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Melting Moments: Understanding Carburettor Icing

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 there's 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 icing a 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 aren't 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 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 from ERSA.

A full listing of METARs is also available, by State or Territory from the Bureau of Meteorology on the internet at www.bom.gov.au/reguser/by_prod/aviation/ and using the user ID and password provided on that page.

Type: Educational Fact Sheet
Author(s): Mike Watson
Publication date: 8 October 2001
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Last update 23 March 2016
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