Volcanic ash clouds tend to settle in the atmosphere at an altitude of 11,000 metres, which is an aeroplane’s normal cruising height.

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The dust is so thin that it is basically invisible, and the only way to clearly identify this type of cloud is from a satellite in space.

So pilots can fly into such clouds unknowingly, allowing the invisible particles of volcanic glass and dust to get sucked into the plane’s engine, where they melt, coagulate together and eventually stop the engine. As a consequence, the plane will suddenly find itself with a four-engine failure, effectively becoming an engineless glider.

What is volcanic ash

Volcanic ash consists of small tephra, which are bits of pulverised rock and glass created by volcanic eruptions. It can be as fine as talcum powder.During a volcanic eruption, the ash can be breathed deep into the lungs and cause irritation even in healthy people. But once it falls from a greater distance — like from the cloud currently hovering above Europe — its health effects are often minimal, experts say.

Power failure

Very fine volcanic ash particles, particularly glass-rich if from an eruption under ice, sucked into an engine melt at about 1,100 degrees, fusing on the blades of other parts of the turbine, which operates at about 1,400 degrees. They can erode and destroy parts, drive it out of balance and cause jams in rotating machinery. The effect on the operation of a jet engine is often failure of all engines.

Emergency procedure

The standard emergency procedure is to throttle back the engines, and to lose height to drop below the ash cloud as quickly as possible. The inrush of cold, clean air is usually enough to cool, solidify and shatter the glass, unclogging the engines.

Sandblasting effect

Ash can also blind pilots by sandblasting the windscreen requiring an instrument landing, damage the fuselage, and coat the plane so much to add significant weights and change balance.The sandblasting effect can also damage the landing lights, making their beam diffuse and unable to project in the forward direction.

Clogging of plane’s sensors

Accumulation of ash can also block an aircraft’s pitot tubes, an instrument used to measure airspeed. This can lead to failure of the aircraft’s air speed indicators.

Electromagnetic wave insulation

Volcanic ash particles are charged and disturb communication by radio.

Near-disasters causes by ash clouds

An accident in 1982 was the earliest reported mid-air engine stalling as a British Airways Boeing 747 had all four engines stalled after having flown through a volcanic ash cloud over Indonesia.

A KLM Boeing 747 from Kuala Lumpur ran into a nearly identical incident in 1989 when it dashed in a volcanic ash cloud over Alaska. In both cases, the crew managed to re-start the engines after their planes dropped below the ash clouds.

Flight cancellations

The incidents prompted the aviation industry to rethink the way it prepared for ash clouds, resulting in international contingency plans being activated this month. When a cloud is identified by a satellite, air space is closed as a precautionary measure.

Iceland volcanoes particularly dangerous

The British Geological Survey says volcanoes in Iceland are the types of volcanoes that tend to produce plumes of harmful ashes, as they are often covered by glaciers.There are now more than 100 active volcanic mountains in Iceland.

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