Carburettor icing: when it happens and what to do

Why carb icing can form on a warm summer day, how to recognise it, and how to use carb heat correctly.

The short answer

Carburettor ice forms because fuel evaporation and the pressure drop in the venturi cool the air sharply. It can form at outside temperatures well above 20 °C when the air is humid, and it is most likely at low power settings such as descents and approaches.

The signs

• Fixed pitch propeller: a slow drop in RPM
• Constant speed propeller: a drop in manifold pressure
• Rough running if it gets worse

Using carb heat

Apply full carb heat, not partial. RPM will drop further at first as the warm, less dense air enters, and the engine may run rough while ice melts. Leave it on until power recovers, then return to cold. Follow your flight manual, which usually calls for carb heat before reducing power for descent.

High risk conditions

Risk is highest when the temperature and dew point are close together, for example in the morning, after rain, near clouds or over water. Many flying schools use a carb icing chart that plots temperature against dew point, which is worth keeping on your kneeboard.