"Neil" wrote in message
...

For those interested, here is the reason why the planes have been grounded
. . .

http://tinyurl.com/y394h58


Good link, and well explained. However, in this case, the situation is
slightly different.

Under normal circumstances, when a volcano blows, the larva and ash are
expelled directly into the air, cool slowly, and the ash ends up somewhat
like pumice, a very soft rock, that although being dangerous to engines, is
fairly benign stuff otherwise.

In this particular instance, the larva and ash is being ejected through a
glacier, partially melted, but the majority still being solid frozen ice.
This causes the larva and ash (detritus) to cool very, very quickly, and in
doing so forms a glass like substance, as has been mentioned on the news.
These particles are also a lot smaller than the softer type.

From Neil's link, the damage caused by this glass is well explained, but
this is after the ash has been through the guts of the engine. In the
current case, the detritus is already glass before it hits the plane, and
this glass is very similar to emery dust, or sand paper, and will strip
engines bare before it is even half way through them, not to mention the
effect it will have on the body of an aircraft, an effect that is not
present with the softer pumice type of ash. Bear this in mind. The aircraft
will hit these particles at several hundred miles an hour, so it is like
putting the aircraft in front of a sand blaster. And I think everyone here
knows the effect that has on any material!

jim, Northampton