The International Air Transport Association says disruptions to European air travel from the volcanic ash cloud have cost the industry at least $1.7 billion.
IATA says airlines lost revenues of $400 million each day during the first three days of grounding, AP reported.
The group's chief executive says the scale of the disruption was worse than after the Sept. 11, 2001, terrorist attacks in the United States.
Giovanni Bisignani told a news conference in Berlin that at one stage 29 percent of global aviation and 1.2 million passengers a day were affected by the airspace closure ordered by governments fearful of the risk ash clouds posed to airplanes.
Germany's air traffic controllers gradually reopened the country's airspace - the busiest in Europe - Wednesday after days of closures and limited activity, as stranded passengers finally saw hope of getting home.
But severe delays were still expected across Europe and the world as airlines moved their planes around and tried to cope with the backlog caused by volcanic ash floating across the continent.
German skies will be gradually reopened for regular flights during the morning hours, the German government agency Deutsche Flugsicherung said in Langen near Frankfurt.
"There is a good chance that the airspace above Germany and all international airports will remain available until late in the evening," the agency said.
Air traffic control agency Eurocontrol said it expects at least 15,000 flights to go ahead across Europe on Wednesday, with the likelihood that the total number for the day may be much higher.
Agency spokeswoman Kyla Evans said about 28,000 flights are normally scheduled on a weekday.
Some restrictions remained in force Wednesday morning over parts of Britain, Ireland and France, as well as over parts of central Europe and the Baltic states.
In Germany, airspace had been closed but on Tuesday airlines were permitted to operate a limited number of flights to and from all airports under so-called visual flight rules. Up to 800 flights operated on Tuesday.
Deutsche Lufthansa AG's chief executive on Wednesday welcomed the government agency's decision to reopen the skies.
The quantity of ash from Iceland's volcano in German airspace is so low that there's "absolutely no danger," Wolfgang Mayrhuber told broadcaster ARD on Wednesday.
"We will restart our system as quickly as possible," he said. Germany's biggest airline planned to operate some 500 flights on Wednesday, comparing with 1,800 on a normal day.
"Our prime concern is security," he added.
On Tuesday, the company operated some 200 flights under visual flight rules.
With huge parts of Europe's airspace reopened, air traffic is lurching back to life. But the gridlock created by Iceland's volcanic ash plume is far from over: Officials say it will take days if not weeks before operations are back to normal and all stranded travelers are be brought home.
Mayrhuber, meanwhile, reiterated his criticism on how the flight disruption was handled, shutting down wide swaths of Europe's air space based on what he said were forecasts of questionable reliability.
"From the beginning, we had the suspicion that the forecasting model could not be all right," Mayrhuber said.
Lufthansa is Europe's largest airline group by sales. It owns or holds stakes in carriers including Swiss International Airlines, Austrian Airlines and British Midland.
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