For airplanes with fixed-pitch propellers, the first indication of carburetor icing is loss of RPM. For airplanes with controllable-pitch (constant-speed) propellers, the first indication is usually a drop in manifold pressure. In both cases, a roughness in engine operation may develop later. There will be no reduction in RPM in airplanes with constant-speed propellers, since propeller pitch is automatically adjusted to compensate for the loss of power, thus maintaining constant RPM.

Use of Carburetor Heat

 The carburetor heater is an anti-icing device that preheats the air before it reaches the carburetor. This preheating can be used to melt any ice or snow entering the intake, to melt ice that forms in the carburetor passages (provided the accumulation is not too great), and to keep the fuel mixture above the freezing temperature to prevent formation of carburetor ice.

 When conditions are conducive to carburetor icing during flight, periodic checks should be made to detect its presence. If detected, full carburetor heat should be applied immediately, and it should be left in the “on” position until the pilot is certain that all the ice has been removed. If ice is present, applying partial heat or leaving heat on for an insufficient time might aggravate the situation.

 When heat is first applied, there will be a drop in RPM in airplanes equipped with fixed-pitch propellers; there will be a drop in manifold pressure in airplanes equipped with controllable-pitch propellers. If no carburetor ice is present, there will be no further change in RPM or manifold pressure until the carburetor heat is turned off; then the RPM or manifold pressure will return to the original reading before heat was applied. If carburetor ice is present, there will normally be a rise in RPM or manifold pressure after the initial drop (often accompanied by intermittent engine roughness); and then, when the carburetor heat is turned “off,” the RPM or manifold pressure will rise to a setting greater than that before application of the heat. The engine should also run more smoothly after the ice has been removed.

 Whenever the throttle is closed during flight, the engine cools rapidly and vaporization of the fuel is less complete than if the engine is warm. Also, in this condition, the engine is more susceptible to carburetor icing. Therefore, if the pilot suspects carburetor-icing conditions and anticipates closed-throttle operation, the carburetor heat should be turned to “full-on” before closing the throttle, and left on during the closed-throttle operation. The heat will aid in vaporizing the fuel and preventing carburetor ice. Periodically, however, the throttle should be opened smoothly for a few seconds to keep the engine warm, otherwise the carburetor heater may not provide enough heat to prevent icing.

 Use of carburetor heat tends to reduce the output of the engine and also to increase the operating temperature. Therefore, the heat should not be used when full power is required (as during takeoff) or during normal engine operation except to check for the presence or removal of carburetor ice. In extreme cases of carburetor icing, after the ice has been removed, it may be necessary to apply just enough carburetor heat to prevent further ice formation. However, this must be done with caution. Check the engine manufacturer’s recommendations for the correct use of carburetor heat.

 The carburetor heat should be checked during the engine runup. To properly perform this inspection, the manufacturer’s recommendations should be followed.

Carburetor Air Temperature Gauge

 Some airplanes are equipped with a carburetor air temperature gauge which is useful in detecting potential icing conditions. Usually, the face of the gauge is calibrated in degrees Celsius (C), with a yellow arc indicating the carburetor air temperatures at which icing may occur. This yellow arc ranges between -15° C and +5° C. If the air temperature and moisture content of the air are such that the carburetor icing is improbable, the engine can be operated with the indicator in the yellow range with no adverse effects. However, if the atmospheric conditions are conducive to carburetor icing, the indicator must be kept outside the yellow arc by application of carburetor heat.
 

 Certain carburetor air temperature gauges have a red radial which indicates the maximum permissible carburetor inlet air temperature recommended by the engine manufacturer; also, a green arc which indicates the normal operating range.

Outside Air Temperature Gauge

 Most airplanes are equipped with an outside air temperature gauge (OAT) calibrated in both degrees Celsius and Fahrenheit. It is used not only for obtaining the outside or ambient air temperature for calculating true airspeed, but also is useful in detecting potential icing conditions.

Fuel Injection System

 Fuel injection systems have replaced carburetors on some engines. In this system, the fuel is normally injected either directly into the cylinders or just ahead of the intake valve. The fuel injection system is generally considered to be less susceptible to icing than the carburetor system. Impact icing of the air intake, however, is a possibility in either system. Impact icing occurs when ice forms on the exterior of the airplane and results in clogging openings such as the air intake for the injection system.
 There are several types of fuel injection systems in use today. Although there are variations in design, the operational methods of each are generally similar. Most designs include an engine-driven fuel pump, a fuel/air control unit, fuel distributor, and discharge nozzles for each cylinder.

 Some of the advantages of fuel injection are:

• Reduction in evaporative icing.
• Better fuel flow.
• Faster throttle response.
• Precise control of mixture.
• Better fuel distribution.
• Easier cold weather starts.
 Disadvantages are usually associated with:
• Difficulty in starting a hot engine.
• Vapor locks during ground operations on hot days.
• Problems associated with restarting an engine that quits because of fuel starvation.

The air intake for the fuel injection system is somewhat similar to that used in the carburetor system. The fuel injection system, however, is equipped with an alternate air source located within the engine cowling. This source is used if the external air source is obstructed by ice or by other matter. The alternate air source is usually operated automatically with a backup manual system that can be used if the automatic feature malfunctions.