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Cloudy With a Chance of Danger: Airplane vs. Ash

Here's aviation's $1.7 billion headache: How much volcanic ash is in the air, and how much is too much for an airplane engine?

Lacking definitive answers and fearing a crash, airlines and regulators have responded conservatively to the Icelandic volcano that started erupting in March, canceling more flights than may have been necessary.

"No one had sat down to test the level" of ash density that is unsafe until this spring, says Jonathan Nicholson, spokesman for the U.K.'s Civil Aviation Authority. "And that's because no one had needed to do so before."

Regulators initially barred travel when there was any detectable ash—fine particles of pulverized rock and glass—since it can clog engines. That threshold rose to two milligrams of ash particles per cubic meter of air in late April. This week it was bumped up to four for some flights.


These shifts haven't come fast enough for airlines, which have lost more than $1.7 billion in revenue from canceled flights, according to the International Air Transport Association, an airline trade group. Airlines, in turn, have lashed out at the statistical models that predict how much ash is in the air and where it is. The U.K. model of volcanic ash clouds is "substantially fictitious," Irish airline Ryanair said in a statement.

Don't blame the modelers. They're just combining rough estimates of how much ash the volcano is generating with weather forecasts to predict where wind will take the stuff.

The United Nations' International Civil Aviation Organization has divided the task of forecasting ash cloud movement among nine Volcanic Ash Advisory Centers, or VAACs, world-wide. The U.K. Met Office, the government's weather agency, runs the London VAAC, which tracks ash above the U.K., Ireland and Iceland, and has taken the lead on producing maps of the ash spread.

Some airlines have criticized its forecasts as overly conservative, particularly on Sunday, when the Civil Aviation Authority closed airports because of a Met Office forecast of ash. Met Office planes on Monday showed the ash had been dispersed sufficiently to be traversable.

"Quite rightly, airlines want this inaccuracy in forecasting to be urgently resolved," says Amy Spenlove-Brown, spokeswoman for the International Air Carrier Association.

Representatives of other VAACs, however, say their models haven't been producing appreciably different predictions from London's. "There is good agreement, generally speaking," says Jean-Marie Carrière, director of forecasting at Météo-France, the French national meteorological service, which oversees the VAAC in Toulouse.

Weather Services International, a sister company to the Weather Channel, produces its own forecasts, based in part on its satellites. "Their science is top-notch," Roy Strasser, vice president of meteorological services for WSI, says of the London forecasts.

At the same time, the forecasts generally don't include numbers indicating how uncertain they are. One reason VAACs tend to produce similar results is that they all rely on the same estimates of volcanic output. Mr. Carrière says these estimates may be off by as much as a factor of 10.

Weather forecasts themselves, despite their vagaries, can at least be compared to actual weather. Analogous checks are much tougher with ash models. Satellite imagery can indicate whether there is ash, through its telltale dark clouds, but not its concentration, which is what's used to decide on airspace closures.

So weather agencies, researchers and airlines have sent test pilots to fly in or near the cloud in empty planes with measuring devices. They generally have found results consistent with the models. There haven't been nearly enough test flights, though, to provide a clear numerical accuracy score, or error rate. Moreover, different planes and engines could react differently to the ash.

Small changes in allowable ash levels can have big effects. Flybe, a British airline, managed to push the threshold higher this week by conducting tests with its engine and aircraft makers. Had it done so just days earlier, Flybe would have had to cancel 21 flights this week, rather than 380.

Airlines generally haven't tested planes and engines for the impact of ash. Most recent eruptions have been far from crowded airspace, and airlines generally didn't send test flights into clouds from those eruptions to gauge reliability.

The current limit "is certainly too conservative," says Ms. Spenlove-Brown. "But it will take time for testing and certifying a significantly higher limit."

Given all that is unknown about the ash concentration in the air, and how much is too much, airlines are asking for the final say. Ms. Spenlove-Brown says her trade group wants airlines to be able to "assume responsibility to make their own decisions where and when to fly."

The models, test flights and satellite data all should help airlines and regulators make better decisions next time around. That doesn't help airlines much this time.

"Right now we are very busy running the model," says Météo-France's Mr. Carrière. "We don't have time to make fundamental progress on the model itself. When the eruption is finished, we will take time to learn about and improve the model."

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