Civil Aviation Authority chief executive Andrew Haines on lifting flight ban
The UK Met Office is still detecting an ash cloud in European airspace, so why is it now safe to allow planes to fly?
The Civil Aviation Authority (CAA) has confirmed that the guidelines regarding flying through volcanic ash have been changed, following six days of discussions between aviation engineers and experts to "find a way to tackle this immense challenge, unknown in the UK and Europe in living memory".
A spokeswoman from the CAA told BBC News: "Air manufacturers, both engine and airframe, were asked to look at the scientific evidence from test flights and at the Met Office data, to understand how much volcanic ash in the atmosphere jet engines could tolerate [without being] damaged."
Now, scientists and engineers have agreed a safe threshold - a concentration of ash of 0.002g per cubic metre of air. At or below this concentration, there is no damage to the engine.
Current data suggested that concentrations of ash in UK airspace were around 100 micrograms (or 0.0001g) per cubic metre, explained Dr Grant Allen from the Centre for Atmospheric Science at the University of Manchester.
"Analysis of those atmospheric measurements is early and still ongoing and being supplemented with new measurements all the time," he said.
"Two research aircraft will fly [on Wednesday] to record ash size distributions to assess how near to the new tolerance concentrations actually are, which will also be compared to previous days."
The CAA has opened airspace where the concentration of ash in the air is below this new threshold.
So does this mean the ban was ever necessary? Was the ash cloud ever sufficiently dense to prevent safe flight?
The answer to this is still unclear, but Dr Allen said that early analyses of the research flight data suggested that the plume that had been measured over the past four days contained only 0.0004g of ash per cubic metre at its peak.
But the regulations before this event were set out by an international body called the International Civil Aviation Organisation (ICAO). It set a limit of "no tolerance" for any concentration of volcanic ash.
Dr Colin Brown, director of engineering at the UK's Institution of Mechanical Engineers, explained that previously, scientists were not able to detect ash at the very low concentrations that modern analytical equipment is capable of measuring. So the advice was to avoid it all together.
"Previous to this, the regulations were, if you see ash, you fly 100 miles away from it," he told BBC News.
"But now we have this blanket over the biggest airports in northern Europe. It's an unprecedented situation."
So over the past six days, the Natural Environment Research Council (Nerc) has run four test flights with its Dornier 228 research aircraft, to sample different layers of the plume.
The UK's Facility for Airborne Atmospheric Measurements also carried out a research flight on Tuesday. Each of these flights carried analytical instruments to detect particles of ash in the cloud, much of which is not visible to the naked eye.
Back in the air: Planes left contrails over the Met Office on Wednesday
Commercial test flights, such as the one that was operated by British Airways, have simply checked for engine damage following the flight.
It is the results of these test flights that have been translated into the updated regulations.
The Met Office fed the new safe threshold figure into its "Name" model (Numerical Atmospheric-dispersion Modelling Environment) and mapped the density of the ash cloud.
"The model is routinely verified against satellite and aircraft and balloon observations of the ash cloud, along with other ground based observations such as lidars [a type of radar], and other visual observations," The Met office told BBC News.
The CAA spokeswoman said: "Volcanic ash has not left UK airspace - it's still here, and it's a very fluid situation. There are still 'black blobs' [on the map] where the ash is more dense, and the co-ordinates are being updated every six hours."
A statement from the CAA explained that for these new guidelines to be implemented, airlines would also be required to carry out "intensive maintenance ash damage inspection before and after each flight".
"Nats [the UK air traffic authority] is responsible for operating controlled airspace, so they are responsible for communicating Met Office data to pilots," explained the CAA spokeswoman.
This means that Nats, via air traffic control, will alert each pilot to the ash forecast at a particular time. The same information will be provided regularly to airlines, enabling them to plan flights as appropriate.
"All airlines always consider the weather," said the CAA. "So these new Met Office charts are just being brought in as part of that planning process."
The right ash
How ash can stop a jet engine
The data has also been supplemented by engine tests on the ground - allowing different concentrations of ash to flow through a jet engine and then assessing the damage.
But Dr Brown explained that the cloud itself had to be analysed in order to come up with an official safety threshold.
"You have to test the stuff that is actually there," he told BBC News. "You can't assess safety properly if you're carrying out tests on the ground with the wrong ash."
Aviation consultant Chris Yates added that it often takes a major incident such as this to effect reforms.
"As with so much in the aviation world, the wheels of regulatory reform move at a glacial pace," he said.
"Manufacturers have been taken to task, and in the face of evidence have been convinced to revise their thinking on the issue. Over the coming months, we will now see a flurry of activity on this operationally important issue and redefining of the regulations."
He added that although planes were now allowed to fly, the CAA would close airspace again if ash concentrations rose.
The government's chief scientific advisor, John Beddington, said that he was confident that the new regulations "guaranteed safety".
Only the beginning?
Stephen Wright, a lecturer in the University of Leeds School of Process, Environmental and Materials Engineering, said that the UK's impeccable safety record in aviation would also have delayed the decision to lift the ban.
He explained: "As well as the test flights, the authorities will have been trawling through historical data from episodes such as the first Gulf War, where the mass movement of troops in Saudi Arabia and Kuwait stirred up all the ultra fine sands in the region resulting in damage to the UK tornado aircraft fleet."
Mr Wright added that the difficulties in operating commercial aviation "are likely to be felt for many months to come", and that this ash cloud and the regulatory upheaval would have a significant impact in the cost of flying aircraft.
"Engines are just the start of the problem," he said. "There are many other components on aircraft that are equally sensitive to particulates and maintaining these in the current conditions is likely to be very expensive."
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