Brussels:
Major airlines that sent test flights into European air space found no damage on Sunday from the volcanic ash that has paralyzed aviation over the continent, raising pressure on governments to ease restrictions that have thrown global travel and commerce into chaos.
Is it safe to fly yet? Airline officials and some pilots say the passengerless test flights show that it is.
Meteorologists warn that the skies over Europe remain unstable from an Icelandic volcano that continues to spew ash capable of knocking out jet engines.
European Union officials said air traffic could return to half its normal level on Monday if the dense cloud begins to dissipate.
Germany allowed some flights to resume.
Eighty percent of European airspace remained closed for a devastating fourth day on Sunday, with only 4,000 of the normal 20,000-flight schedule in the air, said Brian Flynn, deputy head of operations for Eurocontrol, which supports the air traffic control network across the European Union's 27 states.
The test flights highlighted a lack of consensus on when to reopen the skies.
The microscopic but potentially menacing volcanic grit began closing airports from Ireland to Bulgaria on Thursday, stranding countless passengers and leaving cargo rotting in warehouses.
"This is not sustainable. We cannot just wait until this ash cloud dissipates," EU Transport Commissioner Siim Kallas told reporters at the European capital in Brussels.
KLM Royal Dutch airlines, the national German carrier Lufthansa, Air France and several regional airlines sent up test flights, probing altitudes where the cloud of ash has wafted over Europe since the volcano turned active on Wednesday.
British Airways planned an evening flight over the Atlantic from Heathrow, one of Europe's busiest hubs.
None of the pilots reported problems, and the aircraft underwent detailed inspections for damage to the engines and frame.
"With all the flights, also with the flights of other airlines, there were no (problematic) findings whatsoever in the planes," said Lufthansa spokesman Klaus Walther.
"The planes had no dust deposits in the engines, there was no damage in the body shell, also the pilot heads have not been damaged. This information should be taken into account by the politicians," he added.
Civil aviation authorities in each country must decide whether to resume commercial traffic, but the 27-nation EU said if weather forecasts are correct it expected half its flights to operate normally on Monday.
While it was still unclear how the dust would affect jet engines, the EU said it was encouraged by promising weather predictions, at least for the next 24 hours.
"Probably tomorrow there will be half of EU territory influenced by this ash cloud and... the forecast is that there will be half of the flights possibly operating in Europe," said Diego Lopez Garrido, state secretary for EU affairs for Spain, which holds the rotating EU presidency.
He did not provide details about which flights might resume.
France's transport minister, Dominique Bussereau, said there will be a meeting on Monday of European ministers affected by the crisis to coordinate efforts to reopen airspace.
Meanwhile in London it was announced late on Sunday that there would continue to be no flights in the UK until at least Monday evening.
"They will be giving us a further forecast in the morning and on that basis we will make further information available," British Transport Secretary Lord Andrew Adonis told UK broadcaster Sky.
The comments came after a high level meeting of government ministers in the capital.
Adonis said international regulators were continuing to analyse evidence from a number of test flights including several in Britain.
Business Secretary Lord Peter Mandelson said the government was continuing to look at new ways to help the estimated 150-thousand Britons stranded abroad by the UK flights ban.
"We need to look at every single logistical option for getting our people back home and that's what we are going to do," Mandelson said, raising the possibility of an involvement by the Royal Navy.
Meteorologists warned that the situation above Europe was constantly changing because of varying winds and the continuing, irregular eruptions from the Icelandic volcano.
That uncertainty is bumping up against Europe's need to resume flights.
A spokesman for the European Aviation Safety Agency said there was currently no consensus as to what consists an acceptable level of ash in the atmosphere.
The ranks of stranded passengers, meanwhile, were growing, and many would be stuck for days even if restrictions were fully and immediately lifted.
A spokesman with the Civil Protection Agency in Iceland, said on Sunday the eruption is continuing and there are no signs that the ash cloud is thinning or dissipating.
German air traffic control was the first on Sunday to loosen its ban on passenger flights, allowing some traffic from Frankfurt and airports in the north, but only for northern destinations.
Eastward-bound flights were permitted from Berlin, Hannover, Erfurt and Leipzig.
The Swiss Federal Office of Civil Aviation also began allowing some flights on Saturday.
AccuWeather.com said the top of the ash plume had dropped to about 10,000 feet from 33,000 earlier in the week, putting it in the flight path of even low-flying aircraft.
Shifts in the wind will increase the risk for the Netherlands and Germany on Tuesday and Wednesday, the forecaster said.
Ash and grit from volcanic eruptions can damage a plane in various ways.
The abrasive ash can sandblast a jet's windshield, block fuel nozzles, contaminate the oil system and electronics and plug the tubes that sense air speed.
The greatest danger is to the engines, where melted ash can then congeal on the blades and block the normal flow of air.
There are no recorded instances of fatal aircraft crashes involving volcanic ash, though several have suffered damage and some temporarily lost engine power.
Scientists say that because the volcano is situated below a glacial ice cap, magma is being cooled quickly, causing explosions and plumes of grit that can be catastrophic to plane engines.