Why ash is so dangerous to flights
VOLCANIC ash is one of the most serious problems that aviation faces and there have been a number of incidents in recent years when passenger planes have been in peril.
Not only can volcanic ash reduce visibility for pilots, but it can cause jet engines to fail.
Sometimes aircraft many miles from the scene of an eruption can be in danger, with giant plumes of ash rising high into the sky and then travelling vast distances.
When Mount St Helens erupted in Washington state in America in 1980 the plume reached as high as 90,000ft in just 30 minutes.
In 15 hours, it had travelled 600 miles downwind and within two weeks ash had circled the earth.
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One of the biggest difficulties facing flight crews is the problem of distinguishing ash clouds from ordinary clouds, both visually and on radar.
In June 1982, a British Airways Boeing 747 jumbo jet flight from Heathrow to Auckland flew into a cloud of volcanic ash caused by the eruption of Mount Galunggung in Java, Indonesia.
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At first the crew were unaware of the exact nature of the problem but within minutes all four engines had failed. With cockpit windscreen visibility seriously reduced, the plane managed to glide far enough out of the ash cloud for three of the four engines to restart.
The crew, who were to receive various awards, made light of a number of difficulties in landing at Jakarta and put down safely.
It was later discovered that as the ash cloud was dry it did not show up on the weather radar designed to detect the moisture in clouds.
The cloud sandblasted the windscreen and clogged the engines which restarted when enough of the molten ash broke off after solidifying.
In December 1989 a KLM Boeing 747 flight from Amsterdam to Anchorage in Alaska lost power in all four engines after entering a cloud of ash from the erupting Mount Redoubt volcano in Alaska.
The plane dropped more than two miles before the crew were able to restart the engines. The plane landed safely in Anchorage but needed millions of pounds of repairs, including the replacement of all four engines.
More than 20 aircraft were damaged by the ash cloud from the June 1991 eruption of Mount Pinatubo in the Philippines. The cloud travelled more than 5,000 miles to the east coast of Africa.
There have been various international conferences on the problem of volcanic ash and its effect on aviation safety.