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. 

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. 

## Bleed Air Heating Systems 

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. 

## Electrical System 

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: 

• Alternator/generator 

• Battery 

• Master/battery switch 

• Alternator/generator switch 

• Bus bar, fuses, and circuit breakers 

• Voltage regulator 

• Ammeter/loadmeter 

• Associated electrical wiring 

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. 

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. 

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: 

• Position lights 

• Anticollision lights 

• Landing lights 

• Taxi lights 

• Interior cabin lights 

• Instrument lights 

• Radio equipment 

• Turn indicator 

• Fuel gauges 

