When news emerged that the Grimsvötn volcano in Iceland had begun erupting at 19:00 GMT on 21 May, the aviation industry could have been forgiven for fearing the worst.
The initial explosion sent an ash plume over 65,500ft (20,000m) into the atmosphere, twice as high as the eruption of Eyjafjallajökull in April 2010.
Although flights into airports into the north of the UK suffered two days of disruption as the ash cloud drifted south, with German airspace also affected in the following days, a repeat of the total shutdown of European airspace experienced last year did not occur. According to Eurocontrol, 900 flights from a total of 90,000 between 23 and 25 May were cancelled.
Although there were differences in the two events - notably in the duration and strength of the eruption, which had ceased by 02:00 GMT on 25 May - experts are agreed that Europe's systems coped markedly better on this occasion.
In a joint statement, the Association of European Airlines and the Airports Council International - Europe generally endorsed the advances since last year and the systems now in place. Olivier Jankovec, ACI-E director general said: "For those feeling a sense of déjà vu, I can tell you that this latest eruption is being handled in a very different manner. Lessons have been learned from the previous volcanic ash shock in April 2010.
"This past year, the European Commission, Eurocontrol and the European Aviation Safety Agency have worked intensely to devise an alternative procedure for flight operations, safeguarding the highest possible level of safety, while minimising disruption. This procedure is at the disposal of national governments. It now needs to be applied promptly and consistently throughout Europe."
There were disenting voices, however, perhaps the loudest of which was Ryanair. The Irish budget carrier described restrictions on flights to and from Scottish airports as unnecessary. It cancelled 36 flights to or from Scottish airports on 24 May on the advice of the Irish Aviation Authority.
It pointed to a 1h verification flight it conducted over Scotland on the morning of 24 May at an altitude of 41,000ft, which it said found no traces of ash, as proof of its position.
The flight's route took it from Glasgow Prestwick over Inverness, Aberdeen and Edinburgh, all areas cited by UK Met Office predictions as being in the "red zone" of high-density concentrations of ash. It described the zone as "a misguided invention by the UK Met Office and the CAA". However, the Civil Aviation Authority later disputed the course taken by Ryanair's aircraft.
A later verification flight by British Airways using an Airbus A320, in co-operation with the CAA, was flown into the ash "red zone" in Scottish airspace for about 45min and suffered no observable damage, the airline reported. However, it has yet to test various filters that were renewed specifically for this flight.
The Met Office described the BA flight results as "useful", and said that there have also been numerous pilot reports of visible ash in UK airspace and over the North Sea. All data from all sources is useful, the Met Office said.
The Met Office has, however, come in for criticism for the absence of the only test aircraft available to it, a BAe 146 operated by the Natural Environment Research Council. The aircraft is at work in North Africa on a project that it was booked to carry out a long time in advance, said the Met Office.
The International Air Transport Association's director general Giovanni Bisignani voiced his anger about the lack of a research aircraft to the UK's Department for Transport. He said: "It is astonishing and unacceptable that Her Majesty's government cashes £3.5 billion [$5.7 billion] a year in air passenger duty but is incapable of using a small portion of that revenue to purchase a Cessna to use as a back-up aircraft. I ask please that you ensure that all possible efforts are made to get the existing aircraft operational in the shortest possible time."
The UK Met Office has pointed out that airborne monitoring is only one of the many ways in which ash can be tracked and its concentration measured, suggesting we "shouldn't be too hung up" about the lack of available aircraft-mounted sensors. A test aircraft is only one of an arsenal of ash analysis weapons, says the Met Office, and it is not guaranteed to be any more useful than the alternatives because it can only "take a snapshot" of conditions in a small part of the vertical and horizontal airspace affected by ash.
Airborne reporting in real time can also be achieved by weather balloons. Ground-based sensor tools that the Met Office uses include light detection and ranging (lidar), laser cloud base recorders (LCBR) - which can analyse ash layers - and radar. It has 36 ground-based atmospheric monitoring stations including lidar and LCBR, with 14 more LCBR installations shortly to be installed. A mobile radar in Iceland is also available, said the CAA.
Source: Flight International