"T o d d P a t t i s t" wrote in message
...
"Peter Duniho" wrote:

I can't speak for all airplanes -- maybe there's one out there that uses a
filter with carb heat -- but yes, generally speaking carb heat means the
air
is not filtered.

I've never quite figured out why carb heat is designed this
way.

I suspect it's a matter of keeping the design simple. FAR 23.1091(b)
requires an alternate air source. FAR 23.1093(a) requires an induction air
heating source. It makes some sense to ensure that the alternate air source
would not be compromised by the same thing that required its use (such as
icing of the air filter), and it also makes sense to combine the two systems
into a single system.

I've always got carb heat on during landing, and as
soon as I land, I've got to start worrying about pulling
dust and debris into my expensive engine. I mean really!

I suspect that at the usual power settings used for landing (ie idle) that
sucking dirty air into the engine is less of a worry. Of more concern to me
is during the runup, when you are standing still (not getting a fresh supply
of air) and have the engine power set higher than idle.

[...]
I recognize that there are rare circumstances when one would
want to bypass the filter (impact iced over). Nonetheless,
it seems to me that accidents due to fiddling with the carb
heat after landing are at least as much of a concern.

How so? Induction icing can cause a forced landing, if not a crash.
Assuming the pilot has their priorities straight, fiddling with the carb
heat after landing should pose no risk at all.

Yes, there seems to be a window of opportunity for unfiltered air to get
into the engine, but that should not cause the pilot to forget about
controlling the airplane (and I think most airplanes have the carb heat
control in a more practical place anyway...I wouldn't denigrate the basic
idea of combining the carb heat and alternate air source on the sole basis
of one single aircraft type).

Is there some risk with the design as is? No doubt. But I have seen no
evidence that suggests that using the carb heat as the alternate, unfiltered
air source leads to any significant reduction in engine life, never mind any
traumatic failure of the engine.

Oddly enough, in reviewing the certification requirements, I found two
paragraphs that my own airplane doesn't appear to meet. My automatic
alternate air source (not carb heat, since I have no carb) does not provide
for a manual override by the pilot, nor is there any indication in the
cockpit to show that the alternate air source has been activated. Now I'm
curious how the airplane got certified. The type is new enough to be under
Part 23, but perhaps elements of the TC got grandfathered in via the
previous type on which this airplane was based. Or maybe there's a waiver
of some sort? I don't know. A new mystery...fun.

Pete