Flying Smart

Carburetor Ice

Preflighting your airplane on a hot and humid summer day, the only ice you're likely to think about is that which floats inside a nice glass of cold soda in the airport caf�. Humidity in the air, however, is moisture that can collect inside your trainer's carburetor and form ice while the aircraft is aloft.

You learned in ground school that your airplane's carburetor mixes incoming air from the induction system with fuel and sends it to the combustion chamber. Air enters at the carburetor's venturi, whose narrow shape produces an area of low pressure. Fuel vaporization and the decreased air pressure create a drop in temperature.

If water vapor condenses when the temperature inside the carburetor is at or below freezing, ice can collect around the throttle valve. The ice chokes off the flow of the fuel/air mixture to the engine. This is when you're likely to notice the drop in rpm and a rough-running engine that are the first signs of carb ice in an aircraft with a fixed-pitch propeller. If you don't apply carburetor heat - which will send heated air into the carburetor - the ice can build up and the engine will quit.

In some trainers, like most older Cessnas, you will be taught to apply carburetor heat as a precautionary item when landing, performing power-off stalls, or any other time the throttle is at idle. That's because at reduced rpm your airplane's engine is operating at a cooler temperature, and any water vapor present in the carburetor is more likely to freeze. Prime conditions for carburetor ice formation are temperatures between 20 degrees and 70 degrees Fahrenheit, but it can form at other temperatures. Carb icing is not a concern with newer Cessnas or other aircraft that have fuel-injected engines.

Some engine makes in piston-driven aircraft are more prone to carburetor icing than others. Just because you fly a Lycoming instead of a Continental, however, doesn't mean you're in the clear. As pilot in command you must recognize the prime conditions for carb ice formation, know the warning signs that an engine will make, and be able to remedy the situation.

When you do apply carb heat, leave it on for several minutes - even if the engine seems to run worse. The roughness is caused by melting carb ice as it goes through the engine. Your instructor can tell you when carburetor heat should be applied in the airplane that you fly, but you should also know what the aircraft pilot's operating handbook requires.