24 May 2011 Last updated at 15:51 GMT Eurocontrol's Brian Flynn explains how the ash cloud is being monitored as passengers wait at airports in northern EuropeThousands of passengers have had their flights cancelled because of drifting ash from an Icelandic volcano.
Airports affected include Londonderry, Glasgow, Edinburgh, Prestwick, Durham Tees Valley, Newcastle and Carlisle, air traffic services company Nats said.
The UK's emergency response committee, Cobra, met to discuss the knock-on effects of the ash cloud.
Air traffic management body Eurocontrol said about 500 flights were cancelled across Europe on Tuesday.
UK air traffic control service Nats said the ash cloud would continue to affect flights from some airports in Scotland and northern England from 1900 BST on Tuesday until 0100 BST on Wednesday.
Transport Secretary Philip Hammond, who led the Cobra talks, said although there would be widespread disruption for Scottish airports, it was unlikely to last long.
"At the moment the model suggests that disruption later in the week is likely to be limited, but of course the weather patterns are changing all the time," he said.
The Met Office said the ash had reached northern Scotland and would spread across much of the UK by the end of the day.
But forecasters said changing wind patterns made it hard to predict its exact path and concentrations would vary between regions.
The following airlines have announced cancellations:
British Airways will not operate any flights to and from Glasgow, Edinburgh and Newcastle on TuesdayKLM cancelled flights to and from Aberdeen, Glasgow, Edinburgh, and Newcastle as well as from Durham Tees Valley AirportAer Lingus cancelled 12 flights to and from Glasgow, Aberdeen and EdinburghFlybe cancelled 11 flights to and from ScotlandBMI has cancelled all flights from Edinburgh and Glasgow for the rest of TuesdayLoganair, based in Glasgow, has cancelled 38 flights. Only inter-island routes in Orkney are unaffectedEastern Airways will not be operating any services in or out of Scottish airspaceEasyjet has cancelled flights to and from Edinburgh, Glasgow, Inverness, Aberdeen and Newcastle until 1900 BSTRyanair has cancelled all flights to and from Scotland for the rest of Tuesday
Minor air traffic disruptions were also reported in Norway and a small part of Denmark.
Despite later cancelling its flights, the Irish carrier Ryanair claimed it had made a test flight through ash over Scotland and challenged a ruling some flights should be grounded.
Ryanair said its 90-minute flight at 41,000ft showed there was "no visible volcanic ash cloud or evidence of ash on the airframe, wings or engines".
Ryanair said the "red zone" over Scottish airspace where ash has been classified "high-density" was invented by the Met Office and the Civil Aviation Authority (CAA).
Continue reading the main story During last year's disruption, the advice was for planes to avoid the ash at all costsSince then, the CAA has worked towards a better understanding of what engines can takeThere are now three recognised levels of ash concentrationLow: 0 to 0.002 grammes per cubic metreMedium: 0.002 to 0.004 grammes per cubic metreHigh: Over 0.004 grammes per cubic metreThere are no restrictions on flying in low ash concentrationHowever if an airline wants planes to fly through medium or high concentration ash, it must put forward a "safety case" to its aviation authority showing it has assessed whether the aircraft will be able to copeThe safety case includes information from a series of tests and from consultations between the airlines and plane manufacturersBut a CAA spokesperson said: "The CAA can confirm that at no time did a Ryanair flight enter the notified area of high contamination ash over Scotland this morning."
BBC transport correspondent Richard Scott said the CAA confirmed Ryanair were being, at best, "misleading".
The cancellations come just over a year after another volcanic eruption in Iceland caused widespread disruption across Europe, including the closure of UK airspace, amid concerns about the damage volcanic ash could cause to engine aircraft.
This year, in the UK, the decision on whether to fly or not in ash cloud conditions is down to individual airlines subject to aviation authority approval.
The CAA said procedures were "totally different" to last year and although no airlines had applied to fly in high-density ash, some had applied for, and been given, permission to fly in medium ash.
The Grimsvotn volcano in Vatnajokull National Park began erupting on Saturday and closed Iceland's airspace for a period.
Experts say the eruption is on a different scale to the one last year and ash particles are larger and, as a result, fall to the ground more quickly.
Frances Tuke, from travel industry body Abta, urged passengers to contact their airlines, which he said had legal obligations to their customers.
He said passengers could have a claim under European "denied boarding" regulations.
These state that if a flight is cancelled or delayed for more than five hours, passengers are entitled to be either re-routed, given a replacement flight, or a refund.
Since last year, the CAA has graded ash levels as low, medium or high, and airlines are notified if levels reach medium or high.
All British aircraft can fly in medium-density ash but the airlines need to consider whether to fly, according to risk assessments.
The Foreign Office is advising passengers to remain in regular contact with their travel agent or airline for the latest news on the status of flights and bookings.
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