The recent eruption of another Icelandic volcano (not the one with the unspeakable name which erupted last year) means air traffic throughout Europe has again been disrupted - but not as badly as last time. So, if the eruption and ensuing ash cloud is much larger, why is there less disruption? Well, according to some reports it's simply that the policy regarding air travel has changed - allowing flights in denser ash conditions. Hmmm. Makes you wonder what led to the policy being developed in the first place - and how much is "too much" ash - and does it very for different aircraft, different engines, etc?