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HOW EFFECTIVE ARE AVALANCHE AIRBAGS? FIELD TESTS OF AVALANCHE SAFETY EQUIPMENT

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  • GEOPRAEVENT

Abstract and Figures

Avalanche transceiver-shovel-probe. This still is the standard equipment recommended for touring in the backcountry. More and more, off-piste and backcountry recreationists carry additional avalanche safety gear such as avalanche airbags. In a series of field tests with four artificially triggered avalanches, we compared the effect of additional safety equipment. We measured burial depth and visibility of dummies equipped with two different brands of avalanche airbags (ABS and Snowpulse), the avalanche ball and of dummies with no additional equipment. The burial depth of dummies equipped with an airbag was significantly lower compared to dummies which carried an avalanche ball or no additional equipment. Moreover, based on a qualitative validation the airbag systems were rated better than dummies without airbag. Both brands of airbags and the avalanche ball were visible in all cases on the surface of the avalanche deposits-partly due to the avalanche size and the path topography. Acceleration measurements at the head of the dummies suggest that the risk of injury may be reduced with an appropriate form of the airbag.
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HOW EFFECTIVE ARE AVALANCHE AIRBAGS? FIELD TESTS OF AVALANCHE SAFETY
EQUIPMENT
Lorenz Meier *, Stephan Harvey
WSL Swiss Federal Institute for Snow and Avalanche Research SLF, Davos, Switzerland
ABSTRACT: Avalanche transceiver – shovel – probe. This still is the standard equipment recommended
for touring in the backcountry. More and more, off-piste and backcountry recreationists carry additional
avalanche safety gear such as avalanche airbags. In a series of field tests with four artificially triggered
avalanches, we compared the effect of additional safety equipment. We measured burial depth and
visibility of dummies equipped with two different brands of avalanche airbags (ABS and Snowpulse), the
avalanche ball and of dummies with no additional equipment. The burial depth of dummies equipped with
an airbag was significantly lower compared to dummies which carried an avalanche ball or no additional
equipment. Moreover, based on a qualitative validation the airbag systems were rated better than
dummies without airbag. Both brands of airbags and the avalanche ball were visible in all cases on the
surface of the avalanche deposits – partly due to the avalanche size and the path topography.
Acceleration measurements at the head of the dummies suggest that the risk of injury may be reduced
with an appropriate form of the airbag.
1. INTRODUCTION
„Your Whole Life Fits into this Backpack“,
„Head On Top Technology (H.O.T.)“ or „the fastest
system to locate buried people in avalanches“ –
With such slogans additional safety equipment
gets presented to backcountry and off-piste skiers.
Manufacturers justify these statements by referring
to own tests and also to tests which were done by
the SLF in winter 1994-1995 (Tschirky and
Schweizer, 1997) and 2000-2001. (Kern et al.,
2001) showed that in a flowing avalanche a
segregation takes part so that larger particles tend
to stay closer to the surface than smaller particles.
Airbag systems take advantage of this physical
process when skiers release a balloon at the
moment they get caught by an avalanche. In the
last 20 years, a number of commercially available
products have been developed. This additional
safety equipment complements the standard
equipment (Avalanche transceiver, shovel and
probe) and is used more and more often in
practice. Therefore the magazine “K-Tipp” (a
Swiss magazine for consumer protection) wanted
to do new field tests with current safety equipment
(Cetojevic et al., 2011). SLF developed the test
concept and coordinated the field tests that were
carried out in winter 2010-2011 near Davos.
______________________
* Corresponding author address:
Lorenz Meier, WSL Swiss Federal Institute for
Snow and Avalanche Research SLF,
Flüelastrasse 11, CH-7260 Davos Dorf,
Switzerland, phone: +41 81 417 02 50, fax: +41 81
417 01 10; email: meier@slf.ch
2. METHODS
The tests included the airbag systems
ABS and Snowpulse (SP) as well as the
avalanche ball (LB). The products were carried by
full-sized rescue dummies with a weight of 75 kg.
In total, four avalanches were released artificially.
For the first two avalanches, 8 dummies per
avalanche were placed on the slope. After a
dummy has been destroyed in the second
avalanche, there were only 7 dummies left for the
last two experiments. Four dummies in a row were
placed by helicopter in the avalanche slope. One
row was higher up in the slope, the other row
100 m below (Fig. 1). Each product was placed
once in each row and was therefore tested twice
per avalanche. Two dummies (one per row) were
only equipped with transceiver and no additional
equipment (Nix). The dummies in the upper row
were equipped additionally with an acceleration
logger attached to the back of the head. These
loggers recorded the three dimensional
acceleration at a rate of 1600 Hz. To protect the
logger, these dummies carried a helmet.
Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska
756
Figure 1: One row of four dummies placed in the
avalanche slope. The order of the safety
equipment was changed each time.
Figure 2: Position of the two rows with 4 dummies
each in the avalanche slope.
Towards the end of February 2011 the
conditions were ideal to trigger avalanches. During
a dry and cold period from 26 January onwards
the surface of the snow cover had transformed
into facets with low cohesion. This weak layer was
covered by 20-30 cm of new snow on 21-22
February. Accompanying northwesterly winds
additionally loaded the northeast-facing slopes
near the Flüelapass above Davos. The tests were
carried out on 26 February and 1 March 2011.
Table 1 describes the four triggered avalanches.
The fracture line of the avalanches was in steep,
rocky terrain and could not be visited and
characterized in detail due to a lack of time. The
fracture depth was more than 1 m. The outline of
the debris was measured with a GPS-based laser
rangefinder. The position of the dummies was
determined before and after the avalanche release
by GPS (Fig. 3).
At all four avalanche sites the terrain was similar
without sudden changes in terrain features and
slope angle. There were hardly any possibilities for
the debris to accumulate in deep trenches or
depressions. The avalanche runout zone was thus
rather favorable for a victim and not representative
for all possible avalanche burial situations. This
also reflects in the average burial depth of 53 cm
of completely buried dummies in comparison with
the long-term mean of 100 cm (Harvey et al.,
2002).
Table 1: Characteristics of the four avalanches
Avalanche no. 1 2 3 4
Date 26 Feb
2011
26 Feb
2011
1 Mar
2011
1 Mar
2011.
Number of dummies caught 4/8 8/8 6/7 7/7
Not caught all in
bottom row
- LB top row -
Mean burial depth of dummies without
airbags (cm)
18 43 63 45
Difference in elevation dummies were carried
down by avalanche (top row/bottom row (m)
120/0 220/60 200/140 200/120
Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska
757
Figure. 3: Outlines of the avalanches 1, 4, 2, 3 (from left to right) and the positions of the dummies before
() and after (×) the avalanche. Dummies of the upper row before the avalanche are denoted by the suffix
‘1’, the ones from the lower row by ‘2’. ABS = Airbag ABS, SP = Airbag Snowpulse, LB = Avalanche ball,
NIX = without additional equipment.
3. RESULTS
3.1 Burials
The following parameters were recorded
from each dummy after the avalanche: Visibility
of part of the body and of safety equipment,
position of dummy, burial depth of airways, head
and body.
The airbag systems ABS und SP as well
as the avalanche ball were visible on all
avalanche deposits. Mean burial depth and
position are summarized in Tab. 2.
The limited number of tests makes
statistical comparison of burial depths
challenging. Therefore, we performed a
bootstrap simulation (Davison et al., 1997) on
the basis of the poor data to get reliable mean
burial depths and to do meaningful statistical
testing. Fig. 4 and 5 show the distribution of the
measured airway burial depths of dummies with
and without airbags. The larger scatter for the
burial depth without airbag is obvious. The 95%
confidence interval is between 25 and 63 cm,
whereas for burial depths with airbag it ranges
from 8 to 26 cm.
With ai rb ag
Airway burial depths [cm]
Frequency
0 20406080100
0246810
Figure 4: Histogram of airway burial depths for
dummies with airbag systems. The red line
shows the mean of the data, the doted red line is
the 95% confidence interval calculated from
2000 bootstrap simulations.
Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska
758
Without airbag
Airway burial depths [cm]
Frequency
0 20406080100
0246810
Figure 5: Histogram of airway burial depths for
dummies without airbag systems. The red line
shows the mean of the data, the doted red line is
the 95% confidence interval calculated from
2000 bootstrap simulations.
To check if these differences of burial
depths were statistically significant we tested the
hypothesis that airbag systems do not reduce
burial depths (H0).
We used the non-parametric Wilcoxon
test for testing the null hypothesis with the poor
amount of measured data. To improve the
estimate and confidence interval, it was checked
with a bootstrap test whether the difference of
mean and median as test statistics for the
bootstrap sample was significant. Tab. 3 shows
the probabilities (p-values) under the null
hypothesis.
Considering p-values of less than 0.05
as statistically significant, the results show that
the burial depths were lower for the dummies
carrying an airbag system compared to the
control dummies. There are no significant
differences in burial depths between the two
airbag systems ABS and Snowpulse (SP) on the
one hand, as well as no significant differences
between the avalanche ball (LB) and the
dummies without any additional safety
equipment (Nix). There was also no correlation
between the position of the dummies
(upper/lower row) and the tested equipment.
Table. 2: Mean burial depth, standard deviation and position of dummy in the debris.
Airways
(cm)
Head
(cm)
Body
(cm)
Position of body:
back/belly/lateral (counts)
Nix (n=5) 49 ± 31 47 ± 34 51 ± 30 4/1/0
LB (n=6) 37 ± 22 27 ± 24 40 ± 22 3/3/0
ABS (n=7) 12 ± 14 7 ± 15 26 ± 22 3/2/2
SP (n=7) 19 ± 13 7 ± 11 22 ± 22 3/3/1
With airbag (ABS/SP, n=14) 15 ± 13 7 ± 13 24 ± 21
Without airbag (nix/LB, n=11) 42 ± 26 36 ± 30 45 ± 25
Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska
759
Tab. 3: p-values of Wilcoxon tests and bootstrap
tests concerning the null hypothesis.
Wilcoxon
test
Bootstrap
difference
mean
Bootstrap
difference
median
Burial
depth
airways
0.002 <0.001 0.012
Burial
depth
head
0.004 <0.001 0.01
Burial
depth
body
0.025 0.01 0.023
3.2 Evaluation of burial
Burial time is crucial for surviving in an
avalanche. After 18 minutes, 91% of completely
buried people in avalanches are still alive
(Brugger et al., 2001). Therefore it is important
to minimize the burial time. From this point of
view we assigned to each dummy credits for
criterions which reduce burial time:
a) 1 credit point if equipment or dummy was
visible from far away.
b) 1 credit point if the head was visible
c) 1 credit point if the airways were buried less
than 10 cm under the surface.
The number of credit points was higher
for the dummies equipped with an airbag system
than for the dummies equipped with the
avalanche ball or with no additional equipment
(Table 4).
Table. 4: Given credits for buried dummies with
mean and +/- one standard deviation. If only one
or two credits were given, the additional letters
indicate which credit points were given.
Credit points median
Nix (n=5) 0/0/3/0/0 0
LB (n=6) 2ab/1a/3/1a/1a/1a 1
ABS (n=7) 3/3/3/1a/2ab/3/3 3
SP (n=7) 3/3/2ab/1a/2ab/3/3 3
In the same way as for the burial depths
we tested differences between the safety
equipment by the given credits. Tab. 5 shows
the p-values under the null hypothesis stating
that there are no differences between the
systems.
From the results in Tab. 5 we conclude
that:
- Airbag systems (ABS and SP) were rated
significantly better than systems without
airbags.
- Airbag systems were rated significantly better
than the avalanche ball (LB).
- The rating for the avalanche ball (LB) was
only marginally better than for dummies
without any additional equipment.
Table 5: p-values of Wilcoxon tests and
bootstrap tests of the credit point data.
null hypothesis Wil-
coxon
test
Bootstrap-
test
difference
mean
rating for airbag system
(ABS and SP) is not
higher than for control
group (Nix)
0.007 0.002
rating for airbag system
(ABS and SP) is not
higher than for
avalanche ball (LB)
0.02 0.008
rating for avalanche ball
(LB) is not higher than
control group (Nix).
0.041 0.09
3.2 Measurements of the head acceleration
Fig. 6 shows the acceleration which was
measured on the dummies back part of the head
in avalanche no. 2. Depending on the direction
of the impact, the acceleration logger measured
at maximum between 16 and 27 times the
gravitational acceleration (g). In avalanche no. 2,
such high values were measured during some
short peaks for all dummies except for the
Snowpulse system (SP). Also in the other
avalanches, isolated peaks exceeded this
threshold.
Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska
760
010 20 30
0
5
10
15
20
25
Nix
Tim e (s)
Acceleration (g)
010 20 30
0
5
10
15
20
25
LB
Tim e (s)
010 20 30
0
5
10
15
20
25
ABS
Tim e (s)
Acceleration (g)
010 20 30
0
5
10
15
20
25
SP
Tim e (s)
Figure 6: Accelerations measured during
avalanche flow (avalanche no. 2).
How dangerous are these
accelerations?
The car industry uses expensive
dummies where acceleration is measured in the
centre of the head. The head is connected with a
flexible neck to the body. To describe the
severity of a collision, a so-called “head injury
criterion (HIC)” is used (Cichos et al., 2008).
=
5.2
2/3
12
,
2
1
21 )(
)(
1
max
t
t
tt dtta
tt
HIC ,
where the maximum is searched for during a
period (t2-t1) 36 ms.
Although there is no direct correlation
between the HIC and the expected injury, it is
considered as a measure of possible harm
(Marjoux et al., 2008).
Figure 7. HIC calculated from the acceleration
data (top). The bottom graph shows the HIC
normalized by the mean acceleration of all
dummies per avalanche. The vertical lines show
one standard deviation of the data.
In our tests the head was fixed rigidly to
the body. Therefore, the acceleration data would
have to be corrected by the ratio body head to
body mass. This would give approximately 15
times higher acceleration values. Due to the
construction of our dummies, the injury risk
could not be determined absolutely. Therefore
we confined ourselves to a relative comparison
between the tested systems. The top panel of
Fig. 7 shows the HIC-values for all four
avalanches. Due to technical reasons there was
Nix LB ABS SP
100
101
102HIC (36ms)
Nix LB ABS SP
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
HIC (36ms) normalized
Avalanche 1
Avalanche 2
Avalanche 3
Avalanche 4
Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska
761
no data measured for the avalanche ball in
avalanche no. 3 and for the ABS airbag in
avalanche no. 4. The values scatter strongly and
are highest for avalanche no. 2. To compare the
results among all avalanches, the HIC values
were normalized by the mean HIC per
avalanche (Fig. 7, lower panel). The dummies
without additional equipment (Nix) and the ones
with the avalanche ball (LB) had similar force on
the head, whereas the force for the ABS airbag
was higher and the one for the Snowpulse
airbag lower. However, to draw any sound
conclusions, more measurements would be
necessary. Theoretically, the higher force for the
ABS airbag can be explained: People with
airbags stay closer to the surface of the
avalanche where the velocity is highest (Kern et
al., 2004). If you get caught by an avalanche
from above, as in the tests, you stay at the
turbulent front of the avalanche with the highest
stresses. This also applies to the Snowpulse
airbag, but the different shape of the airbag
seems to protect the head and reduce the
acceleration.
The impact on the airbags is large
enough to nearly tear off the backpack from the
body. We often observed the breast strap to be
right below the chin of the dummies after the
avalanche release. The use of leg loops is
therefore recommended although not often used
in practice and also not in our tests.
3. CONCLUSION
For the first time since 10 years, tests
were done with additional avalanche safety
equipment available on the market. In four
typical avalanches for backcountry and off-piste
skiers, we tested the avalanche balloon packs
ABS and Snowpulse as well as the avalanche
ball. The results show that dummies with airbag
systems were buried significantly less deep than
dummies with the avalanche ball or without any
additional equipment. The airways of dummies
without avalanche balloon packs were buried
42 cm on average with a 95% confidence
interval from 25 to 63 cm. The mean burial depth
of dummies with airbag systems was 15 cm with
a 95% confidence interval from 8 to 26 cm.
There were no significant differences of burial
depths between the two airbag systems ABS
and Snowpulse.
The avalanche runout zone was rather
favorable for a victim as specific terrain features
which would promote deep burials did not exist.
The four avalanches triggered for the tests are
therefore not representative for all avalanche
burial situations. Still they allowed finding
differences between the tested systems.
Furthermore, we evaluated each burial
with a rating system. The airbag systems
performed best since they both increased the
visibility as well as reduced the burial depth
significantly. The avalanche ball was also always
visible after each avalanche, only the dummies
without any additional equipment where not
visible in 4 out of 5 burials.
Preliminary results suggest that the only
difference between the two airbag systems ABS
and Snowpulse may be the force acting on the
head as indicated by the acceleration
measurements. With the ABS airbag system the
head seems to be more exposed to
accelerations than with the Snowpulse system.
5. ACKNOWLEDGEMENTS
We would like to thank the Vali Meier
and Daniel Kistler from the avalanche control
and rescue service Jakobshorn for the triggering
of the avalanches and in general their help with
the field tests. Thanks also go to the insurance
company SUVA for financial support and to
MSR-Electronics for the special programming of
the acceleration logger and of course to the
magazine K-Tipp for the good collaboration.
6. REFERENCES
Brugger, H., Durrer, B., Adler-Kastner, L., Falk,
M. and Tschirky, F., 2001. Field
management of avalanche victims.
Resuscitation, 51(1): 7-15.
Cetojevic D., Jaggi D., Birmele Ch., Airbags
können Tourengängern das Leben
retten, K-Tipp Nr. 6/2011.
Cichos D., de Vogel D., Otto M., Schaar O.,
Zölsch S., 2008: Arbeitskreis Mess-
datenverarbeitung Fahrzeugsicherheit,
Crash-Analyse: Beschreibung der
Kriterien, Ausgabe Mai 2008.
Davison, A.C., Hinkley, D.V., 1997: Bootstrap
Methods and their Application.
Cambridge: Cambridge University.
Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska
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Etter, H.J., Schweizer, J. and Stucki, T., 2009.
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2002. Avalanche incidents in
backcountry terrain of the Swiss Alps:
New investigations with a 30 years
database. In: J.R. Stevens (Editor),
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Snow Science Workshop, Penticton BC,
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Kern, M., Tschirky, F. and Schweizer, J., 2001.
Feldversuche zur Wirksamkeit einiger
neuer Lawinen-Rettungsgeräte. In: H.
Brugger, G. Sumann, W.
Schobersberger and G. Flora (Editors),
Jahrbuch 2001. Österreichische
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Höhenmedizin, Innsbruck, Austria, pp.
127-145.
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R., 2008: Head injury prediction
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ULP criteria, Accident Analysis and
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Tschirky, F. and Schweizer, J., 1997. Avalanche
balloon - preliminary test results.
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Workshop, Banff, Alberta, Canada, 6-10
October 1996. Canadian Avalanche
Association, Revelstoke BC, Canada,
pp. 309-312.
Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska
763
... A third study used a series of planned avalanches to test dummies equipped with and without airbags. 8 Of the 5 dummies without airbags, burial depth was a mean of 43 cm, and only 1 of 5 was visible from the surface. In contrast, of 14 dummies with airbags, burial depth was a mean of 15 cm, with all 14 visible from the surface. ...
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Avalanche incidents in backcountry terrain of the Swiss Alps: New investigations with a 30 years database
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Kern, M., Tschirky, F. and Schweizer, J., 2001. Feldversuche zur Wirksamkeit einiger neuer Lawinen-Rettungsgeräte. In: H. Brugger, G. Sumann, W. Schobersberger and G. Flora (Editors), Jahrbuch 2001. Österreichische Gesellschaft für Alpin-und Höhenmedizin, Innsbruck, Austria, pp. 127-145.
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