Pall Einarsson, a geophysicist at the University of Iceland, said the 2010 eruption was a rare event.
"The ash in Eyjafjallajokull was persistent or unremitting and fine-grained," he said.
"The ash in Grimsvotn is more coarse and not as likely to cause danger as it falls to the ground faster and doesn't stay as long in the air as in the Eyjafjallajokull eruption."
BBC News - Iceland volcano: Grimsvotn eruption hits local flight
So, wrong kind of ash. Ash is to airlines as snow is to British Rail !!!!!!