Aircraft in some parts of northern Europe grounded by Icelandic volcanic ash have gradually started flying again.
But major flight restrictions remain in place across most of the UK, Ireland, Finland, Germany and Poland.
Here, BBC specialists answer your questions about the volcano ash.
YOUR TRAVEL QUESTIONS
Answered by Richard Scott, BBC transport correspondent
We are due to fly out on Mon 26 April to Orlando, Florida. If the flights are open by then do we still have our seats or are they given to people who have had their flights to Orlando cancelled? We aren't being given that sort of information.
Mrs Joyce, Wokingham, Berkshire, UK
Priority will be given to those who have tickets booked on the flights that leave. Any remaining space will be given to people in the backlog. But remember even if airspace is open, it doesn't necessarily mean flights will leave - many planes and crews are in the wrong places.
Why are non-jet engine aircraft not flying?
Janey Simpson, Bridlington, UK
Some propeller based planes have been flying, but only on a limited basis. Although they don't suffer from the same problems as jet planes, the ash can still damage the planes with a sandblasting effect caused by flying through it. And they'd have to fly out of controlled airspace to be allowed to fly anyway - which generally means at much lower altitude (which can cause its own problems) and from smaller airports.
With the potential dispersion of the ash clouds by the weekend and moving towards eastern Canada, what is the possibility of flights departing from London and arriving in Toronto? Is there an estimated time on when flights from London to Canada will take place?
Brandon, Liverpool, UK
No estimate, I'm afraid. Toronto is unlikely to be affected by any ash drifting towards Canada - the problem really is whether the flight can take off from London. And we don't know when that will be possible.
My daughter and I are stranded in Australia. We have been rebooked on flights to the UK for nearly three weeks' time. How are people going to manage who don' t have someone to stay with?
Rebecca Weir, Norwich, UK
A lot depends on which airline you're flying with, which you don't mention. Your rights are much better if it's an EU airline - but have a read of a couple of articles written by one of my colleagues which go into more detail:
YOUR SCIENCE QUESTIONS
Answered by Richard Black, News website environment correspondent
Eyjafjallajoekull erupted in December 1821 and lasted over a year until January 1823 (from reports I've heard). Can eruptions have times forecasted? And if so, is it possible that this eruption could be shorter or longer?
Phil Oates, Blackpool, UK
Scientists are not hazarding guesses about the potential duration of this eruption. It appears to be quietening now, but there's a chance it could get lively again, or continue producing quite small amounts of material for a while. Another possibility is that the magma body will find another outlet, possibly at nearby Katla, as has happened before.
Why is it that I can see nothing but blue sky, can see the sun, and yet the south is 'covered' by the volcanic cloud?
Barry Smith, London, UK
The word "covered" is a bit misleading in this context as it suggests a thick blanket. The "cloud" isn't like that and the particles in it are tiny - mainly between the size of talcum powder and sand grains. But the impact they have on aeroplane engines is still very real.
Other volcanoes around the world have erupted recently (last 50 years). Have there been problems with aviation in other parts of the world?
Christopher Wills, Whiteley, UK
Yes. The biggest two of recent times - Mount St Helens and Pinatubo - have both led to flights being grounded. However, many planes did fly through the Pinatubo cloud and more than 25 of them were damaged as a result, two of these incidents involving all four engines shutting down. Those potentially fatal incidents are why there is greater caution nowadays.
Answered by Victoria Gill, BBC science reporter
I understand that big jets can't operate at low level but couldn't they fly under 20,000 ft whilst in the area of the cloud and then gain height to operate normally when clear of the danger of the cloud?
Alexander York, Malmesbury, UK
Previously, regulations from the International Civil Aviation Organisation stated that no matter how low the concentration of ash, no commercial aircraft should fly as long as it is in its path. So with a blanket of ash in UK and European airspace, flights were grounded.
This was based on previous flights and engineering tests that showed that the fine dust could enter and clog the tiniest of holes or vents in a jet. The engines rely on having air cooling to keep the metal in the engine from melting. If that air is blocked off, the metal melts and the engine melts.
Now though, based on test flights and analysis of the cloud over recent days, the Civil Aviation Authority has been able to set a safe threshold for the concentration of dust that aircraft can fly through without risking engine damage.
Aren't any of the airlines or aerospace companies developing retro fit filters to protect the aircraft engines from the potentially dangerous dust particles? Or other solutions to negate any effects of these particles in the atmosphere?
This is a consideration for the industry, but it would be costly. Dr Ranjan Vepa, a lecturer in avionics at Queen Mary, University of London has said that in future it would be possible, in principle, to filter the air entering the jet engines. But, he said, as well as the cost of the filter and the retrofitting, this would lead to loss of efficiency - as some energy would be needed to drive the filter. The increase in operating costs could be "quite significant".
I'd like to know what levels the dust is at. Could the aircraft take-off, head towards the clear air and then climb to cruising level above or below the dust level and head down over Spain?
Peter Murray, Crete, Greece
The are layers of dust from ground level up to 20,000 feet throughout the UK's and some of Europe's controlled airspace. What the Met Office and other research flights have now been able to measure though, is the concentration of this ash in the air.
In the past six days manufacturers (of both jet engines and airframes) were brought together by the civil aviation authority to look at the scientific evidence and establish a safe threshold for a concentration of ash below which jet engines will not be damaged.
This threshold has been set at 0.002g per cubic metre of air and the latest data indicate a concentration in UK airspace of 0.0001g per cubic metre.
There are still pockets where the ash density is higher than the new threshold, and these detailed Met Office forecasts are being provided to pilots and airlines so they can safely plan their routes.
What is going on in Iceland? I was told that historically when Eyjafjallajokull erupts Katla also erupts. How is this affecting the local people?
Takhisis, Portsmouth, UK
In terms of domestic upheaval, Iceland is very well-prepared for volcanic eruptions - there are 130 volcanic mountains in the country, up to 22 of which are potentially active and 18 of which have erupted since the settlement of Iceland in about 900 AD.
The country sits on top of the North Atlantic ridge, where two tectonic plates meet and are stretching the Earth's crust apart - slowly moving the UK and US away from each other. As the Earth's crust is stretched, cracks and fissures emerge and the liquid magma underneath seeps or bursts through. Dissolved gases in this liquid rock contribute to an explosive eruption. In this case, the steam generated by the melting ice cap on top added to this, resulting in a tall plume of volcanic ash.
Volcanologists have said that they are concerned that Katla, a volcano very close to Eyjafjallajokull, may also erupt. Three out of four previous eruptions at Eyjafjallajokull have been associated with an eruption at Katla. This is a much larger volcano with a bigger magma chamber, so scientists are monitoring the seismic activity in that area of Iceland very closely.
Based on previous eruptions by this volcano it can continue erupting for several months, possibly for a year or more; so what happens then? Surely air space cannot close for months?
Robert E, London UK
With the new regulations, carriers are now being asked to inspect engines before and after every flight. The Met Office is also providing a forecast to airlines and to the Civil Aviation Authority, showing the predicted density of ash in the air.
The volcanic ash is still in European and UK airspace and there are some pockets where the concentration of ash is over the new safe threshold. But by planning flight paths according to the density map, carriers are able to keep flying. The analysis of the cloud, and of its effects on engines is continuing though, so there could be further regulatory changes.
YOUR VOTING QUESTIONS
Answered by the
How will people trapped abroad be able to vote?
Chris, Surrey, UK
If you are already registered to vote and think you will not be able to get back to the country before 6 May, you can apply to vote by proxy. This means you have to get someone you trust to vote on your behalf.
The deadline to apply for a proxy vote is 5pm 27 April. You can fax or email the form, taking care to make sure that the signature is legible.
However, you must already be registered to vote to be able to apply for a proxy vote.
The deadline to register to vote was midnight Tuesday 20 April.
The postal vote deadline has also passed - 1700 BST on 20 April.
Is there a contingency plan in place in case the disruption to flights is prolonged enough to prevent stranded Britons being back in the UK for the polls on 6 May? Surely the sheer numbers of people absent from voting would invalidate any outcome?
Lisa, Cambridge, UK
The election cannot be cancelled. Election results can be challenged by the individuals standing for election once the results come in.