AMSTERDAM—Dutch airline KLM said it safely flew aircraft without passengers through a window in the cloud of volcanic ash over Europe Sunday, and pressed for an end to the total ban on commercial air traffic that has paralyzed travel across the continent.
Other airlines including Lufthansa and Air France said they, too, were conducting test flights. Authorities, however, extended airspace restrictions across Europe and said there was no end in sight to the plume spewing out of a volcano in Iceland, which they insist is dangerous to planes.
A KLM spokeswoman said four flights completed the short flight from Duesseldorf in western Germany without incident and four more planes were due to return to their home base at Schiphol Airport. The airline had permission from Dutch and European aviation authorities before sending the planes aloft.
Engineers immediately took the aircraft for inspection as they landed.
Steven Verhagen, vice president of the Dutch Airline Pilots' Association and a Boeing 737 pilot for KLM, said he would have no qualms about flying today and his 4,000-member organization was calling for a resumption of flights.
"With the weather we are encountering now - clear blue skies and obviously no dense ash cloud to be seen, in our opinion there is absolutely no reason to worry about resuming flights," he said.
"We are asking the authorities to really have a good look at the situation, because 100 percent safety does not exist," Verhagen said. "It's easy to close down air space because then it's perfectly safe. But at some time you have to resume flights."
But meteorologists said the situation above Europe was unstable and constantly changing with the varying winds - and the unpredictability was compounded by the volcano's irregular eruptions spitting more ash into the sky.
The cloud "won't be present at all parts of the area at risk at all times, you can see clear area, but it will change, it won't stand still," said John Hammond of the British Meteorological Office.
Millions of passengers have had plans foiled or delayed because of a ban on air travel that has gradually expanded over large swaths of Europe since Thursday.
The aviation industry, already reeling from a punishing economic period, is facing at least $200 million in losses every day, according to the International Air Transport Association.
KLM, a subsidiary of Air France, began test flights Saturday with a Boeing 737 flying up to 41,000 feet (13,000 meters), the maximum altitude at which the aircraft is certified to fly.
"We observed no irregularities either during the flight or during the initial inspection on the ground," said Chief Executive Peter Hartman, who was aboard Saturday's flight.
The airline planned to return seven airplanes without passengers to Amsterdam from Duesseldorf on Sunday.
"We hope to receive permission as soon as possible after that to start up our operation and to transport our passengers to their destinations," Hartman said in a statement.
German airlines also conducted flights, but at a lower altitude. German restrictions allow such flights, so long as no passengers are on board.
Lufthansa flew 10 empty long-haul planes to Frankfurt from Munich at heights of between 9,800-26,200 feet (3,000-8,000 meters Saturday under so-called visual flight rules, in which pilots don't have to rely on their instruments, spokesman Wolfgang Weber said.
He stressed that the flights weren't tests, and were merely intended to get the planes in the right place at the Frankfurt hub for when restrictions are lifted.