At high altitudes, temperatures can drop below zero degrees Fahrenheit. Combined with seasonally cold temperatures, heating an aircraft’s cabin interior becomes a necessity, not a luxury. Pressurized aircraft use air cycle conditioning systems that mix bleed air from the engines with cold air produced by the air cycle machine expansion turbine to obtain warm air for the cabin. Some turbine-powered aircraft not equipped with air cycle systems still use engine compressor bleed air to heat the cabin, by mixing it with ambient air, or cabin return air, and distributing it back throughout the aircraft via ducting. These bleed air heating systems are simple and function well, as long as the valves, ducting, and controls work well.
Electric heating systems are another option. Electricity flowing through a heating element makes the element warm, and a fan blows air over the elements and into the cabin to transfer the heat. Other floor or sidewall elements simply radiate heat to warm the cabin. These require a significant amount of the aircraft’s generator output however, and therefore are not very common. They can, however, be used on the ground when powered by a ground electrical source before passengers board.
Most single-engine light aircraft use exhaust shroud heating systems to heat the cabin. Ambient air is directed into a metal shroud or jacket that encases part of the aircraft’s exhaust system. The air is warmed by the exhaust and directed through a firewall heater valve into the cabin. This requires no electrical or engine power and makes use of heat that is otherwise wasted. The largest concern with exhaust shroud heat systems is the possibility of exhaust gases contaminating the cabin’s air. A crack in the exhaust manifold can send carbon monoxide into the cabin, so strict inspection procedures are mandatory to prevent this from happening.
Combustion heaters are also used on small to medium-sized aircraft. It is a heat source independent from the aircraft’s engines, although it does draw fuel from the main fuel system. Similarly, to exhaust shroud systems, they use ambient air that is heated and sent to the cabin, the source in this case being an independent combustion chamber located inside the cylindrical outer shroud of the heater unit. Fuel and air are ignited inside an airtight chamber (the exhaust of which is funneled overboard), while ambient air is directed between the combustion chamber and the outer shroud. There, it absorbs heat by convection, and is channeled into the cabin.
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