What is Carburettor Icing?
The carburettor is the area inside a piston engine where the air mixes with fuel before entering the engine’s cylinders. When the fuel and air mix together, it causes the fuel to evaporate and air pressure to drop – both of which cause the temperature to decrease (think of the water on your body as you get out of the shower – as it begins to evaporate it makes you feel cold. The same temperature drop occurs when fuel evaporates and mixes into the air in your carburettor).
A temperature drop of 25°C to 30°C inside the carburettor is common, which can easily cause the air temperature to drop below 0°C. If that is below the dew point temperature, ice will start forming on the carburettor walls and the fuel/air mixture flow to the engine will be restricted. If the pilot does nothing, this will progressively reduce engine power and can lead to complete engine failure.
Since there is a large temperature drop inside the carburettor, the outside air temperature (OAT) doesn’t need to be below 0°C for ice to form. In fact, as warm air can hold more moisture than cold air, the highest risk of carburettor icing is on warm days with high humidity!
Be aware that when you apply carburettor heat to remove any ice, it may cause your engine to run rough at first. If any carburettor ice has built up, it will start to melt as soon as you apply carb heat and water will flow through your engine. The rough running that this causes should only be experienced for a few minutes at the most. Once the ice has melted and you turn the carburettor heat off, you should see the RPM increase to a value higher than you started with (assuming you haven’t moved the throttle).
The below short video has some good footage of ice forming and melting inside a carburettor:
Carburettor Ice and Power Setting
At low power settings, the throttle valve restricts the flow of air into the engine. This restriction of airflow again decreases pressure, so it has the same temperature reducing effects and further increases the risk of carburettor icing.
Therefore, carburettor icing is more severe at low power settings.
Application of carburettor heat should quickly remove any ice buildup. However, carburettor ice continues to be a factor in many aviation accidents, often because icing isn’t on a pilot’s mind on a sunny day with scattered cumulus clouds (suggesting high humidity) and temperatures of 20°C!
Determining the Risk of Carburettor Icing
Graphs showing the risk of carburettor icing based on temperature and dew point can be seen pinned to the wall of many flight schools and clubs.
It is a good idea for you to be familiar with using this graph to determine the risk of carburettor icing.
Engine Intake Icing
Carburettor icing is not the only type of icing that your piston engine can suffer from.
Flight in icing conditions (visible moisture, such as drizzle, and temperatures close to or below 0°C) can also cause a layer of ice to form over the airframe and engine air intake. A partial blockage of the intake will significantly reduce engine power and if the ice continues to form, the engine can eventually be starved of air and stop completely.
It’s difficult to spot, but the white line around the engine air intake here is ice. I quickly descended below the cloud & into positive temperatures before it became excessive!
Propeller Icing
If you do find yourself flying in icing conditions where ice is forming on the engine intake and the rest of the airframe, then you have to expect it is also forming on the propeller. However, propeller ice doesn’t form evenly, leading to a weight imbalance across the blades which are rotating around 40 times per second. This can cause severe vibration and the ice layer disrupts the airflow over the propeller, reducing thrust.
