There is a special procedure in our company in case of volcanic ash encounter. The first actions to be taken are memory actions, and must be known by heart:
Escape Maneuver - INITIATED
Auto Throttle - DISENGAGED
Throttles - IDLE
Descent - INITIATED
ENG IGN OVRD Sw - OVRD ON
ANTI-ICE Sws - ALL ON
Pack Sws - CHECKED ALL ON
ECON Sw - OFF
So the basic aim is to get out of the ash cloud ASAP. General recommendation is a 180° turn, as you cannot be shure how far the cloud extends in front of you.
The descent is of course to keep the engines at, or as close to idle as possible, and thus keep the temperature within the engine as low as possible to keep any glass particles contained in the volcanic ash from melting and then solidifying in a cooler part within the engine. Glass particles in volcanic ash usually have a melting temperature of around 600-800°C, while the christalline parts usually melt at around 1100-1200°C. The effect of molten particles on the engine is much more severe than just the contamination with particle dust.
At times it might be a good idea to shut down one or two engines until you have left the ash cloud - but I hope never to be in a situation that asks for something like this.
Finally, switching on all available bleed sources is said to increase the stall margin of the engines.
Hope we all never have to use a procedure like this one.
Regards,
DBate