Volcanic ash – an ongoing aviation hazard

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As ash from the Sangeang Api volcano disrupts flights around South Asia, Flight Safety Australia looks back at the chaos caused by the 1982 eruption of Mount Galunggung and the now legendary flight of a British Airways 747.

It was one of the great understatements in the history of aviation. After flying through a volcanic ash cloud, British Airways Boeing 747 captain, Eric Moody, told his passengers: ‘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.’

Volcanoes create ash in three ways: gas release under decompression; thermal contraction from chilling on contact with water, and ejection of particles during steam eruptions. The principal ingredient of volcanic ash is silicon dioxide, the most common material in the Earth’s crust and the major component of sand.

Silica is extremely hard, and when it occurs as volcanic ash is also sharp and irregularly shaped, making it very abrasive. Volcanic ash is accompanied by gases, which in the atmosphere are converted into sulphuric acid and other corrosive and poisonous substances.

Another important property is its melting point. Being made up predominantly of glassy silicates, the melting temperature (~1100°C) of which is below the temperature of jet engines operating at normal thrust (1400°C), volcanic ash can melt and be deposited in the hot section of the jet engines.

Because it was a dry substance, the ash spewed from the eruption of the Mt Galunggung volcano was invisible to the BA 747’s weather radar, which detected moisture. The first sign of trouble was the glow of St Elmo’s fire flickering around the nose and leading edges, quickly followed by a sulphuric smell and dry mist in the cabin as the polluted air entered.

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Then, as ash coated their compressor and turbine blades and blocked their finely constructed cooling ducts, the engines failed one by one.

The skill of Captain Moody and his crew in restarting the engines, turning to avoid the high Javanese mountains, and making a night landing with an almost opaque windscreen (and an inoperative ILS) has passed into aviation legend. Following Flight 9’s experience, volcanic ash entered the repertoire of aviation hazards.

It’s for these reasons and more that the Civil Aviation Safety Authority (CASA) released an Airworthiness Bulletin in 2011 detailing the risks associated with flying in such conditions.

To read more of Flight Safety Australia’s 2010 article, ‘Volcano fires debate’, click here

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