A carburetor mixes fuel and air before delivering that mixture to the piston for ignition. The fuel is mixed with air as it travels through a venturi, which has a double-ended funnel shape that decreases air pressure and increases its speed. This causes the fuel to atomize (form tiny droplets that ignite more easily), and it also decreases the temperature of the mixture. If any moisture is in the fuel-air mixture (for example, in humid conditions), ice may form as a result of that rapid temperature decrease. Ice in the venturi will reduce flow of fuel and air, which, in turn, causes a decrease in engine power, ranging from minor to extreme—complete engine stoppage being the extreme.

To determine how likely this is to occur on a given day, carb ice probability charts are readily available and are useful in determining probability based on a combination of relative humidity, dew point, and temperature. As a general rule of thumb, conditions conducive to carb ice include temperatures below 70 degrees Fahrenheit and humidity above 80 percent. However, it can occur in temperatures as high as 100 degrees Fahrenheit and humidity as low as 50 percent. This is something to be alert to for much of the year, for much of the country.

To counter this hazard, carb heat is available and serves as both an anti-icing and a deicing system. By preheating air before it enters the carburetor and preventing ice formation, it is an anti-icing system. By melting ice that has already accumulated inside the carburetor, it is a deicing system. Remember, though, that carb heat cannot eliminate a large ice accumulation. In addition, use of carb heat results in decreased performance since the warmer air is less dense. With carb ice, the initial indication of a problem will be the engine running rough. At that first sign, the carb heat should be applied and kept on. Initially, the engine may run rougher as the ice melts, but it will run smoothly once the ice has been eliminated.

So, how do you check to ensure the carb heat works before you actually need it to remove ice? During the runup, before your flight, one of the checklist items is to turn the carb heat on and verify it works based on a drop in rpm (due to decreased air density). Some digital engine monitoring instruments will show a digital readout of the carb temperature increasing as soon as heat is applied.

Always check and follow the approved flight manual and checklists for the specific aircraft regarding the use of carb heat. In some airplanes, the prelanding checklist includes a carb heat check while others require carb heat to be turned on and left on during descent and landing.

Learn more about carburetors and carb ice in Chapter 7 of the Pilot's Handbook of Aeronautical Knowledge.