European airports are struggling to help thousands of passengers stranded after severe wintry weather paralysed parts of Europe's transport network.Freezing temperatures, snow and ice has grounded flights, trapping travellers returning home for Christmas.Airport operators defended their handling of the crisis amid criticism from the European Commission.Officials at the worst-affected airport, London's Heathrow, rebuffed offers to bring in the UK Army to help.Since Saturday, when 12.7cm (five inches) of snow fell in just one hour, Heathrow airport - the world's busiest - has cancelled hundreds of flights.The airport said it planned to operate two-thirds of its scheduled flights on Wednesday.Delays at Heathrow also had a knock-on effect on other northern European airports.In Ireland, the Dublin Airport Authority said the airport would be closed until at least 0800GMT on Wednesday, the The Irish Times reports.'Unacceptable disruption'In Germany, Frankfurt airport cancelled 550 of almost 1,300 flights on Tuesday because of the bad weather.Air France said that some 5,000 people spent the night at Paris' Charles de Gaulle airport on Tuesday - 4,000 of whom were trying to fly into London, according to Le Monde.While in Sweden, Scandinavian airline SAS said flights to London's Heathrow were the most problematic."It is the absolute worst there," SAS spokeswoman Elisabeth Manzi told The Local."From Frankfurt and Paris, things are running smoothly and we are getting passengers out. In London, we currently have about 5,000 to 6,000 SAS customers."Many passengers sought to travel by rail instead of plane, causing Eurostar to recommend that passengers trying to leave from London should cancel their tickets and stay at home.But rail expert Christian Wolmar said the real problem was not bad weather but bad management."Eurostar ought to be ashamed of themselves," he told Associated Press."It would seem possible to put on extra trains, but they can't get the crews or they can't get the trains in place. It's inexplicable."The European Commission said it was "extremely concerned" about the level of disruption caused by the severe snow, saying that it was "unacceptable and [...] should not happen again".But Airports Council International (ACI), the professional association of European airport operators, said 88% of flights to and from European airports had been operating.It said airports in northern Europe found it easier to cope with severe weather because the temperatures there remained largely below freezing, so the condition of the runways did not change, whereas the fluctuating temperatures in western Europe had caused problems.UK Prime Minister David Cameron said he was "frustrated" it was taking so long to get Heathrow fully operational again."If it's understandable that Heathrow had to close briefly, I'm frustrated on behalf of all those affected that it's taking so long for the situation to improve."Inside the terminal, passengers who had spent several days waiting to catch their flights home were equally angered. Some were being accommodated in tents on the edge of the terminal."It's not even snowing!" 19-year-old Candie Sparks, who was trying to get back to Santa Fe, New Mexico, told AP. "It's crazy."Source: BBC