On Thu, 18 Mar 2004 19:01:54 -0800, "Peter Duniho"
wrote in Message-Id:
:

"Larry Dighera" wrote in message
.. .
I only see that as an issue if you are downwind of an active volcano,
and the ambient atmosphere is dust laden. It's my understanding that
most aircraft induction systems have a spring-loaded door that admits
air in the event of an interruption of flow through the filter
element.

Huh? Many do not.

Of the 5 POHs I checked, only the Rockwell International 112A
mentioned using carb-heat as an alternate air source. Neither the
1974 Cessna Centurion, nor 1973 Cessna Cardinal RG, nor 1974 Cessna
Stationair, nor Piper Archer II, nor Bellanca Super Viking POH/Owner's
Manuals mention alternate air. Or, at least I couldn't find it.

All aircraft that I've flown DO have an alternate air
system; most of the C172-class planes I know of use carb heat as their
alternate air, not a spring-loaded door.

In the C-172S there is no carb-heat (it's injected).

My own airplane has a
spring-loaded door, but that's most likely because it's also fuel-injected,
so there's no carb heat installed.

But regardless of how the alternate air system works, it's still an issue to
be aware of. And in all cases I know of, the alternate air system means
unfiltered air. In cruise flight, not necessarily a problem, but if it also
means turning on the carb heat, it means a) you need to be aware of the
potential for the need to use carb heat, and b) you'll want to remind
yourself of the various implications of operating with the carb heat turned
on (like, need to readjust the mixture, fuel efficiency changes, maximum
power changes, that sort of thing).

You don't need to be downwind of an active volcano to care about the air
filter getting clogged.

Right. What I was attempting to imply was that at cruise altitude the
air is generally free of impurities normally filtered unless there is
a source of such impurities upwind spewing to significant heights.