Major airlines that sent test flights into European air space found no damage 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.
"Today it has been, I would say, the worst situation so far," Flynn said.
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.
"It is clear that 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.
"Not the slightest scratch was found" on any of the 10 empty long-haul planes Lufthansa flew Saturday to Frankfurt from Munich, spokesman Wolfgang Weber said. The planes flew at low altitude, between 3,000 and 8,000 meters (9800 and 26000 feet), under so-called visual flight rules, in which pilots don't have to rely on their instruments.
Steven Verhagen, vice president of the Dutch Airline Pilots Association, said he would not hesitate to fly an aircraft today carrying his own family.
"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," said Verhagen, a pilot of Boeing 737s for KLM. "We are asking the authorities to really have a good look at the situation, because 100 percent safety does not exist."
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 one half of EU territory will be influenced. This means that half of the flights may be operating," 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.
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.
"There is currently no consensus as to what consists an acceptable level of ash in the atmosphere," said Daniel Hoeltgen, a spokesman for the European Aviation Safety Agency. "This is what we are concerned about and this is what we want to bring about so that we can start operating aircraft again in Europe."
The ranks of stranded passengers, meanwhile, were growing, and many would be stuck for days even if restrictions were fully and immediately lifted.
Mike Parker, trying to return to London from Milan, had made it as far as Paris by Sunday. He stood in line for hours for a bus ticket, only to be told "there's nothing available, there's no trains, there's no planes, there's nothing."
"This is my third day and it looks like it might be another three days before I get back," Parker said.
Rognvaldur Olafsson, a spokesman with the Civil Protection Agency in Iceland, said Sunday the eruption is continuing and there are no signs that the ash cloud is thinning or dissipating.
"It's the same as before," he said. "We're watching it closely and monitoring it."
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 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.
"Normally, a volcano spews out ash to begin with and then it changes into lava, but here it continues to spew out ash, because of the glacier," said Reynir Bodvarsson, director of Swedish National Seismic Network.
Bodvarsson said the relative weakness of the eruption in Iceland also means the ash remains relatively close to the earth, while a stronger eruption would have catapulted the ash out of the atmosphere.