senses ventilation air temperature) turns on the fuel solenoid  allowing fuel to spray into the burner chamber. Fuel mixes  with air inside the chamber and is ignited by the spark plug,  producing heat.  The by-product, carbon monoxide, leaves the aircraft through  the heater exhaust pipe. Air flowing over the outside of the  burner chamber and inside the jacket of the heater absorbs  the heat and carries it through ducts into the cabin. As the  thermostat reaches its preset temperature, it turns off the fuel  solenoid and stops the flow of fuel into the burner chamber.  When ventilation air cools to the point that the thermostat  again turns the fuel solenoid on, the burner starts again.  Engine-driven alternators or generators supply electric  current to the electrical system. They also maintain a  sufficient electrical charge in the battery. Electrical energy  stored in a battery provides a source of electrical power for  starting the engine and a limited supply of electrical power  for use in the event the alternator or generator fails.  Most DC generators do not produce a sufficient amount of  electrical current at low engine rpm to operate the entire  electrical system. During operations at low engine rpm, the  electrical needs must be drawn from the battery, which can  quickly be depleted.  This method of heat is very safe as an overheat switch is  provided on all combustion heaters, which is wired into  the heater’s electrical system to shut off the fuel in the case  of malfunction. In the unlikely event that the heater fuel  solenoid, located at the heater, remains open or the control  switches fail, the remote fuel solenoid and/or fuel pump is  shut off by the mechanical overheat switch, stopping all fuel  flow to the system.  As opposed to the fuel fired cabin heaters that are used  on most single-engine aircraft, it is unlikely for carbon  monoxide poisoning to occur in combustion heaters.  Combustion heaters have low pressure in the combustion  tube that is vented through its exhaust into the air stream. The  ventilation air on the outside of the combustion chamber is  of higher pressure than on the inside, and ram air increases  the pressure on the outside of the combustion tube. In the  event a leak would develop in the combustion chamber, the  higher-pressure air outside the chamber would travel into the  chamber and out the exhaust.  Alternators have several advantages over generators.  Alternators produce sufficient current to operate the entire  electrical system, even at slower engine speeds, by producing  alternating current (AC), which is converted to DC. The  electrical output of an alternator is more constant throughout  a wide range of engine speeds.  Some aircraft have receptacles to which an external ground  power unit (GPU) may be connected to provide electrical  energy for starting. These are very useful, especially  during cold weather starting. Follow the manufacturer’s  recommendations for engine starting using a GPU.  Bleed air heating systems are used on turbine-engine  aircraft. Extremely hot compressor bleed air is ducted into  a chamber where it is mixed with ambient or re-circulated  air to cool the air to a useable temperature. The air mixture  is then ducted into the cabin. This type of system contains  several safety features to include temperature sensors that  prevent excessive heat from entering the cabin, check  valves to prevent a loss of compressor bleed air when  starting the engine and when full power is required, and  engine sensors to eliminate the bleed system if the engine  becomes inoperative.  The electrical system is turned on or off with a master switch.  Turning the master switch to the ON position provides  electrical energy to all the electrical equipment circuits  except the ignition system. Equipment that commonly uses  the electrical system for its source of energy includes:  Most aircraft are equipped with either a 14- or a 28-volt direct  current (DC) electrical system. A basic aircraft electrical  system consists of the following components:  Bleed Air Heating Systems  Electrical System  •  Battery  •  Master/battery switch  •  Alternator/generator switch  •  Bus bar, fuses, and circuit breakers  •  Voltage regulator  •  Ammeter/loadmeter  •  Associated electrical wiring  •  Position lights  •  Anticollision lights  •  Alternator/generator  •  Fuel gauges  •  Turn indicator  •  Radio equipment  •  Instrument lights  •  Interior cabin lights  •  Taxi lights  •  Landing lights  7-30 