Originally Posted by
foxfire42
I think I'm right in saying that the Met Office has changed things since 2007 at least once, if not twice, as new data becomes available to them.
The difference in approach between London VAAC and (say) Washington VAAC is still there. London is still using the same NAME model and setting hard limits, rather than allowing any interpretation of the model ouput.
The changes that have occurred due to "new data" have been:
1. a 10x raising (to 2 mg/m3) of the original NOAA table threshold of 0.2 mg/m3 for the April 2010 eruption, as an immediate result of the airspace closures. No additional engine test or forecasting data was involved.
2. an additional 2x raising of the threshold to 4 mg/m3, for those who signed up to it in May 2010. Again, no additional engine test or forecasting data was involved.
The original 0.2 mg limit for the April 2010 eruption was the direct output of the quoted NOAA table, with no interpretation (as explained by the 2007 paper). According to the paper, Washington VAAC could have potentially gone to a figure 1000x greater i.e. 200 mg/m3, in the same scenario, without anyone raising concerns about flight safety.