter what kind of rescue gear you use. There have been a number of prominent accidents in which the victim with a deployed airbag died because he was either strained through thick trees and rocks, deposited in a terrain trap, buried deeply, or went over a cliff. In zero-tolerance terrain, airbags don’t work, beacons don’t work, Avalungs don’t work. Nothing works.
In other words, having a load of equipment doesn't change the fundamental rules of off-piste skiing. Read the avalanche forecast, look at a map and choose the safer places to ski before you leave home. Once you're up on the hill, look around and notice how the snow and weather have been changing. When choosing your descent, think about its aspects, angles and shape changes. Doing this stacks the odds more in your favour and if things do go wrong and there's a better chance that you'll be able to use your safety equipment to sort out the mess.
The Brazil Nut Effect
How do airbags actually get people up on top of avalanches? What is the physics behind this phenomenon? A common misconception is that airbags help people float to the top of avalanches, in the same way that life jackets float shipwrecked sailors on the water. The actual mechanism is known as granular convection, inverse segregation or the Brazil Nut Effect. The following demonstration was especially filmed for this article:
Avalanche airbags work on the same principle as shaking a box of muesli to get all of the nuts to rise to the top. It is a sort of ratcheting mechanism, with smaller particles falling under the bigger ones as the mixture is shaken about. The greater the difference in surface area/volume between the big particle (a person) and the rest of the mixture (the chunks of snow), the better this mechanism will work. However, the sorting mechanism has to be powered by something. In the case of the muesli it's you, shaking the box. In the case of an avalanche it's the mixture of snow and skier tumbling down the mountain.
If we understand how a tool works (transceivers for example) we can use it more effectively and also be aware of the situations where it might not work.
For example if you are the bottom of a terrain trap, such as a steep sided valley, an avalanche backpack will not be effective. The snow will come down on top of you and stop, giving no time for the sorting process to occur. On a slope, depending on where you start in the avalanche, the size of chunks in it and how energetically it is travelling, there will be a minimum distance required to shoogle you to the surface.
The airbag might keep you on top of the avalanche but there are still the hazards of cliffs and trees that might injure you. Some brands of airbag are designed to unfold around the head of the wearer in order to protect it from trauma.
If the user fails to activate the airbag for whatever reason then it is just deadweight in their pack. Problems can also occur if the bag fails to inflate or is punctured. Part of the thinking behind battery powered systems is that they can continue pumping air into the balloons and keep them inflated even if air is leaking out through a hole. One other potential advantage of a battery & fan combination is that it can be packed away and reused immediately, whereas gas bottle systems require replacing or recharging after one use.
Now that we understand a bit about how avalanche airbags are supposed to work, are there any statistics to show that they are useful in real avalanches? Is it worth spending your cash on this expensive bit of off-piste kit?
Gathering data in this area is quite difficult. There are many variables involved, plus avalanche incidents (with or without airbags) that are never reported and so they never make it into the statistics. Different countries collect the data in different ways. When looking at the numbers, it's probably best to bypass the claims on manufacturers' websites and search for some independent data. Even then, we need to think about who asked the questions, what were the questions they asked and who did they ask. We need to go back to our GCSE history skills and remember to evaluate our sources.
The main article in the field was published in Resuscitation, the journal of the European Resuscitation Council, in September 2014. It was co-authored by researchers from around the world, using datasets from Austria, Canada, France, Norway, Slovakia, Switzerland and the United States. Its results showed that,
The adjusted risk of critical burial is 47% with non-inflated airbags and 20% with inflated airbags. The adjusted mortality is 44% for critically buried victims and 3% for non-critically buried victims. The adjusted absolute mortality reduction for inflated airbags is −11 percentage points (22% to 11%; 95% confidence interval: −4 to −18 percentage points) and adjusted risk ratio is 0.51 (95% confidence interval: 0.29 to 0.72).
However the results also showed a non-inflation rate of 20% for various reasons, 60% of which were attributed to people not pulling their handles to trigger the airbag. The team concluded that airbags were effective, so long as they were activated in the first place. Like Anatoly Kvochur in his MiG-29, you have to quickly decide to pull the handle, get your hand to it (despite the various forces acting you) and pull the handle with enough time for the airbag to inflate and do its job. This problem has been recognised for a long time with ejection seats in military aviation.
A version of the paper, aimed more at the general public, was also published in The Avalanche Review and you can read its full text by clicking the link.
Yes, there is data that having an airbag will help you if you're caught in a moving avalanche and have time to deploy it and be shuffled up to the top of the flow. Researchers want more, better quality, more consistent data but for us off-piste skiers, the message is that it is worth spending our money. However, the fundamental message of avalanche safety hasn't changed: get educated and use that knowledge to stay out avalanches in the first place. Choosing to ski in the right terrain is as crucial as ever.
Why do I climb for hours for a handful of turns in untracked snow? Why do I grin & dance afterward? Why is fun such an anaemic answer to the questions above? Powder snow skiing is not "fun". It is life, fully lived in a blaze of reality.
Dolores LaChapelle, "Deep Powder Snow",
40 Years of Ecstatic Skiing, Avalanches, and Earth Wisdom.