Airlines and authorities should refrain from "experimenting" by flying planes through skies filled with ash from Iceland's erupting volcano, the author of a study on the impact of last year's ash cloud on aircraft said Tuesday.

"Neither you nor I would like to get on a plane where there would be any experimenting," said Susan Stipp, a geochemist at the University of Copenhagen who has headed a Danish-Icelandic study published last month on the dangers posed by the Eyjafjoell volcano's ash cloud.

She insisted airlines and aviation authorities should learn from the lesson of the Eyjafjoell eruption last April, which caused the planets biggest airspace shutdown since World War II, grounding more than 100,000 planes and stranding more than eight million passengers.

"There were a lot of people frustrated and a lot of money lost, but there were no lives lost as a result of plane failure," Stipp said.

"The air authorities made the right decision last time based on the fact that these particles were dangerous in certain proportions," she insisted.

Many critics, especially in the airline industry which was hard-hit by the 2010 airspace closures, have said the danger was exaggerated and called for more lenient rules, but Stipp and her team's study concluded the measures taken were largely warranted.

The ash particles from the Eyjafjoell eruption "were small so they went high and far. They were sharp, so they were a danger to airplane windows. It's like sand-blasting the airplane," she told AFP last month after publishing the study.

While there has yet to be a full analysis of the ash from Grimsvoetn, Stipp pointed out that the fact that the plume this time peaked at around 20 kilometres (12 miles) -- double last year's eruption column -- suggested the ash particles were as fine.

"I suspect that these particles are also dangerous," she said.

Several experts have described the first ash gathered as coarser and heavier than during last year's blast, and therefore less likely to be carried far afield and less dangerous to aircraft.

University of Iceland geophysicist Magnus Tummi Gudmundsson told AFP Tuesday the ash from Grimsvoetn was "not the same type as from the Eyjafjoell eruption," pointing out the ash causing trouble now "is typical basaltic ash."

However, he said, the ash "is quite light and fine-grained, but not quite as fine-grained as it was in Eyjafjoell."

Stipp meanwhile insisted aircraft makers do not have enough information to properly assess the danger of fine-particled ash.

Most of "their testing has been made in places like deserts, where the sand is rounded and the dust is rounded. It's a different composition" from ash, she said, pointing out that "volcanic ash particles are extremely sharp, like if you drop a beer bottle on a concrete floor."

There is some good news though: measurements showed Tuesday that Grimsvoetn's plume had shrunk to between three and five kilometres, indicating according to Stipp that the magma may no longer be coming in contact with ice and water, making the eruption less explosive and meaning far less ash.

earlier related report
Ash cloud will not ground Scandinavian flights: experts
Helsinki (AFP) May 24, 2011 - The giant cloud of volcanic ash spreading from Iceland will reach southern Sweden Tuesday night, but is not expected to be thick enough to prevent air travel, Finnish experts said Tuesday.

"At the moment it looks like the ash cloud will spread into southern Sweden at 2100 Finnish time (1800 GMT), and it looks as if it will be moving east, but we don't have an estimate as to when it might hit Finland," Raine Luojus, a spokesman for the country's aviation safety authority Finavia, told AFP.

"Most likely, it won't be so thick that it would prevent flights" in Scandinavia, he added, referring to estimates based on data from the Volcanic Ash Advisory Centre in London.

Airlines meanwhile halted dozens of flights to and from Scotland Tuesday as the volcanic ash cloud blew over Britain, even forcing US President Barack Obama to revise his travel plans and leave Ireland for Britain a day early.

In Sweden, flights were expected to run almost as normal Tuesday, except for possible cancellations to and from Scotland, according to a spokesman for the Swedish airport operator Swedavia.

"There are some indications that there might be (some ash) in western Sweden but those prognosis are still very uncertain," Anders Bredfell told AFP, adding though that Swedish authorities were preparing "for the worst-case scenario."

Iceland, which was forced to close its airspace a day after its Grimsvoetn volcano began erupting Saturday, had by Monday evening reopened its four international airports, including its main Keflavik airport near the capital Reykjavik.

On Tuesday, all flights from Keflavik appeared to be on schedule except the cancellation of one flight to London Heathrow and one to Manchester/Glasgow.

When Grimsvoetn, Iceland's most active volcano located at the heart of the country's biggest glacier, Vatnajoekull, in the southeast began erupting late Saturday, it shot up a plume of ash and smoke as high as 20 kilometres (12 miles) into the air.

In April 2010, a massive cloud of ash from the nearby Eyjafjoell volcano caused the planet's biggest airspace shutdown since World War II, with more than 100,000 flights cancelled and eight million passengers stranded.

The costs in terms of lost revenue and compensation to passengers were a body blow to the airline industry, particularly in Europe.