Page 190 - Fundamentals of Physical Volcanology

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VOLCANIC HAZARDS AND VOLCANO MONITORING 167

Fig. 11.4 An accumulating pyroclastic
fall deposit from the 1991 eruption of
Mount Pinatubo volcano caused the
collapse of part of the Officers’ Club at
Clark Air Base, Philippines. (USGS
Photograph by T.J. Casadevall,
courtesy of U.S. Geological Survey.)

the level of the top of the cloud, and so unless the
pilot is warned, the aircraft may fly into the cloud of
particles. Aircraft jet engines work by forcing air
into the turbine system and using it to burn fuel.
If that air contains silicate particles, they not only
reduce the air flow but they also melt in the burning
fuel and some of the silicate liquid is smeared onto
the inside of the engine casing, where it solidifies
into a glassy coating (Fig. 11.6). As this layer builds
up, it further restricts air flow and reduces the
efficiency of the engine. The immediate reaction
of the pilot, noticing the loss of power, may be to

Fig. 11.5 A World Airways DC-10 aircraft tilted onto its tail throttle up the engine to recover power, but this
by the weight of air fall pyroclasts during the June 15, 1991 will raise its temperature and make the problem
eruption of Mount Pinatubo volcano, Philippines. worse. In the extreme case, the engine may stop.
(Photograph by R.L. Rieger., courtesy of U.S. Geological
Indeed, several events occurred in the 1990s in
Survey.)
which commercial aircraft had all of their engines
shut down in flight in this way. Fortunately, all of
ground (Fig. 11.5), causing a great deal of structural them managed to restart at least one engine before
damage. crashing. However, in some cases, a final problem
for the pilots landing these planes was to discover,
on nearing the ground, that the cockpit windows
11.2.3 Ash in the atmosphere
had been “sand-blasted” nearly opaque by the im-
It is not just the accumulation of pyroclasts on the pacts of small pyroclasts. The effects of small parti-
ground that matters. The region from tens to hun- cles in the atmosphere are felt on the ground and
dreds of kilometers downwind of an eruption col- at low levels too: surface vehicles and helicopters
umn and eruption cloud will contain small falling being used to evacuate people during eruptions
silicate particles. The number density of these par- find that their engines cannot function.
ticles may not be great enough to make them easily There is a second hazard from ash-size particles
visible from the cockpit of any aircraft flying below settling to the ground. They can be inhaled by people