While many think that aircraft icing only occurs on colder days, this is not the case. In fact, the airframe exterior and many internal components are susceptible to icing for a number of reasons, all of which jeopardize passenger and aircrew safety. For example, as air and fuel are vital for an aircraft engine to run optimally, if they are blocked or restricted due to icing, it can have detrimental consequences. According to the NTSB, carb icing is responsible for at least 250 of the aircraft accidents that have happened in the last 10 years. To minimize this risk, there are several things one can do, some of which will be outlined in this blog.
Belonging to the aircraft throttle system, the carburetor is the section of the engine where the fuel-air mixture is created before being drawn in by the cylinders and ignited. The more air and fuel being sucked in by the carburetor, the greater the power generated by the engine. Carburetors operate using the Venturi effect, which states that as air or fluid is drawn through a restricted passage, the speed of the air will increase and the pressure of the air will drop. At the narrowest part of the Venturi, there is a fuel line. When there is a drop in pressure, the Venturi draws in fuel and mixes it with air to form a vapor that makes its way into the cylinders.
The air is drawn through the carburetor at a rate controlled by a butterfly valve that is directly linked to the throttle controls. It is important to note that full throttle indicates that the butterfly valve is fully open, while the throttle at idle means that the butterfly valve is nearly closed. As previously mentioned, the carburetor is a Venturi, and a similar effect can be observed around the butterfly valve, since airflow narrows and is restricted in this area as well. As such, icing can form in two places within the carburetor: the Venturi and around the butterfly valve. The former will lead to air starvation, while the latter will lead to throttle control degradation. In most cases, however, carburetor icing forms around the butterfly valve.
How Carburetor Icing Forms
Carburetor icing typically happens at low power settings, such as when making an approach. If the throttle is idle upon final approach, the pilot may be unaware that icing has formed until they attempt to apply power for a missed approach. Additionally, carburetor icing forms due to a drop in temperature in the carburetor. A loss in temperature may source from the evaporation of fuel, combined with a pressure drop. If the carburetor is obstructed, this can lead to fuel and air starvation in the engine, which can cause the engine to stop completely.
More than that, carburetor icing is more likely to occur on warm and humid days. This is because warm air has a greater capacity to hold more moisture, causing condensation or ice to form. When water vapor condenses into liquid, this is called the dew point. On days where the dew point is not far from the ambient air temperature, even the slightest drop in temperature in the carburetor can lead to ice formation.
To prevent ice from forming, one can take advantage of carb heat. It works by sucking in warm air from around the exhaust manifold and directing it through the carburetor. As the warm air raises the temperature, the ice melts. It is worth noting that when carb heat is used, the air is warmer and less dense, which can lead to degradation in performance because the fuel-air mixture will not be optimal. For this reason, one can expect changes in pitch and power for a short period of time. Since engines are only meant to run on fuel and air, if ice particles are present and they melt, it will result in engine issues.
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