Below is an extract from an ICAO experts meeting where it indicates that no one knows the safe level of ash concentration, and presumably no entity has funded serious research to determine this. Echos of icing and pitot tubes.
INTERNATIONAL AIRWAYS VOLCANO WATCH OPERATIONS GROUP (IAVWOPSG)
FOURTH MEETING
Paris, France, 15 to 19 September 2008
1.1 Secondly, the past paragraph of the workshop summary suggested that ‘clear limits of ash content are required from both the manufacturers and aviation licensing authorities’. This refers to an indisputably difficult and longstanding problem; that there is no defined lower limit on ash concentration. As remote sensing techniques improve, it is likely that the aggregate areas where ash is sensed or inferred will increase, possibly leading to over-warning for ash and cost-blowouts for airlines. It should be recognized that progress on this issue would considerably aid the future operation of the IAVW, including in relation to the development of future satellite sensors, dispersion modeling, eruption notification procedures, and so on.
1.2 The group may agree that this “safe concentration of ash” problem should properly be advanced in both IAVWOPSG and WMO fora, with scientific studies informing a decision-making process about the standards required from the warning system. As it has proven difficult to get formal aviation representation at science-focused workshops such as Rotorua, the input of the aviation industry to this problem might formally be sought through IAVWOPSG processes. A draft conclusion in this regard is given below.