From jonfr.is Geology Blog today:
Is it going to create a flight chaos as happened with Eyjafjallajökull in 2010?
No, that is unlikely to happen. While disruptions should be expected once an eruption starts under the glacier that appears to be powerful enough to break trough the glacier. The volcano ash from Bárðarbunga volcano is going to be different, since the magma is basalt, it is going to explode on contact with water, creating volcano. No lava is not going to be erupting while water gets into the erupting craters. This also means that the volcano ash that forms is larger and heavier as result. That means it doesn’t stay as long up in the air and does not go long distances.