Originally Posted by
brooksjg
I thought it was generally agreed that in many situations, even CAVOK, it was possible, even likely that flight-deck crew will see NOTHING on instruments or outside the aircraft that will give any clue AT ALL of the presence of ash! If you've got an instrument that images diffuse ash-clouds, you're sitting ona gold-mine!
It is generally stated, although it does not appear to be true. It is clear that if the crew can not see out (eg. it is moonless night over ocean or they are in IMC) they can't see ash. However, in all of the documented encounters it was clear after the fact that the ash was visible (Eg. the stars become obscured but the crew couldn't tell if they were in or under cloud because it was night - the NASA DC-8).
To JFs point - the above applies to the immediate flight safety incidents that have been documented by USGC or ICAO. It is quite possible (even probable) that flying in invisible ash could significantly reduce the life of engine components and cause premature failure - I don't think anyone in this thread is arguing otherwise. However, that appears to be much more of an operational economic and maintenance inspection question than a flight safety issue.
PS
It's interesting that there WERE symposia and publications (eg. by USGS) after Pinatubo in the early 90s but not much got done about it!
There conclusion appears to have been don't carry on in an ash cloud - (determined by the cabin fills with haze, you smell sulphur, St Elmo's fire appears, your windshield gets sand blasted). Also, there appeared no evidence of any long term ill effects of operating in the general environment. How much more study should they have done?