Aircraft Pneumatic Systems — Page 203, Lesson 266

I want to walk you through the pneumatic systems used for non-pressurized flight. We're starting with ram air systems, which are the simplest way to get air into a cabin. These systems are found on unpressurized piston-engined aircraft. The key idea is that ambient atmospheric air — the air outside the aircraft — is introduced into the cabin through forward-facing air intakes. Because the aircraft is moving forward, the air is literally "rammed" into those intakes, which is why we call it ram air. Now, that ram air can be cold. So in a typical light aircraft system, some of that ram air can be heated by exhaust or combustion heaters. Then that heated air is mixed with the cold ambient air in varying proportions. That mixing is what gives you a comfortable cabin temperature — you control the blend of hot and cold. Here's the critical safety point, and I want you to remember this as a professional: it is of extreme importance that the supply ram air does not come into contact with, or is contaminated by, the exhaust gases or the air used for combustion. That's a hard rule. The air you breathe in the cabin must never be mixed with combustion products. Let me describe the components of a typical light aircraft system. We have the heater muff, also called the exhaust muff. This is a close-fitting cowl around the exhaust pipe. It allows ram air to come into close contact with the hot exhaust pipe, which provides hot air for heating the cabin. So the exhaust pipe itself is the heat source — the muff just wraps around it to capture that heat. We also have a fresh air blower, which is used on the ground when there's no ram air. When the aircraft is stationary, there's no forward motion to ram air in, so the blower provides the airflow instead. There are also hot windscreen demisters in this system, which direct heated air onto the windscreen to keep it clear. Fresh cold air can be allowed into the cabin through the ram air inlets on the wing leading edge. And after the air has been used, it's dumped overboard through a vent on the underside of the aircraft. So the flow path is: ram air in through the intakes, mixed with heated air, used in the cabin, then dumped out the bottom. Now let's move to the combustion heater, which is a different way of producing heat. The fuel used in this heater is normally the same fuel used in the aircraft's engines. The heater works by burning a fuel/air mixture within the combustion chamber. Air for combustion is supplied by a fan or blower. The fuel is supplied via a solenoid-operated fuel valve. A solenoid is an electrically operated valve — when you energize it, it opens or closes. That fuel valve is controlled by duct temperature sensors, but it can be manually overridden. So the system normally regulates fuel flow based on the temperature in the duct, but the pilot can take manual control if needed. The system is designed so that there is no possibility of leaks from inside the chamber contaminating the cabin air. That's the same safety principle as the ram air system — combustion products must never reach the cabin. In addition, the system must be provided with a number of safety devices, and these are mandatory. Let me list them for you: First, automatic fuel shut-off in the event of any malfunction. If something goes wrong, the fuel supply is cut automatically. Second, adequate fire protection in the event of failure of the structural integrity of the combustion chamber. If the chamber itself fails, the system must be able to contain or suppress a fire. Third, automatic shut-off if the outlet air temperature becomes too high. If the air leaving the heater gets too hot, the system shuts itself down. So to summarize what we've covered: ram air systems use forward-facing intakes to bring ambient air into the cabin, with exhaust or combustion heaters providing heat, and strict separation between combustion products and cabin air. The combustion heater burns a fuel/air mixture, uses a solenoid-operated fuel valve controlled by duct temperature sensors, and has three mandatory safety devices: automatic fuel shut-off on malfunction, fire protection on structural failure, and automatic shut-off on high outlet temperature.