Volcanic ash

As passengers stranded by volcanic ash back in April publish a collaborative magazine, we take a look at how ash affects jet engines.

The makeup of ash
Volcanic ash is made up of small pieces of rock and glass no more than 2mm across. It is formed by eruptions, the force of which shatters solid rock, and by expanding gases breaking up magma and causing it to become airborne. Explosive eruptions eject ash high into the atmosphere, and, given enough energy, the fine particles can reach the jet streams and be carried for long distances.

It is at these altitudes that commercial airliners fly, and ash can pose a risk to their safety, mainly by affecting the performance of jet engines.

A jet engine’s weak spot
Jet engines and rockets work on the same basic principle. Burning a fuel in oxygen generates a fast-moving jet of fluid, which is expelled backwards from the exhaust to generate forward thrust in accordance with Newton’s laws: equal and opposite action and reaction.

The components of a jet engine

However, while rockets carry an oxidising agent with them, jet engines used in planes have to get oxygen from the air. In practice, this means jet engines are made up of several stages. An air intake fan passes air to a compressor, where its pressure is increased. The now high-pressure air is then delivered to the combustion chamber where it is used to burn fuel.

The exhaust gases reach around 2000 ˚C, and leave the engine at high speed. On their way they are used to drive a turbine, which works much like a windmill and is used to power the compressor and some other electrical systems. It is here that volcanic ash can cause most damage.

Engine trouble

Ash buildup inside the turbine blade of a NASA-owned DC-8 that flew through a cloud of ash near Iceland in 2000

Ash sucked into a jet engine may melt in the high temperatures it encounters, causing it to stick to turbine blades and other machine parts. This is particularly true of ash that is rich in glass, such as that caused by eruptions underneath a glacier.The particles of ash can also erode the engine’s components or cause jams in rotating parts.

There is also a risk caused by large intake of ash preventing air from getting to the engine. Any of these things can cause the engines to shut down. In 1982, a Boeing 747 operated by British Airways flew through a cloud of volcanic ash released by Mount Galunggung in Indonesia, and all four of its engines failed. After descending below the cloud, the engines cooled enough to allow the melted ash to solidify, and it broke off, allowing air to flow and the engines to restart.

The plane landed safely in Jakarta, but had to rely on its instruments as the ash had “sandblasted” the windscreen. A similar incident in Alaska in 1989 caused $80m of damage to a 747 owned by KLM Royal Dutch Airlines.

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