# Why is full carb heat recommended when reducing power below normal setting?

In my first flight lesson today, my instructor asked me to apply full carburetor heat when we had to reduce power to 1900 RPM in our Cessna 152.

I was surprised by this as I didn't remember learning this in ground school, and I later checked the Jeppesen textbook I am using, and referring to the section on carburetor icing, I saw that when power is reduced below normal operating limits, full carb heat is recommended (must have missed it when studying the ground material).

My understanding is that the cause of carb icing is the sudden drop in temperature in the venturi due to drop in pressure as the fuel speeds up. If I am reducing power, I am reducing fuel flow, so shouldn't this reduce the chances of icing? So my question is why is full carb heat recommended when decreasing power below normal operation?

• relaxing the throttle might result in a reduction in engine-generated ambient heat as well. – Erich Apr 29 '15 at 5:42
• You might want to read the answers and comments to this question which is related. – Simon Apr 29 '15 at 6:23
• Icing comes from the moisture in the air being drawn into the carb, not from the fuel. – GdD Apr 29 '15 at 8:10
• Not enough time for a full answer but: fuel is a liquid with a much higher specific heat capacity, thus it has "more energy" per temperature then air. When you have more fuel flowing into your engine, the fuel heats up the air, reducing carb icing. Less fuel doesn't heat up the air as much, thus you need carb heat – Maverick283 Apr 29 '15 at 9:02
• As power is reduced, so is the manifold pressure, therefore there is a greater pressure drop across the butterfly (more suck) which results in a temperature drop and lowering the dew point. As temperature drops, the moisture in the air will condense out. – Simon Apr 29 '15 at 13:06

At full throttle, manifold pressure is high, close to ambient pressure at that altitude. At partial throttle or at idle, manifold pressure is low. It is the transition from higher pressure (ambient) to low pressure (in the induction manifold) that causes cooling. Think of why ambient temperature is cooler at 10,000 feet MSL than it is at 1,000 MSL. So moving the throttle to a lower power position will cool the carburetor.

When you close the throttle for descent, the airflow is constricted. This constriction is going to cool the air more than it is during cruise configuration, and may increase icing.

From This AOPA document:

• Throttle ice is formed at or near a partly closed throttle valve. The water vapor in the induction air condenses and freezes due to the venturi effect cooling as the air passes the. throttle valve.

At lower power settings in particular, the butterfly valve of the throttle has a smaller opening, which effectively act as venturi, making the carby more conducive to icing.

The other answers are good, but I'll contribute the physical reason.

When a gas is expanding due to pressure differences, the temperature of the gas will drop. This is stated in the Ideal Gas Law. While air is not an ideal gas, it's close enough that we can use the law to calculate the temperature drop - within reasonable limits.

PV=nRT, where P is pressure, V is volume, n is number of moles of gas, R is the ideal gas constant (8.314 J⋅mol−1⋅K) and T is temperature in Kelvin.

When you close the throttle, pressure in the manifold will drop, and the air entering from ambient pressure to manifold will have to expand. This cools it down - and as it cools it can hold less moisture as well, so you risk water vapor condensing out of the air, and potentially very cold air.

I learned that applying carb-heat when throttling down has nothing to do with Carb-Icing, but instead is to prevent shock-cooling.

When the engine is running at full power, it is creating a lot of heat. When you throttle down rapidly, suddenly much less heat is being generated, but the engine is still being rapidly cooled. This rapid cooling can create stress as the metal parts of the engine contract at different rates.

Applying Carb-heat directs available heat into the carburetor, and by association, the overall engine, which slows down the cooling of the engine and reduce the stress from rapid cooling.

• Carb heat may or may not help with shock-cooling, but even if does, 1.) I suspect there are limits to how much it can help and 2.) shock cooling is not generally a huge concern (though a good pilot will always be mindful of it and do everything practical to reduce even that slim possibility) in aircraft with smaller and non-high performance engines. Effectively preventing shock cooling requires closing the cowl flaps (if installed and currently open) and ensuring that throttle reductions are both smooth and gentle (don't want to say slow lest someone assume 1" of M.P. reduction per hour :P ) – habu Apr 29 '15 at 15:04
• @abelensky: You need to ask your instructor who told you that carb heat is used to prevent shock cooling for a reference to any legitimate source that backs up this statement. Please post it here. I strongly don't believe that carb heat is useful in reducing shock cooling, but hey, we are all always learning, right? – Skip Miller Apr 29 '15 at 15:25

The final authority on the use of carb heat is the POH, unless the POH itself leaves it to the pilot to make a judgement call, in which case you have to reference your knowledge of carb icing formation, the specifics of the aircraft's system, your intended operating parameters, and current atmospheric conditions.

Your instructor asking you to use carb heat when reducing power is likely both in line with the POH (can't remember for certain, it's been years since I've seen a -152 POH) as well as general prudence when operating carbureted aircraft, especially something with an engine that small.

• Very true, although I think that "pilots discretion" is unlikely and potentially dangerous. I was taught POH and keep the carb gauge in the green which was above (from memory) 15c and below -2c and at all times in the the final descent or sustained low power. The point being that adding carb heat can cause icing if not used properly. – Simon Apr 29 '15 at 18:02
• Simon, two points: First, very few Cessna 152s have a carburetor temperature gauge. It was optional equipment coming out of the factory. Second, adding heat can cause icing? Well, no but there is a dangerous truth to what you say. Using partial carb heat can melt ice, with the water flowing further into the induction system where it can refreeze. From this point full carb heat will not melt it. So use full carb heat if you suspect ice. If you do have a carb temp gauge, then and only then is it safe to use partial carb heat. – Skip Miller Apr 29 '15 at 19:31
• @Simon There are quite a few POH's out there that will have something to the effect of "only use carb heat if icing is suspected to be a concern". This was, to give but one example, the case with the Lycoming-powered Husky I've flown for both work and pleasure around Florida for several years. You are correct that "pilot's discretion" can be dangerous when used improperly, but, with proper knowledge and understanding of the factors involved (and a healthy dose of operational conservatism), it often makes the difference between efficient execution and simple resource and performance waste. – habu Apr 29 '15 at 19:52
• @SkipMiller Adding heat in very low temperatures can indeed induce icing. Start here pprune.org/archive/index.php/t-218441.html – Simon Apr 30 '15 at 8:19