Posted Thursday, May 7, 2015 @ 06:52 AM
CARBURETOR ICING is caused by a temperature drop inside the carburetor, which can happen even in conditions where other forms of icing will not occur.
The causes of this temperature drop are twofold:
1. Fuel icing-the evaporation of fuel inside the carburetor. Liquid fuel changes to fuel vapor and mixes with the induction air causing a large temperature drop. If the temperature inside the carburetor falls below 0°C, water vapor in the atmosphere condenses into ice, usually on the walls of the carburetor passage adjacent to the fuel jet, and on the throttle valve. Generally, fuel icing is responsible for around 70% of the temperature drop in the carburetor.
2. Throttle icing the temperature loss caused by the acceleration of air and consequent pressure drop around the throttle valve. This effect may again take the temperature below 0°C and water vapor in the inlet air will condense into ice on the throttle valve.
Two criteria govern the likelihood of carburetor icing conditions: the AIR TEMPERATURE and the RELATIVE HUMIDITY.
The ambient air temperature is important, but not because the temperature needs to be below 0°C, or even close to freezing.
This temperature drop in the carburetor can be up to 30°C, so carburetor icing can (and does) occur in hot ambient conditions.
It is no wonder carburetor icing is sometimes referred to as refrigeration icing. Carburetor icing is considered a possibility within the temperature range of -10°C to +30°C.
The relative humidity (a measure of the water content of the atmosphere) is a major factor.
The greater the water content in the atmosphere (the higher the relative humidity), the greater the risk of carburetor icing.
That said, the relative humidity (RH) does not have to be 100% (i.e., visible water droplets-clouds, rain) for carburetor icing to occur. Carburetor icing is considered a possibility at relative humidity values as low as 30%. Herein lies the real danger of carburetor icing, that is can occur in such a wide range of conditions. Obviously the pilot must be alert to the possibility of carburetor icing at just about all times. Flight in or near clouds or in other visible moisture (i.e., rain) might be an obvious cause of carburetor icing, but visible moisture does not need to be present for carburetor icing to occur.
In this aircraft, fitted with a fixed pitch propeller, the symptoms of carburetor icing are straightforward. A loss of RPM will be the first symptom, although this is often first noticed as a loss of altitude. As the icing becomes more serious, engine rough running may occur.
Apart from the normal check of carburetor heat during power checks, it may be necessary to use the carburetor heat on the ground if carburetor icing is suspected. Safety considerations apart, the use of carburetor heat on the ground should be kept to a minimum, because the hot air inlet is unfiltered, and sand or dust can enter the engine, increasing engine wear.
Very few operators recommend the use of anything other than FULL carburetor heat. A normal carburetor icing check will involve leaving the carburetor heat on (hot) for 5 to 10 seconds, although the pilot may wish to vary this, dependent on the conditions. The use of carburetor heat does increase fuel consumption, and this may be a factor to consider if the aircraft is being flown towards the limit of its range/endurance in possible carburetor icing conditions.
With carburetor icing present, the use of carburetor heat may lead to a large drop in RPM, with rough running. The instinctive reaction is to put the carburetor heat back to cold (off), and quickly. This is, however, the wrong action. Chances are this rough running is a good thing, and the carburetor heat should be left on (hot) until the rough running clears and the RPM rises. In this instance, the use of carburetor heat has melted a large amount of accumulated ice and the melted ice is passing through the engine causing temporary rough running.
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RF4D flying in the Afican Sky!