November 16, 1998 Eye of Experience #8: Carb Ice Demons

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by	Howard Fried

About the Author ... Howard Fried started flying with the Army Air Corps in WWII, where he served both as a multi-engine instructor pilot and in combat piloting B-17s. After a stint teaching sociology and on-the-air and management jobs in the radio business after the war, he turned to teaching flying again full-time. Over 40,000 general aviation hours later, he is still instructing and running his own flight school. Along the way he administered over 4,000 flight tests as a Designated Examiner until victimized by rogue FAA officials. He has authored two popular flying books aimed at student pilots and instructors, Flight Test Tips and Tales and Beyond The Checkride, and a series of audio tapes, Checkride Tips from Flying's Eye Of The Examiner.

Reader Jerry McKissack sent me an email requesting that I do a column on the subject of carb ice. He wrote, "Could you do an article on carburetor ice and when to use the carburetor heat. What planes are more susceptible and why? I've heard the 150 is notorious for carb ice—why? Can you do anything to reduce it 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. that say 'icing can occur in temperatures as high as 70 degrees F with high humidity, etc., etc.' 'Can' and 'will' are two different things. Should I take the precaution and turn it on, or simply wait and see if the symptoms occur? Can't icing occur during takeoff and climb—but we're told to not have carb heat on because it reduces power."

Jerry concluded, "I would like some of your experience along these lines."

Some Answers

The purpose of the carburetor is to create a fuel/air mixture that is in the most efficient state of volatility. It does this by using Bernouli's Principle—by forcing the air as it enters the throat of the carburetor through a narrow opening in the intake, called a venturi, into which the fuel is drawn and a volatile mixture is created. This is a result of a reduced pressure at the narrow point in the tube. A side effect of this reduction in pressure is a concurrent reduction in temperature, and if there is moisture in the air and the temperature in the throat of the carburetor is below freezing, ice can and will form.

Carb ice, like structural ice, is weird stuff. It does indeed form at unexpected times. Your information is correct in that the Cessna 150 is particularly susceptible to ice in the carburetor. So are the older Skyhawks, those Cessna 172s with the Continental six-cylinder engine. Even newer Skyhawks with the 0-320 Lycomings are much more prone to suffer from ice in the throat of the carburetor than Piper Cherokees with the same engine (the new 172R has an IO-360 fuel-injected engine).

When Needed?

I believe this results from two causes: one, the Pipers are more tightly cowled, and two, the carburetor is positioned differently on the engine (next to a warmer part of the mill). You will note that in the manuals for the Cherokee series, Piper recommends that you check for carb ice in the pattern, then remove the heat. They imply that you should only use heat if you detect the presence of ice, even in a reduced power glide, whereas Cessna recommends that any time you are operating below the green arc on your tachometer, you should apply full carb heat. This is not to say that you cannot get carb ice in a Cherokee, you most certainly can, although I have personally never experienced it, while I have in a Cessna 170, 172, and 150. And, although I have never experienced it in a Beechcraft with a carburerated engine, I know of a Beech Model 95 (Travelair) with two of those great 0-360 engines that was brought down by carb ice in both engines. In the Model 23 (Musketeer) series Beechcraft, the manual recommends the use of carb heat whenever needed. The problem for the pilot is knowing just when it is needed.

It is important to remember that carburetor heat is anti-ice rather than de-ice in nature. That is, it is meant to prevent the formation of ice, not to remove ice that has already formed. It is to be used as a preventive measure whenever conditions are favorable for the formation of ice, before any ice starts to form. It is not meant to get rid of ice that already exists in the throat of the carburetor. Ice can form in the carb in temperatures up to and even above seventy degrees Fahrenheit whenever there is visible moisture in the air, and as I once experienced, in clear air on a warm day.

Detecting Ice

Is there anything we can do, or is it simply a fact of life for those of us who fly with which we must live? Well, it used to be "just a fact of life," but there are two instruments available today to help the pilot. First, there is the Carburetor Air Temperature Gauge. This device operates by placing a probe in the venturi and measuring the temperature at that point in the air intake (the place where ice will form if it is going to do so). The gauge in the instrument panel in front of the pilot is 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 apply heat until the temp rises above the yellow arc. I have some experience flying behind this kind of instrumentation, and it works quite well. Another carb-ice detecting device is called the Iceman Probe. I have no personal experience with this one, but unlike the Carb Temp Gauge, the Iceman Probe is said to alert the pilot to the actual formation of ice itself. It is designed to actually detect 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 your airplane is equipped with a carb air temperature gage with a probe in the throat of the carburetor, apply carb heat if you have the slightest suspicion that conditions may be conducive to ice forming in 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 to use it all, if you are going to use it at all, because the application of partial carb heat could cause the formation of ice when otherwise it might not have done so. I have never experienced this phenomenon but I've certainly heard the advice enough times. Perhaps it is just an OWT (Old Wives' Tale) with no basis in fact, but to be on the safe side I have treated it as gospel. This goes back to my earlier statement, if there is the slightest suspicion of the 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 adopt it. If not, it is OK by me. When using carb heat, to restore the lost power caused by its application, I lean the mixture. Works for me. I do this on takeoff, climb, and cruise, anytime I'm using carb heat.

As you pointed out, can and will are two different things. If it 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, ice is weird stuff. On those rare occasions when I've actually had ice form in the carburetor of an airplane which I was flying, it happened at the most unexpected times. Of course, if I'm expecting it, it won't happen because I will have already applied heat.

On one of these occasions I was taking off in a light rain shower in a Cessna 150, with full power of course. Like you, I had been taught that you don't get ice in the carb in a full power takeoff and climb. Therefore, I certainly did not expect to encounter carb ice. However, after climbing some two to four hundred feet after takeoff, the engine began to lose power. I applied full carb heat, and after sputtering for a few seconds, the engine regained full power operation (with the mixture slightly leaned). My reaction time was somewhat longer than it might have been because I was in a state of denial. I simply couldn't believe that I had ice in the carburetor. After all, I had always been told that it doesn't happen in a full power climb. But I'm here to tell you that it can happen and it does happen, because it did happen to me. There goes another OWT, shot to pieces. Just goes 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 power climb, students are always told not to do so because of the loss of engine power with the addition of carb heat. My answer to this is that unless a pilot is required to get out of a short strip with an obstacle at the end of the runway, the slight loss of power won't substantially detract from his ability to take off and climb. Also, leaning the mixture will compensate for the slight loss of power, so there's nothing really wrong with using carb heat on the takeoff.

On another occasion, at seven thousand five hundred feet msl in a Cessna 170 (a great old taildragger), on a perfectly clear summer day (blue skies and sunshine), my engine began to run a bit rough and gradually lose power. After trying everything else (switching tanks, etc.) I applied carb heat. Once again I was in a state of 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 up at times I certainly wasn't expecting to find ice growing in the throat of my carburetor, so I was a bit slow to react properly by applying heat. Because of my policy of using full heat when I 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 following the manufacturer's recommendations where carb ice (or anything else for that matter) is concerned. In fact, if you're flying one of those airplanes in which the manual advises the use of carb heat only "as needed," I suggest erring on the side of caution and using full heat anytime you have the slightest suspicion that ice might form.

Just to Be Safe

Reader McKissack says that during his private checkride in a PA28-151 Piper Warrior, in which he has never experienced carburetor ice, on a long power-off glide simulating a forced landing due to engine failure, the Designated Pilot Examiner reached over and applied carb heat. Although Warriors are not particularly susceptible to carb ice, I would have done the same thing—just to be on the safe side. A power-off glide sets up an excellent condition for the formation of ice in the throat of the carburetor because 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 few seconds to insure that the engine will respond when we need it to.

If I were to own and regularly fly an airplane with a carbuerated engine, I would invest in either an Iceman Probe or 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 full carb heat anytime there is the slightest suspicion of the possibility of carb ice. This is what experience has taught me.

In most cases, someone else has already gained the experience you need the hard way—keep an eye out!