Why can't jet aircraft fly beneath -- or above -- Europe's cloud of volcanic ash?

That's a question being asked by many, especially those stranded at airports around Europe waiting for flights disrupted by the Eyjafjoell volcano to resume.

The answer, say experts, is based partly on safety concerns but also on cost.

Kjetil Toerseth, director of regional and global pollution at the Norwegian Institute for Air Research, explained that the cloud varies in height and thickness according to location and is not a single plume that is the same density everywhere.

The thickness varies according to belches of the volcano over the past six days and the strength and direction of the winds.

A simulation of the plume on his institute's website (http://www.nilu.com/index.cfm?ac=news&text_id=33469&folder_id=4316&view=text) shows waves of dust that head towards northwestern Europe and then gradually fan out.

These waves range in altitude from roughly 5,000 metres (16,000 feet) -- although some passes through at heights lower than this -- to about 11,000 metres (36,000).

"A lot of (ash) transport has been at lower levels, quite often at 5,000, 6,000 metres (16,000, 20,000 feet)," Toerseth told AFP.

"Lower is difficult, because you will always have particulate matter. There is no altitude which is completely safe below 11,000 (metres, 36,000 feet). Above 11,000 (36,000 feet) of course is safer."

Flying above 11,000 metres (36,000 feet) is a problem because planes may well have to ascend and descend through the cloud, he said.

Other factors are whether aircraft types are able to fly at this height, and what the added cost would be, he said.

For flying at low altitude -- on the assumption that the air corridor is clean of ash -- the big factor is cost.

At lower altitudes, the atmosphere is denser, which means a plane needs more power to "push" through atmospheric molecules.

"It is possible, but it's not ideal because jet aircraft do not operate so well at low altitude. It makes them burn more fuel and it's expensive," said David Kaminski-Morrow, a journalist with the British aviation publication Flight.

earlier related report
Sighs of relief as Paris flights resume
Paris (AFP) April 20, 2010 - Relieved passengers boarded planes as others touched down at last in Paris where air traffic inched back to normal Tuesday after being shut down for five days due to volcanic ash.

"I've never been so happy in my life going back home," said Shahriar Ravari from San Diego, waiting for a flight to Los Angeles. "I love France but to be going home is something else.

"I drove all the way from Gothenburg in Sweden, all the way through Amsterdam to here," he said.

"It's my daughter's birthday and unfortunately the aeroplane got closed as soon as I arrived here so I had to wait. Finally, thank God, I got the ticket back to LA. So I missed by daughter's birthday but that's OK. That's life."

Nine in 10 scheduled long-haul flights ran Tuesday out of Paris's main international airports, Charles de Gaulle and Orly, along with a third of medium-haul services, Environment Minister Jean-Louis Borloo said.

French airports hope to ensure 100-percent of long-haul flights and 60 percent of medium-haul services on Wednesday, Borloo told reporters following a meeting with aviation and tourism industry professionals.

Virtually all of the estimated 20,000 French travellers still stranded abroad should be home by Thursday, he added.

Tourism Minister Herve Novelli said the lockdown had so far cost France's air and tourism industries 200 million euros (270 million dollars), with each passing day adding 10 million euros to the bill.

He said France was looking into possible financial aid for the sector such as delays to employer welfare contributions, with the specifics to be announced early next week.

The first load of passengers took off from Charles de Gaulle at 8:00 am (0600 GMT) bound for Algiers, as others waited to board delayed flights or meet loved ones finally returning home.

"I am happy, I'm going to see my wife again," said one young man, Conrade, after stepping off a flight from Abidjan.

An Air France A380 superjumbo landed on Tuesday afternoon, bringing 541 passengers, tired but largely grateful, back from New York.

"It's a bit tense at JFK (airport in New York). It's very disorganised," said one of its passengers, Philippe Corcuff. He spent three days waiting for a flight out of New York with his wife and two children.

Other reopened French airports such as Nantes were bustling meanwhile as they helped clear the backlog.

Officials warned that uncertainties remained over the lingering presence of ash from the still-rumbling Eyjafjoell volcano in southern Iceland.

"We are flying planes under special safety conditions, but we remain in a crisis situation," Borloo warned earlier.

He rejected as "scandalous" claims that the government had ceded to pressure to open the skies from airlines who say they are losing tens of millions of euros a day.

Test planes were meanwhile flying reopened routes ahead of the commercial aircraft, to check for floating ash.

"Like you send minesweepers through a minefield, they are going to clear part of the route," said Stephane Durand, a senior official of the national air traffic controllers' union.

"They will say, 'you can go on this route, but if you stray off it, I can't guarantee anything'."