Carb ice

I was flying with an instrument student yesterday (she is almost done) and
she noted that the engine was running rough and turned on the carb heat.
Sure enough, the engine began running so rough that the cowl was shaking. I
hoped it would go away as we continued toward Tacoma Narrows, but it did not
and we decided to just proceed direct and land instead of flying another
approach. The temperature was in the low 70s and (this being Puget Sound)
the humidity was high.

By the time we landed the engine was just barely rough, probably because the
carb heat had made the mixture a little too rich. We had managed to get two
hours of practice in, though, so it was still a good flight. I expect to put
the student up for a check ride as early as next week if we can get her
partial panel work to acceptable levels (still a little rusty after the
cross countries).

I have seen carburetor icing in temperatures approaching 100 degrees in this
area. This has to be one of the worst places in the world for it. This was a
172M. These airplanes seem especially susceptible anyway, but I have never
understood the reason for it. Maybe they have Freon cooled carburetors or
something. :-)

--
Christopher J. Campbell
World Famous Flight Instructor
Port Orchard, WA


If you go around beating the Bush, don't complain if you rile the animals.

Argh. Wrong group. I meant to post this over on the student group.

"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?

I have never
understood the reason for it. Maybe they have Freon cooled carburetors or
something. :-)


You joke about the Freon, but you have come pretty close to the answer.
The reason air conditioners work is because of evaporation. The Freon
is compressed into a liquid. The condenser, the 'radiator' up front,
does serves two purposes. 1, it condenses the compressed freon gass
into liquid; and 2, it cools down the hot liquid freon. Then the liquid
is sprayed through an expansion valve into a low pressure "evaporator".
The evaporating freon cools simply through the act of evaporating.
Take a spray bottle filled with water and spray it at your face on a hot
day. The evaporating water will cool your face.

You mentioned that a carburetor can freeze even on 100 degree days. A
refrigerator can also create ice even on a 100 degree day. Much like
the freon, your gas is being sprayed into a low pressure area of the
carburator and the evaporation of the gas can cool the surrounding air,
and humidity, to a temperature below freezing.

I once knew the physics numbers behind evaporating gas, but that was
many years ago. Suffice it to say, icing can occur on hot days.

"m pautz" wrote

I once knew the physics numbers behind evaporating gas, but that was
many years ago. Suffice it to say, icing can occur on hot days.

Right. But people need to pay attention to the amount of water in the air,
'cause it is the water vapor in the air that freezes. You could cool air
down to relative zero, and it would not freeze, *if* it had no water in it.
:-)
--
Jim in NC


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.762 / Virus Database: 510 - Release Date: 9/13/2004