As a plume of ash once again drifts towards Scotland from the north Atlantic, experts insist that this time will be different, and new rules will permit more aircraft to brave the cloud.
But as aviation authorities admitted they did not yet know how badly flights would be affected, airlines including BA, Aer Lingus, KLM, easyJet and Flybe grounded services due to take off today.
Barack Obama and his entourage took no chances, preferring to skip a planned one-night stay in Dublin and fly in to London last night for the US president's state visit.
What is known is that the densest parts of the ash cloud from the Grimsvötn volcano are expected to exceed a new safety threshold set for airlines, and services at a dozen airports, including Glasgow and Edinburgh, are already under threat.
A string of carriers have announced changes to their schedules, with BA stating that it will not operate any flights between London and Scotland before 2pm today.
KLM said 16 flights scheduled for today to and from Glasgow, Edinburgh, Aberdeen and Newcastle would be cancelled, while Eastern Airways, based in Lincolnshire, axed all flights. Loganair, a regional carrier based in Glasgow, cancelled 36 flights.
The air safety watchdog for British airlines and airports, the Civil Aviation Authority, said particles from Grimsvötn could affect transatlantic journeys by reaching western England on Thursday or Friday, depending on wind direction.
High winds had already caused disruption to rail and road users in Scotland and northern England , with 100mph winds disrupting motorists, rail users and homeowners in Scotland, causing mass cancellations of rail services, and power cuts. Eastbound traffic on the M62 between Leeds and Manchester was brought to a standstill, while three racing yachts were rescued after high seas disrupted an event between Whitby and Scarborough. The winds are expected to affect ash cloud forecasts. The Met Office warned of difficulties in tracking the cloud's progress because of erratic shifts in wind direction. If airspace in western England, Ireland and the Atlantic is affected by the plume, US-bound flights from Heathrow could see delays later this week as planes are diverted around the densest parts of the cloud.
But the CAA said it was confident that a new Europe-wide safety regime introduced after the Eyjafjallajökull eruption last year would reduce disruption and avoid the continental shutdown that stranded millions. Under the new operating procedures, it is understood that the effect of last year's plume on commercial routes would have been reduced by 75%.
Andrew Haines, the CAA's chief executive, said: "Our number-one priority is to ensure the safety of people both on board aircraft and on the ground. We can't rule out disruption, but the new arrangements that have been put in place since last year's ash cloud mean the aviation sector is better prepared and will help to reduce any disruption in the event that volcanic ash affects UK airspace."
Under previous guidelines, aircraft were summarily grounded if there was any volcanic ash in the air. Now, airlines can fly through ash plumes if they can demonstrate that their fleets can handle medium- or high-level densities of ash.
The Met Office's volcanic ash advisory centre will track the cloud, aided by satellite images, weather balloons and a radar specially installed in Iceland last year. Once those zones are relayed to airlines, they will need to prove that they can fly through them by producing "safety cases" that will include information from aircraft and engine manufacturers on the airline's tolerance to volcanic ash.
The transport secretary, Philip Hammond, said that the ash had already caused "modest delays" to flights, including some Atlantic services.
"Clearly, this is a natural phenomenon which we cannot control, but the UK is now much better prepared to deal with an ash eruption than last year," he said.
"Our investment in new equipment to better monitor ash concentrations and our development of new operating procedures for airlines will all help to lessen the impact the eruption will have on UK aviation.
A CAA spokesman said: "Safety will still be paramount, but we will be able to drastically reduce disruption compared to last time, provided there is not a huge amount of high-density ash." The spokesman said a similar level of ash to the Eyjafjallajökull incident would not result in a mass-grounding. "It will be a different picture." However, jets would still have to divert around high-density clouds, causing delays on some routes, because no UK airline has submitted a safety case for flying through heavy ash plumes.
BAA, owner of Heathrow, Stansted, Edinburgh, Glasgow and Aberdeen airports, has convened a crisis team to prepare for a cut in flights, as airlines and airports await a briefing from Eurocontrol and the UK air traffic controller, Nats..
Under the new ash guidelines, cloud densities are split into three levels: low, medium and high. Once the Met Office assigns a particular density of ash to a section of airspace, airlines must prove they have the safety case to fly through it. A low density cloud is 2g of ash per 10 cubic metres of air, with medium being 2g to 4g of ash per 10 cubic metres. Anything above 4g is deemed high density.
The Grimsvötn volcano began erupting at the weekend, causing flight cancellations at Keflavik airport after it sent a plume of ash, smoke and steam 12 miles into the air. Helen Chivers, a Met Office spokeswoman, said: "At the moment if the volcano continues to erupt to the same level it has been, and is now, the UK could be at risk of seeing volcanic ash later this week. Quite when and how much we can't really define at the moment."
She said the weather situation was likely to be different from last year, with the wind direction set to change continuously. She added: "If it moves in the way that we're currently looking, with the eruption continuing the way it is, then if the UK is at risk later this week, then France and Spain could be as well."
While the ash has grounded aircraft in Iceland, it is not anticipated that it will have a similar impact in the rest of Europe.Dr Dave McGarvie, volcanologist at the Open University, said the amount of ash reaching the UK was "likely to be less than in the 2010 Eyjafjallajökull eruption", and the last two times Grimsvötn erupted it had not affected UK air travel.
"In addition, the experience gained from the 2010 eruption, especially by the Met Office, the airline industry, and the engine manufacturers, should mean less disruption to travellers," he said.
The eruption of the Eyjafjallajökull volcano in south-east Iceland in April 2010 caused the worst disruption to international air travel since 9/11. Flights across Europe were cancelled for six days, stranding tens of thousands of people, and the eruption was estimated to have cost airlines £130m a day.
Eurocontrol said in a statement: "There is currently no impact on European or transatlantic flights and the situation is expected to remain so for the next 24 hours. Aircraft operators are constantly being kept informed of the evolving situation."