Aircraft Systems

by changing the airspeed or the power output of the engine. Some exhaust systems have an EGT probe. This probe High engine temperatures can be decreased by increasing the transmits the EGT to an instrument in the flight deck. The airspeed and/or reducing the power. EGT gauge measures the temperature of the gases at the exhaust manifold. This temperature varies with the ratio of The oil temperature gauge gives an indirect and delayed fuel to air entering the cylinders and can be used as a basis indication of rising engine temperature, but can be used for regulating the fuel-air mixture. The EGT gauge is highly for determining engine temperature if this is the only accurate in indicating the correct fuel-air mixture setting. means available. When using the EGT to aid in leaning the fuel-air mixture, fuel consumption can be reduced. For specific procedures, Most aircraft are equipped with a cylinder-head temperature refer to the manufacturer’s recommendations for leaning the gauge that indicates a direct and immediate cylinder fuel-air mixture. temperature change. This instrument is calibrated in degrees Celsius or Fahrenheit and is usually color coded with a green Starting System arc to indicate the normal operating range. A red line on Most small aircraft use a direct-cranking electric starter the instrument indicates maximum allowable cylinder head system. This system consists of a source of electricity, wiring, temperature. switches, and solenoids to operate the starter and a starter motor. Most aircraft have starters that automatically engage To avoid excessive cylinder head temperatures, increase and disengage when operated, but some older aircraft have airspeed, enrich the fuel-air mixture, and/or reduce starters that are mechanically engaged by a lever actuated by power. Any of these procedures help to reduce the engine the pilot. The starter engages the aircraft flywheel, rotating temperature. On aircraft equipped with cowl flaps, use the the engine at a speed that allows the engine to start and cowl flap positions to control the temperature. Cowl flaps maintain operation. are hinged covers that fit over the opening through which the hot air is expelled. If the engine temperature is low, the cowl Electrical power for starting is usually supplied by an onboard flaps can be closed, thereby restricting the flow of expelled battery, but can also be supplied by external power through hot air and increasing engine temperature. If the engine an external power receptacle. When the battery switch is temperature is high, the cowl flaps can be opened to permit turned on, electricity is supplied to the main power bus bar a greater flow of air through the system, thereby decreasing through the battery solenoid. Both the starter and the starter the engine temperature. switch draw current from the main bus bar, but the starter will not operate until the starting solenoid is energized by Exhaust Systems the starter switch being turned to the “start” position. When Engine exhaust systems vent the burned combustion gases the starter switch is released from the “start” position, the overboard, provide heat for the cabin, and defrost the solenoid removes power from the starter motor. The starter windscreen. An exhaust system has exhaust piping attached motor is protected from being driven by the engine through a to the cylinders, as well as a muffler and a muffler shroud. clutch in the starter drive that allows the engine to run faster The exhaust gases are pushed out of the cylinder through than the starter motor. [Figure 7-20] the exhaust valve and then through the exhaust pipe system to the atmosphere. When starting an engine, the rules of safety and courtesy should be strictly observed. One of the most important safety For cabin heat, outside air is drawn into the air inlet and is rules is to ensure there is no one near the propeller prior to ducted through a shroud around the muffler. The muffler is starting the engine. In addition, the wheels should be chocked heated by the exiting exhaust gases and, in turn, heats the and the brakes set to avoid hazards caused by unintentional air around the muffler. This heated air is then ducted to the movement. To avoid damage to the propeller and property, cabin for heat and defrost applications. The heat and defrost the aircraft should be in an area where the propeller will not are controlled in the flight deck and can be adjusted to the stir up gravel or dust. desired level. Combustion Exhaust gases contain large amounts of carbon monoxide, During normal combustion, the fuel-air mixture burns in a which is odorless and colorless. Carbon monoxide is deadly, very controlled and predictable manner. In a spark ignition and its presence is virtually impossible to detect. To ensure engine, the process occurs in a fraction of a second. The that exhaust gases are properly expelled, the exhaust system mixture actually begins to burn at the point where it is ignited must be in good condition and free of cracks. 7-18