"Philip Sondericker" wrote in message
...
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?

The Northwest Mountain Division has rearranged things, and I can't find
their article anymore. But sometime back in the 90's, I read a description
on their web site about some consequences of the St. Helens eruption in
1980. The description included a jet that suffered the failure of all
engines, and a lightplane that barely survived.

Here are some articles that address the turbine side of things:
http://www.boeing.com/commercial/aer...nic_story.html
http://www.nw.faa.gov/releases/volash.html
I found them looking for the other article I remember.

Anyway, I don't know how long it would take for the air filter to become
clogged, but probably not too terribly long if you flew through the meat of
an ash cloud. Obviously, it depends on how close you get to the densest
part; just skim a side of the cloud, and you might not notice a problem at
all. Go through the middle, and you might clog the filter in a matter of
minutes.

Of course, the next step at that point would be to use the alternate air;
carb heat for carbureted engines, and whatever alternate air source for
fuel-injected (often is an automatically deployed, spring-loaded door that
bypasses the filter when it's clogged). One ingested, the ash would start
eating up the engine, but I would guess that you'd at least have enough time
to get on the ground to make an emergency landing. I think that the plugs
getting fouled would be the least of your worries, but who knows? Maybe the
heat of the spark would cause a nice, insulating glass barrier to form at
the sparkplug gap.

That is assuming, of course, you can still see out of the windows, since
they'll have been sandblasted the whole time. And that you find an area of
visibility high enough to make a landing. If the engine fails before you
can land, then you obviously will be forced to land wherever you happen to
be, regardless of whatever visibility you have.

I gather that generally speaking, other than trashing the airplane, pilots
usually make it out of ash clouds alive. But not all do, and it's certainly
not something I'd want to try, given the option.

Pete