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Carburetor Ice and Heat
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Contents:
Carburetor Heat; ...Occurrence; ...Three kinds of Carburetor Ice; ...Recognition; ...Using Carburetor Heat; ...Application; ...Results;...Questions and Answers; Carburetor Ice Lesson and Opinion); Carburated different from Injected; ...About Carburetor Heat; ......One in 35 in O4: ...

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.

Recognition
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'. Carburetor 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.

I am no longer surprised when a pilot does not show awareness of carburetor ice. The onset can be very subtle. The effects very subdued, and the wrong reaction very common. Contrary to some manuals, the National Transportation Safety Board is now recommending that heat be applied for all power reductions below cruise. I teach my students to keep their right hand and indexing finger on the throttle at all times below 1000' except when trimming or making radio changes. This helps determine if any power change can be classified as 'unexplained' hence attributable to carburetor ice.
You know if you made a throttle change or not or at least you should know.

The greatest temperature drop occurs in the throat of a carburetor. 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 Carburetor Heat
1. Even in warm weather
2. All or nothing at all, not just an old song.
3. Use before takeoff; not during takeoff
4. The engine talks; you listen and feel.
5. Not for continuous operation on the ground.
6. Check proper operation and maintenance

Application
When you pull the aircraft carburetor 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 carburetor 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.

There are no complex operations in flying for which there are not several simple, straight forward, and WRONG ways to perform. One such combination of operation and solutions is the use of carburetor heat. The warning indicators for the need of carburetor heat are deceptive, variable, and inconsistent. The actual application of carburetor heat will produce results that are deceptive, variable, and inconsistent. Every student pilot is goings to have opportunities to make carburetor heat mistakes. The learning process will consist of both successes and mistakes in the use of carburetor heat. The training process is designed to reduce the probability of a mistake resulting in an unpleasant event. It takes an act of faith to stay with your initial application of carburetor heat when it only makes things worse. Very often the worst thing that can happen to a pilot is to 'get away' with a mistake. This applies to use of carburetor heat as well as any other aspect of flying.

Carburetor heat is intended as a preventative rather than a cure. Heat should be applied early and fully. Carburetor heat is best used in anticipation of carburetor ice. Put it on before a potential icing situation occurs. Going into a slow flight configuration will increase the effectiveness of carburetor heat by decreasing the cooling air over engine. In addition, carburetor 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 carburetor 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 carburetor 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 carburetor 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 carburetor 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.    Carburetor 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. (I find my personal allergic reaction to pollen ceases above 500') Carburetor heat should not be used when maximum power is required such as on takeoff.

The application of carburetor 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 carburetor heat. Carburetor heat enriches the mixture. If you intend to fly with carburetor heat on you should also plan to lean your mixture since with carburetor 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 carburetor heat. Avoid extended low power or no power descents.

Using the carburetor 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 carburetor air temperature gauge to warn if the internal temperature of the carburetor is conducive to icing. This serves notice to use carburetor heat as the icing preventative which is its primary purpose. Carburetor 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 carburetor heat as an icing preventative prior to any power reduction. The use of carburetor 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.

I often use carburetor heat as a cruise descent device. It will decrease power by about 200 rpm and allow a fuel saving leaner mixture throughout the descent. You can reduce some power as well. Don't mess with the trim. You will probably be leveling off at pattern altitude on arrival and by just removing carburetor heat or adding power you are already trimmed for level. When conditions warrant, I use carburetor heat when rolling into position on the runway for takeoff. There are exceptionally cold times when you should not use carburetor heat. Partial heat applications are never recommended unless performed in conjunction with a carburetor heat temperature gauge.

Carburetor heat use should be limited to operational checks while on the ground as a standard procedure since air going to the carburetor will be unfiltered and allows dust and abrasives into the internal engine. However, some ground temperature dew point conditions may require the use of carburetor heat on the ground, regardless. Having a carburetor air temperature gauge is highly recommended to increase awareness and accuracy in the use of carburetor 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 carburetor 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 your life's ambition is to become an old pilot, I would become sensitized to the conditions causing carburetor icing; sensitized to the first unexplained drop in rpm; and start using anticipatory carburetor heat. If you think any loss of power is due to ice you should apply heat and try to enter a climb with any remaining power to conserve and create as much heat as possible.

Any suggestions regarding the use of carburetor heat should be qualified by reference to the specific model of aircraft. Cessna usually recommends carburetor heat in operations below the green arc, Piper does not; instead, Piper's usual recommendation is to use only when indicated. The reason behind the operating differences has to do with the way the carburetor is mounted below the engines and operational experience.

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 carburetor heat in high moisture conditions just prior to takeoff while entering the runway. Always use full C.H. since partial applications can actually cause carburetor ice. Any time you have carburetor 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.

Results
The concept of relative safety extends itself to all matters of flying.
The emergency procedures for engine failure in all carburetor aircraft includes the application of carburetor heat, immediately and fully. Required pilot knowledge should be knowing why you use carburetor 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 carburetor 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 carburetor ice. Apply full carburetor 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 carburetor heat to become effective may cause the butterfly to jam the ice and stop the engine.

When carburetor heat is turned on there is normally a slight drop in rpm because heater muff heats the air going into the carburetor. 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 carburetor 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 carburetor 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 carburetor can drop to freezing level in the carburetor venturi making ice a possibility in all but the driest air conditions.

The initial effect of adding carburetor 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 carburetor heat is removed. A pilot should be aware that a reversal of the results of carburetor 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 carburetor 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 carburetor 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 carburetor ice. Over 18 out of the 35 stoppages cause accidents annually

Questions and Answers
--What
is carburetor ice?
May form at the fuel discharge nozzle, in the venturi, on or around the butterfly valve, or in the passages from the carburetor to the engine.
--Why is carburetor ice dangerous?
It restricts the power output and may stop engine by depriving it of air.
--What causes carburetor 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 carburetor.
--What are ideal carburetor 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 carburetor ice detected?
First sign is loss of rpm or drop in manifold pressure (no drop in rpm}. Engine roughness occurs. Carburetor air temperature in the yellow.
--How is carburetor ice prevented?
Carburetor heat is a preventative. It may not melt existing ice. Periodic checks advisable when favorable conditions exist. Always apply full heat. Always apply full heat during partial throttle situations.
--What is the most important reason for not taxiing with carburetor 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 carburetor heat is optional to the extent that ice must be recognized before applying carburetor heat?
The mounting of the carburetor to the Lycoming oil pan allows the engine heat to reach the carburetor. Continental engines have their carburetor separated from the engine so engine heat does not give this beneficial side effect. Since Continental has a cooler carburetor they tend to get more power than an equal displacement Lycoming.

Carburetor Ice Lesson and Opinion
Speaking of carb heat, I had a very interesting flight lesson the other day. My instructor leaned back the engine until it barely ran and asked me what could I do to fix it. I applied carb heat and it started to run smooth again. Then with the carb heat on he leaned it back even more again until it started to barely run and said fix that now. This time I had no ideas of what to do. The asked me to pump the primer and I ended up flying the plane on the primer. I don't know if I could have maintained altitude if he had fully leaned it out, but he explained to me that in the event that I had applied carb heat to late, I could still bide some time and keep the fire lit enough to get the carb heater working and eventually thaw out a frozen carb. I thought it was a worthwhile lesson.

If the carb heat doesn't seem to be doing the job, or if the carb heat cable comes out in your hand, or you otherwise seem unable to deal with the ice buildup, this technique has worked for me.

Open the throttle wide open and proceed to do a good demonstration of minimum controllable airspeed at full power available. This technique minimizes the blockage in the intake and provides maximum air, while at the same time producing maximum heat.

Several times I had severe carb icing with the carb heat control inoperative for one reason or another and was able to clear the ice and continue in this manner.
HighFlyer

Carbureted Different from Injected
--Carburetor 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..

About Carburetor Heat
You may not be familiar with is a NTSB study showing that there are 35 'unexplained' engine failures every year. By the time the airplanes are inspected there is no 'explanation' for the engine stoppage. NTSB suspects carburetor ice. If your life's ambition were to become an old pilot, I would become sensitized to the conditions causing carburetor icing; sensitized to the first unexplained drop in rpm; and start using anticipatory carburetor heat.

Carburetor icing occurs when the air/moisture/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. The initial effect of adding carburetor 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 carburetor heat is removed.

I often use carburetor heat as a cruise descent device. It will decrease power by about 200 rpm and allow a fuel saving leaner mixture throughout the descent. You can reduce some power as well. Don't mess with the trim. You will probably be leveling off at pattern altitude on arrival and by just removing carburetor heat or adding power you are already trimmed for level. When conditions warrant, I use carburetor heat when rolling into position on the runway for takeoff.

There are exceptionally cold times when you should not use carburetor heat. Partial heat applications are never recommended. Carburetor heat use should be limited to operational checks while on the ground as a standard procedure since air going to the carburetor will be unfiltered and allows dust and abrasives into the internal engine. However, some ground temperature dewpoint conditions may require the use of carburetor heat on the ground, regardless. Having a carburetor air temperature gauge is highly recommended to increase awareness and accuracy in the use of carburetor heat.

Carburetor heat is intended as a preventative rather than a cure. Heat should be applied early and fully. 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.

When carburetor heat is turned on there is normally a slight drop in rpm because heater muff heats the air going into the carburetor. 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 carburetor 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 carburetor 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 carburetor can drop to freezing level in the carburetor venturi making ice a possibility in all but the driest air conditions.

There is more to becoming an old pilot than just being lucky. The concept of relative safety extends itself to all matters of flying. The emergency procedures for engine failure in all carburetor aircraft includes the application of carburetor heat, immediately and fully. Required pilot knowledge should be knowing why you use carburetor 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 carburetor heat is removed. Failure to understand what is happening and what to expect can be a fatal deficiency.

There is one condition where the application of carburetor heat produces no effect. Using today's fuel practically all pilots will lean while taxiing… Lycoming has stated to the effect that it is all right to do your runup while the mixture is still lean since you are not going to full power. However, if you have leaned aggressively while taxiing the engine will be running hotter than normal. The effect of this increase in engine temperature will negate the ability of a carburetor heat application to cause an RPM drop. When the full application of carburetor heat fails to cause an RPM drop a pilot is apt to look for something wrong. Enriching the mixture and then pulling on the heat will give the usual response. The first time it happened to me I was ready to take the plane into the maintenance shop.

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.

In Continentals the 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.

Any recommendations regarding the use of carburetor heat should be qualified by reference to the specific model of aircraft. Cessna POH’s usually recommends carburetor heat in operations below the green arc, Piper does not; instead, Piper’s usual recommendation is to use only when indicated. The reason behind the operating differences has to do with the way the carburetor is mounted below the engines and operational experience.

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.

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. Some aircraft are equipped with a carburetor air temperature gauge to warn if the internal temperature of the carburetor is conducive to icing. This serves notice to use carburetor heat as the icing preventative which is its primary purpose. Carburetor 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 carburetor heat as an icing preventative prior to any power reduction. The use of carburetor 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.

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.

There are two very important training aspects related to carburetor ice and the use of carburetor heat. First, 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. Recognition is important but it comes after the fact. Depending on the circumstances, after the fact, may be too late. Like 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’. Carburetor 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.

There are no complex operations in flying for which there are not several simple, straight forward, and WRONG ways to perform. One such combination of operation and solutions is the use of carburetor heat. The warning indicators for the need of carburetor heat are deceptive, variable, and inconsistent. The actual application of carburetor heat will produce results that are deceptive, variable, and inconsistent. Every student pilot is goings to have opportunities to make carburetor heat mistakes. The learning process will consist of both successes and mistakes in the use of carburetor heat. The training process is designed to reduce the probability of a mistake resulting in an unpleasant event. It takes an act of faith to stay with your initial application of carburetor heat when it only makes things worse. Very often the worst thing that can happen to a pilot is to ‘get away’ with a mistake. This applies to the use of carburetor heat as well as any other aspect of flying.

One of 35 in O4
About three hours into the until-then uneventful night flight, the pilot contacted Mansfield, OH approach control, told them that he was experiencing a partial power loss, and requested to land at Mansfield. Mansfield runway 32 was straight ahead and four miles, and the pilot was cleared to land. According to the passenger, the pilot applied carburetor heat for about ten seconds, and with no change in engine operation, returned it to cold and said that he thought they were experiencing vapor lock. Shortly thereafter, the pilot informed Mansfield approach that he was going to land on a road, and that the engine had quit.

Investigation revealed that as the flight progressed, the temperature and dew point spreads were getting closer together. At the time of the accident, the temperature in Mansfield was minus one degree Celsius (30.2 F) and the dew point was minus four degrees Celsius (24.8 F).

The NTSB determined the cause of this accident to be the pilot's improper use of carburetor heat, and subsequent forced landing.

The pilot might have been able to prevent this accident had he taken advantage of the free advice in the ASF Aircraft Icing Safety Advisor. It says, "At the first indication of carburetor ice, apply full carburetor heat and LEAVE IT ON. The engine may run rougher as the ice melts and goes through it, but it will smooth out again."

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