Airspace throughout northern and central Europe are continuing to gradually reopen following days of disruption triggered by a volcano in Iceland that spewed out huge ash clouds and endangered air travel.
But as flights resumed, some airports in northern Europe had to close again on Wednesday due to the returning ash cloud.
"There are still some areas where it is dangerous for planes to enter them, and that will be prohibited, but they are very much smaller," Andrew Haines, chief executive of the Civil Aviation Authority, told Sky News.
Some airports in Sweden and in Greenland remained closed early on Wednesday, and Ryanair cancelled all flights to and from the Slovakian capital, Bratislava, until 11:00 GMT on Thursday.
Al Jazeera's Imran Khan, reporting from Heathrow Airport in London, said that the airport was far from becoming fully functional.
"Only 422 flights were expected to depart, compared to the 655 that the airport would see on a normal day," he said from Europe's busiest airport.
Arrivals are also not back to normal, with around 343 planes expected, compared to the 657 on a normal day.
"The problem is getting the planes back where they need to be, but the government is trying to help and has allowed an extra 16 flights overnight."
Many travellers are still stranded as airlines try to shift a huge backlog after about 95,000 flights were cancelled in the last week.
The Eurocontrol air traffic agency in Brussels said it expected at least 15,000 of the continent's 28,000 flights to go ahead on Wednesday across Europe, and possibly much more.
Eurocontrol predicted that the number of take-offs and landings in Europe would be close to normal by Friday.
Volcano ash chaos
The ash cloud from a volcano eruption on Iceland caused air traffic authorities across Europe to close airspace over most of the continent and millions of people were affected.
The Eyjafjallajoekull volcano has been spewing ash and lava for days, but since the weekend, the eruption has lost nearly 80 per cent of its intensity, a spokeswoman for Iceland's civil protection agency said on Wednesday.
"If the volcano becomes active again, new closures might happen," Axel Raab, spokesman for German government agency Deutsche Flugsicherung, said.
He said that the situation would be reviewed based on meteorological data.
But concerns have been raised about a second volcano in the area.
"Eyjafjallajoekull is a fairly mild volcano but is next to a very powerful volcano, Katla Volcano, 20km away," Pall Einarsson, a volcanologist from Reykjavik in Iceland, told Al Jazeera.
"We are not saying that Katal is going to erupt, but we are on guard.
The last three times Eyjafjallajoekull volcano erupted, Katla volcano, which it is 20 times more powerful, also erupted.
"If one volcano expands it can push the next one," Einarsson said.
The airport shutdowns have cost the airline industry $200m a day, according to the International Air Transport Association (IATA).
Giovanni Bisignani, the IATA chief, told an Italian television that more than five medium- and small-sized European airlines risked bankruptcy in the fall-out from the ash cloud shutdowns and called for EU compensation.
With more than 95,000 flights cancelled across the world, it will be some time before the airlines are able to clear the backlog of passengers.
British Airways, which cancelled about 500 flights a day in the past five days, said it was trying to clear its backlog on a case-by-case basis.
It said travellers could either rebook online or claim a full refund, and it also urged travellers booked to fly this week to consider cancelling their trips so that it could maximise space to fly people home.
Spain's Iberia said it was coping with the backlog by using bigger aircraft and adding extra flights.
"We've never had a backlog like this before," Laurie Price, the director of aviation strategy at consultant Mott Macdonald, said.
"After 9/11 airspace was shut for three days, and then the US airlines were bailed out by the government."