Australian Aviation — January 2018

68 AUSTRALIAN AVIATION

T

he ominous ash clouds that

spewed forth from Mount

Agung hung ominously over

the island of Bali, threatening

the residents and their homes. Those

same clouds impact upon the world

of aviation in a variety of ways and for

many it stirs memories of the British

Airways Boeing 747 Flight Nine, or

‘Speedbird 9’, which lost thrust on all

four engines one night over Indonesia

in 1982.

From the strategies put in place

by airlines to the tactics employed

by pilots should an encounter with

volcanic ash occur, the underlying

message is that when airliners are

concerned, prevention is the best

policy.

The finest cut

Weeks, or even months, before

molten rock bursts through the

earth’s surface, small tell-tale jets of

steam can signal the approaching

eruption. When the explosive event

occurs, it has the potential to blast the

volcanic material high enough into

the atmosphere to create a hazard for

airliners cruising as high as 45,000ft

within minutes of the eruption. The

column’s vertical path ultimately slows

and begins to spread as gravity and

atmospheric temperature changes take

effect, but the hazards have only just

begun.

The ejected volcanic ash is very

fine and is easily carried upon the

wind and, in the upper atmosphere,

by the jetstreams. Consequently, the

region affected by the eruption can

reach far beyond its immediate area

as was seen in 2011, when the ash

from Chile’s Puyehue-Cordon Caulle

volcano circled the globe and impacted

upon flights operating in Australia.

Furthermore, it can take years for an

ash cloud to totally vanish.

Even though the particles within

this ash are very fine, less than two

millimetres in diameter, it can have

a significant effect on aircraft. From

engines and windscreens to air-

conditioning and avionics, very few

components are immune and each

year airliners are written off by the

damage they suffer at the hands of

volcanic ash.

The silent enemy

It seems at odds with logic that

mammoth high-tech machines such

as airliners can be held to ransom

by elements that can be measured

in microns. However, it is partly

that very issue of size that allows the

volcanic ash to infiltrate an airliner so

effectively.

When wood is reduced to ash, the

by-product is soft, but this is not the

case with volcanic ash. Volcanic ash

is comprised of small rock fragments,

sulphuric acid droplets, minerals, and

silica. This form of ash is abrasive in

nature and not broken down by water

• it absorbs it. With an airliner’s jet
  engine core reaching temperatures of
  1,400 degrees Celsius, the silica melts,
  forming glass and bonding with the
  blades and vanes within the engine.
  This disrupts the airflow through the
  engine, which in the first instance may
  result in surging, but can ultimately
  result in the engine, or engines,
  flaming out.
  In turn, the hydraulic, electrical
  and pneumatic systems can fail,
  leading to the aircraft relying on

OUT OF THE

ASHES

Volcanic ash

WRITER: OWEN ZUPP

Airliners and volcanic ash don’t mix

‘On the

flightdeck

the same

light show

can be seen.’