Originally Posted by edsailing
... Now it seems that airlines are questioning the scientific advice and continuing closure of airspace - possibly governments and their agencies are over reacting?
If I was real cynic I would start think conspiracy theory!
Encounters with volcanic plumes are rather like in-flight icing. The pilots may be unaware that they've penetrated an ash cloud, and the effects can range from immediate to delayed. The physical effects include erosion of paint
and windscreens, eroded leading edges of engine fan blades, clogged cooling
holes in engines, buildup of a glasslike material in the hot section (engine core
temperatures being hot enough to melt the ash), and clogged pitot tubes (with conflicting/erroneous airspeed readings), failed flight management computers
, electronic engine control failures, cockpits filled with volcanic dust, and multifarious other effects. For example, repeated exposure to volcanic gases, such as sulfur dioxide, can create acids when combined with water
vapor that adhere to the aircraft's skin and penetrate microcracks in the metal, causing structural deterioration. Ash particle in the air conditioning
system can abrade ductwork and clog filters.
In the worst case, engines can fail outright. Capt. Eric Moody of British Airways [BAB Flight 9] was the first to experience a total engine failure from volcanic activity on a night flight on June 24, 1982. On his four-engine B747, Moody had five engine shutdowns in 20 minutes (having to shut down one badly damaged engine after a successful in-flight restart). Moody said the sulfur dioxide and carbon dioxide in an ash cloud could starve the engines of oxygen, the ash can make the air less compressible, and the glasslike contamination can gum up the inner works. After an unpowered glide to an altitude below the bottom of the ash cloud, Moody was able to restart his engines and make a successful emergency
landing at Jakarta.
Two other B747s have experienced total engine loss and last-minute recovery from encounters with ash clouds, and seven cases of partial temporary engine failure have been recorded. Repairs
have been costly, to include total engine change-outs costing millions of dollars per engine. So far, no aircraft have crashed from these ash cloud encounters. Better yet, since the early 1990s there have been no cases of in-flight engine failures from ash encounters. Faster alerting has helped. Today, a worldwide network of nine volcanic ash advisory centers (VAACs) provides warning of eruptions, and these are translated into reports of significant meteorological activity (SIGMETS).
Here's what Airbus has to say on the subject