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At what concentration is ash in the atmosphere an ash cloud?
Someone posted that it was measured at over 2000ppm. The NASA aircraft that was damaged, flew through 600ppm I believe.
I have just been giving the NASA DC8 report a once over. There is no statement of the ash cloud's particulate parts per million during the 7 minute encounter but there are two graphs, 7a and 7b, of what happened. 7a shows the sulphur dioxide concentration against time which peaks at about 800,000 parts per trillion by volume. 7b shows the aerosol number density against time which peaks at about 28,000 per cubic centimeter. Can anybody explain how these figures translate to the concentration of particulates in parts per million (physics a bit rusty!).
One bit of good news for the brave souls flying the test flights is the following quote from page 13 of the NASA DC8 report:
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.
Sounds like an on-the-wing de-coke!