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Flight Safety - Carb Heat Why?

By Jack Dueck, Editor

carb heat
Carb heat installed on a Rotax 912; Ref. Light Sport Aircraft Store

An American friend, building a Sonex amateur-built aircraft, is at the building stage where his efforts are directed at the firewall-forward, fitting an AeroVee (Volkswagen) engine. I recently asked him if he was providing carb heat for his aircraft. His reply: “No. It’s not a requirement in the U.S., and besides, it’s not necessary!”

Many American amateur-built aircraft with carbureted engines are imported into Canada each year, and surprisingly, many do not have provision for carb heat installed.

I see two important reasons to include a carb-heat installation on your aircraft:

The first is simply that it’s a regulatory requirement. Our Exemption from Section 549.01 of the Canadian Aviation Regulations and Chapter 549 of the Airworthiness Manual – Airworthiness Standards-Amateur-built Aircraft states in Part III, Design Standards, Para. 25: Engines equipped with carburetors shall have means to minimize the likelihood of carburetor icing, unless this can be shown to be unnecessary, either by actual test or by documentation from the kit manufacturer, engine manufacturer or aircraft designer.

The second is simply that of risk mitigation. Carburetor ice can occur in one of two manners. The first is the simple accumulation of heavy snow into the air intake of the carburetor that chokes off air supply entering the carburetor, known as impact or atmospheric icing. The second is the accumulation of ice on the throttle plate and in the venturi of the carburetor, and is due in part to the fuel vaporization process. Let’s look at this second and more insidious form of carb ice.

The combination of Boyle’s and Charles’ gas laws states that PV/T is a constant. In any given condition, a finite amount of air passes through the carburetor. As this constant volume of air increases in velocity passing through the venturi of the carburetor, it’s accompanied by a reduction in pressure which sucks the gasoline into the airstream. So, back to our law of gasses—if V (volume) is kept constant, and P (pressure) is reduced, for the law to hold, the T (temperature) must also be reduced in accordance with the reduction in pressure. Without going into absolute values and the increased specific gravity of fuel mixed with air, the result is a reduction of temperature of the air-fuel mixture through the throttle body of the carburetor. This temperature drop can be as great as 70° F. If moisture is present, this can easily cause ice. And ice is not nice!

Carburetor icing is insidious and can occur at ambient outside air temperatures from below freezing to upward as high as 70° F. It can creep up unexpected, and when it does and isn’t checked, it can and will ruin your whole day. I have never had an aircraft engine fail due to carb ice. I have had engines quit in automobiles from carb ice. I have at times while flying, suspected its presence, and after applying carb heat, watched as the engine’s rpm increased back to normal with the assuring increase in the smoothness of the engine’s operation confirming carb ice as the culprit. The problem is that by the time carb ice builds up and the engine begins to choke off, the application of carb heat may no longer be able to melt the accumulation of ice already present.

When I first learned to fly decades ago, my instructor established a rule that remains with me today. Whenever you’re going to pull back on the throttle, first the carb heat is applied, no matter what the day’s climatic condition might be. Carb heat is also left on during the glide or descent. And during the descent, at regular intervals, the engine is cleared (throttle advanced for a few seconds) to ensure no icing is present.

Certain engines are more likely to pick up carb ice. These include the Continental small block series. I have found a lesser tendency to find carb ice with the Lycoming engines that run the induction manifold through the oil sump, thereby picking up some residual heat. But beware—all carbureted engines are susceptible to carb ice.

Back to risk mitigation. One U.S. import that I encountered was a Cassutt powered by a modified Continental C-85. The owner had purchased this aircraft to race in the Reno air races, and he was disinclined to add anything to the basic weight of the aircraft. He researched the problem and found an excerpt from the National Research Council of Canada, dated July 1970, that stated: “The use of a teflon-coated throttle plate to prevent ice adhesion was studied and found to virtually eliminate any ice formation on plate.” He submitted this to Transport Canada and received permission to apply this alternative.

But this above situation is exceptional. Most aircraft have an exhaust system that allows a heat muff to provide sufficient heat for the carburetor. Some liquid-cooled engines provide a heat exchanger at the inlet to the carburetor that allows the hot coolant to add heat to the incoming airstream.

My friend doesn’t expect to experience an off-field landing in his Sonex due to carb ice. I hope he’s right. If I were to buy his aircraft, I would immediately install a reliable carb-heat system. It is simple risk avoidance that I would deem necessary.

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