Day 34 - Lesson Cancelled

So what is 'carburettor heat' anyway?

Posted by Owain Abraham-Williams on Saturday 13th February 2016

The weather today was rubbish, so another week would have to pass before I could consolidate my first solo. Therefore I’m going to write about carburettor heat, as it’s something I mention in almost every blog post, but haven’t gone into much detail before.

NATS publish a series of Aeronautical Information Circulars (AIC), the pink ones are related to safety and in particular P 077/2009 discusses ‘Induction System Icing’, commonly referred to as ‘carburettor icing’.

When fuel is vaporised as it’s being injected into the engine, there’s a sudden drop in temperature. If the air is humid this rapid cooling can cause ice to start forming. If the air’s humid enough this can even occur with outside air temperatures over 20°C. If the engine isn’t producing much heat (e.g. because it’s idling while on the ground waiting for permission to taxi, or when descending towards the runway) then ice can form causing a decrease in engine power; in it’s most severe form it can even cause the engine to stop.

To help remedy this, the exhaust heat exchanger can be used to heat the intake air before it reaches the carburettor. Sounds complicated, right? Well, all you have to do as a pilot is move the carburettor heat control to its HOT position!

When you do this, the engine will drop in power by about 100 RPM. You should leave it on HOT for about 30 seconds and then switch it back to COLD. If the engine returns to its previous power setting (e.g. you get the 100 RPM back) then no ice had formed. If, however, you get back more than the 100 RPM, then this indicates that ice was there before and was reducing the total amount of available engine power. The carburettor heat has melted the ice and you’re now back to full engine capacity.

If this has happened then you would need to regularly apply carburettor heat to ensure no build-up of ice, especially when the engine is idling, e.g. while descending.

I hope this post helps demystify another aspect of learning to fly.