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.
November 16, 1998
|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
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 icewhy? Can you do anything to reduce
it or prevent itor 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 climbbut we're told to not have carb heat on because
it reduces power."
Jerry concluded, "I would like some of your experience along
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 Principleby 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).
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.
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.
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.
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.
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 thingjust
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
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 waykeep an eye out!