FRANKFURT Europe's skies were open for business on Wednesday but, with so many planes having been grounded by the volcanic ash from Iceland, it could take days or even weeks to clear the backlog.
About 75 percent of flights in Europe will operate on Wednesday, some 21,000 of the 28,000 flights normally scheduled each day, European air traffic agency Eurocontrol said.
Flights resumed after scientists and manufacturers downgraded the risk of flying in areas of relatively low ash concentrations, Britain's Civil Aviation Authority said.
"The major barrier to resuming flight has been understanding tolerance levels of aircraft to ash. Manufacturers have now agreed increased tolerance levels in low ash density areas," CAA head Deidre Hutton said.
"A return to normal will take another 48 hours," French junior Transport Minister Dominique Bussereau predicted. "I think the situation will be normal before the weekend."
Britain had lagged behind its European neighbors in downgrading the threat to airplanes from the ash, which can potentially scour and even paralyze jet engines.
In 1982, a British Airways jet lost power in all four engines after flying through an ash cloud over the Indian Ocean.
With aircraft having flown successful test flights for several days, recriminations have begun over why governments took so long to give the green light to the aviation industry, which the International Air Transport Association (IATA) says has lost some $1.7 billion in revenues.
"For an industry that lost $9.4 billion last year and was forecast to lose a further $2.8 billion in 2010, this crisis is devastating," IATA Director General and Chief Executive Giovanni Bisignani said in a statement on Wednesday.
He urged governments to examine ways to compensate airlines.
"It is an extraordinary situation exaggerated with a poor decision-making process by national governments," he said.
The two biggest travel companies in Europe, Thomas Cook and TUI Travel issued a joint statement criticizing the British government's response to the volcano.
"The government's response to the crisis has been a shambles," said TUI Travel Chief Executive Peter Long.
"It is clear that they underestimated the severity of the consequences of the decision for a blanket closure of the airspace for such a protracted period of time."
But Henri Goudru, a UN scientific adviser who has studied volcanic eruptions for 40 years, defended the closures.
"In the absence of reliable facts, the only thing to do was to stop aircraft from flying, given the risk to planes of suffering serious engine damage, perhaps crashing," Goudru, who is also president of the European Volcanological Society, told a news conference in Geneva.
"This was not an overreaction. We...do not know enough about these clouds and what can happen to planes flying into them."
The progressive reopening offered stranded passengers and the airline industry some relief, but timetables are wrecked.
"To get back to normal levels of operation from an industry point of view will take weeks," British Airways Chief Executive Willie Walsh told BBC television.
For some passengers who have faced epic journeys and huge financial outlay since no-fly zones were imposed on Thursday, the decision came too late. Others were more lucky.
A hotel in Taiwan organized an impromptu wedding for a Slovakian couple stranded on the island and unable to get home for their long-planned ceremony.
When Juraj Hegyi, 32, and Zuzana Marcekova, 31, were spotted crying in the Orchard Park Hotel near Taiwan's main airport, the manager offered to organize a wedding, invited clergy and packed the venue with 100 people including other stranded Europeans.
"This is not our normal line of work, but the boss here learned they were crying, that they wanted to go back urgently and had planned the wedding for more than a year," assistant hotel manager Chou Chia-hui said on Wednesday.
The economic impact of the cloud has already hit parts of the supply chain and could potentially dent the fragile recovery from the global recession.
PricewaterhouseCoopers said a week of disruption could destroy 0.025-0.05 percent of annual British gross domestic product. The same could be true of other European countries, though Germany said the effect on its economy would be limited.
The volcano under the Eyjafjallajokull glacier was still erupting on Thursday, but producing much less ash.
"There is ongoing activity in the volcano and we don't see any signs of it coming to an end. There is less ash production, it is probably the same as yesterday," Icelandic meteorological office official Gudrun Nina Petersen told a news conference.
"The plume is very low, so most of the ash is falling here and keeping itself under 20,000 feet," she said.
A low pressure weather system moving into Iceland should also help clear the ash cloud within days, an expert from the World Meteorological Organization said in Geneva.
(Additional reporting by European and Asian bureaux; Writing by Jon Hemming; Editing by Sonya Hepinstall and Lin Noueihed)