from me recently, and the temperature on the ground
was +1C, with a dewpoint of +1C. Hmmm. If you look
at the following chart:
That's definitely in the "serious icing at cruise power"
region!
Keeping in mind that the airframe manufacturer's POH/AFM
is golden, and supercedes everthing else ....
Without a carb heat temperature gauge (uncommon in
light singles) I don't use partial carb heat - I use it as a binary
switch, either off or on. Whenever the RPM is below the
green arc, as a rule of thumb I use full carb heat.
Now, different airframes and different engines will plumb
the pipes differently. For example, the Continental O-200
(in Cessna 150) and O-470 (in Cessna 182) hang the carburetor
out in the breeze, and these engines are notorious for building
carburetor ice.
Lycoming boxer engines, on the other hand, like the O-320 or
O-360 or O-540, bolt the carburetor to the bottom of the engine,
where there engine oil sits. So as soon as the oil warms up, the
Lyc is generally much more resistant to the formation of carb ice.
But it won't make as much power. You don't get something for nothing.
Note that a Lyc is more likely to form ice when the oil is cold, which
is often the first takeoff of the day. It gets worse if the engine is idling,
because ice can form around the nearly-closed throttle.
What I do, on a cool day with no dewpoint spread (see chart above)
is position the engine on the runway for takeoff, run it up to at least
1500 RPM, apply carb heat and lean the mixture. Wait 10 seconds,
then carb heat and mixture in, release the brakes and full throttle.
This simple procedure burns off any carb ice which has formed as
you have been taxiing out.
Anyways, back to the theory. The reason a carburetor builds ice
is because it is actually a little refrigerator. I know kids these days
don't have much experience with carburetors - nearly all cars are
fuel-injected these days - but the venturi in the carburetor uses the
bernoulli principle, just like a wing, to create a low pressure area to
suck the fuel out of the carburetor float bowl.
This low pressure area gets cold, because you have the same
amount of energy in a greater volume of space. Ever let the air
out of a tire and had the valve get cold on your hand? If you put
your hand on the outside of a carburetor of a running engine, it
would feel cold to the touch. You might see condensation and
even ice on the outside of it, depending upon the conditions.
All certified aircraft must have some means of dealing with carburetor
ice. I suppose you could electrically heat the carburetor, but that
would take a lot of amperage and there would be questions about
the reliability of the source of electricity.
Instead, what nearly every airframe manufacturer does is wrap
some sheet metal around the hot exhaust, and route it to the
carburetor, which has a flapper valve at it's intake, to select
either cold air (normal) or hot air (carb heat on).
Running carb heat bypasses the normal filter, so you should be
careful about using it on the ground, depending upon the runway
and taxiway surface. That sounds like a bad thing, and it is on
the ground, but in the air it's a good thing, because another form
of icing you can experience is impact icing, where the air filter or
intake on the cowling can plug up with ice. This blocks the air
from getting even into the carburetor, and is another time that
carburetor heat comes in handy.
I should mention that even fuel-injected systems can suffer
from intake impact icing, and are legally required on certified
aircraft to have some kind of alternate air input. It can either
be automatically selected (via some kind of spring/magnet
mechanism) or there might be a manual alternate air selection
in the cockpit that you as the pilot might need to select someday.
Now onto one of the most important considerations about the
use of carb heat: when you select carb heat on, immediately
lean the mixture for max RPM. No one does this, and no FTU
checklist will tell you to do this - in a descent, they want you
to jam the mixture full rich - but that's not what you want.
Full rich mixture cools the engine - that's why you use it for
takeoff - and that's not what you want, when you select
carb heat. You want the engine to make heat, so that the
air going past the exhaust is warmed up. To do that, you
need to lean the mixture. Remember, when you selected
carb heat, that automatically enriched the mixture all on it's
own, because the hot air is less dense and has less oxygen
in it, but the same volume of air is flowing, so the same
mass of fuel is added to it by the carburetor. Less oxygen,
same fuel is richer mixture, which is colder.
In a descent where the engine isn't making much power,
the last thing you want, if you need carb heat, is additionally
enrichening the mixture by jamming the mixture control full
in.
Every time you select carb heat, immediately move your hand
to the mixture and pull it out for max RPM. Or, if you have
a constant speed prop, max airspeed. I don't need an EGT to
lean any normally-aspirated engine - I just lean to best power,
which with a fixed-pitch prop is max RPM, or with a constant
speed prop is max airspeed. Best power is 50 or 100F rich
of peak, and that's a good place to be.
Yesterday I was flying a very weird old piston twin with geared
Lycomings and no EGT's, and that's exactly how I leaned it -
for max airspeed:
The best way to lean an engine, IMHO, is via fuel flow. For
example, on the C421B I just set 120 pounds per hour per
side with my normal cruise MP/RPM and I know that's going
to work out ok when I look at the EGT's.
One last sup-topic - what the heck is a "pressure carburetor"?
If you look at the icing probability chart above, even a pressure
carb can ice up, if the conditions are severe enough.
A pressure carburetor (somewhat rare) is a very early simple
form of fuel injection. It uses a fuel pump to feed fuel under
pressure to the carburetor, and in fact functions during zero
and negative G, because it has no float bowl/needle valve
arrangement, which requires positive G to function correctly.
It is entirely possible that you will fly your entire career and
never encounter a pressure carburetor, but I was flying with
two of them yesterday (on the GO-480 Lycs) and there's also
one on the 450hp Stearman that I fly.
Anyways. Executive summary:
Any time the carb heat goes on, always lean the mixture
for max RPM for maximum carb heat effectiveness.
It also gives you best power, for whatever particular engine
power setting (MP/RPM) that you have at the time, which could
come in really handy if you want to climb, or even avoid descending.
