Another quick question: Carb heat

M15A Guest

What is carb heat and what should I do with it while flying vfr?
In none of the lessons in FS2004 I found out anything about it...


9 Responses

kfitz Guest

Carb heat on diverts warm air into the intake therefore preventing icing in the carby's when flying at low engine power. ie - when throttle back on approach, carb heat on. This does however reduce engine power (C172 by approx 100 rpm) due to the fact that warm air is less dense than cool air

For C-172, roughly revs at 2100 or less (below the green) carb heat on

M15A Guest

Thanks! Just a few more questions...

First of all: Whats a carby... 😕
And: How do I turn carb heat on in a C172?

M15A Guest

oh...and what is icing?

Thanks! 😎

M15A Guest


Pro Member First Officer
ARD-DC First Officer

I'll take a shot....although it won't be perfect ❗

Carby, shorthand for carburator;
It's the the engine component that mixes fuel with air. That mixture is then injected through nozzles into the cylinder chamber, where the spark plugs ignite the mixture.
At this moment of ignition, the piston is at or near it's highest position. The explosion of the fuel/air mixture pushes the piston downwards. The piston is connected in a specific way to an axle (crankshaft?), and that method converts the downward movement of the piston, into a rotation of the axle. And presto, that's how your propellor spins around.

* Excuse the technical imperfecties, I'm no expert at the topic, but the principle should be just about right 🙂

The principle of Ice forming inside the engine (again, probably a little off, but you'll get the jest of it Wink ) the watermolecules present in air condensate because of air temperature dropping, since cold air can contain less watermolecules then warm air.
When the watermolecules condensate and the ambient temperature is below zero, I would assume the now liquid watermolecules freeze, forming ice within the carburator.
I am guessing the carb heat is designed to keep the temperatures within the carburator above zero, so that the water won't freeze. Otherwise the ice that is built op over time would eventually obstruct the air intake of the carburator, causing the engine to stall.

How to turn on the carb heat ❓ ...I could swear there is a switch on the Cessna's instrument panel somewhere... Dont Know

M15A Guest

Thanks man! 😎 So it seems to me it is quite important during flight to keep the temperature right to prevent icing.
Maybe someone knows how to deal with it during flight in a cessna...

Maybe in FS you don't deal with it at all?

Pro Member First Officer
ARD-DC First Officer

I'm sure it is carefully considered in real life, but you need not worry about it in FS, this aspect of piston engines just isn't included in FS9.

Pro Member Chief Captain
CRJCapt Chief Captain

Most aircraft in FS9 don't have carburetors. The Cessna 172 model in FS9 does not have a carburetor, it's fuel injected. The historic aircraft do have carburetors. Normally the carb heat is turned on whenever you operate the engine at low power such as landing approach.

Pro Member First Officer
Tartanaviation First Officer

Carb Icing can actually form well above Zero Degrees Celsius. Up to a maximum OAT of +25 Degrees Celsius.

A carburettor venturi causes the air passing through it to increase in velocity which results in a pressure drop. Any pressure reduction will produce adiabatic cooling of the air that in turn through conduction will lower the temperature of the carburettor venturi. When air is cooled its relative humidity increases.

Fuel entering the venturi is vaporised. This will require the fuel in its liquid state to gain heat in order to evaporate.

The fuel takes latent heat from both the air passing through the venturi and the carburettor body setting up a cooling process that may lower the temperature of the carburettor body and the airflow within it by as much as 30 degrees C.

Conditions most favourable to carburettor icing are

Warm weather and high humidity as this will provide an abundant supply of water vapour. The greater the relative humidity the smaller the temperature drop required to reach the dew point temperature.

Low power settings such as idle in the descent when the throttle butterfly is only partially open will produce a large velocity increase and corresponding large pressure drop as the air accelerates to pass the restrictive space created between the partially open throttle valve and the carburettor wall. The consequence is a significant cooling effect which may cause moisture present to form as ice on the downstream side of the throttle valve.

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