The most common form of engine icing is carburettor icing and it’s also the hardest to detect in flight. 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 (remember the exchange of heat from The States of Water lesson?).
A temperature drop of 25°C to 30°C inside the carburettor is common. This often causes the air to drop below 0°C and if that is below the dew point temperature, ice will start to form on the carburettor walls. The flow of the fuel/air mixture to the engine will be restricted. If the pilot does nothing, this 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, since warm air can hold more moisture than cold air, the highest risk of carburettor icing is on warm days with high humidity!
At low power settings, the throttle valve restricts the flow of air into the engine. However, this restriction of the airflow again reduces pressure, so it has the same temperature reducing effects, further increasing the risk of carburettor icing.
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!
Graphs showing the risk of carburettor icing based on temperature and dew point can be seen pinned to the wall of many flight schools.
You need to be familiar with using this graph to determine the risk of carburettor icing.
Flight in icing conditions can also cause a layer of ice to form over the engine air intake. A partial blockage will significantly reduce engine power and if ice continues to form, the engine can eventually be starved of air and stop completely.
The conditions that cause airframe icing are also likely to produce ice on the propeller. This ice doesn’t form evenly, leading to a weight imbalance on the propeller, which is rotating around 40 times per second. This can cause severe vibration and the ice layer disrupts the airflow over the propeller, reducing thrust.