"Larry Dighera" wrote in message
...
Of the 5 POHs I checked, only the Rockwell International 112A
mentioned using carb-heat as an alternate air source.

See George's post. If there's carb heat, there's usually alternate air,
even if the manual doesn't say so.

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).

So? What's your point? Are you claiming the 172S has no alternate air
source?

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.

I understand what you were trying to imply. But the point here is that
filtration is not the only concern with many alternate air systems, since
they combine two completely different functions (carb heat and alternate
air).

Pete