Carburetor heat

Carburetor (or Carburettor) heat (usually abbreviated to 'carb heat') is a system used in piston-powered light aircraft to help prevent or clear carburetor icing. It is usually manually controlled by the pilot. It consists of a flap which diverts warm air from around the exhaust manifold into the engine's air intake. The warmer air will usually clear any icing present within the carburetor.

Related Topics:
Light aircraft - Carburetor icing - Exhaust manifold

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Icing occurs in certain conditions due to the venturi within the carburetor, which raises the velocity of the air in the carburetor, which lowers its pressure (see Bernoulli effect) and hence temperature (Boyle's Law). If the outside air is already at a low temperature, the temperature in the carburettor can drop below the freezing point of water, and if the air is humid, ice can form inside the carburetor, narrowing the aperture of the throat, which can create an even stronger venturi effect, and so forth. If left unchecked, the carburetor will eventually malfunction which will cause an engine failure, an emergency situation. Temperature drops of 20 degrees C or more are often encountered within the carburetor, and so icing can occur even on relatively warm days. Also, the adiabatic lapse rate (temperature drop) is around 4 degrees C per thousand feet, so it is really the humidity of the air which is the more important indicator of potential icing conditions. Perhaps paradoxically, winter flying is often less prone to icing, since cold weather is rarely associated with high humidity, and the air temperature can drop so far below freezing that there can be little or no water vapour in the air to begin with.

Related Topics:
Venturi - Carburetor - Bernoulli effect - Boyle's Law - Freezing point - Humid - Ice - Adiabatic lapse rate

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Diverting warm air into the intake will usually clear any icing present, though in some conditions still may not be sufficient. The wise pilot will not attempt to fly into known icing conditions if his aircraft is not equipped to deal with it.

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The diversion of warm air into the intake reduces the thermodynamic efficiency of the engine, which will be manifest as a slight reduction in power while carb heat is applied. The reduction in power indicates to the pilot that there is no icing present, a reassuring piece of information. If there is icing, applying carb heat may not show this initial reduction, and as the ice clears there may be an increase in power. Again, the pilot will note this as evidence that icing conditions are present.

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If carb icing results in an engine stoppage, one of the first things the pilot will do is apply carb heat in an attempt to clear the icing, though as the engine will not be running, it is possible that the exhaust will cool sufficiently quickly that clearing the icing will not be possible. In any case the pilot will be carrying out the emergency landing procedure, including a possible engine restart. The outcome depends on the conditions, prompt action and skill. Again, avoiding icing is far better than trying to clear it in an emergency.

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Applying carb heat as a matter of routine is built into numerous in-flight and pre-landing checks (e.g. see BUMPH).

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Engines equipped with fuel injection do not require carb heat or an analogous system as they are not prone to icing.

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