"Philip Sondericker" wrote in message
...
in article fT27d.3595$mS1.2578@fed1read05, BTIZ at

wrote on 9/30/04 7:00 PM:

that would be one heck of a thermal ride... if it weren't for the
rocks...
and "cough cough" the ash..

BT

I was thinking about this recently, and I was trying to determine exactly
what the effects would be on a small plane flying into an ash cloud. How
long would it take for the air filter to become completely clogged? And at
that point, assuming the plane had a carburetor, what would be the effect
on
the engine? Would the plugs become fouled? Would they fire at all?

Volcanic ash is very fine, like talcum powder. It is the same abrasive stuff
that they sell in pumice soap that removes anything, including your skin,
from your hands. It gets into everything. Worst case scenario if you flew
into a cloud of it your air filter would become clogged almost instantly. A
lot of the ash might work its way through the filter and into the engine,
where it is going to start abrading all the moving parts very quickly. Using
unfiltered air will just suck huge quantities of ash into the engine. The
cylinders will quickly overheat and the engine will quit. Odds are, though,
it would not be that bad. There are plenty of films of people driving around
in the ash cloud. It isn't smart and most of them suffered engine damage,
but their cars kept running. You might be able to keep a piston airplane
engine running long enough to make it to a safe landing area, but it will
probably suffer some damage.

Perversely, just when you would like to enrichen the mixture in order to
cool the engine and maybe wash some ash out of it, the ash is displacing air
and causing the mixture to be too rich already. As you set up for approach
and landing, you will find brakes, flaps, landing gear, propellers, and
every other moving part clogged with ash, even if you are no longer inside
the cloud. Your alternator may be ground to a pulp and your battery dead, so
you will not have any electrical system anyway. Your prop governor will also
be damaged, as will the oil cooler and anything else that moves. Control
surfaces and cables will be coated with abrasive, sticky ash.

The ash will also simultaneously reduce lift and increase drag. Stall speed
will increase as significant amounts of ash accumulate on and in the
airplane.

You also will be IMC and the cabin will also fill with ash to the point that
you will have difficulty breathing. All surfaces will be sand blasted and
you won't be able to see out the windows once you leave the cloud. It is
very dark inside a volcanic cloud, even darker than night. You cannot see
your hand in front of your face. The whole effect is one of driving around
at night in a very heavy snowstorm and dense fog.

Did I mention the poisonous gases? They aren't concentrated enough to hurt
most people, but asthmatics and others with impaired breathing can be
killed. There will be unhealthy concentrations of CO2, various sulfur
compounds, and acids. They can also damage the airplane. Fortunately they
are pretty well dispersed except in the immediate vicinity of the volcano.