Dear all,
A few comments from a non-pilot but VAAC insider... (not in London....):
1) The threat is real in general - there have been many documented encounters with volcanic ash clouds and while the focus tends to go on the BA, Singapore Airlines and KLM incidents there have been many others. Eg: after Miyakejima in 2000 there were 2 aircraft needing all engines replaced. Longest time between eruption and (minor - pitot tube and abrasion) damage appears to be 20 days for a cross-Pacific cloud drifiting from Ecuador to north of New Guinea. Obviously though the greatest risk is going to be where the cloud is densest.
2) While VAAC operations tend to be conservative (and have been strongly encouraged to be by IATA and IFALPA) they are not ridiculously so. The analysis of an area of ash cloud tends to rely heavily on satellite imagery analysis - while a particular algorithm that identifies silicates is used most of all, other tools such as Sulphur Dioxide detection, straight infrared imagery, satellite or ground based lidars etc are also used. If the cloud is not observed for some time then the VAAC will tend to want to cease the event pretty quickly. The comments on the London VAAC blog
Met Office: Icelandic volcano blog give an idea of what the analysts are thinking.
3) Dispersion modelling is approximate - it depends on the source term, the dispersion model physics, and the met model used - but past verifications show the models to usually be pretty good.
4) Pilot reports are
always useful. Don't assume the satellites see everything, particularly on a cloudy day with a high overcast. Observations of volcanic ash and volcanic encounters *must* be reported. Personal follow up after flight if you've seen something interesting is always appreciated too - meteorologists are human and that personal contact is a golden way to give feedback (even including photos). Also, not all volcanoes are monitored on the ground - particularly in the developing world, and pilot reports might be the only information available.
5) The safe concentration of ash issue has been ongoing for a while and it's not simple. Most of all it's the users (airline industry) that needs to set a warning standard that the VAACs aim to achieve - at the moment it's avoid all ash, which in practice means avoid all 'visible' (reasonable evidence that it's there) ash.
6) The World Meteorological Organisation and International Civil Aviation Organisation have just had meetings on the volcanic ash issue. The reports are just being finalised. If you want to look at the results of the previous science meeting on the issue, go to
http://www.caem.wmo.int/moodle/course/view.php?id=27 and particularly the meeting summary at
http://www.caem.wmo.int/moodle/file....Summary_1_.pdf Things have moved since then but you get the gist of it.
Hope this is helpful and please remember that we're all in this mess together...!