Carb Ice Versus Carburetor Heat
The Real Cause of Many Unexplained
By Pete Humphrey (From FAA
No matter how many
hours we have logged, that "carb ice" gremlin
can sneak up and catch us by surprise. Engine
runup on the ground is by far a better place to
discover it than during flight. But many times
it is during flight that carb ice rears its ugly
head. And when it happens it may have progressed
to the point where the only way is down; that
is, an immediate landing with little or no
available engine power.
But first, let's review this ever-present
problem, look at means of detection, and share
some timely methods for staying ahead of engine
What Is Carb Ice and What Causes It?
There is always some degree of moisture
(humidity) in the air that flows into and
through an aircraft engine for every unit of
fuel burned. A carburetor provides the explosive
air/fuel mixture to each cylinder in the engine,
where your power is generated.
As air is drawn into the small throat of a
carburetor, the venturi effect accelerates the
air and cools it. It cools even further when
mixed with vaporized fuel. When this moist air
reaches the freezing level of 32°F, the ice
particles that begin to form deposit themselves
on the throttle plate. The carburetor can then
become choked up by this ice to the point that
the engine receives less air than is required
for full power. The once-explosive air/fuel
mixture becomes so rich from excess fuel that
the engine ceases to fire.
What Conditions Are Conducive to Carb Icing?
Basically: Whenever the cooling effect of the
air flowing through the carburetor is sufficient
to bring the temperature of the carburetor
throat down to 32°F or colder AND there is
sufficient moisture in the air.
- If the outside air temperature (OAT) is
between about 20°F and 30°F with visible
moisture or high humidity
- If the relative humidity of the outside
air is high, even in a cloudless sky, with
an OAT as low as 15°F and as high as 100°F
- In the spring and fall, especially just
after a rain
In other words, carbureted engines are
susceptible to icing almost anytime.
Are All Internal Combustion Engines Prone to
No. In pressure carburetors, fuel is admitted at
a point past the throttle plate and downstream
from the air inlet throat. It is well on its way
to the hot engine before ice can form. In
fuel-injected engines, the air/fuel mixture
sprayed into the engine cylinders is metered
into the hot cylinder at temperatures near that
of a volcano. So, there is no potential for carb
ice on either of these types.
Our concern is the mixture of evaporating
fuel and moist air before it gets near the heat
of the running engine, and this only occurs in
the float-type carburetors found on most light
Detecting Carb Ice
There are two opportunities to detect the subtle
indication of developing carb ice. The subtlety
is a gradual, small drop in RPM on a fixed-pitch
prop aircraft, even though the pilot did not
retard the throttle. On a constant-speed prop
aircraft, carb ice is manifested by a gradual,
small drop in manifold pressure (MP) while in
If detected early and dealt with correctly
you can easily prevent an untimely engine
During Pre-flight Engine Run-up
On the ground during engine run-up, ice is easy
to identify positively and remove. On a Cessna,
for example, at 1,700 rpm the carburetor heat
control is pulled out fully to the hottest
position. Because air entering the carburetor
after application of carb heat is warm (from the
engine compartment) and less dense, you will
notice an rpm decrease of 100 to 300 rpm, and
the rpm should remain low until the carburetor
heat control is pushed all the way back in.
However, if the 100 to 200 rpm decrease is noted
but slowly begins to increase so that when the
carb heat control is pushed back in the rpm
reads more than the original 1,700 rpm, you had
If it happened on the ground, it can happen
again during takeoff. While lined up on the
centerline, just before takeoff, I heartily
recommend another carburetor heat check.
In Flight at Constant Cruise Throttle
An often asked student question during their
ground school training is, "If the engine falls
in flight because of carb ice, why not just
apply carburetor heat to melt the ice?" That is
a reasonable question, but let's look at why
that action is more than likely too late to
Carburetor heat is obtained essentially from
within the engine compartment (rather than
directly from the intake air filter on the front
of the aircraft). If the engine has cooled
sufficiently because of an excessively rich
air/fuel mixture, there may not be sufficient
hot air in the engine compartment to melt the
accumulation of carb ice even with the carb heat
control to full "hot." That is the point when
the engine will cease developing enough power to
keep you airborne.
It is of. the utmost importance to keep a
sharp eye on engine performance at all times.
At the slightest hint of deteriorating power
(from decreasing rpm or MP) use carburetor heat
for at least eight to 10 seconds or for however
long the aircraft's manual recommends.
Many accidents and off-airport landings may have
occurred because unsuspecting pilots — having
noted decreasing engine power during cruise —
pulled on carburetor heat.
Their engine immediately began running very
roughly and/or backfiring. In response to such
noise, they removed carburetor heat and
continued on their way. Continued, that is,
until the engine continued to slow and
What happened? When the pilot applied
carburetor heat at the first sign of decreasing
power, the warm air from the engine compartment
did just what it was supposed to do.
It melted the ice from the carburetor throat
and throttle plate. Where did the melted ice
(now water) go? Right into the engine that wants
to burn gasoline, not water; hence, a
momentarily rough and perhaps backfiring engine.
If carburetor heat had been kept on long, the
hot engine and warm carburetor heat air would
have kept the carburetor ice-free.
How to Fly Safely in Carb Ice Conditions
When icing conditions exist, apply carburetor
heat often at your cruise throttle setting.
Never use partial carburetor heat unless the
aircraft is equipped with a carburetor
temperature gauge, ice light, or similar
instrumentation. Partial heat can possibly make
matters worse, unless you know exact carburetor
Remember, full heat or nothing. When below
the green band on the rpm or MP gauge, apply
carburetor heat. It is good practice to use it
on any descent where you have reduced power.
If recommended by the manufacturer, use that
carburetor heat control on aircraft at reduced
power settings and especially for landing.
In conditions conducive to icing or if ice
was noted during engine run-up, perform another
carburetor heat check immediately before
takeoff. DO NOT leave carburetor heat on for the
Check your POH or aircraft manual for general
To be totally knowledgeable, I recommend that
everyone read FAA Advisory Circular 20-113
concerning precautions and procedures for
preventing induction and fuel system icing.
Carburetor heat will keep carb ice from forming
if applied early enough, often enough, and long
AC 20-113, Pilot Precautions and Procedures
to be taken in Preventing Aircraft Reciprocating
Engine Induction System and Fuel System Icing
Problems, is free from U.S. Department of
Transportation, Subsequent Distribution Office,
Ardmore East Business Center, 3341 Q 75th
Avenue, Landover, MD 20785.
Mr. Humphrey is from Hagerstown, MD and is an
Aviation Safety Counselor for the FAA's
Baltimore Flight Standards District Office. This
article appeared in the Baltimore FSDO's
Aviation Safety Program newsletter,
Mid-Atlantic Flight Safety Monitor