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the gremline digest — carburettor icing in aircraft
Understanding Carburettor Icing
A high proportion of accidents to GA aircraft in the United Kingdom are either caused, or contributed to, by carburettor icing. Perhaps a better understanding of carburetion, carburettors and how and why carburettor icing forms would help pilots to avoid that sinking feeling as the engine splutters and the power fades away.
Most pilots have probably looked at the classic graph illustrating the relationship between dewpoint, temperature and relative humidity that shows areas of different rates of carburettor icing likely to be encountered at various power settings. This illustration is included later in this article, but the graph alone does not seem to get the message across. A deeper understanding of the mechanics of carburetion and of ice formation within the carburettor may help.
More powerful piston engines, as fitted to
ex-military aircraft for example, may have an aneroid controlled needle
within the carburettor jets so as to maintain a constant weight of fuel to
weight of air at all altitudes. These engines may also have an automatic
mixture control to bring in additional jets to enrich the mixture at high
power settings by an interconnection between either the throttle lever or the
The Mechanisms of Carb Icing
Anticipating Carburettor Icing
Now that we know why carburettor icing occurs
we need to examine when it will be most likely to occur. The amount of
carburettor icing encountered depends on the relative humidity of the outside air. The
higher the relative humidity, the more likely is the occurrence of
carburettor icing, because of the amount of water vapour in the air. The
actual temperature of the air has very little to do with the risk of carb
icing. For example, air having a relative humidity of 50% at +10C will have a
relative humidity of 100% (saturated) at 0C. Any further drop in air
temperature will cause condensation to take place below freezing point, and
ice will form. In general, the lower the temperature of the outside air, the
smaller will be the amount of moisture it contains. That is, cold air is dry
--- and warm air is moist. Therefore, you are more likely to suffer
carburettor icing when flying in clear, cloudless warm air than you are in
clear, cloudless cold air. That is an important point not always appreciated by
private pilots. Perverse as it may seem, you are more likely to
suffer carb icing on a warm day than on a cold day.
Dealing With Carb Icing
The picture below shows a typical system for providing hot air to a carburettor.
This is the installation in a Cessna 182 and
illustrates the simplicity (crudeness?) of a typical general aviation
aircraft. The baffle is wrapped around part of the engine exhaust pipes, thus
collecting hot air when the exhaust pipes are hot. This air is then fed
through the (red) hot air pipe towards the carburettor. A simple gate valve,
manually controlled by the pilot, may be opened or closed to allow either hot
or cold air into the carburettor. The weaknesses of this system include the
fact that the pilot has to recognise either the possibility of ice formation,
or catch the beginning of ice formation, and then take the correct action to
either prevent ice forming or to get rid of ice that has already formed.
Another obvious disadvantage of this system is the fact that it will only
provide air at a temperature dependant on the heat of the exhaust pipes. This
means that a system like this may not be efficient for some time after start.
It should also alert pilots to the need to select hot air BEFORE reducing power for a
descent or approach. It is bad airmanship to close the throttle
first and then select hot air to the carburettor.
It was interesting to examine a de Havilland “Domine” in a Haverfordwest Airport hangar. This aircraft was built as a de Havilland “Dragon” in 1929 and later modified to become a “Domine.” It still retains its original 200hp de Havilland Gipsy Queen engines. These are equipped with a fully automatic carburettor icing prevention system. Also, the air driven flight instruments are driven by suction in the engine exhaust system, so there is no possibility of the loss of flight instruments due to impact icing of the instrument system venturi. The “Domine” has almost exactly the same capabilities and performance as the Britten-Norman “Islander” built many decades later. If de Havilland could provide fully automatic ice protection for the carburettors of their Gipsy Queen engines in 1929, why are we still flying about with bits of wire connected to tin ducts that the pilot has to remember to operate? Answers on a ten dollar note, please!
There are some conditions in which the use of hot air
may actually start the formation of carburettor icing instead of preventing
it. If the refrigeration and throttle icing cooling effects (see above) in
the carburettor are high and are assisted by the OAT and dewpoint, the final
temperature drop may be so high that the temperature inside the carburettor
is so low that ice will not stick. In these conditions, the use of hot air
may raise the temperature back into the range at which the ice will stick and
thus induce carburettor icing.
The above diagram shows the degree of carburettor icing likely to be encountered at various combinations of outside air temperature and dewpoint. It may be worth reminding pilots that the closer together are the OAT and the dewpoint the greater is the saturation of the air i.e. the more moisture there is in the air.
The red area on the diagram indicates that there will be SERIOUS ICING AT ANY POWER SETTING.
The orange area indicates that there will be MODERATE ICING AT CRUISE POWER and SERIOUS ICING AT DESCENT POWER.
The blue area indicates that there still will be SERIOUS ICING AT DESCENT POWER.
The green area indicates that there will be LIGHT ICING AT CRUISE OR DESCENT POWER.
The dotted horizontal line at about +22C dewpoint indicates that this is the probable upper limit of dewpoint encountered in NW Europe.
I suggest that your pre-flight planning should always include a careful look at the OAT and dewpoint relationship. Armed with that knowledge and a clear understanding of the carburettor hot air system on YOUR aircraft you should avoid problems with carb icing and we will have fewer GA accidents related to carburettor icing.
Glossary (For our more
comprehensive Glossary of aviation and technical terms,
PS. I was surprised to find a hand-held battery-operated digital combined thermometer and hygrometer that gives instant readings of air temperature and relative humidity, simultaneously. The ranges of this gadget are from 0 to +50C and relative humidities from 25% to 95% so it covers the areas where carburettor icing is likely to be encountered. It is made by Digitron and runs off AAA batteries. Perhaps this would be a useful gadget for your flying club or flying school to have available – so long as the readings are taken outside!
Text and Photographs © 2008 Gremline & Hill House
Publications, unless otherwise stated.
Text and Photographs © 2008 Gremline & Hill House Publications, unless otherwise stated.