From Document 9974 of the International Civil Aviation Organization,
Flight Safety and Volcanic Ash
indicators that an aircraft is encountering volcanic
ash
are related principally to the following:
Odor: smoky or acrid odor that can smell like electrical smoke,
burnt dust or sulfur.
Haze: within the aircraft cockpit and/or cabin; fine
ash
can settle on surfaces.
Changing engine conditions: surging, torching from the tailpipe,
and flameouts can occur; engine temperatures can change
unexpectedly, and a white glow can appear at the engine inlet.
Airspeed: if volcanic
ash
fouls the pitot tube, the indicated airspeed can decrease or
fluctuate erratically.
Pressurization: cabin pressure can change, including possible loss
of cabin pressurization.
Static discharges: a phenomenon similar to St. Elmo's fire or glow
can occur; in these instances, blue-colored sparks can appear to
flow up the outside of the windshield or a white glow can appear
at the leading edges of the wings or at the front of the engine
inlets.