Airframe and Powerplant: Count On It

Float carburetors--a most reliable accessory

June 1, 2008

A float carburetor is a simple, yet extremely effective and reliable device that automatically meters fuel at the proper ratios for existing flight conditions. The carburetor that metered fuel for the Wright brothers’ historic 1903 flight consisted of nothing more than a tube that dripped fuel from the small, quart-and-a-half tank attached to one wing strut into a shallow chamber next to the cylinders, where it mixed with incoming air. Heat from the combustion process vaporized the fuel/air mixture as it passed into the intake manifold and cylinders. Fuel flow was controlled by a small valve in the copper feed line. Charles Lindbergh depended on a carburetor to keep his Wright J-5 engine running smoothly during his historic nonstop flight across the Atlantic Ocean in 1927.

Carburetors have metered fuel far and wide, in airplanes flown by the famous and obscure, and there’s no reason this most dependable of engine accessories shouldn’t still be metering fuel when aviation celebrates its bicentennial. Until very recently, only Maule Aircraft and Aviat Aircraft have been selling new carburetor-equipped FAA-certified airplanes. But the carburetor seems to be making a comeback. Cessna’s C-162 SkyCatcher, and many of the airplanes being flown in the strong LSA market, are carburetor-equipped.

Carburetor strengths...

Carburetors are dependable. How often have you heard of a flight being canceled because the carburetor wasn’t metering fuel? The reason is that there are only a few moving parts on even the most complex carburetor. Even when wear does takes place, it’s so gradual that it rarely causes a flight cancellation.

With only basic maintenance, carburetors stoically deliver the correct fuel-to-air ratios for a long time. According to Precision Airmotive service bulletin MSA 3 Rev. 1, the recommended carburetor TBO (time between overhauls) is identical to the TBO of the engine it’s installed on, or 10 years, whichever occurs first.

In addition to being dependable and long-lasting, carburetors are the least expensive of all accessories to overhaul. Overhauled carburetors are available from reputable parts supply houses. They range in price from $560 to $710.

For decades the Marvel-Schebler Products Division of Borg Warner supplied the majority of the carburetors used on light airplanes. Facet purchased the brand in 1982 and maintained production until 1990, when Precision Airmotive of Everett, Washington, purchased the line. Within weeks, Precision announced a shipping suspension because of an inability to purchase product liability insurance. In early November 2007, Aero Accessories/Tempest announced that it had reached a tentative agreement to buy Precision’s line of MSA carburetors and parts. According to Tempest, the MSA line will be moved to the company’s headquarters and manufacturing plant in North Carolina. The carburetor line will be marketed under the Tempest name and will complement the company’s other products, which include oil filters, magneto parts, dry vacuum pumps, fuel pumps, and other accessory-style products. In addition to Tempest, two other companies, Consolidated Fuel Systems (a division of Kelly Aerospace) of Montgomery, Alabama, and AvStar of Jupiter, Florida, market FAA-PMA approved parts and carburetor-overhaul parts kits.

...And weaknesses

In operation, carburetors meter fuel based on the volume of airflow through the throat. The rule from physics that makes float-type carburetors work is the same one that explains how a wing generates lift. Bernoulli’s principle states that as the velocity of a gas—in this case the gas is inlet air—increases the pressure decreases. An insert, called a venturi, is installed in the carburetor throat to reduce the cross-sectional area of the carburetor throat.

This reduction causes the inlet air that’s moving through the carburetor throat to accelerate, which in turn causes the air pressure in the venturi area of the throat to decrease. This decrease in pressure enables atmospheric pressure bearing on the surface of the fuel supply in the carburetor float bowl to push the fuel out the main fuel discharge nozzle. The main fuel discharge nozzle is located in the lowest pressure area of the throat.

So far, all is simple and dependable. Unfortunately carburetors are prone to icing. After inlet air passes through the venturi the air temperature drops rapidly because of expansion. This cooling causes water vapor in the inlet air to freeze. Carburetor icing may occur anytime OATs are between 32 degrees F and 100 degrees F with a relative humidity of 50 percent or greater. Be especially wary for signs of carb ice when the temperature-dew point spread is 20 degrees F or less and the relative humidity is 50 percent or greater.

Research in the AOPA Air Safety Foundation accident database shows 63 accidents attributable to carburetor ice during the five-year period between March 2002 and March 2007. These carburetor-icing accidents took place during every month of the year, as far north as Alaska and as far south as Puerto Rico.

There are three categories of carburetor ice. Impact ice is very similar to airframe icing and can only occur when flying in clouds or precipitation at outside air temperatures (OATs) of 32 degrees Fahrenheit or colder. As impact ice builds it blocks the air inlet. Unlike impact icing, flying in clear air does not prevent the other two categories of carburetor icing. Fuel ice, also called fuel evaporation ice or fuel vaporization ice, forms downstream from the main fuel discharge nozzle. Throttle ice, also called expansion ice, builds up at or near the throttle butterfly valve of the carburetor. It is likely to occur in conjunction with fuel icing.

Carburetor heat hints

The first indication of carburetor ice in an airplane with a fixed-pitch propeller is a loss of rpm. The indication in an airplane with a constant-speed propeller is a loss of manifold pressure. If carburetor ice is suspected, apply full carburetor heat. Melting ice will cause a momentary engine roughness because the engine has just ingested a big slug of water and because application of full carburetor heat will richen the mixture.

Carburetor icing is most likely to take place during reduced power operations such as pattern work or descent from cruise altitude in preparation for landing. Get in the habit of “clearing” the engine during these periods by advancing the power for short periods. When atmospheric conditions are conducive to the formation of fuel and throttle icing, test the carburetor heat system during pretakeoff checks by making sure a marked power drop-off takes place when full carburetor heat is applied. Pulling the carburetor heat control moves a flap in the carburetor air box that closes off the cold filtered inlet air to admit heated unfiltered air.

Federal Aviation Regulation 23.1093 spells out the requirements for carburetor heating systems. This regulation says that, depending on whether the engine is a sea-level engine (normally aspirated), or an altitude engine (turbocharged or turbonormalized) the carburetor heat system must be capable of creating a temperature rise of 90 or 120 degrees F, respectively, when the inlet air is 30 degrees F and clear of moisture.

When full carburetor heat is applied, the engine will lose power and may run rough. Here’s why: Hot air is less dense than cold air. Therefore, when carburetor heat is applied there are fewer air molecules flowing into the carburetor throat. At the same time the fuel flow remains the same, so the mixture gets noticeably richer to the point that the engine may run rough. Hence the pilot should be prepared to lean the mixture to restore the proper ratio and return the engine to smooth operation.

Remember a carburetor measures the volume of air, not the density of the airflow. One reason many of today’s new airplanes are fuel-injected is because these systems eliminate the possibility of fuel and throttle icing because fuel-injection systems (and pressure carburetors) add the needed fuel downstream of the venturi.

Heat up the mixture—it’ll run better

Float carburetors must vaporize the raw fuel from the fuel tanks to produce a fuel-air mixture that the engine can use. Because of factors such as low induction air temperatures, rapid cooling of the induction air as it expands after passing through the venturi, and rudimentary induction systems often found on carburetor-equipped engines, the mixture strength and quantity of the fuel-air mixture varies widely between the individual cylinders of carbureted engines.

Savvy pilots will no doubt wonder if the partial application of carburetor heat will prevent carburetor throat ice. The answer is yes, and that’s not all it will do. In many cases the judicious application of partial carburetor heat will aid the vaporization of the fuel/air charge, making the engine more efficient.

An excerpt from Textron Lycoming’s service instruction 1148C, “Use of Carburetor Heat Control,” dated October 12, 2007, says, “In those aircraft equipped with a carburetor air temperature gauge, partial heat may be used to keep the mixture temperature above the freezing point of water (32 degrees F).”

Many owners who have experimented with the partial application of carburetor heat during cruise flight report seeing a decrease in the spread between the highest and lower cylinder exhaust gas temperatures and have concluded that the application of heat improves fuel vaporization, creating a more even fuel distribution between the individual cylinders of their engine. The no-free-lunch rules applies here, since the application of partial carburetor heat reduces the engine power output. It’s also important to remember that carburetor heat air is not filtered, so this practice should only be used during cruise conditions in clear air.

Both carburetor temperature gauge systems and Iceman ice detection system are approved for installation in most airplanes under supplemental type certificates (STCs) and are helpful in both detection and prevention of carburetor ice and during leaning.

Leaning that carburetor

Each pilot has the key to reducing the costs of flying and it’s sticking out of the instrument panel. It’s that red mixture knob. That knob connects directly to the mixture metering valve on the carburetor. That knob controls every drop of fuel that’s metered by the carburetor. Contrary to the “never lean below 5,000 feet” suggestion in some manuals, there’s absolutely nothing wrong with leaning at any altitude at reduced power settings. Both Lycoming (Service Instruction 1094D) and Continental (pilot operating handbooks) have published generic engine leaning parameters for their engines. Lycoming allows leaning to a peak EGT value whenever the power setting is 75 percent or less.

Continental allows leaning to a peak EGT value any time the engine power setting is at 65 percent or less. Precise leaning requires instrumentation that displays exhaust gas temperature (EGT) values for every cylinder simultaneously. To lean effectively, this advanced instrumentation will be required on all but the newest aircraft, which are equipped with monitors from the factory.

Advanced engine instrumentation that displays EGTs, cylinder head temperatures (CHT), oil temperatures and other engine data are called engine monitors and are available from more than six manufacturers. Instrumentation that simultaneously displays the EGTs of all cylinders permits the pilot to determine the instant when the first cylinder reaches peak EGT—which is signaled by a drop off or decrease of its EGT value as the mixture is continuously leaned. Many modern monitors are equipped to flash or otherwise signal the pilot when the first cylinder reaches peak EGT. At this point the engine is considered to be leaned to peak EGT.

Don’t get confused about this. The first cylinder to reach peak EGT as the mixture is slowly leaned has nothing to do with how hot the EGT is when compared to the other cylinders—the actual temperature is irrelevant.

If an engine monitor is not installed, optimum leaning at high power settings isn’t possible. Simply reduce the cruise power to 65 percent and slowly pull the mixture control to lean the engine until engine roughness is felt. Then richen the mixture until the engine again runs smoothly. There’s one more time when leaning is important and that’s before a takeoff from a runway when the combination of the airport altitude above sea level and outside temperature result in a density altitude of 5,000 feet msl or higher.

In these cases Lycoming recommends running the engine to full power on the ground and leaning to obtain maximum rpm for fixed-pitch propeller/engine combinations, and for smoothest operation for constant-speed propeller/engine combinations. High-power ground running should be kept to a minimum. Turbocharged and turbo-normalized engines are not leaned for high-altitude takeoffs.

A 1970 study, “Aircraft Carburetor Icing Studies,” by L. Gardner and G. Moon—now available from the Society of Automotive Engineers International —concluded that the incidence of carburetor throat icing can be prevented by applying a Teflon coating to the throttle plate to prevent ice adhesion, and by adding ethylene glycol monomethyl ether (EGMME) to the fuel at a rate of .10 to .15 percent by volume. This additive depresses the freezing temperature of water.

Unfortunately Teflon-coated throttle plates have never been introduced by any carburetor manufacturer, but diethylene glycol monomethyl ether (DEMME), a fuel additive similar to EGMME, is available as PRIST at some pilot supply stores.

E-mail the author at steve.ells@aopa.org.