[Applause] the carburetor is a simple and reliable device it mixes air and fuel in the correct ratio for internal combustion of an aeroplane engine as air is sucked into the engine firstly it passes through the air filter it then flows through the carburetors narrow section called the venturi at this point the static air pressure Falls and fuel is drawn into the air stream the mixture enters the engine cylinders for combustion and flows out of the exhaust is hot gas if you are old enough to have seen a carburetor in a motor vehicle you may be wondering why the airplanes carburetor is mounted upside down and below the engine one advantage is to prevent the engine from flooding with excessively rich mixture any surplus fuel simply flows down out of the system another advantage is that if the carburetor was mounted on top of the engine the pilot would have a good view of it but would not see what's in front of the aeroplane a typical training aeroplanes carburetor is susceptible to ice forming in the venturi ice can restrict the airflow and possibly stop the engine you don't even need to be flying through a snowstorm for the ice to form even on a warm day if there is moisture in the atmosphere it can form into ice inside the carburetor the temperature of the air drops dramatically in the venturi because the static air pressure drops the air temperature drops some more because fuel that is drawn into the air stream evaporates just like the sweat that evaporates off your skin to cool you down so does the air inside the carburetor ice is most likely to form at low engine power at a low power setting the carburetors butterfly valve is nearly closed which means that the temperature drop is significant because the venturi effect is significant a low engine power setting also means that the normally hot engine block is cooler therefore the attached carburetor is cooler these factors combined to create the perfect recipe for ice to form ice formation in the carburetor can be observed by rough engine running lowering of the engine power and subsequently a total engine failure so how can you prevent carburetor ice from forming or remove it once it's already formed the engines hot gases pass through the heat exchanger before they exit out of the exhaust by switching carburetor heat on the airflow is redirected through the heat exchanger where it's temperature is raised the warmer air flowing through the venturi prevents ice from forming or it can melt ice away if some had already formed since hot air is less dense than cold air the engine power drops when carburetor heat is applied if ice is present the engine power will drop initially then increase considerably once ice is gone the engine may experience momentary rough running as the melting ice is ingested into the engine apply carburetor heat only as directed by the pilots operating handbook applicable to your aeroplane type generally carburetor heat is not used on or low to the ground because the heated bypass air is not filtered to remove contaminants from the airflow carburetor heat should not be used for extended time as it can overheat the engine and even burn a hole in your exhaust