Reporting from London — LONDON Air traffic over Europe was on track to return to about 75% of normal capacity Wednesday, aviation officials said, but they warned that clearing the backlog of flights could take days.
The continent's biggest airports were expected to be open and running flights by the end of the day. That included London's Heathrow Airport, one of the world's busiest hubs, which the British aviation authority allowed to reopen late Tuesday.
Problems posed by the volcanic ash spewed by the Eyjafjallajokull volcano in Iceland had not entirely disappeared, however. A corner of Scottish airspace remained sealed off, with potential that the closure could spread further south.
The restoration of air service came as welcome news to thousands of passengers who have been stranded far from home since last Thursday. Airlines were also relieved at the lifting of the restrictions over Europe, which a trade group said cost the industry as much as $1.7 billion.
The regional aviation agency Eurocontrol said it expected 21,000 flights through European airspace would take place Wednesday, 75% of the usual tally of 28,000.
But a full return to normal will take days as airlines scramble to get grounded planes into position across the continent and to accommodate passengers whose flights were canceled. Eurocontrol said 100,000 flights have been canceled since Thursday.
"We've now got to start the difficult task of getting our stranded customers back home," said British Airways Chief Executive Willie Walsh, who had been increasingly critical of the continued closure of most of British airspace. "This has been a difficult period for everybody."
Recriminations have already begun over whether aviation officials across the continent had overestimated the threat from the volcanic ash and had imposed a blanket ban on flights unnecessarily. In Britain, aviation authorities abruptly opened all airports Tuesday night, leading critics to question what had changed to allow such a sudden rollback of flight restrictions.
On Tuesday, the British government had said Spain, relatively untouched by the travel crisis, had offered to make Madrid a hub for Britons unable to fly home directly. From the Spanish capital, buses were to deliver passengers to ports from which they could catch ferries to Britain.
A British warship picked up troops returning from Afghanistan and about 200 civilians from the Spanish seaside city of Santander. Two other navy ships were also to transport stranded travelers in the next few days.
The gradual easing of the lockdown of European airspace came amid heavy pressure by airlines to get their idled fleets back in the air. Aviation authorities said safety remains of paramount concern, but growing gaps in the ash cloud allowed for a rollback of some of the flight restrictions.
Eyjafjallajokull continues to erupt, belching dust and glass particles that could cause jet engines to seize up. More grit has been creeping south and east, and air traffic regulators say restrictions on airspace could be revived.
Meteorologists say, however, that the most recent plumes of ash are not reaching such high altitudes as before and that winds strong enough to disperse the ash could pick up in a few days.