The formation of carburettor ice is most likely when the temperature is below 20 degrees Celsius and the relative humidity is above about 80%. What does that statement mean to us? It means we are operating at a time of year where conditions are continually favourable for the formation of carburettor Ice.
In aircraft with a fixed pitch propeller, symptoms of carburettor icing can be a loss of engine power indicated by a drop-in engine rpm and possible rough running. If this were to occur it would require us to fully apply carburettor heat. Failure to do so could lead to a worsening situation where the engine can be effectively choked of any air. But what if applying carburettor heat caused an even bigger rpm drop and even rougher running? Should we switch it off as it’s making the situation worse? The answer is no. It means the carburettor heat is doing its job. As the hotter less dense air goes through the engine, we get a slight decrease in performance. As the carburettor ice melts and passes through the cylinders as water it will give us even rougher running until all the ice has been removed. This is normal and not something to be concerned about. As the majority of engine power is restored once all the ice has cleared what will we do with the carburettor heat then? We’ve fixed the problem so should we put it back to cold? We could, but we’ve already been flying in conditions conducive to icing. While those conditions still exist it’s best to leave the carburettor heat on so we don’t get a reformation of any ice.
Prevention of carburettor ice is always better than the cure so we need to remember anytime we are flying outside of the green arch on the tacho we will always select carburettor heat on first. Also, if we consider we are flying in possible icing conditions we can select carburettor heat on.
See Icing Chart here: carb_icing
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