Originally Posted by
KBPsen
Some insight into how the limits were developed can be found
here
Thanks KBPsen! This only covers the 2 mg limit though:
"The manufacturers were clear that although they were content that 2x10-3g/m3 represented a tolerable density level for their products, any further increase in this level would require more data on, and analysis of, the effects of ash contamination on airframes and engines" - 23 April 2010. The 4 mg limit (albeit with some riders) was introduced on 18 May 2010 - the data gathering and analysis process must have gone mighty quick in the intervening 3 weeks!
I also agree wholeheartedly with Pace in praising the Met Office's efforts. It's not the forecasting or data that I have a problem with; it's the use the data is put to and the fact that no one seems to have thought to figure out the result of making new rules until confronted with the practical consequences...
The referenced paper also says: "It would seem sensible to consider assessing the tolerability of ash and other particulates at densities of the order of 10^-2 g/m3". I hope that work is progressing as a matter of the utmost urgency, because the new "vertical" rules will be difficult to implement effectively and would be made completely redundant by lateral shrinkage, resulting from a higher density tolerance.