The ash plume from Iceland has grounded planes for good reason – it wrecks jet engines, and it's at the right height to do so

Aircraft avoid any airspace that has volcanic ash in it for a simple reason: the ash can wreck the function of propeller or jet aircraft, because it is so fine that it will invade the spaces between rotating machinery and jam it – the silica melts at about 1,100C and fuses on to the turbine blades and nozzle guide vanes (another part of the turbine assembly), which in modern aircraft operate at 1,400C.

That, in turn, can be catastrophic – as the crew of two aircraft, including a British Airways Boeing 747, discovered in 1982 when they flew through an ash cloud from the Galunggung volcano in Indonesia. On both planes, all four engines stopped; they dived from 36,000ft (11km) to 12,000ft before they could restart them and make emergency landings.

That's not the only problem. Ash can pit the windscreens of the pilot's cabin, damage the fuselage and light covers, and even coat the plane so much that it becomes tail-heavy. At runways, ash creates an extra problem because takeoffs and landings will throw it into the air again – where the engines can suck it in and it will create horrific damage to moving parts that suddenly find themselves in contact.

The Icelandic plume has been thrown to between 6km and 11km into the atmosphere – exactly the height that aircraft would be flying.

Passengers on the BA flight that hit the cloud in 1982 said the engines looked unusually bright: soon after all four flamed out. "I don't believe it – all four engines have failed!" said the flight engineer. The crew were prepared to ditch, and the captain told the 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."

Luckily, three of the engines could be restarted. The plane landed safely, and nobody was injured.

The problem with such ash is that it is extremely fine – less than 2mm in diameter, and in the case of fine ash only 6 microns in diameter – which means that it is easily carried by the wind; and because it is ejected by enormously hot air from a volcano it will often be thrown high into the jetstream at exactly the height that aircraft like to fly. The ash particles' light weight means that they will then remain there, dispersing so slowly it can take two to three years for them to vanish.

The measures taken today – clearing UK airspace from noon until at least 6pm – are a precaution, but a sensible one. Once ash has got into an engine, it is all but impossible to remove because it is so fine; no amount of washing will get every piece out. It pollutes filtration systems, electrical and avionic units – and the accompanying sulphuric acid aerosol can eat into rubber parts.

In all, more than 60 planes have been written off by ash damage. The US National Oceanic and Atmospheric Administration put the benefit to aviation of better avoidance of volcanic ash at around $58m annually.

For that reason, the world is split into nine regions, each with its own volcanic ash advisory centre; the one covering Iceland and the UK is based in London. The London one put out an advisory last night but its forecast for the progress of the cloud suggests that it will have spread widely over northern Europe by the early hours of Friday morning.