Thanks for the much more-definite information from the Real World!
To avoid confusion about quotation from the NASA report, I was quoting from the summary. The relevant part reads
There was no evidence of engine damage in the engine trending results, but some of the turbine blades had been operating partially uncooled and may have had a remaining lifetime of as little as 100 hr
OK -not much different but it SOUNDS more serious. In the same section, there's another interesting sentence:
There are currently no fully reliable methods available to flight
crews to detect the presence of a diffuse, yet potentially damaging volcanic ash cloud.
Elsewhere, it also states that material found in the cabin A/C pack heat exchangers was ash, not sand. I'm afraid that taking this and other statements together, I discounted the previous sand ingestion, probably wrongly.
The only other outstanding questions:
Precision and timeliness of direct observation of the plume(s). If it is indeed the case that satellite observations are possible as frequently as every 15 minutes and that from these sufficient data about the number of (potentially overlapping) plume layers, their density in particles/cu metre and their height can be obtained, then you're quite right. What's all the European fuss about? However, my impression was that satellite data was not that timely or precise, other observations were patchy and plume modelling by computer was not accurate enough for an adequate risk analysis for a given route. I stand corrected.
Cumulative effects. If unreported ash encounters can and will take place, what backstop currently exists in post-flight checks and maintenance to identify cumulative damage due to ash ingestion?
Ps For a Horses Mouth audio report on the NASA incident, see
Volcanic Ash Cloud Encounter.
One important point that emerges is that if ash particles in the plume get covered with ice, the satellite imagery cannot distinguish between ash and normal cloud. What's not made clear is whether the preceding VAAC ash report positions were wrong because of incorrect modelling or because the satellite imagery took precedence but was misleading.