Clouds of ash over Europe from Iceland's Eyjafjallajokull volcano have created chaos for travelers, forcing airlines to cancel 100,000 flights and leaving 8 million passengers stranded. (Watch an al Jazeera report about flghts resuming in Europe.) With economic losses mounting, European authorities have allowed planes to start flying again in much of the previously closed airspace — but the volcano is still erupting, and many passengers are wondering how safe it is to fly when there's volcanic ash in the air:

What does volcanic ash do to planes?
When gritty ash dust gets into a jet-engine turbine, it can congeal and block air flow, causing the engine to shut down. It can also damage aircraft ventilation, hydraulic, electronic and air data systems. "Flying into volcanic ash is as deadly as flying with ice on your aircraft," says former British Airways pilot Eric Moody, who had a nearly fatal run-in with an ash cloud in 1982.

Could ash bring down a jet?
Yes. Over the last three decades, several passenger jets have had to make emergency landings after flying through volcanic ash. Two jets landed safely after being damaged by the eruption of Mount St. Helen's in 1980. In 1982, former British Air pilot Moody's Boeing 747, carrying 263 people, lost all four engines simultaneously after unexpectedly hitting an ash cloud at 36,000 over the Indian Ocean. The jet only regained power after a harrowing dive to 10,000 feet blasted the ash out.

How can pilots avoid the problem?
Just as they've done over the past week in Europe — by erring on the side of caution and staying on the ground when there's ash in the air. After Moody's frightening flight and other near tragedies in the 1980s, scientists started doing more research on the effects of volcanic ash clouds on aircraft, and Volcanic Ash Advisory Centers were established around the world. Now transportation authorities simply shut down any airspace where they've detected plumes of volcanic ash.

Since flights are resuming, are Europe's skies safe?
That's a subject of intense debate. Scientists don't know how much ash it takes to disable a jet engine, nor can they be sure how dense any particular ash cloud is. Some say the decision to resume flights was rushed due to the airlines' huge losses, estimated so far at $2 billion. Moody says you'd have to be a "nutcase" to knowingly fly into air peppered with ash. But airline expert Michael Fabian of Embry Riddle Aeronautical University in Arizona says he relies on the pilot's judgment. "To me if the pilot is willing to risk his own life, I'll go," he says.

Sources: NY Times, Associated Press, Fox News, ABC News, Wall St. Journal