From MSNBC:
The mushrooming cloud of ash from the eruption of the Eyjafjallajoekull volcano in Iceland has resulted in the closure of major airports throughout the U.K. and Scandinavia. The grounded flights make sense, as these super-heated plumes can do more than reduce visibility. They're downright hazards for airplanes. “Basically, planes and volcanic ash don't mix,” Elizabeth Cory, a spokesperson for the Federal Aviation Administration, said today. “When ash is ingested into the engine, it creates problems for the plane, including electrical failure.” The thing that makes volcanic plumes so dangerous is that they look extremely similar to regular clouds, visibly and on radar screens. Even when ash isn't visible to the human eye, it can still pose a threat to aircrafts because of the chemicals floating within volcanic plumes.
Airborne ash makes air travel extremely dangerous and difficult for several reasons, the number one being that when the air that gets sucked through an aircraft's jet engine is mixed with ash, it can cause engine failure. The ash particles that make up volcanic clouds contain powder-size to sand-size particles of igneous rock material that have been blown into the air by an erupting volcano. The tiny particles instantly melt when faced with the internal temperature of an in-flight jet engine, which exceeds 1,832 degrees Fahrenheit (1,000 degrees Celsius).
More here.
