Why ash and aviation don't mix

Most of us can remember the empty skies above Europe when, in 2010, a volcano with an unpronounceable name (Eyjafjallajökull) erupted in Iceland. The eruption injected huge amounts of volcanic ash, essentially pulverised rock and glass, into the atmosphere, with much of it being directly deposited at the levels that are normally home to hundreds of airliners.

We're all used to a pleasant enough environment when flying, but just feet away from the air conditioned comfort we (mostly) enjoy as passengers, the story inside the aircraft's jet engines is very different.

The principle is simple - air is sucked in and compressed by passing it through a series of fan blades, fuel is added and burned, the air expands and rushes out the back creating thrust. The need to burn the fuel as efficiently as possible while creating as little noise as possible means that behind the simple principle and sleek cowling lies a multi-million pound piece of precision engineering.

The hottest parts of the engine will reach temperatures of 1,600° centigrade while parts of the engine turn at over 10,000rpm, and all this is happening while air is rushing through at speeds of up to 600mph.

Introducing volcanic ash into this environment is very bad news - the glass in the ash melts, which then sticks to various parts of the engine. Sensors can fail and the thousands of microscopic holes that direct cooling air through the turbine blades can become blocked.

Apart from causing expensive internal damage to the engines, there's a real risk that exposure to concentrated levels of volcanic ash will result in a partial or total failure. Flying through abrasive ash at airline speeds is like sandblasting the exterior of the aeroplane, and while striping off the paint might not be an immediate safety issue, making the flight crew's windows opaque certainly is.

History has provided us with a couple of inadvertent ash cloud 'experiments'.

In 1982 a British Airways 747 flew through a plume of ash thrown up by the eruption of Mount Galunggung south east of Jakarta. The aircraft was at 37,000ft when one engine failed. The crew performed the required engine shut down drills, but within a minute another engine failed with the remaining two failing just seconds later. The 747 was able to glide clear of the ash cloud, and at 28,000ft the crew made an unsuccessful attempt to restart the engines.

In an aeroplane the engines drive the pressurisation system, and as the cabin pressure decreased, the passenger oxygen masks dropped. With mountains between the aircraft and the emergency diversion airfield it looked like a ditching in the Indian Ocean was inevitable. Fortunately after losing over 25,000ft the crew managed to start one engine and then the other three (although one had to be shut down again). With sufficient power to safely reach the airport, the crew were still faced with the challenge of landing with windscreens that had become almost opaque thanks to the effects of the volcanic ash.

Seven years later in December 1989 a six month old Boeing 747-400, operated by KLM, was descending into Anchorage Alaska. While descending through 24,000ft the pilot flew through a cloud which turned out to be the ash plume from Mt Redoubt, an Alaskan volcano that had erupted the day before. As the the cockpit filled with what the pilots thought was smoke, the crew turned left and tried to climb out of the ash. Seconds later all of the aircraft's four engines shut down. The aircraft lost 14,000ft in height before the captain managed to restart at first one engine and then the others, the crew made an uneventful landing. As a result of ash damage all four engines were scrapped at an estimated cost of $80m.

Not all aviation is, or needs to be, grounded due to volcanic ash.

General Aviation aircraft, including microlights, are commonly powered by piston engines that are far less susceptible to damage, although they too will avoid flying through high concentrations.

Ultimately those who make the decision to suspend operations or to close certain areas of airspace do so after consulting with industry experts including the airframe and engine manufacturers, and it is the manufacturers who will ultimately reduce the disruption caused by volcanic events by designing, testing and certifying new equipment.