There are four well described cases of flameouts caused by volcanic ash clouds which have been extensively studied and recorded. The article linked here by Marianne Guffanti discusses all these episodes and reviews the issue of volcanic ash in detail. One concludes from the article that there is actually very little data available on the subject.
The most famous near-crashes are:
1982, BA 9, Boeing 747, London to KUALA-LUMPUR to PERTH, Mount Galunggung volcano, Indonesia. Distance from Volcano = 100-200 km.
1982, SQ, Boeing 747, probable international flight, Mount Galunggung volcano, Indonesia. Distance from Volcano = Uncertain - but nearby.
1989, KLM, Boeing 747, Near Redoubt volcano Alaska, about 135 km from Anchorage.
2000, Unknown airline, Boeing 737-800, Mijake-jima volcano, Distance from Volcano = about 130 km (near Narita airport).
In all cases the plane was practically in sight of the volcano and flew into a dense cloud.
Mt Redoubt is 135 km from Anchorage so that ash cloud was very fresh. Actually, the KLM pilot described it as a "black" cloud.
The British Airways flight 9 to Perth was 100 km from the active volcano Mt Galunggung volcano and experienced a total flameout. The other episode occurred a few weeks later in the same area and that plane landed on two remaining engines somewhere in Indonesia.
There is only one likely report of engine damage after a faint ash cloud was penetrated by a jet. In the detailed report linked here, Thomas J. Grindle describes the case of a research DC-8 NASA jetplane which flew into a cloud of ash coming from Mt Hecla in Iceland. The cloud was not noticed by the crew but instruments recorded the event and they did not the absence of stars for 7 minutes. Aerosol of 20,000 particles per cm^3 were detected for seven minutes, as well as sulphur. The flight path was 800 miles north of the volcano and supposedly 200 km away from the cloud. The plane flew normally for another 47 hours then was inspected. There was internal abrasive engine damage probably caused by the ash cloud, although it is admitted that it had flown through a sandstorm a few days earlier after which no damage was discovered on internal inspection. This event is taken as evidence that even an invisible ash cloud can damage a plane. However, it is possible that the cloud was much denser than they thought since it was nighttime. Also, it was surprising that the windshield was not abraded, just the engine, which would support the sandstorm hypothesis since "sand blasting" was only seen in the engines. If the inspection after the sandstorm was deficient, would they have questioned it in the official report when at the time, it had little relevance?
Based on these data, should Australian travellers be grounded? The idea that an ash cloud could circumnavigate the globe and then, even though it appears to be a transparent brown stain on a satellite image, could noticeably affect a plane, has been accepted without question by Australian airlines. This is clearly a situation far beyond the actual recorded cases.
Presently, ash from Chilean volcano Puyehue must cross the Atlantic and Southern Oceans, a shortest distance of about 11,000 km and a "cloud path" distance of 19,000 km. The "ash cloud" is visible in thin streaks of light brown transparent air by satellite images. There is no apparent data as to how much ash is actually present. However, it logically would be extremely diluted. Its transparency must indicate a very small amount indeed.
An animated image of the ash cloud is shown below.
Like many things airlines do, cancelling Australian flights because of a Chilean volcanic eruption, has very little basis in fact. It might have about the same safety benefit as preventing people reading their Kindle during landing. The current cancellations of flights in Australia have affected more than 100,000 travellers but no-one has looked again at the evidence base for cancelling flights. Meantime it costs aviation, and the public, tens of millions of dollars (in Australia) and probably hundreds of millions in Europe and the USA (but there they are at least much closer to the volcano - typically planes are flying close to iceland and parallel to the ash cloud).
It would make sense to have research planes fly through the transparent ash cloud and take measurements so that we know how much ash is actually present when a cloud is visible. After that, the engine effects could be correlated with the air sample measurements. These could be posted on the web as ash concentration charts. Then we would know. There are other dissenting voices on the web, read the pilots forums.
June 21st 2011
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