Understanding engine operations — when to use carby heat

I know carburettor heat is used to prevent or clear carburettor icing, but I'm not entirely clear on when it forms. Is it only in cold weather?

No, and this catches a lot of students out. Carburettor icing can occur at outside air temperatures as high as 30°C and in conditions of moderate humidity. The venturi effect in the carburettor causes a temperature drop of up to 20°C, and if the resulting temperature falls below freezing with moisture present, ice forms. It's most insidious on warm humid days at reduced power settings — for example during descent or in the circuit. Symptoms include a gradual loss of RPM (fixed-pitch prop) or manifold pressure (constant-speed prop) with no change in throttle position.

So you could be flying on a perfectly nice day and still get carby ice? That's alarming. How do you handle it?

Apply full carby heat. You'll initially see a further drop in RPM as the hot air is less dense, possibly followed by rough running as the ice melts and passes through the engine. Then RPM should recover to a value higher than where it was before — that confirms there was ice present. Use carby heat as a preventative measure during operations where icing is likely, especially in the circuit at low power settings. Check your POH for specific guidance for your aircraft type.