View Full Version : Carburettor heat and Alternate air

4th Mar 2009, 13:10
Say you have a 2 cherokees an Archer 180hp with carburettor heat and the other an Arrow same engine but fuel injected therefore 200hp it has Alternate air. Why the difference, can somebody explain why one is called carby heat and the other alternate air.?

4th Mar 2009, 15:42
Because one heats the air going to the carb and the other doesn't?

4th Mar 2009, 15:56
A fuel injected engine is not that likely to suffer from carburator icing (as there is none installed), so it does not need a carburator heating.

On the other hand, it might very well get the air filter clogged up - be it via a bird strike, icing or whatever else might sneak up the air path. So past the air filter, there will be some commonly spring-loaded to closed position tabs. They should be opened by suction, if the filter lets too little air pass, or they can be actuated manually. In the F33A I flew some years ago, this was done during pre-flight to ensure the doors were still movable.

So carb heat and alternate air are two fundamentally different things.


4th Mar 2009, 16:37
In practice however, they work in the same way, at least on the 172s I've flown (both with CH and AA). The lever basically controls an airbox which enables you to select filtered outside air or unfiltered air coming from inside the engine compartement, thus being heated.

4th Mar 2009, 16:45
Carb Heat is called Alternate Air.

Pugilistic Animus
4th Mar 2009, 17:09
Don't forget induction icing is still possible on fuel injected engines, although it may have automatic heating provisions----Guppy can explain more of course

I thought at first you meant an alternate static source,... I see you mean alternate induction air

if both Static sources are blocked [in the PS system] or there's only one,..you could break the crystal on the VSI,...that's why I recommend partial panel practice without the VSI


Old Fella
5th Mar 2009, 11:48
Inlet icing and Carby icing are distinctly different. In let icing is when the air inlet before the carby suffers ice build-up. Carby icing is when ice forms in the carby venturi and throttle butterfly area and is what carby heat clears. As a general rule, if you need carby heat go the whole hog, i.e. full heat until the ice clears.

5th Mar 2009, 14:02
Yeh, as Old Fella says, the key to carb heat is the venturi in the carb. Ice in the venturi disrupts the airflow and the fuel is no longer sucked in as efficiently, just as ice disturbs airflow on the wings. Same thing, only different..

Carb heat, in all the lightplanes I've seen, takes heated air from a muff around the exhaust pipes. If the engine dies from carb ice, you're no longer generating heat in the exhaust, so your prospects for a re-start are reduced - a lot.

Injected engines, have no venturi in the intake, of course, just a butterfly throttle, so icing isn't a problem, except when the intake filter gets clogged with it. Alternate air is just that.


5th Mar 2009, 15:05
However as the engine stops, so does the temp drop in the venturi, and chances are that the ice might melt because of residual heat inside the engine compartement.

Pugilistic Animus
5th Mar 2009, 16:29
....And the adiabatic expansion of the fuel could cause icing well above 'typical icing' temperatures if there's also water vapor in the line' even with FI , despite lack of a 'venturi'


5th Mar 2009, 23:36
However as the engine stops, so does the temp drop in the venturi, and chances are that the ice might melt because of residual heat inside the engine compartement.

Hmm, somewhat sceptical!

1) The fire may have gone out, but it's still windmilling, so pulling air through. If it's stopped stopped, you probably have bigger problems
2) In a single, you've probably hit the floor by the time the ice 'might' have melted.

6th Mar 2009, 08:41
True, true. I've never experienced a complete failure due carb icing so I wouldn't know. Just thinking out loud :)