London, England (CNN) -- Volcanic ash clouds are a serious
hazard to aviation, reducing visibility, damaging flight controls
and ultimately causing jet engines to fail.
Encounters between aircraft and volcanic ash can happen because
ash clouds are difficult to distinguish from ordinary clouds, both
visually and on radar, according to the U.S. Geological Survey.
Ash clouds can also drift great distances from their source. The
recent volcanic eruption in Iceland sent a huge plume of ash moving
across the Atlantic, disrupting air traffic across Western Europe.
What happens when aircraft encounter ash clouds?
According to Airbus, flying through an ash cloud should be
avoided by all means. The aircraft manufacturer says experience has
shown costly damage can occur to aircraft surfaces, windshields and
power plants, while ventilation, hydraulic, electronic and air data
systems can also be contaminated.
Critically, it says the ingestion of volcanic ash by engines may
cause serious deterioration of engine performance due to erosion of
moving parts and partial or complete blocking of fuel nozzles.
Volcanic ash contains particles, whose melting point is below
that of an engine's internal temperature. During flight these
particles will immediately melt if they go through an engine. Going
through the turbine, the melted materials rapidly cool down, stick
on the turbine vanes, and disturb the flow of high-pressure
combustion gases.
In the worst case this disorder of the flow may stall the engine,
the manufacturer's operational guidance says.
Can these ash clouds be detected?
Each year millions of passengers fly over volcanically active
regions such as Iceland and the North Pacific, which has more than
100 active volcanoes and four to five ash-producing eruptions each
year, the USGS says.
As weather radar is not effective in detecting volcanic ash
clouds, pilots rely on accurate forecasts of volcanic eruptions
along their air routes.
There are nine Volcanic Ash Advisory Centers (VAAC) around the
world designated by international organizations to provide expert
advice to national meteorological agencies about the location and
expected movement of volcanic ash clouds.
Have there been any fatal incidents?
No. But a number of aircraft have flown into ash clouds over the
years, resulting in a few near-fatal incidents.
In April 1982, British Airways Flight 009 en route to Auckland
from London flew into a cloud of dust and ash thrown up by the
eruption of Mount Galunggung, on the Indonesian island of Java,
resulting in the failure of all four of its engines.
Fortunately the pilots were able to glide the Boeing 747-200 far
enough to exit the ash cloud, before three of the engines were
restarted, allowing the crippled flight to divert to Jakarta and
perform a safe landing.
At the time it was the longest glide in a non-purpose-built
aircraft.
The event was also remembered for the announcement made by the
airliner's remarkably calm captain, Eric Moody.
He told passengers: "Ladies and gentlemen this is your captain
speaking. We have a small problem. All four engines have stopped. We
are doing our damnedest to get them under control. I trust you are
not in too much distress?"
In December 1989, KLM Flight 867 en route to Anchorage in Alaska
from Amsterdam flew into a normal looking cloud, which turned out to
be a volcanic ash cloud -- the result of an eruption of Mount
Redoubt, a volcano in the Aleutian Range.
The pilots increased the power in an attempt to climb out of the
cloud, but all four engines on the Boeing 747-400 failed shortly
after and the standby electrical system failed.
Fortunately the crew eventually managed to restart the engines
two at a time, enabling the airliner to land safely at Anchorage,
despite extensive damage to the windshields, internal aircraft
systems, avionics and electronics.
