"The decision to close the airspace was made exclusively as a result of data from a computer simulation at the Volcanic Ash Advisory Centre in London," Air Berlin chief executive Joachim Hunold told the Bild am Sonntag newspaper.

"Not one single weather balloon has been sent up to measure how much volcanic ash is in the air."

Lufthansa spokesman Klaus Walter told Bild: "The flight ban, made on the basis just of computer calculations, is resulting in billion-high losses for the economy.

"In future we demand that reliable measurements are presented before a flying ban is imposed."

Lufthansa flew 10 aircraft -- without passengers -- from Frankfurt to Munich on Saturday, not as proper test flights but to prepare for air traffic returning to normal, Lufthansa spokesman Wolfgang Weber told AFP.

"The planes were analysed and show no signs of damage, not even the slightest scratch on the cockpit glass, the fuselage or the engines," Wolfgang said.

The aircraft flew at heights of between 3,000 metres and 8,000 metres (10,000-26,000 feet), "and apparently up until 8,000 metres, there is no volcanic ash," he said.

Earlier Germany's national air safety agency DFS said that the country's airspace would remain closed until at least 8:00 pm (1800 GMT) on Sunday.

Lufthansa said it was cancelling all its flights until the same time. All the country's 16 airports were paralysed including Frankfurt, one of Europe's busiest hubs.

earlier related report
KLM runs test flights through Europe's volcanic ash
The Hague (AFP) April 18, 2010 - KLM Royal Dutch Airlines conducted a test flight from Amsterdam to Dusseldorf on Sunday, it said, with eight more to follow during the day to measure the impact of volcanic ash on aircraft.

"A Boeing 737 with 20 crew on board departed from Schiphol airport at 6:30 a.m. (0430 GMT) this morning and landed safely in Dusseldorf in Germany," KLM spokeswoman Joyce Veekman told AFP.

During the course of the day, the crew will fly the test aircraft as well as seven other Boeing 737s which have been stranded in Dusseldorf back to Amsterdam, she said.

Permission had been obtained from the Transport and Water Management Inspectorate (IVW), the Dutch transportation safety agency.

"These are test flights," KLM president Peter Hartman said in a statement.

"This does not mean that normal air traffic has been resumed. This matter will be decided on by the Dutch air transport authorities IVW, in consultation with the European authorities."

The first flight back to Schiphol has already departed from Dusseldorf, said Veekman. The planes would fly at a height of between 10,000 and 13,000 meters (about 41,000 feet).

A test flight conducted with a Boeing 737 over Dutch air space on Saturday evening, with scientists on board, "revealed that no problems had been encountered and that the quality of the atmosphere is in order," KLM said.

Those findings prompted the IVW to authorise Sunday's flights.

Each aircraft will gather data for the decision-making authorities, explained Veekman. Their engines have been fitted with filters which would be checked for ash particles once back on the ground.

"Also, our pilots will conduct visual tests. They will be able to see if the air is clean," she added.

KLM said that if all Sunday's flights went without hitch, it hoped to get permission "as soon quickly as possible" to restart some operations.

"KLM hopes to resume operations as swiftly as possible, so that passengers can be brought to their destination. Naturally, safety remains the chief priority," it said.

The airline said it would work closely with its German counterpart, Lufthansa, "which conducted 10 test flights in Germany" on Saturday.

The IVW announced earlier that Dutch airspace, closed since 6:00 p.m. (1600 GMT) on Thursday, would remain off-limits until at least 2:00 p.m. (1200 GMT) on Sunday.

KLM, which operates 60 percent of flights at Schiphol -- one of Europe's largest airports -- said that all its European flights to and from Amsterdam were cancelled until at least 4:00 p.m. (1400 GMT) on Sunday.

All incoming intercontinental flights remained cancelled indefinitely, while outgoing ones would not resume before 8:00 p.m. (1800 GMT), KLM said.

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