The UK CAA has reviewed the current knowledge on the effects of volcanic ash and the accuracy of the forecasting models. The document describing the rationale for removing the buffer zone can be found here.
This is good news for the aviation industry, not only because it will reduce some of the restrictions that have been placed on flights but, more importantly, because it shows the increasing scientific knowledge that has been gained. While it states that the ash concentration that can cause engine shut down is known with reasonable accuracy, it also admits that, at the lower levels, ‘the exact rate of change of damage with ash concentrations is not known’. Clearly more scientific work is needed in this area.
However, my main reason for optimism is the work being done to determine the validity of the forecast models. This has been done using flight test data gained from specially instrumented aircraft (Do228 and BAe146) during flights in and around the No-Fly Zones. This data has also been correlated with ground based LIDARs and Met Office weather balloons.