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.

The NASA flight

But on the night of Feb. 28, 2000, the crew of a DC-8 NASA used for atmospheric research discovered first-hand that ash plume forecasts are not perfect. The agency ended up paying a $3.2 million repair bill.

The DC-8 was en route to Kiruna, Sweden, for the start of a research project to study atmospheric ozone over the Arctic, according to a NASA technical report written by Thomas Grindell of NASA's Dryden Flight Research Center in Edwards, Calif., and Frank Burcham Jr. of Analytical Services and Materials, Inc., also in Edwards.

Some 35 hours earlier Iceland's Mt. Hekla volcano had sent clouds of ash and steam soaring to altitudes of 45,000 feet.

The DC-8 was cruising at just more than 500 miles an hour at 37,000 feet and some 200 miles north of where the plume was predicted to extend. The sky was generally cloudless with no moonlight. But the highly sensitive research sensors aboard the craft detected a sudden rise in ash particles and sulfur dioxide. For seven minutes, the craft flew through a tenuous ash cloud some 800 miles from the volcano.

The only visual clue they had: They couldn't see stars in the night sky, a common phenomenon when flying through high-altitude cirrus clouds. Cockpit instruments reported no unusual engine behavior. The crew smelled nothing unusual. And they saw no other visual clues that would tip them off to the presence of volcanic ash.

The crew reported the encounter to air-traffic controllers and continued to Kruna.

Damage wasn't immediately evident

Once the crew landed, a cursory inspection of the engines and the plane's exterior showed no evidence of an encounter with volcanic ash. Technicians at the site didn't have sophisticated inspection gear at their disposal, so the plane was pronounced fit and the research project began.

But Mr. Grindle and his colleague write that once the aircraft returned to Dryden, deeper inspections showed that internal cooling passages had been clogged, with some of the engines' areas of highest temperature showing signs of unusual heat stress. One of the aircraft's engines sustained the heaviest damage. It was enough to prompt technicians to dismantle the other three as well.

In essence, all the engine's internal parts were coated with fine white powder. The leading edges of turbine blade were pitted. The build-up of heat from clogged cooling passages blistered coatings on several internal components.

Moreover, some research suggests that if the plane had encountered the ash in daylight, the crew still might have had no visual clue because the ash could well have been encased in ice – looking like high-altitude cirrus clouds. A study of the incident by researchers at NASA's Jet Propulsion Laboratory published in 2002 notes that satellites failed to pick up evidence of ash at the point where the DC-8 encountered it; instead it indicated cirrus clouds.

The researchers posited that the ash in effect took a Trojan Horse approach. It served as seeds around which ice crystals could form. Ice striking the jet would leave no visible signs of collision. But once the crystals entered the engines, they would melt, freeing the ash. To weather satellites, the plume would look like icy clouds, not ash.

"The insidious nature of this encounter and the resulting damage was such that engine trending [readings from in-flight instruments] did not reveal a problem, yet hot section parts may have begun to fail [through blade erosion] if flown another 100 hours," the duo wrote.