Contents:
Carburetor Heat; ...Occurrence;
...Three kinds of Carburetor Ice; ...Recognition;
...Using Carburetor Heat; ...Application;
...Results;...Questions and Answers;
…Carburetor Ice Lesson and Opinion); …Carburated
different from Injected; ...About
Carburetor Heat; ......One in 35 in O4:
...
Carburetor Heat
Somewhere on the front of the engine is an air intake for the carburetor.
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
carburetor 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 carburetor 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 carburetor is attached so as to benefit from the heat of the engine itself.
Carburetor 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 carburetor and air. This air/fuel at lower pressure cools and takes even more heat away from the metal of the carburetor. At some point any moisture in the air will freeze on the metal of the carburetor. The ice that adds to the venturi constriction will both increase the cooling rate, the speed of airflow and lower the pressure. Thus, carburetor ice feeds on itself until the excessively rich mixture kills the engine.
The reason carburetor ice causes a drop in rpm and a rough engine is because of an excessive rich mixture. The addition of hot air by the carburetor 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:
Many years ago I had a totally unexplained engine failure at 800' after taking off from a very remote airport in the Ozarks. I had resigned myself to a treetop landing, and pulled the mixture practically all the way out but not all the way. Suddenly the engine picked up and ran smoothly at full power. No further problem for 2000 miles. Being given a choice between being smart or lucky. Take lucky.
In 1998 I found an article relating to the AOPA Tripacer that explained what
had happened. The carburetor float had stuck so that the rich mixture was
capable of stopping the engine. This is essentially how carburetor ice can
stop your engine unless you pull C.H. and lean until the mixture level adapts
to the venturi throat.
Occurrence
Float type carburetors 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 carburetor will cause vapor expansion
and cooling to create ice from any moisture present. When the ice adheres to
the parts of the carburetor it cuts down the flow of air and 'chokes' the
engine. There is too much fuel for the amount of air available. Carburetor 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 carburetor ice and the use of carburetor heat. The pilot must train to be aware of the weather conditions that have a proclivity for causing carburetor icing. Being aware of the likelihood is the primarily aspect of the anticipation required. Just where icing occurs in the carburetor is a variable. Icing can occur before the butterfly, in the carburetor intake, on the butterfly valve or afterwards. When impact icing blocks the air intake filter as induction icing, then the carburetor heat source serves as alternate air for the engine. The different design of carburetors, engines, and induction systems all make a difference however indeterminate. Some cowling designs are better than others in reducing the occurrence of carburetor 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 carburetor 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 carburetor ice. By keeping both fuel and induction air clean we can avoid these as causes of unusual engine behavior. Carburetor icing occurs when the air, moisture and temperature in the carburetor is modified after ingestion so as to be capable of freezing moisture and adhering it to the internal parts of the carburetor. Carburetor 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 carburetor icing probability chart. In given circumstances carburetor icing can occur at any temperature. Some aircraft models and engines are more susceptible to icing than others. Carburetor 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 carburetor 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 carburetor 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 carburetor 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 carburetor due to adiabatic cooling as the air passes through the venturi by the throttle plate.
The carburetor 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 carburetor heat will prevent the formation of ice in the carburetor and given time it'll melt any existing ice. Even the position of the butterfly valve helps. Carburetor ice can occur at any power setting, it is most likely to occur in the green arc.
Continental's carburetor is suspended below the engine so that there is no residual heat transferred between the engine and the hanging carburetor. Lycomings, on the other hand, have the carburetor secured to the oil pan. The carburetor is heated by the hot engine oil. For this reason Piper recommends carburetor heat be used only as necessary. The pilot determines just where and when necessary exists. Lycoming engines are much less susceptible to carburetor 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 carburetor temperature can drop 60 degrees. Carburetor engines must be able to take 30-degree air through the intake and deliver 120 degrees to the carburetor. 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
Carburetor 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 carburetor 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.
C. H. Reference List
--Ice can form at full power
--Always apply full carburetor heat
--If cruising with C.H. lean the mixture.
--Always use C.H. during descents
Three Kinds of Carburetor 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 carburation system on the throttle valve or
butterfly or on the interior of the venturi system. The water vapor 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
carburetor 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 carburetor heat
in anticipation rather than as reaction. Using the carburetor 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 carburetor 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 carburetor venturi to cool any moisture
present to freezing. Automotive fuel is more likely to cause ice because of its
vapor 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 carburetor 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
carburetor 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 carburetor 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 carburetor heat. Always first with the most.
These forward movements can be accomplished nearly simultaneously in one motion.
The greatest temperature drop occurs in the throat of a carburetor. 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
Carburetor Heat
1. Even in warm weather
2. All or nothing at all, not just an old song.
3. Use before takeoff; not during takeoff
4. The engine talks; you listen and feel.
5. Not for continuous operation on the ground.
6. Check proper operation and maintenance
Application
When you pull the aircraft carburetor 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 carburetor 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.
There are no complex operations in flying for which there are not several
simple, straight forward, and WRONG ways to perform. One such combination of
operation and solutions is the use of carburetor heat. The warning indicators
for the need of carburetor heat are deceptive, variable, and inconsistent. The
actual application of carburetor heat will produce results that are deceptive,
variable, and inconsistent. Every student pilot is goings to have opportunities
to make carburetor heat mistakes. The learning process will consist of both
successes and mistakes in the use of carburetor heat. The training process is
designed to reduce the probability of a mistake resulting in an unpleasant
event. It takes an act of faith to stay with your initial application of
carburetor heat when it only makes things worse. Very often the worst thing that
can happen to a pilot is to 'get away' with a mistake. This applies to use of
carburetor heat as well as any other aspect of flying.
Carburetor heat is intended as a preventative rather than a cure. Heat should be
applied early and fully. Carburetor heat is best used in anticipation of
carburetor ice. Put it on before a potential icing situation occurs. Going into
a slow flight configuration will increase the effectiveness of carburetor heat
by decreasing the cooling air over engine. In addition, carburetor 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 carburetor 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 carburetor 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 carburetor 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 carburetor 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.
Carburetor 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. (I find my personal allergic reaction to pollen ceases above 500')
Carburetor heat should not be used when maximum power is required such as on
takeoff.
The application of carburetor 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
carburetor heat. Carburetor heat enriches the mixture. If you intend to fly with
carburetor heat on you should also plan to lean your mixture since with
carburetor 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 carburetor heat. Avoid extended low power or no power descents.
Using the carburetor 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 carburetor air temperature gauge to warn if the
internal temperature of the carburetor is conducive to icing. This serves notice
to use carburetor heat as the icing preventative which is its primary purpose.
Carburetor 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 carburetor heat as an
icing preventative prior to any power reduction. The use of carburetor 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.
I often use carburetor heat as a cruise descent device. It will decrease power
by about 200 rpm and allow a fuel saving leaner mixture throughout the descent.
You can reduce some power as well. Don't mess with the trim. You will probably
be leveling off at pattern altitude on arrival and by just removing carburetor
heat or adding power you are already trimmed for level. When conditions warrant,
I use carburetor heat when rolling into position on the runway for takeoff.
There are exceptionally cold times when you should not use carburetor heat.
Partial heat applications are never recommended unless performed in conjunction
with a carburetor heat temperature gauge.
Carburetor heat use should be
limited to operational checks while on the ground as a standard procedure since
air going to the carburetor will be unfiltered and allows dust and abrasives
into the internal engine. However, some ground temperature dew point conditions
may require the use of carburetor heat on the ground, regardless. Having a
carburetor air temperature gauge is highly recommended to increase awareness and
accuracy in the use of carburetor 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 carburetor 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 your life's ambition is to become an old pilot, I would become
sensitized to the conditions causing carburetor icing; sensitized to the first
unexplained drop in rpm; and start using anticipatory carburetor heat. If you
think any loss of power is due to ice you should apply heat and try to enter a
climb with any remaining power to conserve and create as much heat as possible.
Any suggestions regarding the use of carburetor heat should be qualified by
reference to the specific model of aircraft. Cessna usually recommends
carburetor heat in operations below the green arc, Piper does not; instead,
Piper's usual recommendation is to use only when indicated. The reason behind
the operating differences has to do with the way the carburetor is mounted below
the engines and operational experience.
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 carburetor heat in high moisture conditions
just prior to takeoff while entering the runway. Always use full C.H. since
partial applications can actually cause carburetor ice. Any time you have
carburetor 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 carburetor aircraft includes the
application of carburetor heat, immediately and fully. Required pilot knowledge
should be knowing why you use carburetor 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 carburetor 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
carburetor ice. Apply full carburetor 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 carburetor heat to become effective may cause the butterfly to
jam the ice and stop the engine.
When carburetor heat is turned on there is normally a slight drop in rpm because
heater muff heats the air going into the carburetor. 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 carburetor 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
carburetor 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 carburetor can
drop to freezing level in the carburetor venturi making ice a possibility in all
but the driest air conditions.
The initial effect of adding carburetor 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 carburetor heat is
removed. A pilot should be aware that a reversal of the results of carburetor
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
carburetor 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 carburetor 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 carburetor ice. Over 18 out
of the 35 stoppages cause accidents annually
Questions and Answers
--What is carburetor ice?
May form at the fuel discharge nozzle, in the venturi, on or around the
butterfly valve, or in the passages from the carburetor to the engine.
--Why is carburetor ice dangerous?
It restricts the power output and may stop engine by depriving it of air.
--What causes carburetor 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 carburetor.
--What are ideal carburetor 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 carburetor ice detected?
First sign is loss of rpm or drop in manifold pressure (no drop in rpm}. Engine
roughness occurs. Carburetor air temperature in the yellow.
--How is carburetor ice prevented?
Carburetor heat is a preventative. It may not melt existing ice. Periodic checks
advisable when favorable conditions exist. Always apply full heat. Always apply
full heat during partial throttle situations.
--What is the most important reason for not taxiing with carburetor 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 carburetor heat is optional to the
extent that ice must be recognized before applying carburetor heat?
The mounting of the carburetor to the Lycoming oil pan allows the engine heat to
reach the carburetor. Continental engines have their carburetor separated from
the engine so engine heat does not give this beneficial side effect. Since
Continental has a cooler carburetor they tend to get more power than an equal
displacement Lycoming.
Carburetor Ice Lesson
and Opinion
Speaking of carb heat, I had a very interesting flight lesson the other day.
My instructor leaned back the engine until it barely ran and asked me what could
I do to fix it. I applied carb heat and it started to run smooth again. Then
with the carb heat on he leaned it back even more again until it started to
barely run and said fix that now. This time I had no ideas of what to do. The
asked me to pump the primer and I ended up flying the plane on the primer. I
don't know if I could have maintained altitude if he had fully leaned it out,
but he explained to me that in the event that I had applied carb heat to late, I
could still bide some time and keep the fire lit enough to get the carb heater
working and eventually thaw out a frozen carb. I thought it was a worthwhile
lesson.
If the carb heat doesn't seem to be doing the job, or if the carb heat cable
comes out in your hand, or you otherwise seem unable to deal with the ice
buildup, this technique has worked for me.
Open the throttle wide open and proceed to do a good demonstration of minimum
controllable airspeed at full power available. This technique minimizes the
blockage in the intake and provides maximum air, while at the same time
producing maximum heat.
Several times I had severe carb icing with the carb heat control inoperative
for one reason or another and was able to clear the ice and continue in this
manner.
HighFlyer
Carbureted Different
from Injected
--Carburetor 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..
About
Carburetor Heat
You may not be familiar with is a NTSB study showing that
there are 35 'unexplained' engine failures every year. By the
time the airplanes are inspected there is no 'explanation' for
the engine stoppage. NTSB suspects carburetor ice. If your life's
ambition were to become an old pilot, I would become sensitized
to the conditions causing carburetor icing; sensitized to the
first unexplained drop in rpm; and start using anticipatory carburetor
heat.
Carburetor icing occurs when the air/moisture/temperature in the carburetor is modified after ingestion so as to be capable of freezing moisture and adhering it to the internal parts of the carburetor. The initial effect of adding carburetor 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 carburetor heat is removed.
I often use carburetor heat as a cruise descent device. It will decrease power by about 200 rpm and allow a fuel saving leaner mixture throughout the descent. You can reduce some power as well. Don't mess with the trim. You will probably be leveling off at pattern altitude on arrival and by just removing carburetor heat or adding power you are already trimmed for level. When conditions warrant, I use carburetor heat when rolling into position on the runway for takeoff.
There are exceptionally cold times when you should not use carburetor heat. Partial heat applications are never recommended. Carburetor heat use should be limited to operational checks while on the ground as a standard procedure since air going to the carburetor will be unfiltered and allows dust and abrasives into the internal engine. However, some ground temperature dewpoint conditions may require the use of carburetor heat on the ground, regardless. Having a carburetor air temperature gauge is highly recommended to increase awareness and accuracy in the use of carburetor heat.
Carburetor heat is intended as a preventative rather than a cure. Heat should be applied early and fully. On first start, there may not be sufficient heat to either prevent or melt any carburetor 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 carburetor heat in anticipation rather than as reaction. Using the carburetor heat every time you reduce power is a good operating procedure and much safer than the POH suggestion for use when required.
When carburetor heat is turned on there is normally a slight drop in rpm because heater muff heats the air going into the carburetor. 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 carburetor 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 carburetor 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 carburetor can drop to freezing level in the carburetor venturi making ice a possibility in all but the driest air conditions.
There is more to becoming an old pilot than just being lucky. The concept of relative safety extends itself to all matters of flying. The emergency procedures for engine failure in all carburetor aircraft includes the application of carburetor heat, immediately and fully. Required pilot knowledge should be knowing why you use carburetor 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 carburetor heat is removed. Failure to understand what is happening and what to expect can be a fatal deficiency.
There is one condition where the application of carburetor heat produces no effect. Using today's fuel practically all pilots will lean while taxiing Lycoming has stated to the effect that it is all right to do your runup while the mixture is still lean since you are not going to full power. However, if you have leaned aggressively while taxiing the engine will be running hotter than normal. The effect of this increase in engine temperature will negate the ability of a carburetor heat application to cause an RPM drop. When the full application of carburetor heat fails to cause an RPM drop a pilot is apt to look for something wrong. Enriching the mixture and then pulling on the heat will give the usual response. The first time it happened to me I was ready to take the plane into the maintenance shop.
The carburetor 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 carburetor heat will prevent the formation of ice in the carburetor and given time it'll melt any existing ice. Even the position of the butterfly valve helps. Carburetor ice can occur at any power setting, it is most likely to occur in the green arc.
In Continentals the carburetor is suspended below the engine so that there is no residual heat transferred between the engine and the hanging carburetor. Lycomings, on the other hand, have the carburetor secured to the oil pan. The carburetor is heated by the hot engine oil. For this reason Piper recommends carburetor heat be used only as necessary. The pilot determines just where and when necessary exists. Lycoming engines are much less susceptible to carburetor icing than Continentals because of their design. However, because of pilot complacency, the icing of a Lycoming is going to be more traumatic.
Any recommendations regarding the use of carburetor heat should be qualified by reference to the specific model of aircraft. Cessna POHs usually recommends carburetor heat in operations below the green arc, Piper does not; instead, Pipers usual recommendation is to use only when indicated. The reason behind the operating differences has to do with the way the carburetor is mounted below the engines and operational experience.
Just where icing occurs in the carburetor is a variable. Icing can occur before the butterfly, in the carburetor intake, on the butterfly valve or afterwards. When impact icing blocks the air intake filter as induction icing, then the carburetor heat source serves as alternate air for the engine. The different design of carburetors, engines, and induction systems all make a difference however indeterminate. Some cowling designs are better than others in reducing the occurrence of carburetor ice.
Using the carburetor heat every time you reduce power is a good operating procedure and much safer than the POH suggestion for use when required. Some aircraft are equipped with a carburetor air temperature gauge to warn if the internal temperature of the carburetor is conducive to icing. This serves notice to use carburetor heat as the icing preventative which is its primary purpose. Carburetor 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 carburetor heat as an icing preventative prior to any power reduction. The use of carburetor 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.
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 carburetor 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 carburetor 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 carburetor heat. Always first with the most. These forward movements can be accomplished nearly simultaneously in one motion.
There are two very important training aspects related to carburetor ice and the use of carburetor heat. First, the pilot must train to be aware of the weather conditions that have a proclivity for causing carburetor icing. Being aware of the likelihood is the primarily aspect of the anticipation required. Recognition is important but it comes after the fact. Depending on the circumstances, after the fact, may be too late. Like 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. Carburetor heat will not allow the time because as you lose power you are losing the engines ability to produce the required heat. Planning your options now is better than a spur of the moment decision that can be wrong.
There are no complex operations in flying for which there are not several simple, straight forward, and WRONG ways to perform. One such combination of operation and solutions is the use of carburetor heat. The warning indicators for the need of carburetor heat are deceptive, variable, and inconsistent. The actual application of carburetor heat will produce results that are deceptive, variable, and inconsistent. Every student pilot is goings to have opportunities to make carburetor heat mistakes. The learning process will consist of both successes and mistakes in the use of carburetor heat. The training process is designed to reduce the probability of a mistake resulting in an unpleasant event. It takes an act of faith to stay with your initial application of carburetor heat when it only makes things worse. Very often the worst thing that can happen to a pilot is to get away with a mistake. This applies to the use of carburetor heat as well as any other aspect of flying.
One of 35 in O4
About three hours into the until-then uneventful night flight, the
pilot contacted Mansfield, OH approach control, told them that he was
experiencing a partial power loss, and requested to land at Mansfield.
Mansfield runway 32 was straight ahead and four miles, and the pilot was
cleared to land. According to the passenger, the pilot applied carburetor heat
for about ten seconds, and with no change in engine operation, returned it to
cold and said that he thought they were experiencing vapor lock. Shortly
thereafter, the pilot informed Mansfield approach that he was going to land on
a road, and that the engine had quit.
Investigation revealed that as the flight progressed, the temperature and dew point spreads were getting closer together. At the time of the accident, the temperature in Mansfield was minus one degree Celsius (30.2 F) and the dew point was minus four degrees Celsius (24.8 F).
The NTSB determined the cause of this accident to be the pilot's improper
use of carburetor heat, and subsequent forced landing.
The pilot might have been able to prevent this accident had he taken advantage
of the free advice in the ASF Aircraft
Icing Safety Advisor. It says, "At the first indication of
carburetor ice, apply full carburetor heat and LEAVE IT ON. The engine may run
rougher as the ice melts and goes through it, but it will smooth out
again."
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