Aircraft Systems and Flight: The Carburetor

What is a float-type carburetor and how does it work?

A carburetor is a critical engine component that is responsible for mixing the fuel and air that is distributed in the engine's cylinders. Pictured below is a diagram of a float-type carburetor. The right side of the diagram consists of a float chamber where the fuel is reserved and metered by a float (PHAK, 2016). In the float chamber lies the mixture needle which regulates the amount of fuel that is drawn in the venturi based on the position of the mixture lever (PHAK, 2016). On the left, you will see the venturi. Fresh filtered air from the air inlet flows through the restriction in the venturi which causes it to accelerate and the pressure drops (PHAK, 2016). The pressure drop creates a vacuum that sucks up the fuel from the discharge nozzle, mixes it with the air, and is distributed to the cylinders through the throttle valve (PHAK, 2016).

Carburetor Icing

Float-type carburetors are susceptible to icing. The NTSB reported that between 2000 to 2011, carb ice was responsible for 250 aircraft accidents, many of which were fatal (2016). As mentioned earlier, as air passes through the venturi, the pressure drops and when it does, the temperature does as well. If the temperature drops to a freezing level and there is sufficient moisture present, ice will accumulate (PHAK, 2016). Ice formation will restrict the fuel/air mixture from reaching the cylinders and will cause power loss or even complete engine failure (PHAK, 2016). If not recognized, it can lead to a serious and fatal accident. 

To combat this problem, aircraft are also equipped with carb heat. When turned on, carb heat will take heated, unfiltered air and distribute it into the venturi (PHAK, 2016). This heated air will keep the temperature above freezing and break up small accumulations of ice (PHAK, 2016). It is important to note that carb heat is anti-ice equipment, not de-icing equipment so it will not remove large accumulations (PHAK, 2016). Because of this, pilots need to be more aware of when accumulation is most likely to occur and use carb heat as a preventative measure.

I have read many articles on carb ice and from the sound of it, it can really happen at any time. According to the Pilots Handbook of Aeronautical Knowledge, the following conditions are most susceptible to the formation of carb ice (2016):

• Temperatures below 70 degrees Fahrenheit  
• Relative humidity above 80%

The video below shows carb ice in flight and how quickly it can accumulate.

Federal Aviation Administration (FAA). (2016). Pilot’s Handbook of Aeronautical Knowledge (PHAK). Retrieved https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/

National Transportation Safety Board (NTSB). (2016, December). Engine Power Loss Due to Icing. 

            Retrieved https://www.ntsb.gov/safety/safety-alerts/Documents/SA_029.pdf