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

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