In 1982 a British Airways jumbo jet flew through an ash cloud from an Indonesian volcano, lost power on all four engines, and dropped from 11,000 to 4,000 m before recovering. This incident led to an assumption that aircraft should not attempt to fly at all when volcanic ash was around, and hence to the stopping of all flights over Britain and much of Europe when the Icelandic volcano Eyjafjallajökull was erupting in April 2010. The cost of this disruption has been estimated at €1.5-2.5 billion.
Rod had felt that this original reaction had perhaps been overdone, as had the initial published views of his own Institution, the IMechE, so he collected and chaired a small working party to look into it further. The temperature in the hottest parts of a jet engine (combustion chamber and first-stage turbine rotor) is in the range 1500-1600 °C. What seems to happen is that any ash ingested melts at this temperature, and then solidifies on cooler parts downstream, thus blocking the very narrow cooling ducts. It also erodes compressor and turbine blades. The engine therefore overheats and shuts itself down. As it cools subsequently, differential contraction breaks the deposits off, so it may be possible to restart the engines (as happened in 1982).
But how much ash in the ingested air can be tolerated? The original view had been "none at all". The protests at the resulting chaos, both for travellers and the air transport industry, led to the limit being raised first to 2 mg of ash per cubic metre of air, and then to 4 mg/m3, with accompanying time limits. But there seems to be as yet no experimental justification for any of these values, and in any case the density can probably not be predicted within an order of magnitude.
He went on to explain the difficulties that meteorologists had in predicting what the ash density would be. The "ash" particles are usually silicates, hard, sharp and angular, of size in the range 1 µm to 1 mm. The larger particles drop to earth quite soon, but the smaller ones can be carried great distances. Volcanologists have generated some existing data about densities and distributions, but the variation is very wide.
Air travel is already very safe, and no one has yet been killed or injured as a result of aircraft flying through ash. The reduced risk to life from the flight ban seemed to be quite negligible compared with the inconvenience caused.
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