Carburetor Ice

The NTSB report on this accident listed contributing factors as: "weather conditions conducive to carburetor icing, unsuitable terrain available on which to make a forced landing and the tree." We all laugh at the tree, but don't miss the first part of that report. This pilot was flying in conditions where serious carburetor icing was likely. The Lycoming Flyer (see link below) says, "Unfortunately, there are many pilots who are not fully aware of what carburetor ice can do or what to do about it when it does occur." There is nothing funny about carburetor ice or pilots' misunderstandings of how to use the carburetor heat. Not every pilot is as lucky as that Cherokee pilot, so let me give you the short version: if you pull on the carb heat in flight, LEAVE IT ON.

"The engine started running rough, so I pulled on the carb heat. It cleared up, so I pushed the carb heat back in."

Don't say that around mechanics unless you want to hear them scream, "NO! Leave it on!!!"

Leave it on? But on the ground, we do that and we get a decrease in performance...and we're taught not to leave it on for very long.

The carb heat diverts air flow from around the muffler shroud into the carburetor, bringing in warm, unfiltered air. Bypassing the air-filter means that you could potentially be sucking dust and other debris into the carburetor -- on the ground, where the dirt is. The key phrase there is: on the ground. In the air, you shouldn't be flying through clouds of dust, so the risk from debris is minimal. Initially, the carb heat can and probably will cause as much as a 15% drop in performance. Lean your mixture and let the carb heat do its job. Remember: carb ice can cause 100% decrease in engine preformance. "The throttle may be advanced and the mixture may be leaned to help get some of the lost power back, but immediately after the application of carburetor heat, the pilot must be patient and keep the airplane flying until the ice has completely melted and normal power returns" (Lycoming, 48).

Let's review what causes carb ice and what carb heat does in non-technical terms.

In your carburetor, air flows in through a venturi, which causes a low pressure area to form around the fuel nozzle, an opening about the same diameter as a pencil. This low pressure pulls the fuel in and feeds the engine. When you pull air through a venturi, there are actually 3 things that happen: you increase the velocity of the air, decrease the pressure, and lower the temperature of the air. Note that last one: you lower the temperature of the air. When fuel vaporizes, it also lowers the temperature. Put these two factors together and you get a significantly colder environment inside your carburetor than the outside air temperature -- in some cases up to 70 degrees lower. "But that means you could get ice in 90 degree weather!" Yes, you can. The average pilot will think about carb ice on a cold day, but not on a beautiful 70 degree spring day. In fact, the probability of carb ice is exactly opposite because icing won't occur if the temperature gets low enough. (There's a wonderful chart in the FAA link below.) That ice is now forming inside what is supposed to be a nicely shaped, smooth chamber so the air and fuel flow will be perfect and undisturbed. Ice disturbs the delicate balance, and the resulting sputtering sound from the engine disturbs the pilot's sense of well being.

Inside your carburetor is a butterfly valve (also known as the throttle plate). This valve is positioned after the venturi and is one of the main areas where carb ice will form. Warm air will melt the ice. This is why when you turn on your carb heat initially, the engine performance will improve fairly quickly. Great. The ice is gone, problem solved. So, why leave the heat on? You want to maximize your engine performance at this moment, right?

Here's the point that causes a bigger problem: what happened to the ice? Well, it melted. So, that butterfly valve is now sitting in...water. Turn the carb heat off right away, and you pump super-cooled air into that water. The water freezes and becomes an ice cube, an ice cube that is probably blocking your air flow as thoroughly as if that throttle plate were closed.

To fully understand the severity of this new type of ice, do the following experiment: you'll need a cup of loosely packed snow and a cup of frozen water in the shape of a block. Take a pot of boiling water and drop in the cup of loosely packed snow: poof, it melts. Now, take the block of ice and drop that into the pot of boiling water: it will take longer for the block of ice to melt. How much longer? Well, depending on your altitude, it could take longer than you have. "How long this will take depends on the severity of the icing, but the pilot should expect a delay of 30 seconds to several minutes. Under the circumstances, this period of time will be stressful and always seems longer than it really is, but the knowledgeable pilot will not retreat from use of carburetor heat. Carburetor heat should remain in the hot position until power returns" (Lycoming, 48).

Lycoming recommends using FULL carb heat whenever flying in conditions where icing can be expected. (Use the chart in the FAA article below to determine how likely icing is. Don't rely on how warm the outside air temperature is!) "An unknown amount of partial heat can actually cause induction ice in the float-type carburetor. This may occur when moisture in crystal form in the incoming air that would ordinarily pass through the induction system without any problem is melted by the partial heat. This moisture then freezes when it comes in contact with the cold metal of the throttle plate." So, you know you've just experienced carburetor ice and used carb heat to melt it off. At this point, you can be fairly sure that you are flying in prime conditions for carb ice to form.

But what if that sputtering isn't being caused by carb ice? There is another bonus to using your carb heat. Suppose something is blocking your air filter. Depending on how thorough you are about maintenance, maybe it has even come apart and has gone...somewhere unhelpful. By turning on the carb heat, you bring air into the system from an alternate route, bypassing the blockage and restoring the air flow. This can buy you the needed time to land safely on an airport.

Take some time to learn about the mechanical and scientific aspects of carburetor icing, then make sure every pilot you know does the same.

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