carburettor
ice and heat
Check for carburettor ice before takeoff
Carburettor Heat
Somewhere on the front of the
engine is an air intake for the carburettor. The air enters through a mesh
filter. Every pilot, in the pre-flight should check security of this
filter and its mounting. The airflow through the carburettor is
controlled by the throttle moving a circular flat rotating plate. Air
passing by this butterfly valve is accelerated by a narrowing throat
called a venturi....
This narrowing reduces
pressure and sucks cooling fuel into the air. The combination of a lower
pressure, gasoline, and moisture in the air can so cool the metal parts of
the intake that the moisture adheres to the parts as ice. The greater the
air's humidity the more ice will be formed. As the airflow is restricted
by the ice, the excess fuel so enriches the mixture that the engine to
flood and misfire. The initial signs of this icing is a gradual decrease
in rpm and an increase in engine roughness. Avoidance of carburettor ice
can be improved by avoiding long, low power descents and by leaning to
make the engine run hotter. Lycoming engines are less susceptible to
ice than are Continental engines since the carburettor is attached so as
to benefit from the heat of the engine itself.
Carburettor ice is caused
by the change in pressure as air passes through the venturi. Fuel atomized
in the throat of the venturi evaporates. This uses heat from the metal of
the carburettor and air. This air/fuel at lower pressure cools and takes
even more heat away from the metal of the carburettor. At some point
any moisture in the air will freeze on the metal of the carburettor.
The ice that adds to the venturi constriction will both increase the
cooling rate, the speed of airflow and lower the pressure. Thus,
carburettor ice feeds on itself until the excessively rich mixture
kills the engine.
The reason carburettor
ice causes a drop in rpm and a rough engine is because of an excessive
rich mixture. The addition of hot
air by the carburettor heat enriches the mixture even more and causes an
additional drop in rpm. Leaning the mixture is one viable option in
trying to improve the engine performance.
It is possible to verify
this procedure by doing the following during runup:
-
Aggressively lean the
mixture.
-
Note that little or no
rpm change will be noted by applying C.H.
-
With the mixture rich,
apply C.H.
4. Note that the rpm drop
is significantly greater than with the lean mixture.
Occurrence
Float type carburettors can create ice any time. Moisture in the
air increases the possibility that the cooling effect of the atomizing of
gasoline in the venturi (narrow throat) of the carburettor will cause
vapor expansion and cooling to create ice from any moisture present. When
the ice adheres to the parts of the carburettor it cuts down the flow of
air and 'chokes' the engine. There is too much fuel for the amount of air
available. Carburettor ice onset is dangerous and insidious.
The first indication of ice is a drop in rpm. This drop may be accompanied
by engine roughness and eventual stoppage. You must sensitize
yourself to rpm changes that you have not induced.
There are important
training aspects related to carburettor ice and the use of carburettor
heat. The pilot must train to be aware of the weather conditions that have
a proclivity for causing carburettor icing. Being aware of the likelihood
is the primarily aspect of the anticipation required. Just where icing
occurs in the carburettor is a variable. Icing can occur before the
butterfly, in the carburettor intake, on the butterfly valve or afterwards.
When impact icing blocks the air intake filter as induction icing,
then the carburettor heat source serves as alternate air for the engine.
The different design of carburettors, engines, and induction systems all
make a difference however indeterminate. Some cowling designs are better
than others in reducing the occurrence of carburettor ice.
We know how/why ice forms
and how to use CH as preventative but we no reliable predictive ability.
You are more likely to get carburettor ice with warm temperatures because
warm air has the ability to hold more moisture. Fahrenheit temperatures
between 20 and 70 accompanied by atmospheric moisture usually trigger the
required venturi temperature drop. Anything less than full CH
application is potentially dangerous. Low fuel pressure and
contaminated fuel can give symptoms similar to carburettor ice. By keeping
both fuel and induction air clean we can avoid these as causes of unusual
engine behaviour. Carburettor icing occurs when the air, moisture and
temperature in the carburettor is modified after ingestion so as to be
capable of freezing moisture and adhering it to the internal parts of the
carburettor. Carburettor heat also performs the function of an alternate
air door in case of induction icing covering the engine air filter as
stated before..
There is no FAA
carburettor icing probability chart. In given circumstances carburettor
icing can occur at any temperature. Some aircraft models and engines are
more susceptible to icing than others. Carburettor ice will get you when
you least expect it. It has occurred on cloudless days and temperatures up
to 100 F. The ambient air temperature in a carburettor can be reduced at
its venturi by as much as 70F. The 10 seconds that it takes C. H. to take
effect can be the longest of your life. Don't make it the longest 12
seconds by failing to immediately apply C. H. Any unexplained power
reduction is a red flag notice. Make all descents with partial
power to retain as much C.H. potential as the situation allows.
Aircraft with an EGT will show a decrease in EGT readings at the onset of
carburettor ice. It is an early warning system.
Most likely icing is at
50%+ humidity from 20 to 90 degrees F. (Some texts give 80% humidity
between 40 and 70 degrees.) Ice is caused by absorption of heat from
air/fuel vaporization as it enters low-pressure venturi of carburettor at
high speed. It can cause drop of 60 degrees causing ice to adhere to
"butterfly" and venturi throat. Even with no visible moisture, ice can
form in the throat of the carburettor due to adiabatic cooling as the air
passes through the venturi by the throttle plate.
The carburettor on a
Lycoming engine is mounted at the very bottom of the engine in such a way
that any heat it gets is from direct contact with the engine and very
little from elsewhere. The engine's putting out enough heat at higher
power where the use of carburettor heat will prevent the formation of ice
in the carburettor and given time it'll melt any existing ice. Even the
position of the butterfly valve helps. Carburettor ice can occur at any
power setting, it is most likely to occur in the green arc.
Continental's carburettor
is suspended below the engine so that there is no residual heat
transferred between the engine and the hanging carburettor. Lycomings, on
the other hand, have the carburettor secured to the oil pan. The
carburettor is heated by the hot engine oil. For this reason Piper
recommends carburettor heat be used only as necessary. The pilot
determines just where and when necessary exists. Lycoming engines are much
less susceptible to carburettor icing than Continentals because of their
design. However, because of pilot complacency, the icing of a Lycoming
is going to be more traumatic and unsuspected.
Many unexplained engine
failures are probably due to carb ice. When humidity is more than 50% and
temperatures range from 20 to 90 degrees Fahrenheit ice will form as the
internal carburettor temperature can drop 60 degrees. Carburettor engines
must be able to take 30-degree air through the intake and deliver 120
degrees to the carburettor. This is done using 75 percent power, which is
far more that is used in most descents. Once again the government
aviation requirements are just minimums. Don't be satisfied with
minimums
Carburettor icing during
takeoff is not as rare as some would like to believe. The best
preventative is to apply C.H. on leaving the run-up area and removing it
at just before full power is applied. Every descent made at reduced
power should be done with full carburettor heat on. The use of C.H.
decreases the power of the engine slightly less than 10% and causes the
mixture to be over-rich. Leaning is advised for best engine operation and
to maintain the required heat for C.H. No use of leaning or C.H. is
advised for engine operations over 75% of maximum power.
Ice can form at full power
Always apply full carburettor heat
If cruising with C.H. lean the mixture.
Always use C.H. during descents
Three Kinds of
Carburettor Ice
1. Impact ice is caused by
moist air on the air filters and air intakes are impacted as rain, snow,
or sleet. Forms from 15 to 32-degree F but is worst at 25F.
2. Fuel ice is caused by vaporization where fuel enters the
manifold system. Will occur when relative humidity is 50% and between 40
and 80deg;F.
3. Throttle ice forms in the carburetion system on the throttle
valve or butterfly or on the interior of the venturi system. The water
vapour from the air intake freezes due to the venturi effect. Effective
temperature drop is about 5deg;F and ice is most likely between ambient
temperatures from 32 to 37deg;F.
On first start, there may not be sufficient heat to either prevent or melt
any carburettor icing. Leaning will raise engine temperature. The
more moisture in the air, the greater the likelihood of icing. A humid hot
day is just as likely to cause icing as is a cold day with water on the
ground. When conditions indicate that icing is likely, the prudent
procedure is to apply carburettor heat in anticipation rather than as
reaction. Using the carburettor heat every time you reduce power is a
good operating procedure and much safer than the POH suggestion for use
when required.
With the advent of Low
Lead 100 Octane gasoline, leaning during taxiing has become mandatory.
Leaning that is a bit aggressive can cause symptoms of carburettor heat
failure to operate during run-up. Running lean causes the engine to
run hotter when there is no excess fuel for co0ling. When the
atmospheric temperature approaches the engine temperature there is
insufficient differential in the two temperatures to cause an rpm drop in
the engine operation. When this happens just enrich the mixture a
bit and run an additional check of the C.H. operation.
Most of my carb icing
encounters have been while taxiing. The explanation of what occurs
deserves repeating. I demonstrate the cooling effect of gasoline on moving
air by placing some gas on the back of a student's hand and have him wave
it. Under certain atmospheric conditions and power settings it is possible
for the blending of fuel and air in the carburettor venturi to cool any
moisture present to freezing. Automotive fuel is more likely to cause
ice because of its vapour point. (Venturi effect can be demonstrated
by holding two pieces of binder paper vertically about three inches apart
and blowing between them.) This can adhere to the metal parts of the
carburettor particularly the butterfly valve which is the throttle
control. This ice will restrict the flow of air through the venturi and
cause an initial reduction in rpm and subsequent engine roughness to final
failure.
At idle power, in the air or on the ground an aircraft can ice up in a
very short time. There is no logical safety reason behind the concept of
removing carburettor heat on short final as a go-around safety measure.
There is nothing so urgent about a go-around that makes it necessary to
remove carburettor heat prior to landing as a time saving or safety
procedure. The closer to the ground you are when initiating the
go-around the greater will be the ground effect and aircraft acceleration.
The go-around is initiated first with a mixture check, full throttle, and
finally with carburettor heat. Always first with the most. These forward
movements can be accomplished nearly simultaneously in one motion.
Recognition
Recognition is important but it comes
after the fact. Depending on the
circumstances, after the fact, may be too late. As with pitot heat,
prevention is the name of the winning game. Waiting five minutes for pitot
heat to remove ice can be done, not easily but 'do-able'. Carburettor heat
will not allow the time because as you lose power you are losing the
engine's ability to produce the required heat. Planning your options now
is better than a spur of the moment decision that can be wrong.
The greatest temperature
drop occurs in the throat of a carburettor. This is because of the
decreased venturi effect pressure. The vaporized fuel releases heat during
the conversion of liquid fuel to atomized fuel. Humidity increases the
probability of icing. A drop in the venturi temperature of forty or fifty
degrees is possible. Low temperatures reduce the probability of icing
because of inability to hold moisture except a solid state as snow or ice.
Using Carburettor Heat
Even in warm weather
All or nothing at all, not just an old song.
Use before takeoff; not during takeoff
The engine talks; you listen and feel.
Not for continuous operation on the ground.
Check proper operation and maintenance
Application
When you pull the aircraft
carburettor heat you are moving a diverter panel which has been taking
external air through the nose air filter to change to taking unfiltered
air through the heat exchanger of the exhaust system. The heat exchanger
air is usually warm enough to both affect the engine power (reduced up to
15%) and melt any ice that may have accumulated on the carburettor venturi
or butterfly valve. This melting may not occur if the engine has cooled
off so don't waste any time pulling the handle. Taxiing lean tends to keep
the engine hotter. If pulling C.H. during taxi or run-up raises the rpm
the aircraft is improperly leaned. Better yet use C.H. as a preventative
to pre-heat the system and if in doubt continuous use may be required.
Carburettor heat is intended as a preventative rather than a cure.
Heat should be applied early and fully. Carburettor heat is
best used in anticipation of carburettor ice. Put it on before a
potential icing situation occurs. Going into a slow flight configuration
will increase the effectiveness of carburettor heat by decreasing the
cooling air over engine. In addition, carburettor heat makes it possible
for the engine to draw its air from inside the engine compartment. If
the engine air intake filter under the propeller is being blocked by
impact ice or snow, the use of engine compartment air via the carburettor
heater will bypass the blockage and allow continued engine operation.
Don't move the throttle. The ice may break loose and cause instant
stoppage. Apply full carburettor heat and allow the diverter valve to
bring heat in the form of hot air to enter the venturi so as to melt the
ice. This air is unfiltered. It passes through the heat exchanger and
flows to the carburettor as hot air. The engine will react as follows.
The already ice reduced rpm will be further reduced by the application of
heat. (Hot air is less dense and reduces power about 15%.) As the ice
melts there will be a gradual rise in rpm because of increased air flow.
A further rpm increase occurs when carburettor heat is removed.
PROOF
of icing is when the sequence of
initial rpm drop is followed by another decrease in rpm when heat is
applied, followed by a small increase as ice melts, followed by a further
increase when heat is removed.
Carburettor heat admits unfiltered air into the engine. This unfiltered
air can contain particles harmful to the engine. This is particularly true
close to the ground. For this reason we limit the time carb heat is
applied on the ground. Carburettor heat should not be used when maximum power
is required such as on takeoff.
The application of carburettor heat changes the flow of engine air from
the outside air intake to unfiltered hot air from the heater muff. This
hot air causes an additional loss of power. Normally the power loss is
1% for every 10 degrees of hot air differential. With icing this loss
can reach 15%. Due to lack of ram air and a usual 100 degrees of heat
above standard the runup loss can reach 13%. On hot days the heat
differential is less so the apparent drop in power is less. If you have
leaned for taxi there is no need to enrich the mixture during runup since
you are not going to full power and will check carburettor heat.
Carburettor heat enriches the mixture. If you intend to fly with
carburettor heat on you should also plan to lean your mixture since with
carburettor heat the engine will be running rich. In icing
conditions always retain enough power to keep the engine warm. Without
a warm engine you will not have carburettor heat. Avoid extended low power
or no power descents.
Using the carburettor heat every time you reduce power is a good
operating procedure and much safer than the sometime POH suggestion
for use when required. Some aircraft are equipped with a
carburettor air temperature gauge to warn if the internal temperature of
the carburettor is conducive to icing. This serves notice to use
carburettor heat as the icing preventative which is its primary purpose.
Carburettor heat effects engine operation and power only as does a higher
density altitude. No harm to the engine occurs beyond that which may occur
through the ingestion of unfiltered air. It is never wrong to use
carburettor heat as an icing preventative prior to any power reduction.
The use of carburettor heat is too late if the engine has become cooled to
the point where it is unable to melt any existing ice. If you need as
much engine heat as you can get, set up a climb attitude even if you
cannot climb. This will be a much better option than one that would
increase speed and engine cooling. Aggressive leaning and use of the
magneto switch to cause a backfire are emergency measures.
Carburettor heat use should be limited to operational checks while on the
ground as a standard procedure since air going to the carburettor will be
unfiltered and allows dust and abrasives into the internal engine.
However, some ground temperature dew point conditions may require the use
of carburettor heat on the ground, regardless. Having a carburettor air
temperature gauge is highly recommended to increase awareness and accuracy
in the use of carburettor heat.
Some POH do not suggest or recommend CH application at power reductions.
NTSB recommends use on power reduction regardless of POH. A substantial
number of engine failures occur because of failure to recognize
carburettor ice and apply heat immediately and fully. Once the engine has
stopped the rapid cooling caused by the descent limits the effectiveness
of any latent heat remaining in the system.
If engine failure seems imminent induce a backfire by turning the magneto
switch to off and then back on. A backfire may be further induced by
leaning. The backfire can jar any ice loose. Use carburettor heat in
high moisture conditions just prior to takeoff while entering the runway.
Always use full C.H. since partial applications can actually cause
carburettor ice. Any time you have carburettor ice you also have a rich
mixture which of itself will cause a rough engine. The situation
permitting lean the mixture aggressively until the engine begins to fire
again. A running engine will begin to produce the heat necessary to allow
the C.H. to melt the intake ice.
Results
The concept of relative safety
extends itself to all matters of flying.
The emergency procedures for engine failure
in all carburettor aircraft includes the application of carburettor heat,
immediately and fully. Required pilot knowledge should be knowing why you
use carburettor heat in this way, what the effect will be in the near term
and what future benefit is to be expected. The initial effect will be an
even greater loss of power, soon to be followed by a rough running engine,
and an increase in rpm as ice is removed as water. The final increase
occurs when carburettor heat is removed. Failure to understand what
is happening and what to expect can be a fatal deficiency.
Removal depends on availability of hot air from alternate air intake
system. FAA requires that CH be able to provide 90 degree air down to 30
degrees outside temperature with engine at 75% power. If you allow engine
temperatures to drop, as during descent, this heat may not be available
for ice removal. In a C-150 the air intake for C. H. is on the right side
of the engine cooling intake as seen from the cockpit. The intake on
the left is for cabin heat.
Symptoms can be varied but involve initial loss of power, engine
roughness, and stoppage. If ice is suspected do not move the throttle.
Such movement can cause ice to break loose and further clog the Venturi If
possible enter a climb attitude to lessen the flow of cooling air over the
engine. Power loss is shown by lower RPM in fixed pitch and lower manifold
readings in constant speed propellers. It make take 15 or more seconds to
clear the ice. Throttle may be difficult to move. (However, difficulty in
moving the throttle can be caused by congealed grease inside the throttle
cable due to cold temperatures.) If the pilot is not sensitive
to engine sounds the power loss may occur quickly enough to result in
stoppage. I insist that my trainees keep their hands on the throttle below
1000' but never recommend constant removal regardless of altitude. You
need to know if any powerchange is due to throttle movement. Any
unexplained loss of power should be assumed to be due to carburettor ice.
Apply full carburettor heat.
Removal of ice requires application of full CH for as long as it takes to
have the engine rise above its additional lower RPM and roughness due to
the introduction of hot air. Removal of CH will cause an additional RPM
rise. Use of partial heat may make it possible for any moisture to
re-freeze. Use of throttle prior to allowing carburettor heat to become
effective may cause the butterfly to jam the ice and stop the engine.
When carburettor heat is turned on there is normally a slight drop in rpm
because heater muff heats the air going into the carburettor. Hot air has
a lower density which means less oxygen is getting into the engine. The
mixture of air and fuel is made richer. On occasions when the outside
temperature is quite near that of the engine, no drop in rpm may occur.
This is normal because the expected drop in rpm is due to a marked
difference in outside air temperature and engine heated air. Some pilots
will add power prior to the use of carburettor heat so maintain engine
temperature and power should a sudden ingestion of water occur. More
conventional wisdom indicates that such power involves movement of the
carburettor butterfly and may result in sudden blockage and engine
failure. Pilots have been surprised by sudden engine roughness or stoppage
in what they considered to be non-icing conditions. The temperature
inside the carburettor can drop to freezing level in the carburettor
venturi making ice a possibility in all but the driest air conditions.
The initial effect of adding carburettor heat will be an even greater loss
of power, soon to be followed by a rough running engine, and an increase
in rpm as ice is removed as water. The final increase occurs when
carburettor heat is removed. A pilot should be aware that a reversal of
the results of carburettor heat application can and does occur at
altitude, when leaned for taxi, and on takeoff. In these situations any
increase in rpm caused by application of carburettor heat is indicative of
improper leaning.
Tests have shown that cruise power and full C.H. is not damaging to the
engine. The drop of power and even increased roughness often frightens
a pilot into taking off the heat. This is a no-no. Don't remove the C.H.
until the engine smoothes out even though at reduced power. Since an iced
carburettor is running rich, leaning will improve engine operation until
C.H. melts the ice. After removing C.H. be sure to readjust the mixture.
A NTSB study shows that there are a number of 'unexplained' engine
failures every year. By the time the airplanes are inspected there is no
visible 'explanation' for the engine stoppage. NTSB suspects carburettor
ice. Over 18 out of the 35 stoppages cause accidents annually
Questions and Answers
What
is carburettor ice?
May form at the fuel discharge nozzle, in the venturi, on or around the
butterfly valve, or in the passages from the carburettor to the engine.
Why is carburettor ice dangerous?
It restricts the power output and may stop engine by depriving it of air.
What causes carburettor ice?
Forms during vaporization of fuel. Expansion of air causes sudden cooling
of the air fuel mixture. If water is present it may be deposited as frost
or ice inside the carburettor.
What are ideal carburettor ice conditions?
Usually 68F or less where visible moisture exists. When partial throttle
is being used. Most critical operational periods are during partial cruise
or descent. Apply C.H. ahead of such operations and maintain sufficient
power to retain reserve of engine heat.
How is carburettor ice detected?
First sign is loss of rpm or drop in manifold pressure (no drop in rpm}.
Engine roughness occurs. Carburettor air temperature in the yellow.
How is carburettor ice prevented?
Carburettor heat is a preventative. It may not melt existing ice. Periodic
checks advisable when favourable conditions exist.
Always apply full
heat. Always apply full heat during partial throttle situations.
What is the most important reason for not taxiing with
carburettor heat on?
C. H. allows induction air to bypass the air filter. Unfiltered air
contains abrasives that can enter the engine and cause damage.
Why do Lycoming engines often say carburettor heat is optional to
the extent that ice must be recognized before applying carburettor heat?
The mounting of the carburettor to the Lycoming oil pan allows the engine
heat to reach the carburettor. Continental engines have their carburettor
separated from the engine so engine heat does not give this beneficial
side effect. Since Continental has a cooler carburettor they tend to get
more power than an equal displacement Lycoming.
the
difference between and engine with carburettor and one with injection
Carburettor heat uses hot air taken
from the engine exhaust system.
Alternate-air uses heat from the cooling fins of the cylinders.
Both system use a hot air door that shuts down the usual engine air
filter system.
Avoid use of either system for extended periods on the ground because
the air is unfiltered from dust..
