Carburetor Heat

Many light aircraft are equipped with a control which is mysterious to the generation of pilots who grew up after the CD was invented – the carburetor heat control.

These pilots probably never saw or operated a carburetor until they arrived at the airport for flight training.  Their automobiles and even motorcycles are electronically fuel injected.

For these pilots, a little background is in order.  A carburetor is a mechanical device designed to evaporate liquid gasoline into a combustible mixture which is sucked into the engine and burned to produce power.  This is different than fuel injection, where the fuel is pumped into the intake port just before it is sucked into the cylinder to be burned.

Some aircraft – especially newer ones – are equipped with a simple mechanical fuel injection, which is really nice, but much more expensive and complicated than a carburetor.

Fuel injected engines will not have a “Carb Heat” control knob.  They might have an “Alternate Air” control – or they might not.  Some aircraft simply use a spring to automatically open the alternate air when the air intake of a fuel injected engine becomes blocked (e.g. during severe icing conditions).

But if your aircraft has a “Carb Heat” control, it’s got a carburetor, and there are a few really important things you need to learn about it, to avoid losing engine power.

In theory, a carburetor is pretty simple.  It uses something called a venturi, which creates suction and pulls liquid gasoline from a float bowl inside the carburetor, just like you suck on a straw to drink a soda.  The fuel is evaporated and mixes with the air rushing through the carburetor into a combustible mixture for the engine to burn.

In practice, a carburetor is a little trickier than that – it has all sorts of gimmicks and tricks, like an accelerator pump, to deal with transient conditions.

One very practical fact about a carburetor is that it is a little refrigerator.  If you put your hand on a carburetor of a running engine – or right after it stops – you will notice that the carburetor is cold.  The reason for that is because of the venturi decreasing the pressure of the air, and the energy taken to evaporate the fuel.

This is actually pretty easy to understand.  Have you ever pushed a tire valve in, and let the air out of a tire?  The valve will get cold, because the pressurized air in the tire that was at room temperature is now spread out into a larger volume, but with the same amount of energy.  That results in cooling.  That’s exactly what a venturi does, with its pressure drop.  It’s called adiabatic expansion, if you are interested in such arcane details.

Also, it takes energy to evaporate the liquid fuel, just like you have to heat up the element on your kitchen stove to heat up and boil water in a pot or kettle.  That’s an endothermic reaction, in physical chemistry.

Depending upon the conditions, you can actually see condensation on the outside of the carburetor, if it’s cooled below the dewpoint of the air mass.  Heck, you can even see ice on the outside of a carburetor.  And you can even get ice forming inside the carburetor, which is really bad.

Remember that an internal combustion engine is really just an air pump.  And it’s got to be able to get air into and out of the engine.  Any kind of plugging of the intake or exhaust of an engine is really bad news.

That’s why it is a legal requirement for any certified aircraft with a carburetor, to be equipped with a carburetor heat system, to melt the ice which can plug it up.  You would be insane not to have a carb heat system on a homebuilt.  Incredibly, many ultralight aircraft do not have carb heat.  They scare me, for many reasons.

The carb heat system is actually very simple.  It consists of a piece of bent metal around the very hot exhaust tubes.  Air flows around the hot exhaust tubes and is heated up.  Just before the carburetor, there is a flapper box which allows you to select either normal cold filtered air, or to select the heated air.

When you pull the carb heat knob out, a cable moves the flapper and hot air goes into the carburetor to melt any ice that may have formed.

The problem is that it can be hard to tell how much ice is forming, and how fast.  Some aircraft are equipped with a carb heat temperature gauge which will allow you to intelligently select partial carb heat, but they are rare in my experience.

Without a carb heat temp gauge, it is best to use the carb heat knob as a binary control – either the carb heat is fully off (knob in, cold air) or the carb heat is fully on (knob out, hot air).  Generally I like to use full carb heat whenever the RPM is below the green arc.  The POH of course is golden – do what it says.

Different engines and different installations of the same engine will be more or less susceptible to carburetor icing.  The Continental engines (e.g. O-200 or O-470) with the carburetor hung out in space below the engine will make good power, but tend to ice up pretty badly.  The Lycoming engines (e.g. O-235, O-320, O-360) bolt the carburetor right to the oil sump, and run the intake tubes through it, so as soon as the oil is up to temperature, they are pretty resistant to carburetor icing, but it can still happen.  You have to watch Lycomings on the first takeoff of the day, when the oil is still cold.

What I do, when the conditions are ripe for carburetor icing (cool, low dewpoint spread) is after taxiing out for takeoff – when the carburetor may have filled up with ice – is to position for takeoff on the runway and select the brakes and push the throttle in for 1500 RPM and select carb heat and lean the mixture for max RPM.  I count to ten, then all the knobs go into the panel and I take off.  This ensures that I don’t have any carb ice, at least at the start of the takeoff roll.

Let’s look at what I did above.  Something I did which might surprise you, but is very important, is that I leaned the mixture after selecting carburetor heat.  No one does this, but it is very important because when you selected the carb heat, you enrichened the mixture with the hot, less dense air and the same amount of gasoline.  A richer mixture will cool the engine, and that’s not what we want right now.  We want the exhaust to be hot, not cold, to produce maximum carb heat.

So the lesson for you today is that every time you select the carb heat, you must also lean the mixture for max RPM, in order to produce hot air.  This may be counter-intuitive in a descent, when you are used to enrichening the mixture.  And in the circuit, you probably are really uncomfortable with anything less than full rich mixture.  And that’s ok, but just be aware that you are not producing as much hot air as you could be.

Most of the time that doesn’t matter, but if you ever have a real carb icing encounter (which is indicated by a loss of RPM with a fixed pitch prop, or a loss of manifold pressure with a constant speed prop) please do remember to lean the mixture for max RPM with a fixed pitch prop, or max airspeed with a constant speed prop.


--      Jan 2012