Tuesday, October 13, 2009

The Avalanche-pack: How it works

The main objective of these packs is to keep you on top (as in not buried) of the avalanche. The advantages are clear to anyone who has taken even the most cursive look at avy literature. After reading independent test reports (performed on dummies, I guess volunteers for avalanche duty are far and few in between…) from Switzerland and Austria it seems pretty clear that in the overwhelming majority of cases the dummies with bags ended up either on top with clear airways or close to the top with the balloon(s) in clear view allowing a really fast rescue when needed.
The bag to the left is ABS Avalanche Packs by Mountain Safety Systems Ltd. The one to the right is the Lifebag by Snowpulse.

The Swiss Federal Institute for Snow and Avalanche Research (SLF) have compiled some really compelling statistics:

"The most efficient means of preventing avalanche fatalities is to avoid complete burial. Of 1886 persons which where caught in avalanches in Switzerland between 1981 and 1998, 735 persons (39%) where fully buried, and 1151 (61%) were partly buried or stayed completely unburied on the surface. 433 persons (23%) did not survive the avalanche. For the completely buried persons, the mortality rate was 52%. For partly buried victims the mortality rate was only 4.2% […] "

Buried: 52% mortality
On top: 4.2% mortality

The main dynamics of an avalanche that make the air bag idea work are twofold:

1) As the avalanche progresses, it acts like a tumbler sending you down under the snow and then spitting you back up and then back down again and so forth as long as it is moving.

2) The reverse segregation effect that means that as the avalanche moves the small particles go to the bottom (falling through the cracks…) while the largest tend to stay on top.

Point 1) above means that If you are below when the avalanche stops, you are in a bad place, and point 2) means that you want to take up as much volume (and little density) as you ever can in an avalanche situation.

On March 16th, 2001 the Swiss Federal Institute for Snow and Avalanche Research (SLF) conducted a test on 13 dummies, some with some without bags. One of the comments was: […] it is striking to see that the burial depths of the dummies without airbags is significantly higher than those of the dummies with airbags, although the difference cannot be considered statistically significant. (Davos, March 16, 2001, Martin Kern, Frank Tschirky, Jürg Schweizer, Page 10). Why is it not statistically significant? Because the sample size is not large enough. This does not change the fact that I tend to find it very significant that I have here an independent test where all dummies with bags ended up on or near the top (beats a 60% shot at it).
The avalanche bags currently sold inflate to 150L (close to 40 gallons) which in most cases doubles our body volume and it does that in about 3 seconds.

We are repeatedly told that the beacon is the answer and that we have 15 min. to rescue our buddy/buddies. As we can read on the ABS System site “The assumed bench mark of 15 minutes is extremely optimistic”. I tend to agree. Depending on where you get your data from, you are told that within 15 min. of burial you have an 85 – 92% chance of making it alive. I have always been really curious about how many of the guys that are deprived of air for 15 min have some form of brain damage? I don’t think I can hold my breath unaffected for 15 minutes… This is not a knock on the beacon, it remains an indispensible tool in my opinion, even with a bag, but I am not too eager to put all my eggs in that beacon only.

It seems that there are only two alternatives to pick from would you want to buy an inflatable avalanche pack. Next post will be: Avalanche-pack face-off!

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