Carburetor Heat
Somewhere on the front of the engine is an air intake for the carburetor. The air enters through a mesh filter. Every pilot, in the pre-flight should check security of this filter and its mounting. The airflow through the carburetor is controlled by the throttle moving a circular flat rotating plate. Air passing by this butterfly valve is accelerated by a narrowing throat called a venturi....

This narrowing reduces pressure and sucks cooling fuel into the air. The combination of a lower pressure, gasoline, and moisture in the air can so cool the metal parts of the intake that the moisture adheres to the parts as ice. The greater the air's humidity the more ice will be formed. As the airflow is restricted by the ice, the excess fuel so enriches the mixture that the engine to flood and misfire. The initial signs of this icing is a gradual decrease in rpm and an increase in engine roughness. Avoidance of carburetor ice can be improved by avoiding long, low power descents and by leaning to make the engine run hotter. Lycoming engines are less susceptible to ice than are Continental engines since the carburetor is attached so as to benefit from the heat of the engine itself.

Carburetor ice is caused by the change in pressure as air passes through the venturi. Fuel atomized in the throat of the venturi evaporates. This uses heat from the metal of the carburetor and air. This air/fuel at lower pressure cools and takes even more heat away from the metal of the carburetor. At some point any moisture in the air will freeze on the metal of the carburetor. The ice that adds to the venturi constriction will both increase the cooling rate, the speed of airflow and lower the pressure. Thus, carburetor ice feeds on itself until the excessively rich mixture kills the engine.

The reason carburetor ice causes a drop in rpm and a rough engine is because of an excessive rich mixture. The addition of hot air by the carburetor heat enriches the mixture even more and causes an additional drop in rpm. Leaning the mixture is one viable option in trying to improve the engine performance.

It is possible to verify this procedure by doing the following during runup:

1. Aggressively lean the mixture.
2. Note that little or no rpm change will be noted by applying C.H.
3. With the mixture rich, apply C.H.

4. Note that the rpm drop is significantly greater than with the lean mixture.

Many years ago I had a totally unexplained engine failure at 800' after taking off from a very remote airport in the Ozarks. I had resigned myself to a treetop landing, and pulled the mixture practically all the way out but not all the way. Suddenly the engine picked up and ran smoothly at full power. No further problem for 2000 miles.  Being given a choice between being smart or lucky.  Take lucky.

In 1998 I found an article relating to the AOPA Tripacer that explained what had happened. The carburetor float had stuck so that the rich mixture was capable of stopping the engine. This is essentially how carburetor ice can stop your engine unless you pull C.H. and lean until the mixture level adapts to the venturi throat.

Occurrence
Float type carburetors can create ice any time. Moisture in the air increases the possibility that the cooling effect of the atomizing of gasoline in the venturi (narrow throat) of the carburetor will cause vapor expansion and cooling to create ice from any moisture present. When the ice adheres to the parts of the carburetor it cuts down the flow of air and 'chokes' the engine. There is too much fuel for the amount of air available.  Carburetor ice onset is dangerous and insidious. The first indication of ice is a drop in rpm. This drop may be accompanied by engine roughness and eventual stoppage. You must sensitize yourself to rpm changes that you have not induced.

There are important training aspects related to carburetor ice and the use of carburetor heat. The pilot must train to be aware of the weather conditions that have a proclivity for causing carburetor icing. Being aware of the likelihood is the primarily aspect of the anticipation required. Just where icing occurs in the carburetor is a variable. Icing can occur before the butterfly, in the carburetor intake, on the butterfly valve or afterwards. When impact icing blocks the air intake filter as induction icing, then the carburetor heat source serves as alternate air for the engine. The different design of carburetors, engines, and induction systems all make a difference however indeterminate. Some cowling designs are better than others in reducing the occurrence of carburetor ice.

We know how/why ice forms and how to use CH as preventative but we no reliable predictive ability. You are more likely to get carburetor ice with warm temperatures because warm air has the ability to hold more moisture. Fahrenheit temperatures between 20 and 70 accompanied by atmospheric moisture usually trigger the required venturi temperature drop. Anything less than full CH application is potentially dangerous. Low fuel pressure and contaminated fuel can give symptoms similar to carburetor ice. By keeping both fuel and induction air clean we can avoid these as causes of unusual engine behavior. Carburetor icing occurs when the air, moisture and temperature in the carburetor is modified after ingestion so as to be capable of freezing moisture and adhering it to the internal parts of the carburetor. Carburetor heat also performs the function of an alternate air door in case of induction icing covering the engine air filter as stated before..

There is no FAA carburetor icing probability chart. In given circumstances carburetor icing can occur at any temperature. Some aircraft models and engines are more susceptible to icing than others. Carburetor ice will get you when you least expect it. It has occurred on cloudless days and temperatures up to 100 F. The ambient air temperature in a carburetor can be reduced at its venturi by as much as 70F. The 10 seconds that it takes C. H. to take effect can be the longest of your life. Don't make it the longest 12 seconds by failing to immediately apply C. H. Any unexplained power reduction is a red flag notice. Make all descents with partial power to retain as much C.H. potential as the situation allows. Aircraft with an EGT will show a decrease in EGT readings at the onset of carburetor ice. It is an early warning system.

Most likely icing is at 50%+ humidity from 20 to 90 degrees F. (Some texts give 80% humidity between 40 and 70 degrees.) Ice is caused by absorption of heat from air/fuel vaporization as it enters low-pressure venturi of carburetor at high speed. It can cause drop of 60 degrees causing ice to adhere to "butterfly" and venturi throat. Even with no visible moisture, ice can form in the throat of the carburetor due to adiabatic cooling as the air passes through the venturi by the throttle plate.

The carburetor on a Lycoming engine is mounted at the very bottom of the engine in such a way that any heat it gets is from direct contact with the engine and very little from elsewhere. The engine's putting out enough heat at higher power where the use of carburetor heat will prevent the formation of ice in the carburetor and given time it'll melt any existing ice. Even the position of the butterfly valve helps. Carburetor ice can occur at any power setting, it is most likely to occur in the green arc.

Continental's carburetor is suspended below the engine so that there is no residual heat transferred between the engine and the hanging carburetor. Lycomings, on the other hand, have the carburetor secured to the oil pan. The carburetor is heated by the hot engine oil. For this reason Piper recommends carburetor heat be used only as necessary. The pilot determines just where and when necessary exists. Lycoming engines are much less susceptible to carburetor icing than Continentals because of their design. However, because of pilot complacency, the icing of a Lycoming is going to be more traumatic and unsuspected.

Many unexplained engine failures are probably due to carb ice. When humidity is more than 50% and temperatures range from 20 to 90 degrees Fahrenheit ice will form as the internal carburetor temperature can drop 60 degrees. Carburetor engines must be able to take 30-degree air through the intake and deliver 120 degrees to the carburetor. This is done using 75 percent power, which is far more that is used in most descents.  Once again the government aviation requirements are just minimums.  Don't be satisfied with minimums

Carburetor icing during takeoff is not as rare as some would like to believe. The best preventative is to apply C.H. on leaving the run-up area and removing it at just before full power is applied. Every descent made at reduced power should be done with full carburetor heat on. The use of C.H. decreases the power of the engine slightly less than 10% and causes the mixture to be over-rich. Leaning is advised for best engine operation and to maintain the required heat for C.H. No use of leaning or C.H. is advised for engine operations over 75% of maximum power.

C. H. Reference List
--Ice can form at full power
--Always apply full carburetor heat
--If cruising with C.H. lean the mixture.
--Always use C.H. during descents

Three Kinds of Carburetor Ice
1. Impact ice is caused by moist air on the air filters and air intakes are impacted as rain, snow, or sleet. Forms from 15 to 32-degree F but is worst at 25F.

2. Fuel ice is caused by vaporization where fuel enters the manifold system. Will occur when relative humidity is 50% and between 40 and 80deg;F.

3. Throttle ice forms in the carburation system on the throttle valve or butterfly or on the interior of the venturi system. The water vapor from the air intake freezes due to the venturi effect. Effective temperature drop is about 5deg;F and ice is most likely between ambient temperatures from 32 to 37deg;F.

On first start, there may not be sufficient heat to either prevent or melt any carburetor icing. Leaning will raise engine temperature. The more moisture in the air, the greater the likelihood of icing. A humid hot day is just as likely to cause icing as is a cold day with water on the ground. When conditions indicate that icing is likely, the prudent procedure is to apply carburetor heat in anticipation rather than as reaction. Using the carburetor heat every time you reduce power is a good operating procedure and much safer than the POH suggestion for use when required.

With the advent of Low Lead 100 Octane gasoline, leaning during taxiing has become mandatory.  Leaning that is a bit aggressive can cause symptoms of carburetor heat failure to operate during run-up.  Running lean causes the engine to run hotter when there is no excess fuel for co0ling.  When the atmospheric temperature approaches the engine temperature there is insufficient differential in the two temperatures to cause an rpm drop in the engine operation.  When this happens just enrich the mixture a bit and run an additional check of the C.H. operation.

Most of my carb icing encounters have been while taxiing. The explanation of what occurs deserves repeating. I demonstrate the cooling effect of gasoline on moving air by placing some gas on the back of a student's hand and have him wave it. Under certain atmospheric conditions and power settings it is possible for the blending of fuel and air in the carburetor venturi to cool any moisture present to freezing. Automotive fuel is more likely to cause ice because of its vapor point. (Venturi effect can be demonstrated by holding two pieces of binder paper vertically about three inches apart and blowing between them.) This can adhere to the metal parts of the carburetor particularly the butterfly valve which is the throttle control. This ice will restrict the flow of air through the venturi and cause an initial reduction in rpm and subsequent engine roughness to final failure.

At idle power, in the air or on the ground an aircraft can ice up in a very short time. There is no logical safety reason behind the concept of removing carburetor heat on short final as a go-around safety measure. There is nothing so urgent about a go-around that makes it necessary to remove carburetor heat prior to landing as a time saving or safety procedure. The closer to the ground you are when initiating the go-around the greater will be the ground effect and aircraft acceleration. The go-around is initiated first with a mixture check, full throttle, and finally with carburetor heat. Always first with the most. These forward movements can be accomplished nearly simultaneously in one motion.

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