Some perspective on this. The only incident where there is no indication the crew knew about the ash at the time is
...
Can't believe we're quoting from the same document!!!
The one I'm looking at is
The 1991 Pinatubo Eruptions and Their Effects on Aircraft Operations by Thomas J. Casadevall,1 Perla J. Delos Reyes,2 and David J. Schneider3
In that report incident 91-09 is perhaps the best example of an undetected encounter with NO external evidence on the aircraft yet still severely trashed turbines. There are several others where no primary incident location is recorded, for several possible reasons. As the original authors state:
The detail of information was variable, especially concerning the locations of encounters and damage. In some cases, carriers were reluctant to discuss encounters, owing to concerns over possible future liability. In other cases, pilots may have been unaware that their aircraft had flown through an ash cloud, and damage to the aircraft might not have been noticed until the aircraft was later inspected on the ground. This partly explains why there are position data for only 11 of the incidents (fig. 1; table 1).
At this date, I doubt that anyone could say with any confidence which reason accounted for which piece of missing data in incidents in 1991!
BUT the key feature that is common to this AND the later NASA DC-8 incident north of Iceland AND probably some aspects of the Finnish F-16 Hornet ash incident two weeks ago is that there ARE many well-documented cases of significant turbine damage when the pilots were completely unaware of any ash encounter. You may not like these facts but unfortunately they are there in black and white, from multiple independent sources.