By Maria Sheahan
FRANKFURT (Reuters) - Europe's skies were open for business on Wednesday, but with so many planes having been grounded by the pall of volcanic ash spreading 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 neighbours in downgrading the threat to airplanes from the ash, which can potentially scour and even paralyse jet engines.
In 1982 a British Airways jet lost power in all four engines after flying through an ash cloud above the Indian Ocean.
With aircraft having flown successful test flights for several days, recriminations have started about why governments took as long as they did to give the green light to the airline industry, which according to the International Air Transport Association (IATA) lost some $1.7 billion (1.1 billion pounds) 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 which he said would take three years to recover.
"It is an extraordinary situation exaggerated with a poor decision-making process by national governments," he said.
Despite their losses, airlines did also save around $110 million a day on costs such as fuel, IATA said.
The Association of European Airlines, representing 36 major commercial and freight carriers, criticised Britain on Tuesday for not reopening its skies sooner.
WELCOME RELIEF
The progressive reopening, after the European Union agreed on Monday to ease the rules, offered stranded passengers and the airline industry welcome relief.
But with aircraft and crew scattered where they were grounded on Thursday, timetables will be 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.
Steve Ridgway, chief executive of Virgin Atlantic, said: "Whilst the reopening of airspace is good news both for passengers and the industry as a whole, it is likely to take several days to get everyone who has been affected to their destinations."
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.
For Meg Newman, 31, a speech and language therapist, and Harry Speller, 30, both of London, New York was the last leg of a three-month tour through India, Nepal and Malaysia after Speller lost his accounting job.
Each budgeted 3,000 pounds for their travels, and Speller estimates the extended stay in New York will cost at least another 1,500 pounds.
"New York was our five-day treat," Newman said. "We weren't expecting it to be 16 days. Now we haven't got the money."
New York itself is losing about $3 million a day in reduced spending, according to city officials, and the Spanish tourist industry, excluding airlines, lost 252 million euros (219.9 million pounds), the nation's tourist body said.
ECONOMIC IMPACT
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 estimated a week of disruption could destroy around 0.025-0.05 percent of annual British GDP, and the same would probably be true of other European countries. But Germany said the impact on its economy would be limited.
Humanitarian flights were also affected.
The U.S. military is evacuating war-wounded from Afghanistan to a base in Iraq, instead of sending them to Germany. A polio immunisation campaign in West Africa has had to be delayed because the vaccines are stuck at French and German airports.
The volcano under the Eyjafjallajokull glacier was still erupting, 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 (6,000 metres)," 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 Organisation said in Geneva.
(Additional reporting by European and Asian bureaux; Writing by Jon Hemming; Editing by Sonya Hepinstall)
