Ash big threat to jet engines

By Mathew Dearnaley

Enhanced engine technology has magnified the risk of fatal damage to aircraft flying into volcanic ash clouds.

But a global network of nine ash detection centres - one operated from Wellington by the MetService to cover the Western Pacific - means few pilots should find themselves in the nightmare situation of the full engine shutdown of a British Airways Boeing 747 over an Indonesian volcano in 1982.

The failure of all four engines from the unseen invasion of very fine but extremely abrasive and blistering hot silica particles turned the Boeing - bound for Auckland via Perth - into a jumbo glider until three of the four were restarted at a lower altitude.

Even so, landing the aircraft became a hair-raising feat as the pilots were virtually blindfolded by a blanket of ash which had scoured and obscured their windscreen.

It was that near-disaster which alerted the international aviation industry to the severe risk of flying through volcanic clouds, leading to a strict policy of avoiding ash at all costs.

Airline safety manuals advise pilots to look for signs of ash, such as the smell of sulphur on flight decks, and an unwelcome glow from the engine intakes.

They are also advised to throttle back and lose altitude in the event of unexpected loss of power, allowing cold air to be drawn into the engines and hopefully shatter any molten glass formed by the ash so that the engines can be restarted.

"We've learned a lot and the world's more on the ball when it comes to volcanic ash and the consequences it can have on jet aircraft," said Air Line Pilots' Association technical officer Ross Gillespie, a former Air New Zealand international pilot.

"It's a bit like a predicted tsunami - just stay away."

Mr Gillespie likened flying through volcanic ash to throwing sand in the engines.

"It is very abrasive and of course, on a jet engine with multiple blades, that completely ruins all the clearances on the blades."

Wellington's Volcanic Ash Advisory Centre is meanwhile keeping an eye on a volcanic eruption of steam and ash in Vanuatu, although aviation forecaster Micky Malivuk said that was rising to only about 10,000ft (3km) above sea-level, well below the 30,000ft (9km) altitudes flown by jet aircraft.

Eruptions of Mt Ruapehu in 1995 to 1996 caused havoc in domestic airline schedules with multiple airspace closures over large parts of the North Island, extending to Auckland at one point.

The Civil Aviation Authority reported engine and windscreen damage to at least one, and possibly two, aircraft.

The Government's former electricity corporation faced a $6 million bill for repairs to the Rangipo underground hydro power station, after ash dumped from the mountain into the waters of the Tongariro River mangled its steel turbines.

Dr Tim Fox of Britain's Institution of Mechanical Engineers said modern jet engines were susceptible to damage from large amounts of grit or volcanic ash, which interfered with the fan blades and air combustion process that generates thrust.

- NZ Herald

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