-Harold Holmes-

CARBURETOR ICING

More Than Half Of The Failures Are Caused By The Pilot

Engine failure accidents caused by temperature and pressure are de- During my 43 years of flying in all icing are not a frequent oc- creased (see diagram). Conversely, sorts of airplanes, I have experienced currence, however, the threat of icing is after the air exits the venturi area it carburetor icing several times. How- always present. In fact, in all of the ac- slows down and the pressure returns to ever, only once did I encounter engine cidents involving single-engine its former value. At this point, in our failure. My personal experience with en- airplanes, a single-engine failure acci- "Cockpit Classroom" style discussion, I gine failure, because of icing, happened dent occurs about every 20,000 flight asked Jim what happens during the several years ago piloting a 1958 hours according to NTSB statistics. vaporization process. Cessna 172 with a Continental engine. More than half of the failures are Jim's answer was as follows: "The It occurred on a night flight while flying caused by the pilot. The factors ac- vaporization process lowers the tem- from Chicago to Lawrence, KS to visit counting for the pilot-induced failures, perature just as a CO2 fire extinguiser my son Bruce who was then attending in order of significance are IN- would do." He pointed out another the University of Kansas. The weather ADEQUATE PREFLIGHT PREPARA- example of using an aerosol deodorant was VFR, and it was a moonlit night TION AND/OR PLANNING, FUEL MIS- and how it would feel when sprayed on with excellent visibility. I was flying at MANAGEMENT, INCORRECT OPER- the skin. It causes a cooling effect due 8500 feet with Kirksville, MO airport a ATION OF ENGINE AND/OR CON- to the liquid vaporizing. This same type few miles directly ahead of me when I TROLS AND FUEL EXHAUSTION. of temperature drop occurs when the had my first indication of icing, which Carburetor icing falls in the category of fuel is vaporized. The net result of car- was a noticeable detonation in the en- faulty operation and/or knowledge of buretor icing is a loss of power or even gine. I probably should have noted a the proper use of the careburetor heat engine stoppage while in flight. In other small power loss prior to the detonation. control and the entire system. words, the vaporization of fuel com- My first reaction was to pull out the carb bined with the venturi action of air as it heat control. This action on my part Recently I had an interesting discus- passes through the carburetor causes caused the engine to quit altogether. sion about carburetor icing with Jim this sudden cooling of the fuel/air mix- Then I experimented by trying various Vitte, an A&P mechanic for United Air- ture. settings. I found that lines who has an excellent working re- Next, I asked if carburetor can with carb heat set at about mid-range lationship with both corporate and gen- form at temperatures above 80 degrees the engine would run in spurts and, after eral aviation. In the past, I have dis- Fahrenheit. Jim answered, "yes", pro- a short time, it quit completely because cussed carburetor icing problems with viding the temperature and of the rapid cooling. At this point, I was a number of A&Ps and other experts in conditions are just right and the cooling almost directly over the Kirksville Airport general aviation, however, I felt that in the venturi reaches the freezing level. when I began a spiraling approach what Jim Vitte presented to me about He pointed out that the temperature down to pattern altitude which resulted icing from a mechanic's viewpoint was range when carburetor ice normally oc- in an uneventful landing. The next extremely educational and beneficial. curs is between 20 and 70 degrees F. morning I had an A&P check out the Jim pointed out that carburetor ice can (O.A.T.). As the rapid cooling takes problem. He determined it was car- be a real problem if the pilot is not aware place, the moisture in the air (providing buretor ice along with some magneto of and alert to methods of detecting there is a sufficient amount) will freeze icing. I had not heard of mag ice before (preventing) and removing ice when it causing ice to form on the plate this incident. I asked Jim Vitte if there occurs. Furthermore, he theorizes that and venturi (refer to diagram). There- is such a thing as magneto icing. He icing is caused primarily by two events: fore, it is reasonable that the more said that it is possible, but a rare occurr- venturi action and vaporization process. moisture present in the air, the greater ence. The following paragraphs include Venturi action occurs when the air is the likelihood of carburetor ice. The some important points which surfaced accelerated through a restricted area of most common signs of moisture are during our discussion. a tube. As the airflow is increased by high humidity or visible moisture such The first indication of carburetor ice the restriction in the tube, the venturi as clouds or rain. is a drop in rpm, if the aircraft is equip-

SPORT AVIATION 31 effect as carburetor ice, e.g. loss of power. The impact ice is not just com- mon to fuel injected engine aircraft. Your carbureted engine aircraft will have the THROTTLE PLATE same problem. The only way to regain your engine power is to provide an alter- nate route for air to enter your engine's induction system. By applying car- buretor heat you select an alternate route, or in the case of the fuel injected engine, there is either a negative pres- sure door incorporated into the induc- VENTURI tion system or a control in the cockpit to select alternate intake air. In either aircraft, there is no "on board" system FUEL NOZZLE to remove impact ice. 5?. Since we are entering the winter fly- ing months, let's discuss the use of car- buretor heat to improve engine opera- tion. I would like to use as an example a Cessna 182 equipped with a Conti- nental 0-470 engine. t The design of the intake manifold that delivers fuel and air from the carburetor to the cylinders runs totally exposed be- neath the engine. During the winter, ped with a fixed pitch prop. In the case temperture being 45 degrees F. with the when the O.A.T. is below 20 degrees of a constant speed prop, you would presence of moisture. F., there is no heat retension within the see a loss of manifold pressure. Generally speaking, at power setting intake manifold to aid in vaporization. Carburetor heat is used to prevent above 75% rated power the risk of car- The inside wall of the manifold stays and remove ice. When carburetor heat buretor ice is quite low. In actual tests well below freezing and causes the fuel, is turned on, the air delivered to the car- carried out in basic flight training which is partly vaporized and partly buretor is from an alternate air intake airplanes, the condition causing the atomized, to condense out and collect located within the engine compartment most serious carburetor icing was a par- on the walls. If this condition is allowed and passes the air over some part of tially closed throttle during descent at to happen, your engine will receive a the exhaust system to warm it. 30% relative humidity and maximum leaner than normal fuel air ratio and The warmed air is then ducted to the temperature of 34 degrees C. The re- cause a rough running engine. Also, carburetor via the heat box. As warm sults of this study show that as power there will be an imbalance between the air enters the carburetor it will elevate is increased, the risk of carburetor icing cylinders with respect to their proximity the throat temperature above the freez- is decreased but never disappears to the carburetor. (The further away, the ing level to prevent ice from forming or completely. leaner the mixture.) melt any ice that might be present. For example, a relative humidity of With the use of carburetor heat, we The warmed air is less dense so it 90% and temperature of 16 degrees C can create a summer like condition will have a direct effect on your engine's may produce ice at full power. As power within the induction system and keep fuel/air ratio. You will see a marked de- is reduced, the risk becomes greater the temperature of the intake manifold crease in rpm or manifold pressure due due to the temperature of the engine within an acceptable range. In doing so, to the creation of a richer mixture. and its components is lowered, and we have prevented the fuel from con- If there is ice present at the time of partly because the throttle (power) is densing on the manifold walls, and each carburetor heat application, you will see progressively reduced. The main cylinder will get its proper amount of fuel a reduction in power caused by the danger period is during descent when and air. richer mixture, and then as the ice dissi- the ice-producing effect of fuel vapori- I sincerely appreciate the assistance pates, an increase in power will occur. zation is combined with the ice-produc- of Jim Vitte for his input into this article (Note: In my Kirksville incident, carb ing effect of a partially closed throttle. on carburetor icing. Jim is both a pilot heat was applied too late for the ice to During this period, the engine cools and A&P. He owns a Cessna 182. After be removed.) Make sure, when using rapidly creating a serious risk of car- discussing the icing problem with a the carburetor heat, that you leave it on buretor icing. number of different mechanics over a long enough to remove all of the ice or Now, regarding aircraft equipped with period of years, I feel that what Jim has you could aggravate the situation by al- fuel injected engine, there is no car- relayed to all of us through this article lowing the venturi ice to melt and reform buretor and, therefore, no venturi to seems to make sense. down stream on the throttle plate (see lower the air temperature. However, if diagram). your air intake is located within the ram After ice removal has been ac- air area of the engine cowling, you are If you wish to contact the author complished, and you shut off the car- subjected to ice, but of a different na- for additional information, please buretor heat, your engine will return to ture. This type of ice is known as impact write Harold Holmes, Department of its original power setting. Remember ice. Safety Studies Injury Research Lab- oratory, University of Wisconsin- the normal range for ice to form is 20 Impact ice can form and seal off your Whitewater, Whitewater, Wl 53190. to 70 degree F. with the most critical engine's air intake and have the same

32 NOVEMBER 1987