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What Happens When Planes Fly Through Volcanic Ash?

Here's why airlines have cancelled flights in and out of Bali.

Ian Thomas Ackerman

The eruption of Alaska's Mt Redoubt in 1989 almost brought down a 747. Image via Wikicommons

As Mount Raung continues to belch volcanic ash into Indonesian airspace, Jetstar and Virgin have again cancelled all flights in and out of Bali. The problem, of course, is that volcanic ash clogs jet engines and can bring down planes. But that's not reason enough to cancel flights, according to dozens of irate posts over Jetstar's Facebook wall.

Fitri Julicia spoke for the masses when she told Jetstar "This is supposed to be a short holiday for my family. Never have I thought it turns into such nightmare." A similar line came from Tracey McCrea who wrote "Great. Our flight has been cancelled to depart Bali to Brisbane tonight... the earliest flight home is Monday night!"

So why can't planes withstand flying through ash clouds? The problem begins with their composition. Unlike regular clouds, which are all water, volcanic ash is made from tiny pieces of abrasive rock and glass. Flying an airplane into a cloud of this stuff is like blasting your car with sand. All outer surfaces get scoured, and the windshield can become so badly damaged that pilots can't see out. While these are mostly cost considerations for the airline, the main issue is that jet engines are sufficiently hot to melt ash particles into a viscous clumps of glass, which quickly seize up engines.

There have actually been no fatal crashes attributed to volcanic ash, but there have been some very close calls. In 1982, British Airways Flight Nine flew into an ash cloud from Indonesia's Galunggung volcano. As the cabin filled with sulphurous smoke, all four engines on the 747 flamed out, and the pilots set the plane to glide. As they assessed whether they'd have enough altitude to clear Indonesia's mountains, Captain Eric Moody made this beautifully understated announcement to the passengers.

"Good evening ladies and gentlemen. This is your captain speaking. We have a small problem. All four engines have stopped. We are all doing our damnedest to get them going again. I trust you are not in too much distress."

As luck would have it, the inoperable engines cooled, which solidified the molten glass over the engines' walls. This cleared the combustion chamber and all four engines began working again. The plane was able to land but idiotically Indonesian authorities didn't close the flight path. Less than three weeks later a second 747 was forced to shut down three of its engines while flying through the same area.

The most recent case of volcanic ash causing engine failure was KLM Flight 867, over Alaska in 1989. As the plane descended into Anchorage International Airport, en route to Tokyo, the plane entered a cloud of ash coming from Mount Redoubt. All four of the plane's engines died, but the pilot managed to restart two and landed safely.

Off the back of these two events, an organisation was formed specifically to advise airlines about volcanic activity. Called the International Airways Volcano Watch Operations Group, the group has nine offices internationally, each monitoring a section of the planet's skies.

Based in Darwin, the Australian Bureau of Meteorology Ash Advisory Centre monitors air quality over South East Asia, Australia, and Papua New Guinea. "When we find discernable ash we send the information along to the Meteorological Watch Office, which then issues a warning that the airlines can use," explained Emilie Jansons, who is the manager in Darwin.

Surprisingly, whether an airline decides to run flights is somewhat up to the discretion of management, as some stranded passengers have observed. "Why is it that only Australian flights are cancelled!!!???" Fumed Eva Andrianopoulos on Facebook.

As Emilie explained, Australian airlines generally err on the side of caution. Other airlines will probably make it back; it's just a question of how desperate you are.

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