The UK and Irish aviation authorities, with airspace closer to Iceland's volcanic activity than anywhere else in Europe, have doubled the allowable concentration of atmospheric volcanic ash in which it is deemed safe to fly.
All individual airlines are required, however, in association with the original equipment manufacturers of their aircraft and engines, to carry out their own risk assessments to ensure that their fleet can safely operate according to the new limits. The revised densities have been derived by the CAA in association with airframe and engine manufacturers and the US Federal Aviation Administration.
The airspace zoning system, when ash is present, is now divided up into three restricted zones and "normal" airspace (see diagram). The "no-fly" zone is now defined as being airspace in which the concentration of ash is greater than 4 x 10-3g/m3, whereas the previous no-fly zone contained more than 2 x 10-3g/m3. The no-fly zone is surrounded by the newly defined "time-limited zone", in which the concentration is between 2 x 10-3g/m3 and 4 x 10-3g/m3.
Within it, the amount of time the aircraft can spend at that concentration is to be determined by the airline in consultation with the OEMs. The outer zone is designated "enhanced procedures", with ash concentrations between 2 x 10-3g/m3 and 2 x 10-4g/m3, and flight in this area entails a higher frequency of engine inspections as determined by the engine manufacturers, because of the risk of potential cumulative damage.
The CAA says that some particular older engine designs may be affected by flight at this density, but no mandatory occurrence reports have been received indicating damage at that level.