Eye of Experience #8:
Carb Ice Demons

Carburetor ice is an insidious killer, a demon which appears without warning, often when least expected. AVweb's Howard Fried provides some insight into why you might be caught out unexpectedly by carb ice, when to make use of carb heat to prevent it, and introduces some products to combat it. He also blows away some preconceived notions you may have about when to use carb heat. It isn't just for part-throttle operations.


Eye Of ExperienceReader Jerry McKissack sent me an email requestingthat I do a column on the subject of carb ice. He wrote, “Couldyou do an article on carburetor ice and when to use the carburetorheat. What planes are more susceptible and why? I’ve heard the150 is notorious for carb ice-why? Can you do anything to reduceit or prevent it-or is that just a fact of life in the air?”

He continued, “I’ve read all the books and AIM, etc. thatsay ‘icing can occur in temperatures as high as 70 degrees F withhigh humidity, etc., etc.’ ‘Can’ and ‘will’ are two differentthings. Should I take the precaution and turn it on, or simplywait and see if the symptoms occur? Can’t icing occur duringtakeoff and climb-but we’re told to not have carb heat on becauseit reduces power.”

Jerry concluded, “I would like some of your experience alongthese lines.”

Some Answers

The purpose of the carburetor is to create a fuel/air mixturethat is in the most efficient state of volatility. It does thisby using Bernouli’s Principle-by forcing the air as it entersthe throat of the carburetor through a narrow opening in the intake,called a venturi, into which the fuel is drawn and a volatilemixture is created. This is a result of a reduced pressure atthe narrow point in the tube. A side effect of this reductionin pressure is a concurrent reduction in temperature, and if thereis moisture in the air and the temperature in the throat of thecarburetor is below freezing, ice can and will form.

Carb ice, like structural ice, is weird stuff. It does indeedform at unexpected times. Your information is correct in thatthe Cessna 150 is particularly susceptible to ice in the carburetor. So are the older Skyhawks, those Cessna 172s with the Continentalsix-cylinder engine. Even newer Skyhawks with the 0-320 Lycomingsare much more prone to suffer from ice in the throat of the carburetorthan Piper Cherokees with the same engine (the new 172R has anIO-360 fuel-injected engine).

When Needed?

I believe this results from two causes: one, the Pipers are moretightly cowled, and two, the carburetor is positioned differentlyon the engine (next to a warmer part of the mill). You will notethat in the manuals for the Cherokee series, Piper recommendsthat you check for carb ice in the pattern, then remove the heat. They imply that you should only use heat if you detect the presenceof ice, even in a reduced power glide, whereas Cessna recommendsthat any time you are operating below the green arc on your tachometer,you should apply full carb heat. This is not to say that youcannot get carb ice in a Cherokee, you most certainly can, althoughI have personally never experienced it, while I have in a Cessna170, 172, and 150. And, although I have never experienced itin a Beechcraft with a carburerated engine, I know of a BeechModel 95 (Travelair) with two of those great 0-360 engines thatwas brought down by carb ice in both engines. In the Model 23(Musketeer) series Beechcraft, the manual recommends the use ofcarb heat whenever needed. The problem for the pilot is knowingjust when it is needed.

It is important to remember that carburetor heat is anti-ice ratherthan de-ice in nature. That is, it is meant to prevent the formationof ice, not to remove ice that has already formed. It is to beused as a preventive measure whenever conditions are favorablefor the formation of ice, before any ice starts to form. It isnot meant to get rid of ice that already exists in the throatof the carburetor. Ice can form in the carb in temperatures upto and even above seventy degrees Fahrenheit whenever there isvisible moisture in the air, and as I once experienced, in clearair on a warm day.

Detecting Ice

Is there anything we can do, or is it simply a fact of life forthose of us who fly with which we must live? Well, it used tobe “just a fact of life,” but there are two instrumentsavailable today to help the pilot. First, there is the CarburetorAir Temperature Gauge. This device operates by placing a probein the venturi and measuring the temperature at that point inthe air intake (the place where ice will form if it is going todo so). The gauge in the instrument panel in front of the pilotis color coded with a yellow arc in the zone conducive to icing,and if the needle is in the yellow arc, the pilot is to applyheat until the temp rises above the yellow arc. I have some experienceflying behind this kind of instrumentation, and it works quitewell. Another carb-ice detecting device is called the IcemanProbe. I have no personal experience with this one, but unlikethe Carb Temp Gauge, the Iceman Probe is said to alert the pilotto the actual formation of ice itself. It is designed to actuallydetect ice as it begins to form in the venturi tube of the carburetor.

The only thing I know that can “reduce or prevent it”is the application of carb heat early and often. Unless yourairplane is equipped with a carb air temperature gage with a probein the throat of the carburetor, apply carb heat if you have theslightest suspicion that conditions may be conducive to ice formingin the carburetor. I’ve even applied heat while taxiing a Cessna. (I know, I’m running the risk of having the engine ingest dust,sand, and other impurities.) I’ve also always been advised touse it all, if you are going to use it at all, because the applicationof partial carb heat could cause the formation of ice when otherwiseit might not have done so. I have never experienced this phenomenonbut I’ve certainly heard the advice enough times. Perhaps itis just an OWT (Old Wives’ Tale) with no basis in fact, but tobe on the safe side I have treated it as gospel. This goes backto my earlier statement, if there is the slightest suspicion ofthe possibility of carb ice, I apply heat – all of it.

I never tell anyone how to fly. I explain what I do and why,and if the listener (or reader) likes the idea, he/she may adoptit. If not, it is OK by me. When using carb heat, to restorethe lost power caused by its application, I lean the mixture. Works for me. I do this on takeoff, climb, and cruise, anytimeI’m using carb heat.

As you pointed out, can and will are two different things. Ifit can happen, I don’t want to let it, whether it will or not. I believe in erring on the side of caution.

My Experience

You asked for experiences. Well, as I pointed out above, iceis weird stuff. On those rare occasions when I’ve actually hadice form in the carburetor of an airplane which I was flying,it happened at the most unexpected times. Of course, if I’m expectingit, it won’t happen because I will have already applied heat.

On one of these occasions I was taking off in a light rain showerin a Cessna 150, with full power of course. Like you, I had beentaught that you don’t get ice in the carb in a full power takeoffand climb. Therefore, I certainly did not expect to encountercarb ice. However, after climbing some two to four hundred feetafter takeoff, the engine began to lose power. I applied fullcarb heat, and after sputtering for a few seconds, the engineregained full power operation (with the mixture slightly leaned). My reaction time was somewhat longer than it might have beenbecause I was in a state of denial. I simply couldn’t believethat I had ice in the carburetor. After all, I had always beentold that it doesn’t happen in a full power climb. But I’m hereto tell you that it can happen and it does happen, because itdid happen to me. There goes another OWT, shot to pieces. Justgoes to show you can’t believe everything you hear.

Carb Heat on Takeoff

And, on this same subject of using carb heat during a full powerclimb, students are always told not to do so because of the lossof engine power with the addition of carb heat. My answer tothis is that unless a pilot is required to get out of a shortstrip with an obstacle at the end of the runway, the slight lossof power won’t substantially detract from his ability to takeoff and climb. Also, leaning the mixture will compensate forthe slight loss of power, so there’s nothing really wrong withusing carb heat on the takeoff.

On another occasion, at seven thousand five hundred feet msl ina Cessna 170 (a great old taildragger), on a perfectly clear summerday (blue skies and sunshine), my engine began to run a bit roughand gradually lose power. After trying everything else (switchingtanks, etc.) I applied carb heat. Once again I was in a stateof denial. It just couldn’t be ice in my carburetor, but it was. Weird? I’ll say. And, another OWT bit the dust.

In both of these incidents the symptoms showed upat times I certainly wasn’t expecting to find ice growing in thethroat of my carburetor, so I was a bit slow to react properlyby applying heat. Because of my policy of using full heat whenI have the slightest suspicion that there might be ice around,these are really the only times I’ve encountered carb ice. Jerry(and anyone else reading this), I strongly recommend followingthe manufacturer’s recommendations where carb ice (or anythingelse for that matter) is concerned. In fact, ifyou’re flying one of those airplanes in which the manual advisesthe use of carb heat only “as needed,” I suggest erring onthe side of caution and using full heat anytime you have the slightestsuspicion that ice might form.

Just to Be Safe

Reader McKissack says that during his private checkridein a PA28-151 Piper Warrior, in which he has never experiencedcarburetor ice, on a long power-off glide simulating a forcedlanding due to engine failure, the Designated Pilot Examiner reachedover and applied carb heat. Although Warriors are not particularlysusceptible to carb ice, I would have done the same thing-justto be on the safe side. A power-off glide sets up an excellentcondition for the formation of ice in the throat of the carburetorbecause the engine is subjected to substantial excess cooling. This is one reason why, during an extended power-off glide, we”clear the engine” by briefly applying power every fewseconds to insure that the engine will respond when we need itto.

If I were to own and regularly fly an airplane witha carbuerated engine, I would invest in either an Iceman Probeor a Carb Air Temp Gage, but since I fly a variety of aircraft,I will continue to operate as I have in the past, applying fullcarb heat anytime there is the slightest suspicion of the possibilityof carb ice. This is what experience has taught me.

In most cases, someone else has already gained the experienceyou need the hard way-keep an eye out!