To follow up with my last post on the carburetor, I’ll be talking a bit about the carb heat lever and what is happening inside the engine when you engage it.
Icing is a problem that can affect an aircraft in many ways: added weight, change in airfoil shape, increased drag, and reduction of airflow to the engine. While all of these effects of icing are dangerous, this post will focus primarily on the last one.
As I explained in my last post, the carburetor mixes incoming air with fuel and feeds this mixture to the engine. Icing can occur when moist air enters the carburetor and freezes to the walls and butterfly valve. The eventual build up of ice can ultimately cut off the air supply or render the butterfly valve inoperable, and it is for this reason that we use carb heat.
Carb heat works by taking unfiltered, outside air and passing it through tubes wrapped around an exhaust muffler. This effectively preheats the air before it reaches the carburetor. If ice is present in the carburetor, this air will melt the ice.
There are a few side effects of using carb heat however. The C172 POH says that carb heat will reduce engine by 100-225 RPM at full power. This is due to the fact that hot air is less dense than cold air, and with less air molecules to aid in the combustion process, power is reduced. Also, if ice is already present and carb heat is applied, it may be observed that the engine will run rougher than it was previously. As the ice melts, the resulting water enters the cylinders and causes the roughness. It is extremely important that you leave carb heat on and let the water get out of the system. Some people apply carb heat and notice that the engine is running rougher than it was – they then turn carb heat off. Ice builds up even more and engine failure becomes imminent.
The moral of this story is to leave carb heat on and let it do its job! Give it some time and engine power should return to normal, even if it runs a little rough at first.
Below are a few diagrams, courtesy of From The Ground Up that show the effects of icing, the carb heat system, and the carb icing range. Enjoy.
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