- anyone know what maxima are specified by engine suppliers for particles in the airflow (ie. number and size - I'd assumed there'd be datasheet items for a number of different combinations of size, characteristics such as melting point, abrasiveness, ..., and number per cubic metre of atmosphere. However, no-one so far (?) has identified such info.)
It is not possible to establish the "how much" of all the various bits of non-air that come into a gas turbine. Instead the design and regulations attempt to set a standard of a "high jumpers bar" This is based on learned experienece across multiple models of engines over 40-50 years of accumulated operational risk. Thus you get the regulations for so many birds of a given size, so much water from a rain storm, so much time in a defined icing environment. Meet the standard and you are relatively safe. By relatively I don't mean 100% but only that the engine for the specific risk encounter is not contributing appreciably to the overall risk of anything (in the whole aircraft) going bad in a given flight.
The flight risk is taken up mostly by an individual flight encounter and typically does not expect accumulated risk over multiple flights-encounters for abnormal operation. (see examples below) It therfore presumes that such abnormalities are limited in time extent and mitigated by maintainence actions between flights. This is the presumption for birds, dirt, sand, dust, gravel, stones, rumway concrete dust, etc. etc.
For commercial operations the risks of encounters are well defined by experience and thus the abnormal effects of ingestions are expected to be recognized and attended by operational controls in the air for the specific flight, retarding but not necessarily shutting down all engines, etc.
Based on documented experience, the greatest risk in a single-flight for volcanic ash ingestion has been the operating temperature in cruise causing the stuff to melt and plate out on the stationary vanes immediately behind the burner and just in front of the first turbine stage. This effect significantly affects the engine stall line of operation and results in what is commonly known by the pilots as "silent stall" where the engine no longer responds to the throttle, spools down and the EGT goes up. The specific engine than has to be shutdown (turn off the fuel) and restarted (hopefully it can be automated by quik restart). The idea is to ensure that all engines don't get into this in the same seconds of time so you should retard the throttles (lower the temperatures) ASAP on all your engines.
There is little to no commercial experience of other engine "ash" effects, e.g. blade errosion, clogging of cooling holes, oil contamination causing multiple engine power loss in a single flight encounter with volcanic ash. Of course if you continhue to dispatch these deteriorated engines over multiple flights you are going to have problems.