One of the greatest dangers of aircraft operation is ice. Its effect on the operation of the power plant is perhaps far more insidious than on the aircraft. Carburetor or induction ice is not visible to the pilot. The first indication of icing is a loss of power, with the loss increasing at a rapid rate. It is possible to have the power seriously affected in less than a minute after the ice accumulation has started. The engine may cut out completely a short time after this unless corrective steps are taken immediately. Carburetor icing under certain conditions may occur when the atmospheric temperature is as high as 80°F (27°C), although generally it is encountered only between 20°F (-7°C) and 65°F (18°C), when accompanied by high relative humidity (approximately 50% or greater), rain, or overcast. Below 20°F (-7°C), the danger of ice formations is less because the quantity of water vapor in the air is slight.

How can we predict when we should be cautious of icing? A relative humidity of more than 50% is the single best indicator of this hazard. We are not provided relative humidity in our METAR or ATIS reports, but we are given temperature and dew point. Consult the data given and apply it to the Carb Icing Charts, as below, and judge your risk more accurately. Will we be carrying this icing chart while flying? Probably not, therefore, we have to learn to interpret the data reported. The recommendation of this presentation is to apply caution if the temperature is less than 70°F (21°C) AND you determine that the relative humidity probably exceeds 50%. Be “Ice conscious”!

The element of surprise that is attendant to carburetor icing makes it essential that the pilot be conversant with icing symptoms. He must be able to judge and be aware of the weather conditions within which carburetor icing is possible so that he can properly adjust his carburetor heat controls in order to prevent icing. He must be able to recognize the symptoms of icing so that corrective steps can be taken as soon as the first signs of icing have appeared. The ice formation should not be allowed to start, but in the event that it does, it must be checked and eliminated in its early stages. After a severe loss in power, it is almost impossible to check further formation or eliminate the ice unless the aircraft is equipped with de-icing mediums other than heat.

CAUSE OF ICINGS:

Carburetor icing is not a mysterious occurrence but is the result or aftermath of the changes taking place within the carburetor during the course of its intended operation. The carburetor, in performing its function of metering fuel and air under various engine operating conditions, induces the following changes in the fuel and intake air which contribute to the refrigeration of the mixture:

(a) Vaporization of the liquid fuel,
(b) Changes in intake air and mixture velocity and, consequently, pressure,
(c) Evaporation of moisture or water entrained in the intake air.

Of all these factors, the vaporization of fuel contributes the greatest by far to the refrigeration of the mixture and is always present regardless of atmospheric conditions with respect to moisture, water, etc. The vaporization of the liquid fuel requires heat, and this heat is drawn from the intake air. The temperature drop due to vaporization of the fuel may be as much as 40°F. The total drop to all three influencing factors may be as much as 60°F. Under these possible temperature reduction conditions, it can be readily appreciated that even during summer operations the temperature in the carburetor at the throat, throttle, or the adapter can be at 32°F (0°C) or lower, and, if at the same time the atmosphere has a relatively high humidity and/or when flying through clouds or rain, an ice formation will occur.

The temperature drop due to velocity and pressure changes is in most cases small and generally does not exceed 6°F or 7°F. However, when this drop is combined with the refrigeration effect of fuel vaporization, it may mean the difference between an icing or non-icing condition. The cooling induced by the evaporating moisture or water entrained in the air is not as significant as that due to fuel vaporization but, under some atmospheric conditions, may be an important contributing factor to icing.

Air is capable of retaining a certain amount of moisture in the form of water vapor, and the maximum amount it can retain varies directly with the temperature — the higher the temperature, the greater the vapor retaining ability. The maximum amount of moisture that the air can retain at any given temperature is referred to as 100% relative humidity. Air at 60°F (15°C) and 100% relative humidity contains 11.2 lbs. of water vapor per 1000 lbs. of dry air; at 32°F and 100% relative humidity, this is decreased to 3.8 lbs. of water vapor per 1000 lbs. of air. If air at 60°F (15°C) and 100% humidity is cooled to 32°F (0°C), then the moisture in excess of 3.8 lbs. per 1000 lbs. of air is condensed out as free water, and at that temperature, it will freeze.

A 75 H.P. engine at cruising R.P.M. consumes approximately 500 lbs. of air per hour, and, under the conditions outlined above, there would be 4 lbs. of free water per hour passing through the carburetor. This is equivalent to 1.84 cubic inches of water per minute. A portion of this condensed moisture will be deposited on the venturi, butterfly, etc., and with the venturi air temperature at 32°F or lower, ice will be formed. The ice deposited results in a reduction of the venturi opening, blanketing of the carburetor fuel jet, and a possible jamming of the throttle. More than one or all of these conditions may occur simultaneously. Any one is sufficient to cause a serious loss in power and result in complete failure if not corrected immediately.

The rate of ice formation at the throttle is increased with a decrease in throttle setting because of:

(a) the greater expansion effect and consequent pressure drop in the region of the throttle.
(b) the increased rate of fuel evaporation in the region of the throttle.

For this reason, serious icing can occur when maneuvering at low or closed throttle settings. Prolonged glides, simulated landings at part throttle, closed throttle spins, etc., are dangerous under icing conditions and should be avoided.

ICE PREVENTION:

It is well to remember that it takes a considerably greater intake air temperature rise to eliminate an ice formation within a reasonable time interval than it does to prevent the formation. Therefore, the icing problem should be attacked from a preventive point of view rather than an attempted cure after the formation has started.

The most common means of preventing and removing ice formation is heat. No ice can form in the carburetor providing the mixture temperature is held above 32°F and preferably in the region of 35°F (2°C) to avoid making the temperature condition too critical. In engine installations equipped with mixture temperature indicators, the pilot can be guided by temperature and R.P.M. or manifold pressure. Some installations have the temperature indicated in the intake air duct before the carburetor; in those cases, the intake air temperature should be maintained at approximately 75°F (24°C) or at a temperature sufficient to maintain a 35°F mixture. The intake air temperature required to prevent icing varies somewhat with the type of engine and installation. It is advisable to obtain non-icing temperature data from the manufacturer where this type of installation is employed. No matter where the temperature indicator is located, it is essential that either the manifold pressure gauge or the R.P.M. be watched when the outside air temperature is at 70°F (21°C) or lower and the relative humidity exceeds 50%. If a decrease in power or any other icing symptoms are noted, treat it as icing regardless of the mixture or intake air temperature. The temperature indicator may be sufficiently off calibration to falsely indicate a non-icing temperature condition. Where no temperature indicators are installed, as is the case with most low-powered airplanes, it is then advisable to operate with carburetor heat “full on” when the atmospheric temperature is 70°F (21°C) or lower and the relative humidity exceeds 50%. It is permissible to take off with carburetor heat at “full cold” in order to develop the maximum take-off power, but during warm-up and idling and immediately after safe flying speed has been reached, the heat should be applied. Higher inlet air temperatures will richen the mixture, but you are already monitoring the mixture control.

When no mixture or intake air temperature indicators are installed, it is essential that the heat control valve be placed at “full on” and not in any intermediate position, as the pilot has no means of determining at what point adequate heat is being applied. Normally, in low-powered aircraft, very little heat is added to the intake air until the control is almost at its maximum heat position.

In the event that an ice formation and accompanying loss of power do occur because sufficient heat was not applied, then it is advisable to apply all heat available until it is ascertained that all the ice has been eliminated. A check on this can be made by returning the heat to “full cold” for a short period to see whether or not the original power has been regained. After eliminating the ice, the intake heat can be reduced (providing the aircraft is equipped with inlet temperature indicators) to a point where it is sufficient to prevent further ice formations.

If adequate heat is not applied at the early stages of icing, there may be a great enough loss of power so that the available exhaust heat will be insufficient to remove or prevent further formations of ice.

OPERATING RULES:

Carburetor icing can be avoided providing the heating system is adequate, carefully serviced and maintained, and properly used by the pilot. Just a few common-sense rules need to be followed in order to stay clear of engine icing:

(a) Be “Ice conscious” whenever the temperature is 70°F (21°C) or below and the relative humidity exceeds 50% or when visible moisture is present.

(b) Make certain that the intake air preheater is in proper working condition. Check the preheater for cracks and burning through due to heat erosion. Make certain that when the heat control is at “full hot,” the valve completely cuts off the entrance of cold air into the induction system.

(c) Keep the heat control at “full on” when you are “ice conscious,” or maintain a mixture temperature at not less than 35°F, or maintain intake air temperature at 75°F (provided the manufacturer advises that this is sufficient to maintain a 35°F mixture). Always use full heat on all low-power aircraft not equipped with mixture or intake air temperature indicators except during takeoff.

(d) With some engine installations, the available intake air heat may be far greater than that necessary to prevent icing. The continuous use of excessive heat may result in a serious loss of power, engine overheating, detonation, and possibly damage to the (high compression) engine. Under these conditions, be guided by the temperature indicator in the application of heat.

(e) Remember that loss of power means loss in heat available for de-icing and ice prevention. The effect is cumulative. The reduction in heat available may be great enough to make it impossible to remove the ice formed or prevent any further formations and, therefore, result in complete power loss.

(f) From the icing standpoint, it is inadvisable to operate at reduced throttle settings over prolonged periods when icing conditions exist.

This article has been heavily plagiarized from an old 1942 R.C.A.F. pamphlet AFP 14 ICING OF AIRCRAFT, CARBURETTOR ICING AND PREVENTION.

Many other good references were used and are available on the internet. Recommend:

1. http://www.avweb.com/other/gasco-icing.html

2. For Rotax and 2 cycle icing information: http://microlighters.co.za/viewtopic.php?f=13&t=8488&sid=7a32b96f0a9f8e876e86c7cf8fe29616&p=85246 3. FROM THE GROUND UP publication pages 61 through 64. 4. Relative Humidity Calculator: http://www.hpc.ncep.noaa.gov/html/dewrh.shtml