Carb ice

"C J Campbell" wrote in message ...

Did you try leaning aggressively? An iced up engine will run richer
and richer as the air portion of the venturi is closed off with ice.
The exhaust will then cool drastically, and surprise (!) there is very
little heat left anywhere to burn out that ice. The problem thus
feeds on itself.

The only way to get on the other side of this with a badly iced engine
is to quickly lean for best power, raising the EGT until enough ice
burns out. then the mixture can be restored to a more normal setting.
Note that this burnout method may cause chunks of ice to go thru the
engine, making the mixture to suddenly go so temporarily lean that the
engine quits again.

I don't know why this isn't taught to students, or even written in the
aviation rags.

A dropping EGT is an effective way to initially detect ice. Our 172M
also ices very slightly quite easily, but i've never had it progress
to the point of serious roughness. I do glance at the EGT many times
when in the air.

As an aside, expect that when an engine at cruise is properly leaned,
suddenly applying carb heat will make it run richer due to the less
dense hot intake air, as raw gas flowing along the walls of the intake
manifold will suddenly vaporize and go thru the engine. Pushing carb
heat off will make it then make it run so temporarily lean that the
engine may stumble momentarily. It takes a few seconds for the
balance of liquid fuel and vaporized fuel to be re-established.

" wrote:

"C J Campbell" wrote in message ...

Did you try leaning aggressively? An iced up engine will run richer
and richer as the air portion of the venturi is closed off with ice.

Uh .. the section of the carb that ices up is the area around the butterfly valve.
This is located at the junction of the carburetor and the intake manifold. This
section carries the fuel/air mixture. AFAIK, there's no such thing as "the air
portion of the venturi" - the venturi is the section of the carb in which the fuel
mixes with the air.

In any case, the mixture will not change appreciably as the carb ices up, and this is
why it's not taught to students. Carburetor heat changes the mixture, though, and
this *is* taught to students.

George Patterson
If you want to know God's opinion of money, just look at the people
he gives it to.

I understand that icing can occur at either the venturi or at the
throttle plate. The two are close together so there will b e some
interplay, and there is some additional fuel delivery thru the idle
ports adjacent to the throttle plate. It does explain that there are
some differences in icing situations.

Here in Minnesota, I had a Fairlane V8 that iced very badly almost 40
years ago. I know the ice was on the two venturiis when I took off
the air cleaner. They were almost totally choked off.

Do you teach leaning for max power (or better yet still leaner for max
EGT) under severe ice situations? I'm impressed that you teach the
effect of carb heat on mixture.

I understand that icing can occur at either the venturi or at the
throttle plate. If nothing else the two are close together so there
will be some interplay. There is some additional fuel delivery thru
the idle ports adjacent to the throttle plate that would also deliver
a richer mixture under throttle plate icing conditions. It does
explain that there are some differences in icing situations.

Here in Minnesota, I had a Fairlane V8 that iced very badly almost 40
years ago. I know the ice was on the two venturiis when I took off
the air cleaner. They were almost totally choked off.

Do you teach leaning for max power (or better yet still leaner yet for
max EGT) under severe ice situations?