Ash Cloud Historical Event
I found out more precisely why the ash cloud is a danger to jet engines via an air pilots' website forum:
Most jet engines will fail quickly if exposed to that Iceland dust. Why? Because the engines' first stage turbine blades run in gas temperatures above their melting point. They don't melt normally because cooling air is blown through a multitude of small holes in the blades. If the unfiltered cooling air contains the Iceland dust this will rapidly clog up the cooling holes.
Result is melted turbine blades which break away to destroy the turbine causing the hapless pilot of the instant glider to pray while he transmit a May Day.
Piston engines will probably be OK for a while until their air filters choke off the induction air flow.
The only safety answer seems to be test flights followed by furiously tearing apart jet engines. Yesterday, the first test flights took place, and the results from commercial airliners on short flights seemed to indicate that the situation might be safe; the test flights and engine teardown from some F-16 fighters from Finland indicated damage.
Meanwhile, most of the news comes from England, where wild differences exist on how many Britons are stranded abroad. Estimates vary from 100,000 to nearly a million, with the current estimate 400,000+.
NOON UPDATE: It looks as if airline service will be starting up again over the next couple of days. There are already a number of flights in the air in Europe