location, considerable dust is removed as the air is forced  to turn and enter the duct. Since the dust particles are solid,  they tend to continue in a straight line, and most of them are  separated at this point. Those that are drawn into the louvers  are easily removed by the filter. When there is no danger of icing, the heat control is normally  kept in the “cold” position. It is best to leave the control in  this position if there are particles of dry snow or ice in the air.  The use of heat may melt the ice or snow, and the resulting  moisture may collect and freeze on the walls of the induction  system. To prevent damage to the heater valves in the case of  backfire, carburetor heat should not be used while starting the  engine. Also, during ground operation only enough carburetor  heat should be used to give smooth engine operation.  In flight, with air filters operating, consideration must be  given to possible icing conditions which may occur from  actual surface icing or from freezing of the filter element  after it becomes rain soaked. Some installations have a  spring-loaded filter door which automatically opens when  the filter is excessively restricted. This prevents the airflow  from being cut off when the filter is clogged with ice or dirt.  Other systems use an ice guard in the filtered-air entrance. Part-throttle operation can lead to icing in the throttle area.  When the throttle is placed in a partly closed position, it, in  effect, limits the amount of air available to the engine. When  the aircraft is in a glide, a fixed-pitch propeller windmills,  causing the engine to consume more air than it normally  would at this same throttle setting, thus adding to the lack of  air behind the throttle. The partly closed throttle, under these  circumstances, establishes a much higher than normal air  velocity past the throttle, and an extremely low-pressure area is  produced. The low-pressure area lowers the temperature of the  air surrounding the throttle valve. If the temperature in this air  falls below freezing and moisture is present, ice forms on the  throttles and nearby units restricting the airflow to the engine  causing it to quit. Throttle ice may be minimized on engines  equipped with controllable-pitch propellers by the use of a  higher than normal brake mean effective pressure (BMEP) at  this low power. The high BMEP decreases the icing tendency  because a large throttle opening at low engine revolutions  per minute (rpm) partially removes the temperature-reducing  obstruction that part-throttle operation offers.  The ice guard consists of a coarse-mesh screen located a short  distance from the filtered-air entrance. In this location, the  screen is directly in the path of incoming air so that the air  must pass through or around the screen. When ice forms on  the screen, the air, which has lost its heavy moisture particles,  passes around the iced screen and into the filter element.  The efficiency of any filter system depends upon proper  maintenance and servicing. Periodic removal and cleaning of  the filter element is essential to satisfactory engine protection. The induction system should be checked for cracks and leaks  during all regularly scheduled engine inspections. The units  of the system should be checked for security of mounting.  The system should be kept clean at all times, since pieces of  rags or paper can restrict the airflow if allowed to enter the  air intakes or ducts. Loose bolts and nuts can cause serious  damage if they pass into the engine. Dust and dirt can be a serious source of trouble to an aircraft  engine. Dust consists of small particles of hard, abrasive  material that can be carried by the air and drawn into the  engine cylinders. It can also collect on the fuel-metering  elements of the carburetor, upsetting the proper relation  between airflow and fuel flow at all engine power settings. It  acts on the cylinder walls by grinding down these surfaces and  the piston rings. Then, it contaminates the oil and is carried  through the engine, causing further wear on the bearings and  gears. In extreme cases, an accumulation may clog an oil  passage and cause oil starvation. Although dust conditions  are most critical at ground level, continued operation under  such conditions without engine protection results in extreme  engine wear and can produce excessive oil consumption.  When operation in a dusty atmosphere is necessary, the  engine can be protected by an alternate induction system air  inlet which incorporates a dust filter. This type of air filter  system normally consists of a filter element, a door, and  an electrically operated actuator. When the filter system is  operating, air is drawn through a louvered access panel that  does not face directly into the airstream. With this entrance  Since aircraft operate at altitudes where the air pressure is  lower, it is useful to provide a system for compressing the  fuel/air mixture. Some systems are used to normalize the air  pressure entering the engine. These systems are used to regain  the air pressure lost by the increase in altitude. This type of Figure 3-8  provides a general guide to the most common  induction system troubles. On systems equipped with a carburetor air filter, the filter  should be checked regularly. If it is dirty or does not have  the proper oil film, the filter element should be removed and  cleaned. After it has dried, it is usually immersed in a mixture  of oil and rust-preventive compound. The excess fluid should  be allowed to drain off before the filter element is reinstalled.  Paper-type filters should be inspected and replaced as needed. Supercharged Induction Systems Induction System Troubleshooting Induction System Filtering Induction System Inspection and Maintenance 3-5