There are many different types of aircraft heating systems that are available depending on the type of aircraft. Regardless of which type or the safety features that accompany them, it is always important to reference the specific aircraft operator’s manual and become knowledgeable about the heating system. Each has different repair and inspection criteria that should be precisely followed.
Fuel Fired Heaters
A fuel fired heater is a small mounted or portable space-heating device. The fuel is brought to the heater by using piping from a fuel tank, or taps into the aircraft’s fuel system. A fan blows air into a combustion chamber, and a spark plug or ignition device lights the fuel-air mixture. A built-in safety switch prevents fuel from flowing unless the fan is working. Outside the combustion chamber, a second, larger diameter tube conducts air around the combustion tube’s outer surface, and a second fan blows the warmed air into tubing to direct it towards the interior of the aircraft. Most gasoline heaters can produce between 5,000 and 50,000 British Thermal Units (BTU) per hour.
Fuel fired heaters require electricity to operate and are compatible with a 12-volt and 24-volt aircraft electrical system. The heater requires routine maintenance, such as regular inspection of the combustion tube and replacement of the igniter at periodic intervals. Because gasoline heaters are required to be vented, special care must be made to ensure the vents do not leak into the interior of the aircraft. Combustion byproducts include soot, sulfur dioxide, carbon dioxide, and some carbon monoxide. An improperly adjusted, fueled, or poorly maintained fuel heater can be dangerous.
Exhaust Heating Systems
Exhaust heating systems are the simplest type of aircraft heating system and are used on most light aircraft. Exhaust heating systems are used to route exhaust gases away from the engine and fuselage while reducing engine noise. The exhaust systems also serve as a heat source for the cabin and carburetor.
The risks of operating an aircraft with a defective exhaust heating system include carbon monoxide poisoning, a decrease in engine performance, and an increased potential for fire. Because of these risks, technicians should be aware of the rate of exhaust heating system deterioration and should thoroughly inspect all areas of the exhaust heating system to look for deficiencies inside and out.
Combustion Heater Systems
Combustion heaters or surface combustion heaters are often used to heat the cabin of larger, more expensive aircraft. This type of heater burns the aircraft’s fuel in a combustion chamber or tube to develop required heat, and the air flowing around the tube is heated and ducted to the cabin. A combustion heater is an airtight burner chamber with a stainless-steel jacket. Fuel from the aircraft fuel system is ignited and burns to provide heat. Ventilation air is forced over the airtight burn chamber picking up heat, which is then dispersed into the cabin area.
When the heater control switch is turned on, airflow, ignition, and fuel are supplied to the heater. Airflow and ignition are constant within the burner chamber while the heater control switch is on. When heat is required, the temperature control is advanced, activating the thermostat. The thermostat (which 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.