Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02202486 1997-04-11
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Method and device for fighting fires from the air
In recent years, a continual increase in the
number of forest fires and extensive blazes recorded
worldwide has been observed. Simultaneou~ly with this
trend, the size of the area destroyed in each fire is
continually increasing. These fires primarily affect
regions of the Northern hemisphere, in particular the
northern coniferous forests of Canada and the tundra of
the Eurasian land mass, and to a less significant extent
the pine stands in the Mediterranean region. The global
volume of harmful emissions resulting from forest fires
has in the meantime been estimated to be approximately
50% of the overall volume.
Current methods of fighting major fires, in
particular forest fires, which exhibit a strongly chaotic
character and spread rapidly in the manner of an
advancing front, have hitherto been insufficient to
extinguish the fires in question quickly and completely.
Hitherto, even in the most favourable scenario, it has
only been possible to check forest fire~. The reasons for
this are extremely diverse. For example, the ground-based
fighting of major fires is essentially carried out using
the same equipment and in the same manner as when extin-
guishing buildings within a residential area with a fully
developed infrastructure. However, in the case of forest
fires one cannot as a rule assume either a sufficient
supply of water or sufficiently numerous and adequately
constructed approach routes for the fire-fighting
vehicles. The latter deficiency is frequently also
accompanied by a general inaccessibility of the source of
the fire and in each case means that one can only act at
isolated points against the front-like fire, preventing
the complete extinguishing thereof.
Although firefighting from the air, which has
been developed over a relatively long period of time,
does improve the accessibility of the source of the fire,
it has to date not contributed to decisive progress.
R~;n;ng down or dropping water from the air by means of
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aircraft or helicopters has proved inefficient despite
high financial outlay, since, firstly, this type of
fighting is carried out only on an extremely isolated
basis and, secondly, makes dropping extremely inaccurate.
Due to the considerable formation of smoke and turbu-
lence, moreover, there are continual obstacles during
deployment from the air, which makes it impossible to
come clo~e enough to the fire from the air and thus in
many cases to ~ucceed with the extinguishing.
A fundamental problem, which in view of the
scarceness of water when fighting forest fires has a
particularly di~advantageous effect, iB the inefficiency
of the deployment of extinguishing water, because using
the methods customary hitherto only about 10% of the
extinguishing water actually has an extinguishing effect.
A large proportion of the water used drains away, unu~ed,
into the earth or remains inaccessible, in the form of
large pools, for any further extinguishing operations on
the ground. This affects both the deployment of air
support and also that of fire-fighting vehicles.
The object of the invention i8 to improve fire-
fighting from the air ~uch that both a significantly
higher extinguishing effect of the water used and also a
considerable enlargement of the area of the fire
extinguished using one such aerial deployment are
achieved, accompanied by an inherently improved accuracy
and the po~sibility of ensuring a maximal level of safety
for the firefighting personnel required for the aerial
deployment.
This object is achieved by the method according
to the invention in that the extinguishing water is
introduced into the source of the fire by means of air-
craft, preferably helicopters, by being atomized to form
a cloud-like mist of microscopic droplets just above the
source of the fire by means of an explosion. This method
is preferably carried out such that a container filled
with the extinguishing water having a wall which can be
bur~t, in which container an explosive charge with a fuse
is arranged within the extinguishing water, is exploded
- - -
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just above the source of the fire. In this method, the
cont~;ner is preferably a drop-shaped or bubble-shaped
bag made of plastic, which is also referred to below as
"exploding bag".
The exploding bag can be brought to the explosion
location just above the source of the fire by means of
the helicopter, hanging from a long holding cable, and
can then be detonated. However, it is exposed to the heat
of the fire for a relatively long time, with the risk of
being damaged and losing the water even before detona-
tion. Also, fundamental safety regulations may stand in
the way of exploding a load hanging from the helicopter.
It is therefore more advisable to uncouple from the
helicopter the exploding bag, which is hanging from the
helicopter by means of the holding cable, in a safe
position well above the source of the fire, to allow it
to fall freely in the direction of the source of the fire
and then to detonate it just above the source of the
fire.
The invention places airborne firefighting on a
completely new footing. In contrast to all known methods,
in which the extinguishing water is ultimately introduced
into the source of the fire "en masse", the invention
provides for the introduction of the extinguishing water
in an extremely finely divided form. High pressures (e.g.
of several thousand bar) are formed inside the homo-
geneous medium water in the exploding bag as a result of
detonating the explosive charge, which pressures burst
the exploding bag and ~uddenly project the water into the
environment in microscopically fine droplets at a high
initial speed (e.g. of over 5000 m/s). Due to dissipation
of the bla~t wave formed during the explo~ion, due to the
air resistance and possibly due to striking obstacles
situated in the vicinity of the detonation site, these
droplets are decelerated after a short time, 80 that a
completely atomized extinguishing region is formed, in
the centre of which the exploding bag was situated. Owing
to the i =ensely large number of extremely small water
droplets and the associated enormously large total
-
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surface area of these dropletg in the direct vicinity of
the burning substances, there is an an [sic] immediate
heat transfer, that is to say a heat transfer which is
complete even within a few fractions of a second, between
the vaporizing water droplets and the fire. The heating
of the water droplets to their boiling temperature, the
boiling method consuming a high heat of vaporization and
the further heating of the steam formed withdraw such
high quantities of energy from the burning environment
that the temperature suddenly falls by several hundred
degrees. In addition, a considerable proportion of the
atmospheric oxygen which is initially still present in
the detonation environment is forced out of the extin-
guishing region by the quantities of steam formed.
The resulting oxygen deficiency in the
extinguishing cloud has a suffocating action on the fire.
Since the temperature in the region of the suddenly mist-
covered fire is simultaneously forced to a comparatively
low level, the formation in this area of combustible
vapours, for example from essential oils, resins or other
gaseous emissions from the wood, i8 brought to a virtual
standstill and the temperature falls below the ignition
temperature required to (re-)ignite these vapours or
combustible organic materials (wood). Together, all this
leads to a spontaneous successful extinguishing which
makes it possible for ground firefighters to gain access
to the extinguished source of the fire and carry out
secondary extinguishing operations, for example beating
out remaining fire pockets using fire beaters.
The following calculation example is cited in
order to illustrate the excellent extinguishing
efficiency of the method according to the invention:
If it is assumed that an exploding bag with a
capacity of 1000 l of water is used and the explosive
charge is metered such that, following detonation in the
centre of the bag, a cloud of water having a radius of
between 20 and 30 m is formed, then this cloud ha~ a
volume of from 33,500 to about 113,000 m3. The heat of
vaporization of water is 539 kcal/kg, which for 1000 l of
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water results in a heat of vaporization of about
2,256,000 kW/s. A sudden withdrawal of a quantity of heat
of 100 kW/s has been calculated as the requirement to
extinguish a burning forest of small pole trees measuring
100 m long, 6 m wide and 6 m high. This corresponds to
about 0.17 kJ/m2. A water cloud of 30 m radius wets a
surface area of about 2,800 m2, which corresponds to a
mean heat of vaporization required to vaporize the water
of about 789 kJ/m2. It can therefore be assumed that
there is a disproportionately high excess of heat which
can be taken up by the finely atomized water, which
results in a sudden extinguishing of almost 3000 m2 of
burning forest, especially if it is al~o borne in mind
that heat is also required to heat the water to boiling
temperature. Such a high level of extinguishing
efficiency has not been known hitherto.
If the method according to the invention is
carried out in a suitable manner, there is no risk of any
hazard to people or equipment. 3 kg of a highly brisant
and high-dose explosive are required to atomize the
quantity of 1000 1 of water assumed in the calculation
example, "highly brisant" being understood generally to
mean an explosive charge which produces a detonation wave
with a propagation rate of more than 5000 metres per
second, and a ~high-dose" explosive charge being present
if more than two kilograms of explosive are used per
1000 1 of water. The detonation pressure of such an
explosive charge iB dissipated 80 strongly that, at 30 m
distance from the detonation site, it is then only about
0.063 kp/cm2. As a result, it is possible to eliminate
any hazard for the helicopter and its crew. It may be
mentioned, in order to illustrate this low detonation
pressure, that a person st~n~;ng upright at 10 m distance
from the detonation site would only feel a strong squall
and would not be thrown to the ground or injured in any
way.
Preferably, a highly brisant modern explosive,
which is water-resistant, can be stored for almost
unlimited periods of time and is scarcely combustible, is
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used for the explosive charge. The latter property is
important for use in the immediate vicinity of the fire.
Modern explosives of this kind can additionally withstand
strong mechanical effects and are thus insensitive to
~hocks and impact~ even when handled roughly. In
addition, they have a high level of safety during
handling, 80 that the risk of accidents resulting from
unintentional, premature detonation can be virtually
eliminated. In a test, an exploding bag filled with
1000 1 of water and provided with 3 kg of such a highly
brisant explosive charge was allowed to fall to the
ground from a helicopter from a height of 150 m without
prior activation of the fuse without the explosive
detonating on impact. Any hazard to firefighting
personnel and the environment is thus reliably eliminated
- in the event of such an undesired, premature release of
the exploding bag.
The enormously high extinguishing efficiency, the
high level of safety for people and materials when
carrying out the extinguishing operation, and the
possibility of a very accurate deployment of the
extinguishing medium are not the only advantages provided
by the method according to the invention.
The method according to the invention can be used
to fight forest fires and extensive blazes without
causing ecological harm. The extinguishing deployment
leaves scarcely any traces, but by contrast, owing to the
high extinguishing efficiency, the destruction of
biological substance, which can never be completely
avoided in the event of a forest fire or extensive blaze,
is reduced to a minimum. There is no additional
impairment to the eco~y~tem, ~uch as for example ~carring
of the ground when using ground-based explosives.
The scarcity of water which often occurs on site
and the inaccessibility of the site of the fire, which is
frequently encountered, combined with the lack of infra-
structure in forest areas, is optimally counteracted by
the method according to the invention. Due to the high
mobility and ~peed of the helicopters used, the water-
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filled exploding bags can be picked up at a considerable
distance from the site of the fire, at locations which
contain natural sources of water or which are easy to
reach for fire-fighting tenders. In addition, there is no
longer any need for the previously re~uired times to fill
helicopter tanks with water, for which actions the
helicopter was previously always forced to land first.
For the method according to the invention, it is
sufficient if the helicopter flies down to a sufficient
height to be able to suspend the exploding bag from the
helicopter. Equipping the exploding bag with extinguish-
ing water, explosive charge, fuse and holding cable and
setting the fuse can be carried out in the absence of the
helicopter "in stock" and at a safe distance from the
site of the fire, 80 that firstly the time for one
deployment is limited to the flying time alone and the
very short equipping period, and secondly maximum safety
is ensured for the firefighters working on the ground.
The rapid, problem-free loading of the helicopter
with an exploding bag is particularly advantageous for
forest fires, where a relatively narrow fire front
exten~;ng over relatively great lengths is typical. Thi~
is because a single helicopter can effectively extinguish
the fire front in sections along its length with a short
cycle time. Naturally, it is even quicker if two or more
helicopters undertake extinguishing by sections in a
rolling, that is to say successive, deployment. As an
alternative, two or more helicopters may also drop their
extinguishing load simultaneously at the distance of one
effective width apart and detonate it, preferably
coordinated by radio, one effective width being under-
~tood to be the mean diameter of the extinguishing cloud
formed on detonation.
The application of the method according to the
invention is not limited just to forest fires and exten-
sive blazes but may also be used for all conceivable
fires which can be fought from the air. Owing to the very
high extinguishing efficiency and the high level of
safety of the method, it can be used particularly advan-
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tageously for very hazardous fires and those which are
difficult to extinguish. These may be, for example, fires
in fuel tanks, oil tower fires or fires occurring during
aircraft accidents, without seeking to restrict the use
of the invention to these cases. It i8 recommended here
to use for the extinguishing operation in question a
quantity of water of 500 1 (bush fires, buildings,
warehouses), 1000 1 (forest fires) or 2000 1 (tank fires,
refinery fires, oil rig fires).
The water used for extinguishing does not have to
be pure water, but may also contain retarders. These are
chemical substances which inhibit or at least delay the
ignition of combustible solid materials and which are
introduced into the burning material together with the
water on explosion of the extinguishing bag. Subsequent
re-ignition of the suddenly extinguished burning material
as a result of any re~;n;ng pockets of ~hers becomes at
least less likely if a retarder is present. Various types
of retarder are known and act in different manners
dep~n~;ng on their composition. Salt-based retarders form
a crust on the burning material, which on the one hand
inhibits gaseous emission therefrom and on the other hand
denies the oxygen in the ambient air access to the
burning material. Other retarders absorb large amounts of
water in a similar manner to a gelling agent and store
this water. On burning material which is finely coated
with such water stores, "the fire dies out". However,
particularly for forest fires, such retarders are not
normally necessary and are also not advisable, owing to
the inevitably accompanying pollution of the ground. They
may, however, have an extremely positive effect on other
fire~.
The invention is explained in more detail below
in exemplary embodiments with reference to the drawings,
in which:
Fig. 1 diayl~ ~tically shows an exploding bag
suspended from a helicopter,
Fig. 2 diagrammatically shows the dropping of
the exploding bag onto the source of a
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fire,
Fig. 3 diagrammatically shows the triggering of
an extinguishing cloud, and
Fig. 4 shows an example of a detonating device.
Fig~. 1 - 3 diayLI~ ~ tically illustrate the
procedure for an extinguishing attack on the flame front
of a forest fire. A water-filled, drop-shaped exploding
bag 1 is suspended from the lifting hook 7 of a heli-
copter 10 (Fig. 1) by means of a holding cable 4 made of
synthetic material (e.g. nylon) or steel, the length of
which may be 3 - 50 m. The exploding bag has a capacity
of about 1000 1 of water and consists of thin-walled
plastic (e.g. polypropylene), the wall thickness of which
is sufficient to carry the water but small enough to
burst on explosion.
An explosive charge 2, which is provided with a
delayed-action fuse, is expediently likewise drop-shaped
or bubble-shaped in accordance with the exploding bag and
is held in position by a short cable 5, is situated
within the water 3 in the exploding bag, and preferably
centrally. The lower side, facing the fire, of the
exploding bag may be provided with a heat-protective
layer 8 which reflects thermal radiation, e.g. an
aluminium foil, which is intended to prevent the
exploding bag being locally destroyed by the fire in the
event of exposure to excessive heat and the extinguishing
water thus being largely lost for the action of the
subsequent detonation.
The holding cable 4 may have different designs,
80 that it is either dropped together with the exploding
bag 1 or r~-; n~ on the helicopter. In the example of
Figs. 1 and 2, it i8 assumed that the holding cable i8
released from the lifting hook 7 and dropped together
with the exploding bag. In this case, it should be
provided with a spliced loop at the upper end situated on
the lifting hook, in order to prevent twisting of the
holding cable under load on the lifting hook of the
helicopter, since this could possibly impede dropping
from the hook. The load should in no event be taken up by
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means of a clevis on the hook itself, since the
relatively heavy clevis would overtake the exploding bag
in free fall after being released from the hook and thus
would undesirably rotate the said bag through 180. For
even better stabilization during the fall, it is
advisable to fix one or two small braking parachutes 6
(Fig. 2) to the holding cable directly below the upper
end, which parachutes are dimensioned such that they do
not brake the falling motion of the exploding bag but
merely prevent the holding cable from destabilizing the
position of the exploding bag during the fall.
As an alternative, however, the holding cable 4
may also be provided at its lower end with a remote-
controlled lifting hook 18 (only indicated diagrammati-
cally), which make6 it possible to release the explodingbag from the holding cable from the helicopter and to
drop it on its own. As a result, the holding cable
remains on the helicopter and is available for reuse.
Lifting hooks controlled remotely from the helicopter are
known and are u6ed, for example, in forestry for feeding
wild animals.
When the helicopter has reached the site of the
fire and has positioned the exploding bag at a predeter-
mined height above the source of the fire, the exploding
bag (with or without holding cable) is dropped and moves
towards the source of the fire in free fall (Fig. 2). As
soon as it has then reached a height just above the
source of the fire (e.g. a few metres above the tree-
tops), the explosion is triggered. Thi~ may be carried
out by remote detonation which is triggered from the
lo~;ng helicopter 10 or a special fire-chief helicopter
or an observation ~tation on the ground by means of a
radio signal. Since remote ignition requires high
concentration and ~uick reactions from the operator, in
view of the high falling rate of the exploding bag, it is
generally more expedient to effect detonation of the
explosive charge in the exploding bag by means of a
delayed-action fuse, which is activated at the same time
as the exploding bag is dropped and detonate~ the
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explosive charge at a specific time after the drop, when
the exploding bag is situated at the desired height above
the source of the fire.
Preferably, delayed-action fuses with two safety
means are used here in order to satisfy safety require-
ments under aviation law. These U-detonators, which are
nowadays produced on an industrial scale, have tolerances
of only a few milliseconds, 80 that the required dropping
height for the explosive charge can be calculated very
precisely and also maximum safety is provided for the
aircrew. By way of example, a fuse which has proven to be
well suited iB one which can be set in up to 18 intervals
of 250 milliseconds each, 80 that falling times until
detonation following activation of the fuse of from 0.25
to 4.5 seconds can be set; corresponding to a falling
height of half a metre to just under a hundred metres,
disregarding the air resistance. The height of the
helicopter above the detonation site at the moment when
the exploding bag is dropped iB composed of this falling
distance plus the length of the holding cable.
Fig. 4 shows an example of a triggering of the
delayed-action fuse which iB particularly acceptable in
terms of safety. A twin-core detonation cable 11 iB led
from the delayed-action fuse, out of the exploding bag 1,
along the holding cable 4 as far as just beneath the
bottom of the helicopter 10. There, the detonation cable
is connected to a direct voltage source 17, situated in
the helicopter and serving as detonation source, by means
of a plug-in connection 12, for example comprising a flat
plug (on the end of the detonation cable) and a flat
socket (fixed firmly on the bottom of the helicopter). A
tumbler switch 13 is arranged in the line between the
flat socket and the detonation source. The detonation
cable is provided after a relatively short distance,
slightly below the flat plug, with a further switch 16,
which contains two contact springs which are separated
from one another by a plastic flat key and contact one
another when the plastic flat key is removed. The plastic
flat key, which is suspended in the lifting hook 7 or a
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different hook on the helicopter by means of a short
traction cable 14, is inserted into the switch 16 and
thus interrupts the electrical connection of the delayed-
action fuse to the detonation source. The detonation
cable i8 fastened to the holding cable 4 by a clamp 15
just below the switch 16. During the approach flight of
the helicopter from the pick-up point for the extinguish-
ing bag to the drop point, the connection between
detonation source 17 and the plug-in connection 12 is
likewise interrupted by the tumbler switch 13, in order
to prevent the detonator being activated prematurely as
a result of the plastic flat key unintentionally slipping
out of the switch 16.
When the helicopter is situated at the site
intended for the drop, at the height correspo~;ng to the
- length of the holding cable, the duration of the delay
for the fuse and the desired detonation height above the
ground, voltage is firstly applied to the plug-in
connection 12 by actuating the tumbler switch 13. The
holding cable 4 is then unlatched from the lifting hook
7 of the helicopter. Due to the fact that the detonation
cable i8 fixed to the holding cable by means of the clamp
15, the plastic flat key, which i8 retained by its
connection to the helicopter, i8 firstly pulled out of
the switch 16 by the falling movement. As a result, there
is electrical contact between detonation source and
delayed-action fuse, and the fuse i8 activated. A short
time later, the continuing falling movement pulls the
flat plug out of the flat socket, as a reæult of which
the so-called in-cockpit component, comprising detonation
source, tumbler switch, line and flat socket, is not
mechanically loaded by the drop and i8 immediately
available again for the next extinguishing deployment.
Attention merely has to be paid here to the fact that the
traction cable 14 for fastening the plastic flat key
should be selected to be 80 short that the plug-in
contact i~ only initiated after the key has been pulled
out of the contact clamp. Otherwise, the fuse would not
be electrically activated and the exploding bag would not
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be detonated as desired.
As already described, the detonation of the
explosive charge in the exploding bag results in a mist
of extremely finely divided water droplets, also referred
to as an extinguishing cloud. This extinguishing cloud
~Yp~n~ essentially spherically. Movement of water
droplets upwards away from the source of the fire is
generally not desirable, however, and an essentially
hemispherical extinguishing cloud, in which all the water
droplets are directed downwards towards the source of the
fire and its environment, is much more effective. Such an
essentially hemispherical extinguishing cloud is illus-
trated diagrammatically in Fig. 3; it can be achieved by
covering the extinguishing water 3 within the exploding
bag 1 with a thin film 9 of plastic, paper or the like
(Fig. 1). This film may be placed loosely on the
extinguishing water, but may alQo be attached to the
exploding bag at points such that a sufficient opening
re~-;n~ for filling the extinguishing water into the
exploding bag.
