Sand shouldn't stand in for volcanic ash in jet engine tests

by Mary Caperton Morton
Wednesday, July 6, 2016

A DC-10 with its tail weighted down by ash sits on the tarmac at Cubi Point Naval Air Station in the Philippines in June 1991 after the eruption of Mount Pinatubo. Credit: U.S. Navy photo by R. L. Rieger.

In 2010, the ash cloud produced by Iceland’s Eyjafjallajökull Volcano grounded trans-Atlantic and European flights for six days due to fears that the high-flying ash could damage — and stall — jet engines. The eruption, which snarled international air travel and led to billions in economic losses, spotlighted the need for more study of the effects of volcanic ash on jet engines. Many such studies have been done using sand as a convenient stand-in for ash. But a new study shows that some types of volcanic ash behave very differently from sand at high temperatures, suggesting sand is an inadequate analogue.

“One of the grounds for the extensive closure of airspace in 2010 was that very little was known about the melting behavior of volcanic ash under the conditions found inside jet engines,” says Donald Dingwell, a petrologist at Ludwig-Maximilians-University Munich in Germany and an author of the new study, published in Nature Communications. “We know that jet engines sometimes operate at higher temperatures than are found in volcanoes, so we were worried about ash melting and recreating lava inside the engine.”

But when researchers went to examine the problem, many teams turned to an old standby in the engine-testing industry: sand, which is readily available and easy to standardize for repeated tests. Despite its convenience, “it’s hard to follow the logic for why people thought sand was an appropriate stand-in for ash,” Dingwell says. “Sand is dominated by quartz, and when quartz melts, it’s one of the most viscous liquids known to man. It behaves differently from just about any other rock and it’s a terrible analogue for volcanic ash.”

In the new study, Dingwell and colleagues investigated how the chemical composition of different types of natural volcanic ash affects their melting behavior at jet engine temperatures, which can reach temperatures up to 2,000 degrees Celsius. They sourced ash samples from nine different volcanoes from all over the world, spanning the range of chemical variability found in explosive terrestrial volcanism, including Grímsvötn Volcano in Iceland, Da’Ure Volcano in Ethiopia, and four massive stratovolcanoes found in the Pacific Ring of Fire, which are considered the most dangerous eruptions for air travel because they tend to produce massive quantities of ash.

By heating the ash samples to more than 1,650 degrees Celsius — the approximate melting point of sand — the researchers found that ash melting temperatures depended strongly on each ash’s chemical composition. For example, the basaltic ash from Grímsvötn melted between 1,100 and 1,300 degrees, whereas the rhyolitic ash from Da’Ure melted between 1,200 and 1,500 degrees. All of the natural volcanic ashes melted at lower temperatures than quartz-rich sand, meaning that ash is potentially more dangerous to engines than sand.

“We’ve always known that sand wasn’t a good surrogate for ash,” says Fred Prata, an atmospheric scientist at Oxford University in England, who was not involved in the new research. Still, scientists have used sand for years, he says, because ash is not as ubiquitous and quantities large enough for testing can be very expensive. However, he adds, “so is shutting down air travel.” It’s certainly better to use ash, so we can “better understand this problem.” The new study adds valuable data to the growing body of research into how ash affects engines and air travel, he says.

The upside of the 2010 shutdown is that the airline industry is now helping to fund research into the impacts of volcanic ash on aviation, Prata says. “The real question is how the industry will react to the next volcanic event. A lot of work has been done since 2010, but there still isn’t consensus on the safest course of action” when a volcano erupts into active airspace, he says. “It’s one thing to come to conclusions [about airspace closures after an eruption] in research papers, and quite another to make decisions in an operational context.”

Dingwell says his group’s efforts to characterize ash should help in future decision-making in the event of another major eruption. The team developed an empirical model that “describes the melting behavior of volcanic ash as a function of its chemical composition and the rate at which it is heated,” he says. “This model provides the basis for determining the potential effects of the deposition of volcanic ash in turbine engines,” so the next time a big eruption that could affect air travel happens, “the authorities can tap into this database to make better-informed airspace closure decisions.”

Dingwell says his team plans to continue broadening its database, using samples of ash from other eruptions as well as ash samples produced in the lab. They’re also seeking to better understand how ash emerging from a volcano can change composition — and, thus, how the hazard it poses to airplanes in flight can change — throughout the course of an eruption. “People think: one volcano, one eruption, one kind of ash,” he says. But “that’s just not true. … We’re working on developing an effective monitoring system to monitor the flux [and chemical composition] of ash coming out of an eruption, in real time.”

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