Airports across Europe were thrown into chaos today after ash drifting from Iceland's erupting volcano caused several countries to close their airspace, creating delays that that one aviation expert said could linger and strand passengers for weeks.
"This is most significant air traffic control event since Sept. 11 and certainly the most significant that's ever hit all of Europe at one time," said Brent Bowen, the head of Purdue University's aviation technology department.
The potentially dangerous cloud of ash and rock spewed up by the volcano more than 1,000 miles away caused the U.K., Norway, Ireland and Sweden to enforce a nationwide no fly policy, stranding thousands of travelers .
France has also announced some airport closures, and Holland, Belgium, Denmark and Germany are expected to follow suit.
"So we're talking about almost one quarter of the entire European area is closed to aircraft at the moment," Brian Flyn of EuroControl, a European aviation authority, told reporters.
It is not yet clear when the flying restrictions will be lifted, but the Eyjafjallajokull volcano is still erupting and could continue spewing ash into the atmosphere for weeks.
"It is likely that the production of ash will continue at a comparable level for some days or weeks," said Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office. "But where it disrupts travel, that depends on the weather."
As the day wore on some travelers' nerves were fraying.
"It's almost like a joke: Volcano ash? It's almost like, what's the punch line?" said Erin Wilson, an advertising executive stranded at London's Heathrow Airport who was hoping to return to Chicago to celebrate her birthday.
But Bowen said relief is not necessarily on the way. Volcano-related delays that weren't predicted yesterday are now "snowballing seemingly out of control across Europe," he said.
Planes from the U.S., Asia, Africa and elsewhere in Europe are being turned around mid-flight. And British media outlets reported that Eurostar trains between London and Paris and Brussels are sold out.
"You can't get on a train out of England through the Chunnel now to mainland Europe. The ferries are filling up. People are trying to go over to Paris before it gets shut down. Amsterdam is already being affected. Copenhagen is being affected," Bowen said. "This is just having a continuous ripple effect."
Volcanic Ash Could Clog Plane Engines
The United Kingdom and Scandinavia were the first hit by the vast cloud of ash this morning, and it is now heading for western Europe and could reach Russia early Friday morning. While the cloud itself is not harmful, residents of the northern Scottish island of Shetland complained of a strong smell of sulphur in the air.
U.K. controlled airspace is closed until 7 a.m. Friday, according to a statement on the NATS (National Air Traffic Services) Web site. The situation remains fluid, and the airspace could be closed for longer, Deborah Seymour, a NATS spokeswoman told ABC News.
The problem is twofold, according to Seymour. First, the volcano under the Eyjafjallajokull glacier is still erupting and therefore continues to pump out this potentially lethal ash, building up a bigger and bigger cloud. Secondly, the trajectory of the wind is pushing this ash cloud over the Atlantic straight toward the U.K. and northern Europe.
Not only would this volcanic cloud affect visibility, but it could also cause airplane engine failure.
"The large particles of rock, which have been blown into the upper atmosphere, are then ingested into the engines. These engines are delicate machines, the rocks clog the engines which means the engines die," Aviation expert Chris Yates told ABC News.
The U.K. Met Office says the cloud was directly over Heathrow at around 11 a.m. local time.
Wind Pushing Cloud Over Europe
Heathrow is a major air travel hub with more than 1,200 flights and 180,000 passengers passing through it every day. It is one of the world's main connection points, and this event will have major impacts on global travel.
"Heathrow is one of the most important airports in the whole world for connections between North America, Europe, then into Europe and then very importantly from Asia into Europe," Hans J. Weber, president and owner of TECOP International, a San Diego-based aviation consulting firm told ABC News.
"It's really one of the key airports. It's really disruptive."
So far today more than 100 flights from the U.S. to Great Britain have been canceled. And that is just counting those from U.S. carriers.
For the 219 people on Continental Airlines flight 4 last night, the volcano provided them with a seven-hour trip to nowhere. Their Boeing 767 left Houston four hours late at 10:30 p.m., according to airline flight tracking company FlightAware. At 1:55 a.m., the plane turned around and headed back to Houston, landing at 5:32 a.m.
Airline Delays Strand Passengers
The ash cloud Thursday afternoon was estimated to be around three times the size of the U.K, according to the Met Office, which tracks weather for the airlines and government aviation agencies. The U.K. airspace has never been closed before, but the size of the volcano eruption is much smaller than others, such as Mount Pinatubo and Krakatau, the Met Office said.
Passengers at Heathrow were told to contact the airlines directly and those due to travel today have been advised not to go to the airport.
At Heathrow, about every five minutes an announcement explained the situation and apologized for any inconvenience. Similar announcements were made on London public transport systems warning people not to travel to the airport.
British Airways announced it will not be offering accommodation or compensation during the delays, but the airline released a statement saying, "customers booked to travel on a cancelled flight can claim a full refund or rebook their flight for a later date."
"The airlines' cost will be racking up like there's no tomorrow," according to Yates, "we could be talking many tens of millions of pounds."
Not only will passengers have to be compensated, refunded or accommodated, but aircraft will be in the wrong place and will have to be repositioned. "It could take several days for Europe's aviation system to return to normal," he said.
While it is still unclear how long flights will be grounded, at least one airline expects it will take at least several days to clear out any backlog. Delta Airlines is letting passengers booked through Sunday to fly in or out of several European airports including London, Amsterdam, Dublin, Paris, and Brussels -- to change their tickets for free.
Miracle Over Java
There have been other incidents of aviation problems and restrictions caused by volcanic eruptions, but "nothing quite as dramatic as this has happened before" said Yates.
"Aircraft have suffered in the past when they've flown into ash, most notably a BA flight in 1982 which lost all four engines."
That British Airways plane was flying over Indonesia to Australia when it passed through a volcanic cloud.
"Anyone who goes into volcanic ash is a nutcase, it's very dangerous material." Eric Moody, the pilot on the 1982 flight, told ABC News today.
Moody was over the Indian Ocean when suddenly all four engines stopped.
"On the flight deck, the three of us, myself, first officer and flight engineer, admitted that when we got on the ground that we were totally confused," Moody said.
Miraculously, Moody managed to restart the engines once he had exited the ash cloud and land the plane safely in Jakarta.
"I will admit to a bit of apprehension just before they started." he said.
He found out two days later that volcanic ash had caused the engine failure.
A similar incident occurred in 1989 when a KLM Royal Dutch Airlines Boeing 747 flew into an ash cloud from Alaska's Redoubt volcano and lost all power, dropping from 25,000 feet to 12,000 feet (7,500 meters to 3,600) before the crew could get the engines restarted. That plane also landed safely.
ABC News' Lama Hasan and The Associated Press contributed to this report.