We do not know, except for a few datapoints, how airframes, systems and engines behave in volcano-contanimated air.
Unfortunately we DO know. here is a research paper/investigation from a NASA instrumented air quality test aircraft which inadvertently flew through the edges of a volcanic cloud:
http://www.alpa.org/portals/alpa/vol...8AshDamage.pdf
Note the following from the report:
The flight crew noted no change in cockpit readings, no St. Elmo’s fire, no odor or smoke, and no change in engine instruments. They did notice that no stars were visible, but this is typical of flight through high cirrus clouds.
After landing in Kiruna, the engine oil, oil filters, and heat exchanger filters were removed and saved for analysis. Visual inspection of the airplane and first-stage engine fan blades showed no apparent damage or erosion on any parts of the airplane, nor was any ash found in the engine cowlings or other normal access panels. Borescope inspection equipment was not available in Kiruna, and since there was no detected change in engine performance, the research flights continued.
the airplane was ferried back to Edwards. A total of 68 flight hours had accumulated since the ash encounter. At Edwards, engine borescope inspections revealed clogged cooling passages and some heat distress in the high temperature section of the engines. Engine number four appeared to be the most heavily damaged and was removed. Following the number four engine teardown and inspection, the other three remaining engines were also removed and disassembled for inspection.
There is no evidence of significant engine performance change following the ash encounter. In fact, there does appear to be a slight drop in cruise EGT. This is consistent with experience that says that a very mild ash encounter cleans and polishes the compressor blades, slightly increasing their efficiency. Blocked turbine cooling air passages and holes would be expected to reduce HPC bleed flow, which would also slightly improve performance. The increased blade and vane metal temperatures would degrade their service life, but not their performance. A blade with blocked cooling operates at a sufficiently higher temperature so that its service life may be as little as 100 hr as compared to a normal service life of thousands of hours.
All four engines were sent to the General Electric Strouther overhaul facility near Arkansas City, Kansas. Photographs were taken as the engines were disassembled. All engines exhibited a fine white powder coating throughout. There was leading edge erosion on HPT vanes and blades, blocked cooling air holes, blistered coatings, and a buildup of fine ash inside passages. Serial number 692632 (the number four engine on the DC-8) had the most severe damage; this may be partially due to the older hardware still resident in this engine. Figure 11 shows photos of the damaged HPT blades, with clogged cooling air holes, leading edge erosion, buildup of ash in passages, and blistered blade coatings clearly visible. Total cost of refurbishment (to standard flight condition) for all four engines was $3.2 million.
Even though this was a diffuse ash cloud, the exposure was long enough [seven minutes] and engine temperatures were high enough that engine hot section blades and vanes were coated and cooling air passages were partially or completely blocked. The uncooled blades still performed aerodynamically but necessitated expensive overhauls. The insidious nature of this encounter and the resulting damage was such that engine trending did not reveal a problem, yet hot section parts may have begun to fail (through blade erosion) if flown another 100 hr.