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Bill, Pete, Matt, Paul & John
I just watched a "Sixty Symbols" video in which a British "Jet Engine Expert" claims that a USGS report showed that volcanic ash causes flame out by "blocking the airflow through the guide vanes and turbine"

So I found this USGS paper: , which ....

 determined the following mechanisms that can affect aircraft performance due to exposure to a volcanic ash cloud:

(a) deposition of material on hot-section components, 
(b) erosion of compressor blades and rotor-path components, 
(c) blockage of fuel nozzles and cooling passages, 
(d) contamination of the oil system and bleed-air supply, 
(e) opacity of windscreen and landing lights, 
(f) contamination of electronics, 
(g) erosion of antenna surfaces, and 
(h) plugging of the pitot-static system which indicates the airspeed of the aircraft.


The items (a) -->  (h) all make sense to me.  Fine.

What I don't understand is why the engine flames out, yet can be re-started successfully after gliding 10 minutes as it descends from 40,000 ft to ~ 11,000 ft.

I can understand that a restart is far more likely at lower altitudes.  But if the "syrupy glass coats the exit guide vanes and turbine blades" enough to back up airflow so far off-design that the engine flames out, why does 10 minutes of cooling at sub-zero temperatures cause the now-solid glass to evaporate?  Huh?  Where did the blockage go?  When the engine stops, I'd expect glass accretion to continue as the rotation slows for a second or so, until the temperature drops below the glass melting point.  Once the hot parts cool, the ash simply flows through, but not very fast, as 1) the airplane is slowed by the loss of thrust, so it probably slows to some optimal glide-slope speed to give the pilots the greatest chance of re-start, and 2) the airflow through the engine is reduced far below the free-stream speed because, even windmilling, all the blades create a fair amount of blockage.  So I don't expect "sandblasting the glass" can be the explanation.

So how come in the both historical cases of a 747 losing all 4 engines, the pilots were able to re-start 100% of the dead engines?

-- Randy