Should planes fly in Iceland volcano ash? Be careful, study says.

As frustration mounts over the economic cloud an erupting volcano in Iceland has cast over the global airline industry, a handful of European airlines have started to fly empty airliners through Europe's skies to see if it's safe to resume air travel.

But a case study by the US National Aeronautics and Space Administration (NASA) suggests that even tenuous volcano ash clouds can inflict serious damage to an airliner. While the damage may not necessarily threaten the immediate flight, one cruise through an invisible ash plume can run up a multimillion-dollar repair tab.

The plume in the NASA study was so thin that the flight crew had none of the cues they ordinarily might rely on – odd engine readings, the smell of smoke of sulfur in the cockpit, or even outside electrical phenomena such as St. Elmo's Fire – to alert them to a plume's presence. And they had no visual clues on the aircraft after landing to tell them they'd encountered a plume.

The case study concludes that the engines sustained enough damage that key components could well have started to fail with only another 100 hours of flying time.

Iceland's Eyjafjallajökull volcano first sent steam and ash billowing into the sky last Thursday. Since then, canceled flights to, from, and within Europe have affected millions of travelers worldwide and reportedly is costing airlines some $200 million a day.

KLM, Lufthansa, and Air France, reportedly are conducting test flights over the continent, according to wire service reports. A KLM spokesperson noted that the airline had used several different airliners, which flew at altitudes above 10,000 feet.

KLM officials have indicated that an initial inspection of one of the carrier's Boeing 747s showed no "irregularities" during the flight, which reached an altitude of 41,000 feet. The same results held during an initial inspection of the aircraft after it returned.

How to deal with volcano ash: avoid it

Since several highly publicized airliner encounters with ash plumes in the 1980s, airlines have generally agreed that the best way to deal with ash plumes is to avoid them.

In one incident, all four engines of British Airways flight shut down when flying though the ash of an Indonesian eruption in 1982. The same thing occurred in 1989 when a KLM jet flew through a cloud of ash in Alaska. Both flights were able to restart their engines, but only after losing more than 10,000 feet of altitude.

"Even when you set aside things like potential law suits from loss of life, and things like that, the damage to the plane by flying through the ash can run into tens of millions of dollars," says Benjamin Edwards, a volcanologist at Dickinson College in Carlisle, Pa.

KLM, for example, had to replace all four engines on the aircraft, which was less than a year old, at a cost of $80 million.

Ash essentially is pulverized rock. As explosive eruptions occur, the magma is blasted into tiny fragments that cool into jagged particles. Countries have become more skilled at tracking these plumes and alerting flight crews and air-traffic control centers to the plumes' presence.

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