And what triggers escalation of the change from C to A?
A report from the applicable Meteorological Service that a volcanic eruption has occured in the vicinity of flight paths operated or proposed to be operated by the aircraft in question.
SOP is:
Volcano erupts
Met Service reports extent and density of the ash plume
Authorities publish "No Fly" notices for specific high density locations
Airlines re-route to avoid the plume
Airlines implement precautionary measures for flight in areas where lower concentrations of ash may be encountered
Infra-red satellite monitoring of the globe is taking place at roughly 15 minute intervals across the globe and the findings circulated. The failure of the European authorities to adhere to the established methodology resulted in the European "No-Fly" zone being held in operation far beyond the necessary period and led to the subsequent open defiance demonstrated by large, capable and influential European carriers like LH, KLM and BA.
Internationally, such carriers will regularly operate in other volcanic zones. There are, after all as many as 24 volcanoes in various stages of eruption at any time around the globe. A volcanic eruption affecting Europe may be big news but it is an almost routine event around the world.
...could have had a blade failure 'within 100 hours' and NASA decided to part with $3.2m for an engine overhaul, there must have been a lot of damage
No, the report says that on the engine that had the most damage, "hot section parts may have begun to fail if operated for another 100 hours". The report indicates that this engine had flown 388 hours since the last shop visit but does not say what that shop visit was. Total engine hours on each engine were stated as in excess of 5,000 hours, but it is engine cycles that are the more important tracking value for engine maintenance. One would suspect that a NASA research aeroplane would have fewer hours per cycle than an in-service long-haul airliner routinely operating seven to eight hour sectors, would be less likely to use de-rated take-off and climb settings and, with 17 experiment stations would operate at higher all-up weights.
The report also includes evidence of a previous sand ingestion event that was serious enough to warrant a boroscope inspection. Though there were no significant findings, this event must have some bearing on the final condition of the rotating parts. i.e. the total damage was not confined to the ash ingestion.
The repair bill was for all four engines and $3.2 million for one engine overhaul and three engine refurbishments would not be considered unusual.
I guess I'm saying that this one event to an atmospheric research aircraft that seems to have been deliberately flown close to a volcanic plume, leaves as many questions unanswered as it answers and is therefore not a good example to quote with reference to commercial operations in a potential volcanic ash ingestion environment.
I'm personally acquainted with a fleet of B757s that operated in the South China Sea area during and since the Pinatubo eruption that closed Clark Air Force Base permanently due to ash fallout. Local conditions in the rest of SE Asia were similar to those that pertained in Europe recently, yet those engines suffered no adverse symptoms, achieved a normal service life with unimpaired reliability and during their routine shop visits, showed no signs of unusual damage. In fact, although the airframes have long been scrapped, those engines remain in service on other aircraft.