Melting Moments: Understanding Carburettor Icing
By Mike Watson
ATSB's air safety investigator, Mike Watson, in
his unique style, discusses the insidious dangers of carburettor
icing.
The aircraft was on short final for runway 29L when the
pilot made a brief Mayday call. The aircraft was then observed to
land in a car-yard, short of the runway. Both occupants managed to
evacuate without injury.
The pilot later reported that the engine did not respond
when an increase in RPM was required, as the aircraft was
undershooting the approach. The aircraft subsequently collided with
a fence, short of the runway.
Weather conditions at the time were conducive to severe
carburettor icing at descent power. It is likely that carburettor
icing occurred during the low power descent and precluded the
engine accelerating above idle power on the final
approach.
If I were to stuff a gag forcibly down your throat, you would
not be able to get air into your lungs, and after quite a short
time, your body would stop working. The same is true of aircraft
engines: if I were to block their air intakes, they would also stop
working.
The easiest way to block an engines air intake is to freeze
water and simply choke the engine, so that it can no longer
breathe.
Can this happen to my aircraft? Yes. Let us
look at how water can find its way into the air intake when we
least expect it. To do so, we need to examine how water is carried
in the atmosphere and how it can choke a carburettor.
Water is dissolved in the air that both we and our engines
breathe, in much the same way as sugar can be dissolved into a cup
of tea. It is much easier to dissolve sugar into a hot cup of tea
than a cold cuppa, and likewise it is easier to dissolve more water
in warm air than into cold air. Water that has been dissolved into
the atmosphere is actually a gas that you cannot see, and it is
always present in the atmosphere.
Let us take a hot cup of tea, stir in as much sugar as we can,
and then put the cup in the fridge. Once the tea has chilled, you
will see that some of the sugar is no longer dissolved in the tea,
but has formed crystals of sugar in the cup.
In the same way, if you take a cup of warm, humid air, (lots of
water dissolved in it), and cool it, you will see that some of the
water that was dissolved in the air as a gas will change back into
a liquid. Normally, this can be seen is as tiny droplets like those
found in a cloud. Many clouds are formed in exactly this way: as
humid air rises and cools, it cannot hold all its dissolved water,
and some of the water condenses into a cumulus-type cloud.
How will this affect the engine in your aircraft? When air
passes through the carburettor on the way to the engine, fuel is
evaporated into the carburettor. This chills the air, in just the
same way as evaporating water chills a swimmer leaving the ocean
for the beach. If this chilled air was previously humid, then some
of the water dissolved in the air will immediately change into
cloud-type water droplets. If the chilling effect of the fuel was
sufficient to cool the carburettor below freezing level, then when
these water droplets hit the sides of the venturi (the part where
the air passes through), or the throttle valve, the water droplets
will freeze in place. This will start the process of choking the
engine. Eventually, if the process is allowed to continue, it will
no longer be able to breathe, and the engine will stop.
The problem will be more pronounced if the engine is operating
at a low power setting. In this case, the airflow through the
carburettor will be partially impeded by the throttle valve. This
valve not only provides more area for the ice to form: it also
increases the partial vacuum downstream of the valve, and that will
cause a further chilling of the air and the water droplets.
It is interesting to note that although fuel does act as a
refrigerant in a carburettor, it is also needed to keep the engine
running. When your aircraft is flying in cruise, the engine should
normally be leaned with the mixture control. If this is not done,
then not only are you using more fuel than you need to, but you are
also putting more refrigerant into to the carburettor airflow, thus
increasing the likelihood of carburettor icing. This is yet another
good reason for using correct procedures when controlling the
engine!
Even at temperatures exceeding 25 degrees Celsius, air passing
through a carburettor may form ice that can choke your engine. The
more humid the air in which your aircraft is flying, the more
likely it is that ice will form in the air-intake system.
Following a normal climb, the pilot dropped two parachutists
over Hamilton Island. A power-off descent to circuit height
followed. The pilot did not select carburettor heat during the
descent. When on a long final approach, the pilot attempted to
arrest a high descent rate with the use of engine power. The engine
failed to respond. The pilot found that the aircraft was outside
gliding range of the runway. Engine trouble checks failed to
restore power to the engine. The aircraft was ditched in shallow
water and after a successful escape from the cabin, the pilot was
picked up by an island launch.
Bureau of Meteorology data showed that the relative humidity
at ground level was 65 per cent. A carburettor icing-probability
chart showed that serious icing at descent power was to be expected
at such a humidity level.
How do I recognise the start of this problem?
The best solution is to be on the lookout for carburettor icing at
any time the air temperature is less than 30 degrees Celsius. If an
engine is being choked by ice, then its power will be reduced.
However, this is not always easy to detect in the early stages,
particularly if the engine is operating at reduced power settings
or if the air is humid.
Application of carburettor heat for a short time will melt any
ice, and when the carburettor heat is turned off again, you will
see an increase in engine power for the same throttle setting. If
this happens, then apply the carburettor heat, and leave it on!
Textron Lycoming, the engine manufacturer, point out that a
pilot should expect a delay of 30 seconds to several minutes while
ice is melted after carburettor heat is applied. During this time,
rough running and a further reduction in power can be expected. It
is much better to experience a small reduction in power because of
the application of carburettor heat, than to experience a large
reduction in power because of the engine being throttled by
ice!
If you are flying a carburetted engine with a constant speed
propeller, such as a Cessna 180 or 182, then you will not detect
the onset of carburettor icing by a change in RPM. The manifold air
pressure (MAP) is measured between the carburettor and the engine
air inlets, so if the inlet is being blocked by ice and the engine
is still trying to suck-in air, there will be an increased vacuum
in the inlet manifold. This can be seen as a decrease in the
manifold air pressure indication, when there is no other good
reason for it happening.
Its a bit like your lungs being the engine, your lips the
carburettor, and your cheeks the manifold air pressure gauge. If
you breathe normally through your mouth past your lips, the air
pressure in your mouth is nearly the same as atmospheric pressure.
As an analogy, think of when your friendly neighbourhood murderer
sneaks up behind you, and puts his hand over your lips in an
attempt to suffocate you. He is doing the same to you as the block
of ice in the carburettor is to the engine. There will be a
significant vacuum in your mouth, (sucking in of the cheeks) as you
desperately try to suck in your last breath, like your aeroplanes
engine desperately trying to suck in the air it needs past the ice
blockage to the carburettor.
Carburettor icing can sneak up on you when you are cruising
along. In my case, Ive found that can happen as dusk approaches,
and the air cools, making the atmosphere more humid. Its always
worth carefully setting the correct power setting, and noting it,
so that if the RPM or the MAP starts to slowly reduce, and theres
no other good reason for it, like climbing, then you can
immediately suspect icing and do something about it. Its best to
keep an eye out for it.
Carburettor icinga contributing factor? How
does ATSB know if carburettor icing is a contributory factor of an
accident? This is often difficult to answer because ice melts, it
leaves no evidence. It is usually a case of elimination: if the
engine is OK, there is plenty of fuel and all the controls are in
the right place, then the investigators will look at weather
conditions at the time. All we can usually say is that there was no
other good reason for a loss of power. It always seems a shame to
come across such a case, where everything was working fine, only to
find that an aircraft has been downed for such an easily
preventable phenomenon.
During a test flight, on short final approach, the aircraft
encountered windshear. The engine failed to respond to throttle
application. The aircraft landed heavily, ran into a fence and
overturned.
Post-accident inspection of the engine did not reveal any
mechanical reason for the lack of response to throttle application.
Information from the Bureau of Meteorology showed that conditions
were conducive to the formation of serious carburettor icing at any
power setting. The pilot thought that because carby heat was only
applied for about 10 seconds, carburettor ice was the only
reasonable explanation for the loss of power.
Am I likely to experience carburettor icing?
Provided with this article is a chart that will help you to work
out the likelihood of experiencing icing, based on information from
your forecast. You will need to find the temperature and dewpoint,
and these can be found in a meteorological aviation report (METAR),
or a SPECI, or a TTF type forecast. Plot the dew point depression
against the temperature on the chart, and you will see an
indication of the likelihood of experiencing carburettor icing.
Remember, note the air temperature: the most severe icing will
occur at temperatures up to around 20 degrees Celsius, and the
severity will decrease slowly as the temperature increases. The
other major factor is the humidity in the air. If the air feels
muggy, it is humid; if perspiration does not dry rapidly off your
body, it is humid; if a breeze does not cool you on a warm day, it
is humid.
When you are flying, remember that the air gets cooler with an
increase in altitude, and this can increase the humidity. If you
are flying near clouds, then the air is likely to be humid, (the
relative humidity in a cloud is normally 100 per cent).
If you arent sure, check for carburettor icing by applying full
carburettor heat for a short while, and checking for an increase in
power after it is removed.
Prevent carburettor icing at the first indication, rather than
leave it until the engine is choked by ice!
How do I obtain a METAR? Use NAIPS, or the
pilot briefing page on the website http://www.airservicesaustralia.com/, but you
will need to arrange yourself a user name and password first. Some
aerodromes have an aerodrome weather information service (AWIS),
which is available by dialling the approprate phone number from
ERSA.
A full listing of METARs is also available, by State or
Territory from the Bureau of Meteorology on the internet at http://www.bom.gov.au/reguser/by_prod/aviation/
and using the user ID and password provided on that page.