Air traffic controllers were right to close European airspace last year over fears of the ash plume from an erupting Icelandic volcano, a study has found.
Despite protestations from airline bosses last April that it would have been safe to continue flying, samples of ash from the Eyjafjallajökull volcano taken at the time of its eruption have led scientists to conclude that serious structural damage to aircraft could have occurred if passenger planes had continued to fly.
Tests on the ash revealed that it contained minute particles of glass so sharp and abrasive they could have damaged the exposed surfaces of any aircraft, including the engines and cockpit windows.
The volcanic eruption last April caused havoc for up to 10 million airline passengers around the world because of the extensive closures of European airspace, the largest shutdown of air traffic since the Second World War.
Some airlines criticised civil aviation authorities for the shutdown. Ryanair's Michael O'Leary said at the time that "there was no ash cloud. It was mythical. It's become evident the airspace closure was completely unnecessary... none of us could see a bloody thing." He added: "Some idiot in a basement in the Met Office in London spills coffee over the map of Europe and produces a big black cloud."
Willie Walsh, the BA chief executive, described the closure as a "gross over-reaction to a very minor risk" and Virgin boss Richard Branson described the final set of closures as "beyond a joke". It is estimated the airlines lost about £2bn.
The study by Sigurdur Gislason and colleagues at the University of Iceland in Reykjavik was based on an investigation of ash samples sent to a laboratory in Denmark for detailed microscopic analysis.
The scientists found that even after the particles had been mixed continually in water for two weeks, they retained their ability to be dangerous to exposed aircraft surfaces.
"The particles of explosive ash that reached Europe in the jet stream were especially sharp and abrasive over their entire size range," the scientists say in their study published in the journal Proceedings of the National Academy of Sciences.
"The very sharp, hard particles put aircraft at risk from abrasion on windows and body and from melting in jet engines," the scientists said. "Concerns for air transport were well grounded."
The scientists cite the instance of a British Airways 747 which suffered potentially catastrophic damage when it flew through an ash cloud from the 1982 eruption of Mount Galunggung in Indonesia.
All four engines failed as a result of melted ash on the aircraft's turbine blades; the pilot restarted three of them after descending.