WHAT ABOUT CARBURETOR ICE?
IS TRULY amazing how compliI Tcated a simple request can become.
A request to include suggestions, explanations, and advice on how to fly more safely by avoiding the effects of carburetor ice has turned out to be a rather large order — much larger than the label "Pull for Heat" would seem to indicate. The aircraft itself provides one problem — the variety of equipment used to combat ice, the varying degree of effectiveness and, in so many instances, the lack of equipment — and precludes specific recommendations equally applicable to all aircraft. The reader presents another series of problems; being a pilot and an individual, differences exist. His attitude toward safety, his comprehension of conditions outside of the cabin that are more or less likely to produce carburetor ice, the realization of the degree to which his particular machine is sensitive to this malady, his knowledge of the side effects on the engine that corrective action may produce, his habitual qualities of alertness, and the intrinsic value of his experience as applied to a specific problem are all factors that require the utmost caution on the author's part to prevent misunderstanding. In order to present the clearest picture to the "eager to learn" lowtime student, perhaps provide the pilot with a number of years of varied experience a point to ponder, and to help revive memories of instruction long since forgotten by the large plane career pilot who has only recently admitted to the satisfaction and pleasure of sport aviation, I have enlisted the assistance of two current experts in lightplane flying. One is an EAA member and builder who is current and active in flight instruction, and the other is a highly esteemed FAA flight examiner in this area. While exploring the owners' manuals of several well known aircraft as well as a couple of Flight Instructors' Manuals, I discovered the probable reason for this request, namely, a notably significant lack of material on this subject. I don't suppose the authors of those manuals have ever been assigned the task of retrieving a lightplane with wings that were broken by the fence posts on the end of a too short field, or one that flipped on its back in a soft field, most assuredly because of carburetor ice. The requirements governing the use of the carburetor heat or alternate air controls are as varied as the weather and as individual as amateur-built planes, and while specific rules cannot be drawn to cover each and every case an explanation of various aspects of 22
MAY
1971
THE DESIGNEE CORNER By William J. Alston EAA Designee 2 6004 Maplecliff Dr. Cleveland, Ohio the problem and an outline of good practices should help many readers have a better understanding and be able to fly more safely. Technicalities of the conversion of energy through velocity changes of air and vaporization of fuel that explain the ice machine bolted to your engine have been well explained in past issues of this magazine and in easily obtained texts so I see no need to explain that here. The question is, rather, what can the pilot do with the knobs he has in the cockpit, and how and when should he use them to the best advantage? What are the conditions conducive to the formation of ice? At what times should the sport flyer take steps to prevent the accumulation rather than be rudely awakened? What are some of the more important differences in engine installations? What are some vital operational facts behind that placard, "Pull for Heat"? Two types of ice formations can turn off the fans on our flying contrivances — induction ice and impact ice. Both types will receive a share of our attention in this discussion. When float-type carburetors are installed, a provision for providing carburetor heat, and a means for turning it on, is a must! Additional provisions for measuring the heat and controlling the amount applied is desirable in any case, but become increasingly necessary with higher horsepower engines or engines with higher compression ratios. Engines using pressure carburetors or fuel injectors are considerably less susceptible to the effects of ice, and are frequently installed without special heaters for the inlet air. One of the disadvantages of the almost universal use of exhaust type heaters is that the heat source diminishes as engine power is reduced. In either the float carburetor or fuel injector systems, impact ice can close up the air filters and air inlet ducts, and cause the control doors to freeze and become inoperative. For this reason, it is necessary to provide additional sources for carburetor air
supply, and to design sturdy control arms and brackets capable of withstanding force needed to operate an ice sealed door. Suppose that on a beautiful day for flying in the early fall, with the temperature in the high sixties and the distant hills blurred by a slight haze, an EAA homebuilder decides to shoot some landings. The chill in the breeze and the haze in the air should ring a bell with him, alerting him to possible carburetor ice. An extra check under the cowling specifically to make sure the carburetor heat muff is secure and the flex hoses properly in place is strongly advised. Before he starts his engine, he would do well briskly to actuate his carburetor heat control, and listen for it to strike the stops. After starting the engine and taxiing out for take-off, the runup test is made. An important part of the test is a careful trial of the carburetor heater. At a fixed rpm, he notes the loss of rpm as the carburetor heat is turned on, and he watches it rise as the heat control is turned off. From past experience with this aircraft, he knows that the drop is normal. These three pre-flight checks of the carburetor heat system should not be neglected, but rather they should be ardently practiced until they become a habit. Staying close to the field, our
homebuilder got all of' the landing practice possible out of the next hour, but he was careful to apply carburetor heat on the downwind leg well before throttling back for the landing and, while on base, he cleared the engine by revving it up so it would be sure to
produce heat for the carburetor. He knew that an engine may not provide sufficient heat once the power is reduced. He also realized that the probability of carburetor ice forming is actually greater at reduced throttle settings than it is during climb or cruise, and that while shooting landings in a close pattern altitude circuit little time is available to overcome ice, so you can see that he was properly cautious.
The EAA'er on a cross-country has
a somewhat different problem. Different aircraft react to ice accumulations differently, in that the degree of
warning and the rate of power deterioration may be vastly different from one plane to the next. Several variations in weather conditions may be
encountered during any cross-country t r i p , f u r t h e r complicating the
problems. The first indication of ice formation may easily be interpreted as a creeping throttle in a simple aircraft by any but the most alert pilot. Thus we see that the first and very important tool is a combination of understanding and proper attention to the job at hand. While flying any plane that has only a minimum of equipment, an actual test should be conducted frequently even though no signs of carburetor ice have been observed. Outside influences may distract the pilot's attention, or turbulence may make relatively fine readings of a tachometer difficult if not impossible. Observe closely the cruising rpm, making sure that the throttle is locked and cannot creep. Then pull the carburetor heat on slowly but steadily until full heat is on. The rpm should steadily decrease the usual amount as the heat is applied. Allow the heat to remain on for about 30 seconds, and then remove the heat at about the same rate. Recheck the tachometer when the heat is fully off and see if the rpm returns to its original figure. If previous warning signs had been mistaken for a creeping throttle, and rpm was restored at the time by inching the throttle "in" before the check was made, a new and higher throttle setting will result following the test. This then is the proof that ice was present. This test should be performed on any flight at regular intervals of ten minutes or so, as an indicator of the presence of ice until the pilot gains sufficient confidence in his ability to detect safely the early warning signs of ice in a particular plane by other means for that day. Whenever ice is detected, this test may be used as a means of combatting ice by repeating the procedure at intervals as frequently as required. If subsequent checks indicate ice at ever-increasing frequency it might be advisable to apply full heat and leave it on continuously. Remember, though, this action will reduce available power and will also increase fuel consumption if the mixture is not or cannot be adjusted. Perhaps this would be an appropriate place to mention the possibility of a rather confusing sequence of events that can occur during the test.
Following a descent from altitude, a carburetor ice test is made, but things don't seem to work right at all. In fact, things may seem rather backwards. While applying carburetor heat, it seemed that the rpm rose and when turning it off the rpm dropped. Repeating the test out of disbelief only confirms the seemingly mixed up gauges. It doesn't act like ice, but can it be ice? What could it be? How serious is it? This can happen if the engine had been leaned at altitude but not readjusted before descending and at the time of the test was actually adjusted exceptionally lean. Since the application of heat causes a richer mixture, it would present a more proper setting before the engine overheats or other trouble develops. Any builder who will never be satisfied only to fly in "bluebird" weather, or cannot restrain the urge to fly beyond the horizon, should seriously consider the installation of a carburetor temperature gauge. This will permit the application of just the right amount of heat to eliminate any hazard. Use of only partial heat, however, without the benefit of a temperature indicator, is not recommended since, unknowingly, the right amount of heat may be provided to make that ice machine up front work even better. Cross-country trips may involve areas of minimum VFR weather, with low ragged clouds fairly dripping with moisture, where the pilot often follows a zig-zag course over every small airport on his sectional, ever ready to turn back and land. This type of marginal flying condition is frequently the most hazardous type of condition for rapid build-up of carburetor ice. The many necessary diversions that must occupy the pilot's attention under these conditions make it likely that slight rpm losses will not be noticed, the early warnings of ice will be missed, and the pilot may find he is riding a rough engine. Some installations have a rather narrow margin between the first appearance of rough running and engine stoppage. It seems that these conditions would be improved by operating with full heat, or the necessary safe heat if an indicator is installed, as soon as the pilot discovers he cannot efficiently monitor his instruments due to other distractions. Excitedly pulling on carburetor heat very rapidly on a thoroughly iced-up engine may clog the intake with slush or ice, or cover a discharge nozzle, actually causing the engine to quit. Apply the heat fairly slowly but deliberately, listening for its effect on engine operation, and don't be too surprised if it does "blurp" a time or
two during the process, but it will be followed by noticeable improvements. If it does "blurp" a few times you will not be surprised, however, to notice a sudden improvement in your future awareness of the feel and sound of your engine — it only takes one such experience! After successfully clearing the engine, and that may take quite some time, use the proper amount of heat according to the gauge, or full heat if there is no gauge, to avoid duplication of the experience. Bear in mind that the minimum required design heat in a system has been tested with new, not worn, parts and is based on preventing ice rather than removal of ice. Carburetor ice is commonly encountered within a temperature range of 40 to 70 degrees F, but has been experienced at temperatures extending from below freezing to 90 degrees, and at humidity readings as low as 30 percent. If you do pick up ice, don't be surprised, and I do mean caught unawares. A manifold-pressure gauge will show a drop when ice begins to form in the carburetor of a plane equipped with a constant-speed propeller, and some builders install these units in planes with solid clubs specifically for the purpose of ice detection. If a heating unit produces the normally required amount of heat — 90 degrees F temperature rise at an ambient oil temperature of 30 degrees F at cruise power — it should be able to handle any normally expected conditions. On some low-power, lowcompression engines, the maximum recommended air-inlet temperature cannot be reached without a two-stack heater, and continuous operation with full heat would do no more than reduce power. It may be worth a moment's thought when installing the carburetor heat control to locate it in such a position that, while the pilot's hand is on the throttle, an extended index finger could be used to push the heat off as the throttle is opened. Otherwise, it might be good practice to push the heat off, under normal circumstances as you reach the fence in an approach, just in case that guy on the end of the runway pulls out in front of you and you must go around. What about the PS carburetors and injected engines? You have heard that they have no icing problems? This is a fairly common misconception, and I would never go so far as to say that. These types can be subject to ice, perhaps under different conditions, but with a result that may be just as serious. Later in the fall, supercooled moisture might be encountered, the kind Continued on Next Page SPORT AVIATION
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DESIGNEE CORNER . . . Continued from Preceding Page
that freezes on contact with cold metal, and screens and inlets can freeze up surprisingly fast. Scudrunning around the edges and through the ragged edges of cloud bases, or perhaps through a small shower or two, can result in moisture passing through the inlet tubes to freeze on the butterfly and begin to strangle the
engine's only breathing tube.
Suppose, you say, it's well below freezing and the moisture is frozen, snowflakes — does that spell trouble?
I well remember the cold February-
day I stood on an eastern mountain, called to survey the wreckage of a customer's busted up plane on a small
top-of-the-mountain meadow. He had
been flying IFR on top, above the solid layers, but flicking through the fluffy snow-filled cloud tops thrusting upward from the mass of cloud. The engine coughed, then began to run rough and very quickly failed completely, In the minus 20-degree F cold of 11,000 ft. he found that he couldn't operate the throttle, the mixture didn't help in any position,
and he could not complete a successful air start. As he settled into the solid
cloud bank in his $30,000.00 glider, he notified the world by calling "Mayday" and tended to the business of descent through clouds in rugged mountainous country. He emerged a few minutes later, above a high treecovered valley and managed to set it down in this rolling, steeply inclined
meadow. Several days later when our recovery crew arrived, the mountain-top temperature had not yet risen to ten degrees, and one of the crew unbolted the injector airbox and removed a doughnut size and shape chunk of ice. The throttle butterfly was deeply impressed into the sides of the thick ring of ice. It was preserved for some time in an airport freezer. I would suggest that some of the snow forced through the air filter by the 200 mph air flow passed along the edge of the air duct and melted due to the close proximity of the tube to the exhaust stacks, and refroze on contact with the cold metal of the injector throttle assembly. This installation had no manual alternate air control, but had a springloaded door that would be sucked open by the engine if the air filter became clogged, supplying warmer
protected air from the engine compartment. Since the filter did not freeze over, the alternate air door never did open.
While an alternate air door that is automatic has many advantages, I feel the pilot should be able to make his own choice and have an overriding control available. It is easy to see that the development of safe habits, remaining alert, and knowing about the mechanical features of a plane are vital aspects of a safer approach to flying. Some instructors say that if an engine will take the throttle, it is ready to fly. I say — not always! Some say carburetor ice will not form at full throttle operation. I say — beware! Some will tell you that injectors will not ice up and quit. I say — exercise caution! Some will say there is always plenty of warning of carburetor ice. I say — be alert, always! Some will say no chance of ice today. I say — they could be wrong! Lack of understanding can rob the most experienced pilot of a great deal of the pleasure of sport flying.
Midwest Aero Historians To Meet On May 22nd The Spring, 1971 meeting of the Midwest Aero Historians will be held or. Saturday, May 22, at the EAA Air Education Museum in Franklin, Wisconsin. The meeting room will open at 8:00 A.M., and the program will continue all day and evening.
A wide range of interests will be included on the program. Edward K. Crosby of Milwaukee will relate his experiences as one of the crew at Ryan Airlines, Inc. who built the "Spirit of St. Louis," and his varied experiences in the Marine Aviation Corps in the 1920's.
(Photo by Tony Yusken)
Speakers at the Fall 1970 meeting of the Midwest Aero Historians were (from left): George Hardie, AAHS Coordinator; Richard Hill, co-chairman, who spoke on the history of a U. S. Navy "Hellcat" in World War II; Burt
Kemp, who spoke on the career of Ed Heath, pioneer
homebuilder; Charles Boie, aviation artist who spoke on illustrating commercial aircraft; the late Martin Haedtler, who related his experiences as a P-39 pilot on Guadalcanal in World War II; and Ralph Winkel, mechanic on P-39's in World War II, and now in charge of restoring the P-39 in the background for the EAA Air Museum. 24
MAY
1971
Details on plans for a proposed "Museum of Military Air Power" will be presented by Patrick J. O'Hare, President and Head of Research, and Walter L. Ketchum, Vice President and Curator. The museum will cover the period of World War II only and will feature aircraft and related material pertaining to that era. AAHS member John R. Wells of Chicago is well known for his diligent research into the history of the Douglas DC-3. John will present a talk entitled "A Collector's View of the DC-3." The evening program will consist of a slide talk on aircraft used in Viet Nam and Thailand during 1966-1970, presented by Neal Schneider and Dave Hansen, both recently returned veterans. Registration fee will again be $2.00, which includes admission to the EAA Air Museum. „ ©
