carburettor ice and heat

Check for carburettor ice before takeoff

Carburettor Heat

Somewhere on the front of the engine is an air intake for the carburettor. 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 carburettor 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 carburettor 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 carburettor is attached so as to benefit from the heat of the engine itself.

Carburettor 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 carburettor and air. This air/fuel at lower pressure cools and takes even more heat away from the metal of the carburettor. At some point any moisture in the air will freeze on the metal of the carburettor. The ice that adds to the venturi constriction will both increase the cooling rate, the speed of airflow and lower the pressure. Thus, carburettor ice feeds on itself until the excessively rich mixture kills the engine.

The reason carburettor ice causes a drop in rpm and a rough engine is because of an excessive rich mixture. The addition of hot air by the carburettor 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.


Float type carburettors 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 carburettor will cause vapor expansion and cooling to create ice from any moisture present. When the ice adheres to the parts of the carburettor it cuts down the flow of air and 'chokes' the engine. There is too much fuel for the amount of air available.  Carburettor 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 carburettor ice and the use of carburettor heat. The pilot must train to be aware of the weather conditions that have a proclivity for causing carburettor icing. Being aware of the likelihood is the primarily aspect of the anticipation required. Just where icing occurs in the carburettor is a variable. Icing can occur before the butterfly, in the carburettor intake, on the butterfly valve or afterwards. When impact icing blocks the air intake filter as induction icing, then the carburettor heat source serves as alternate air for the engine. The different design of carburettors, engines, and induction systems all make a difference however indeterminate. Some cowling designs are better than others in reducing the occurrence of carburettor 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 carburettor 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 carburettor ice. By keeping both fuel and induction air clean we can avoid these as causes of unusual engine behaviour. Carburettor icing occurs when the air, moisture and temperature in the carburettor is modified after ingestion so as to be capable of freezing moisture and adhering it to the internal parts of the carburettor. Carburettor 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 carburettor icing probability chart. In given circumstances carburettor icing can occur at any temperature. Some aircraft models and engines are more susceptible to icing than others. Carburettor 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 carburettor 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 carburettor 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 carburettor 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 carburettor due to adiabatic cooling as the air passes through the venturi by the throttle plate.

The carburettor 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 carburettor heat will prevent the formation of ice in the carburettor and given time it'll melt any existing ice. Even the position of the butterfly valve helps. Carburettor ice can occur at any power setting, it is most likely to occur in the green arc.

Continental's carburettor is suspended below the engine so that there is no residual heat transferred between the engine and the hanging carburettor. Lycomings, on the other hand, have the carburettor secured to the oil pan. The carburettor is heated by the hot engine oil. For this reason Piper recommends carburettor heat be used only as necessary. The pilot determines just where and when necessary exists. Lycoming engines are much less susceptible to carburettor 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 carburettor temperature can drop 60 degrees. Carburettor engines must be able to take 30-degree air through the intake and deliver 120 degrees to the carburettor. 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

Carburettor 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 carburettor 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.

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

Three Kinds of Carburettor 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 carburetion system on the throttle valve or butterfly or on the interior of the venturi system. The water vapour 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 carburettor 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 carburettor heat in anticipation rather than as reaction. Using the carburettor 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 carburettor 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 carburettor venturi to cool any moisture present to freezing. Automotive fuel is more likely to cause ice because of its vapour 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 carburettor 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 carburettor 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 carburettor 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 carburettor heat. Always first with the most. These forward movements can be accomplished nearly simultaneously in one motion.


Recognition is important but it comes after the fact
. Depending on the circumstances, after the fact, may be too late. As with pitot heat, prevention is the name of the winning game. Waiting five minutes for pitot heat to remove ice can be done, not easily but 'do-able'. Carburettor heat will not allow the time because as you lose power you are losing the engine's ability to produce the required heat. Planning your options now is better than a spur of the moment decision that can be wrong.

The greatest temperature drop occurs in the throat of a carburettor. This is because of the decreased venturi effect pressure. The vaporized fuel releases heat during the conversion of liquid fuel to atomized fuel. Humidity increases the probability of icing. A drop in the venturi temperature of forty or fifty degrees is possible. Low temperatures reduce the probability of icing because of inability to hold moisture except a solid state as snow or ice.

Using Carburettor Heat

Even in warm weather
All or nothing at all, not just an old song.
Use before takeoff; not during takeoff
The engine talks; you listen and feel.
Not for continuous operation on the ground.
Check proper operation and maintenance


When you pull the aircraft carburettor heat you are moving a diverter panel which has been taking external air through the nose air filter to change to taking unfiltered air through the heat exchanger of the exhaust system. The heat exchanger air is usually warm enough to both affect the engine power (reduced up to 15%) and melt any ice that may have accumulated on the carburettor venturi or butterfly valve. This melting may not occur if the engine has cooled off so don't waste any time pulling the handle. Taxiing lean tends to keep the engine hotter. If pulling C.H. during taxi or run-up raises the rpm the aircraft is improperly leaned. Better yet use C.H. as a preventative to pre-heat the system and if in doubt continuous use may be required.

Carburettor heat is intended as a preventative rather than a cure. Heat should be applied early and fully. Carburettor heat is best used in anticipation of carburettor ice. Put it on before a potential icing situation occurs. Going into a slow flight configuration will increase the effectiveness of carburettor heat by decreasing the cooling air over engine. In addition, carburettor heat makes it possible for the engine to draw its air from inside the engine compartment. If the engine air intake filter under the propeller is being blocked by impact ice or snow, the use of engine compartment air via the carburettor heater will bypass the blockage and allow continued engine operation.

Don't move the throttle. The ice may break loose and cause instant stoppage. Apply full carburettor heat and allow the diverter valve to bring heat in the form of hot air to enter the venturi so as to melt the ice. This air is unfiltered. It passes through the heat exchanger and flows to the carburettor as hot air. The engine will react as follows. The already ice reduced rpm will be further reduced by the application of heat. (Hot air is less dense and reduces power about 15%.) As the ice melts there will be a gradual rise in rpm because of increased air flow. A further rpm increase occurs when carburettor heat is removed.
PROOF of icing is when the sequence of initial rpm drop  is followed by another decrease in rpm when heat is applied, followed by a small increase as ice melts, followed by a further increase when heat is removed.    Carburettor heat admits unfiltered air into the engine. This unfiltered air can contain particles harmful to the engine. This is particularly true close to the ground. For this reason we limit the time carb heat is applied on the ground. Carburettor heat should not be used when maximum power is required such as on takeoff.

The application of carburettor heat changes the flow of engine air from the outside air intake to unfiltered hot air from the heater muff. This hot air causes an additional loss of power. Normally the power loss is 1% for every 10 degrees of hot air differential. With icing this loss can reach 15%. Due to lack of ram air and a usual 100 degrees of heat above standard the runup loss can reach 13%. On hot days the heat differential is less so the apparent drop in power is less. If you have leaned for taxi there is no need to enrich the mixture during runup since you are not going to full power and will check carburettor heat. Carburettor heat enriches the mixture. If you intend to fly with carburettor heat on you should also plan to lean your mixture since with carburettor heat the engine will be running rich. In icing conditions always retain enough power to keep the engine warm. Without a warm engine you will not have carburettor heat. Avoid extended low power or no power descents.

Using the carburettor heat every time you reduce power is a good operating procedure and much safer than the sometime POH suggestion for use when required. Some aircraft are equipped with a carburettor air temperature gauge to warn if the internal temperature of the carburettor is conducive to icing. This serves notice to use carburettor heat as the icing preventative which is its primary purpose. Carburettor heat effects engine operation and power only as does a higher density altitude. No harm to the engine occurs beyond that which may occur through the ingestion of unfiltered air. It is never wrong to use carburettor heat as an icing preventative prior to any power reduction. The use of carburettor heat is too late if the engine has become cooled to the point where it is unable to melt any existing ice. If you need as much engine heat as you can get, set up a climb attitude even if you cannot climb. This will be a much better option than one that would increase speed and engine cooling. Aggressive leaning and use of the magneto switch to cause a backfire are emergency measures.

Carburettor heat use should be limited to operational checks while on the ground as a standard procedure since air going to the carburettor will be unfiltered and allows dust and abrasives into the internal engine. However, some ground temperature dew point conditions may require the use of carburettor heat on the ground, regardless. Having a carburettor air temperature gauge is highly recommended to increase awareness and accuracy in the use of carburettor heat.

Some POH do not suggest or recommend CH application at power reductions. NTSB recommends use on power reduction regardless of POH. A substantial number of engine failures occur because of failure to recognize carburettor ice and apply heat immediately and fully. Once the engine has stopped the rapid cooling caused by the descent limits the effectiveness of any latent heat remaining in the system.

If engine failure seems imminent induce a backfire by turning the magneto switch to off and then back on. A backfire may be further induced by leaning. The backfire can jar any ice loose. Use carburettor heat in high moisture conditions just prior to takeoff while entering the runway. Always use full C.H. since partial applications can actually cause carburettor ice. Any time you have carburettor ice you also have a rich mixture which of itself will cause a rough engine. The situation permitting lean the mixture aggressively until the engine begins to fire again. A running engine will begin to produce the heat necessary to allow the C.H. to melt the intake ice.


The concept of relative safety extends itself to all matters of flying.
The emergency procedures for engine failure in all carburettor aircraft includes the application of carburettor heat, immediately and fully. Required pilot knowledge should be knowing why you use carburettor heat in this way, what the effect will be in the near term and what future benefit is to be expected. The initial effect will be an even greater loss of power, soon to be followed by a rough running engine, and an increase in rpm as ice is removed as water. The final increase occurs when carburettor heat is removed. Failure to understand what is happening and what to expect can be a fatal deficiency.

Removal depends on availability of hot air from alternate air intake system. FAA requires that CH be able to provide 90 degree air down to 30 degrees outside temperature with engine at 75% power. If you allow engine temperatures to drop, as during descent, this heat may not be available for ice removal. In a C-150 the air intake for C. H. is on the right side of the engine cooling intake as seen from the cockpit. The intake on the left is for cabin heat.

Symptoms can be varied but involve initial loss of power, engine roughness, and stoppage. If ice is suspected do not move the throttle. Such movement can cause ice to break loose and further clog the Venturi If possible enter a climb attitude to lessen the flow of cooling air over the engine. Power loss is shown by lower RPM in fixed pitch and lower manifold readings in constant speed propellers. It make take 15 or more seconds to clear the ice. Throttle may be difficult to move. (However, difficulty in moving the throttle can be caused by congealed grease inside the throttle cable due to cold temperatures.)   If the pilot is not sensitive to engine sounds the power loss may occur quickly enough to result in stoppage. I insist that my trainees keep their hands on the throttle below 1000' but never recommend constant removal regardless of altitude. You need to know if any powerchange is due to throttle movement. Any unexplained loss of power should be assumed to be due to carburettor ice. Apply full carburettor heat.

Removal of ice requires application of full CH for as long as it takes to have the engine rise above its additional lower RPM and roughness due to the introduction of hot air. Removal of CH will cause an additional RPM rise. Use of partial heat may make it possible for any moisture to re-freeze. Use of throttle prior to allowing carburettor heat to become effective may cause the butterfly to jam the ice and stop the engine.

When carburettor heat is turned on there is normally a slight drop in rpm because heater muff heats the air going into the carburettor. Hot air has a lower density which means less oxygen is getting into the engine. The mixture of air and fuel is made richer. On occasions when the outside temperature is quite near that of the engine, no drop in rpm may occur. This is normal because the expected drop in rpm is due to a marked difference in outside air temperature and engine heated air. Some pilots will add power prior to the use of carburettor heat so maintain engine temperature and power should a sudden ingestion of water occur. More conventional wisdom indicates that such power involves movement of the carburettor butterfly and may result in sudden blockage and engine failure. Pilots have been surprised by sudden engine roughness or stoppage in what they considered to be non-icing conditions. The temperature inside the carburettor can drop to freezing level in the carburettor venturi making ice a possibility in all but the driest air conditions.

The initial effect of adding carburettor heat will be an even greater loss of power, soon to be followed by a rough running engine, and an increase in rpm as ice is removed as water. The final increase occurs when carburettor heat is removed. A pilot should be aware that a reversal of the results of carburettor heat application can and does occur at altitude, when leaned for taxi, and on takeoff. In these situations any increase in rpm caused by application of carburettor heat is indicative of improper leaning.

Tests have shown that cruise power and full C.H. is not damaging to the engine. The drop of power and even increased roughness often frightens a pilot into taking off the heat. This is a no-no. Don't remove the C.H. until the engine smoothes out even though at reduced power. Since an iced carburettor is running rich, leaning will improve engine operation until C.H. melts the ice. After removing C.H. be sure to readjust the mixture.

A NTSB study shows that there are a number of 'unexplained' engine failures every year. By the time the airplanes are inspected there is no visible 'explanation' for the engine stoppage. NTSB suspects carburettor ice. Over 18 out of the 35 stoppages cause accidents annually

Questions and Answers

is carburettor ice?

May form at the fuel discharge nozzle, in the venturi, on or around the butterfly valve, or in the passages from the carburettor to the engine.

Why is carburettor ice dangerous?

It restricts the power output and may stop engine by depriving it of air.

What causes carburettor ice?

Forms during vaporization of fuel. Expansion of air causes sudden cooling of the air fuel mixture. If water is present it may be deposited as frost or ice inside the carburettor.

What are ideal carburettor ice conditions?

Usually 68F or less where visible moisture exists. When partial throttle is being used. Most critical operational periods are during partial cruise or descent. Apply C.H. ahead of such operations and maintain sufficient power to retain reserve of engine heat.

How is carburettor ice detected?

First sign is loss of rpm or drop in manifold pressure (no drop in rpm}. Engine roughness occurs. Carburettor air temperature in the yellow.

How is carburettor ice prevented?

Carburettor heat is a preventative. It may not melt existing ice. Periodic checks advisable when favourable conditions exist.

 Always apply full heat. Always apply full heat during partial throttle situations.

What is the most important reason for not taxiing with carburettor heat on?
C. H. allows induction air to bypass the air filter. Unfiltered air contains abrasives that can enter the engine and cause damage.

Why do Lycoming engines often say carburettor heat is optional to the extent that ice must be recognized before applying carburettor heat?

The mounting of the carburettor to the Lycoming oil pan allows the engine heat to reach the carburettor. Continental engines have their carburettor separated from the engine so engine heat does not give this beneficial side effect. Since Continental has a cooler carburettor they tend to get more power than an equal displacement Lycoming.

the difference between and engine with carburettor and one with injection

Carburettor heat uses hot air taken from the engine exhaust system.
Alternate-air uses heat from the cooling fins of the cylinders.
Both system use a hot air door that shuts down the usual engine air filter system.
Avoid use of either system for extended periods on the ground because the air is unfiltered from dust..