Hi Duncan,

Kudos for wanting more info on the subject!

I don't remember what aircraft you are flying (if you even said), so your instructors
may not have been too far off, depending. If the plane has a Lycoming engine, you are
at less risk for carb ice due to the way the carb is posititioned near hot engine
parts. But Continental (and Franklin, IIRC) engines are quite prone to carb ice.

There are two main problems with your assumptions on carb ice formation.

The first is the assumption that the dewpoint needs to be close to the ambiant
temperature. That is incorrect because the air temperature drops dramtically inside
the carburetor (due to the venturi effect and evaporative cooling of the fuel), which
can lower the air temp well below the dewpoint (as well as the freezing point!). Of
course, a higher relative humidity is more conducive to formation of carb ice.

The second is the assumption that ice formation is unlikely if the carb heat has been
on for most of the approach. The problem here is that ice is most likely to form when
the thottle is only partially open, as on final approach. This is because the air
cools more than ever due to a higher partial vacuum forming in the venturi. Even
though you may have carb heat on for most of the approach, it won't "tide you over" on
final. Once you turn it off, the carb itself cools rapidly. So you can get ice right
away.

Here is a more learned explation than I have given you:

http://www.sacskyranch.com/crbice.htm

By the way, I am a CFI (who doesn't teach anymore) who can tell you with some first
hand knowledge that there are a lot of CFIs who may be able to fly great and teach
great, but don't know diddly squirt about engines. My advice is to do exactly what the
POH says. For all the griping we do about the "stupid manufacturers", they really are
smarter than most of us when it comes to operation of the mechanical systems.

Best regards,

Steve Robertson
N4732J 1967 Beechcraft A2-24 Musketeer

T-Boy wrote:

In article , says...

T-Boy wrote:

I snip

You land with the carb heat ON?

snip

Turning final, carb heat back off. (Any probs in the land, or even
after landing that might necessitate a go round, I'm ready for full
throttle - no messing around with the carb heat).

--
Duncan

Doing it your way, you may find that full throttle produces little power because
the carb could be iced up. Check your POH. I believe you will find it specifies
the use of full carb heat any time RPM is reduced below the green arc on the
tach. There is a reason for this, especially on O-200 and O-300 powered Cessnas.

Let me assure you that most folks without a crippled hand can push the throttle
and carb heat on a Cessna forward at the same time if need be. If physically
unable, go to full throttle and then turn carb heat off. Full throttle with carb
heat on will still get you enough power to go around (unless you are in a C-150
and have 40 degress of flaps down), especially considering you will have the
carb heat off in about 2 more seconds.

Thanks Steve, it's interesting you and Robert Moore (who's obviously an
experienced instructor) say this. I've never been taught it, and I've
had probably ten/fifteen different instructors over the years. They've
all taught me the way I've described already.

I find it somewhat difficult to believe that carb icing could occur
after have flown a base leg with carb heat fully on, on the final. Bear
in mind that this is a VFR/VMC approach - though sure, on some days,
dewpoint is indeed sometimes quite close to temp.

Any more comments - appreciated. Meantime, I might practice the carb
heat off, throttle on - simultaneous thing (sounds easy enough).

--
Duncan