Summary
Highlights
Cabin pressurization enables high-altitude operation and prevents hypoxia by maintaining a higher internal air pressure than the ambient external pressure, creating a cabin differential pressure. This differential pressure causes tension stress on the fuselage, which is designed to withstand a maximum amount. A dump valve can relieve positive pressure, while a negative pressure relief valve prevents cabin altitude from exceeding aircraft altitude by allowing ambient air in. A micro switch on the main landing gear prevents pressurization on the ground.
Pressurization involves stuffing air into the cabin and controlling outflow. Methods include independent cabin air compressors, especially for piston engines, which pull and compress outside air. Modern jet aircraft commonly use bleed air from turbine engines for pressurization and air conditioning due to its cleanliness and abundance. Jet pumps augment airflow in some areas. The pressurization controller requires barometric pressure, cabin altitude, and cabin rate of change as inputs. The cabin pressure regulator, controlled by cabin air pressure, adjusts the outflow valve to maintain desired cabin pressure. The system operates in unpressurized, isobaric (controlling cabin altitude), and differential (maintaining maximum pressure differential) modes. If the cabin climbs too fast, the outflow valve needs to close faster.
Oxygen systems serve as backup or primary sources for unpressurized aircraft. Aviation oxygen is 99.5% pure and water-free, unlike medical oxygen, to prevent freezing and corrosion. High-pressure oxygen cylinders must meet ICC or DOT specifications and are identifiable by their green color and 'Aviator's Breathing Oxygen' labeling. A pressure reducer valve lowers cylinder pressure for the system. If it fails, a pressure relief valve vents oxygen overboard through a thermal expansion disc to prevent system overpressure. A missing thermal disc indicates a past overpressure event.
Continuous flow oxygen systems use a rebreather bag, with flow controlled by a calibrated orifice. Diluter demand systems, common for crew, deliver oxygen only when the user breathes, conserving supply. An aneroid in demand regulators adjusts ambient airflow based on cabin altitude to mix with oxygen. Chemical oxygen candle systems (solid state) offer high storage capacity, are simple, and require little maintenance for emergency use, but cannot be shut off once activated. Mechanically separated oxygen systems use a molecular sieve to filter oxygen from air. Oxygen system maintenance precautions include keeping tools oil and grease-free, displaying 'no smoking' signs, providing firefighting equipment, and avoiding electrical sparks due to oxygen's flammability.
Oxygen cylinders require hydrostatic testing to ensure airworthiness. Standard weight cylinders are tested every 5 years. A cylinder near the neck must be stamped with the test date, DOT number, and serial number. Lightweight DOT 3HT cylinders are tested every 3 years and retired after 24 years or 4,380 filling cycles. Composite DOT 162 bottles have a 15-year service life but 10,000 filling cycles. Oxygen quantity in portable cylinders is determined by the pressure gauge. Maintaining pressure in bottles is crucial to prevent moisture accumulation, which can cause corrosion and freeze, blocking the system. Contaminated systems are purged with oxygen, and Teflon tape is used as a lubricant on pipe threads. Leaks are detected by bubble testing with soap solution. Pressure-temperature correction charts (Figure 14 in FAA Appendix 2) are used to assess allowable pressure drops due to temperature changes, in addition to a 5 PSI allowable leakage in a 6-hour period for a leak test.
Aircraft often use air cycle cooling systems, particularly in jets, for air conditioning. These systems consist of a compressed air source (usually engine bleed air), heat exchangers to remove heat, and an expansion turbine. The expansion turbine is critical, as air passing through it undergoes a pressure and temperature drop, producing cold air for the cabin. A water separator removes moisture, and a mix valve controls the supply of hot, cool, and cold air to achieve the desired cabin temperature.
Freon vapor cycle cooling systems, similar to those in cars, are also used. Key components include: a receiver-dryer (reservoir for refrigerant, removes moisture), an expansion valve (meters and reduces liquid freon pressure, causing a temperature drop), an evaporator (lowers cabin air temperature by converting liquid freon to gas), a compressor (converts low-pressure vapor to high-pressure vapor), and a condenser (transfers heat from freon gas to ambient air, converting gas to high-pressure liquid). Frost or ice on the evaporator indicates inadequate airflow. If freon lines connected to the expansion valve are the same temperature, the valve is not metering properly. Water freezing in the expansion valve is a common reason a system won't take a charge.
When servicing Freon systems, check the sight gauge for a steady stream of bubbles, indicating a low charge. Air bubbles disappearing during charging means the proper amount of Freon has been added. For adding liquid Freon, the bottle must be vertical with the outlet down to prevent compressor damage from liquid refrigerant entering the low side. Goggles are essential when working with Freon 12 due to eye damage risk. Freon 12 combusts into deadly phosgene gas over open flames. If a system loses all Freon, first check/add oil, then evacuate, and then recharge. Evacuating the system removes moisture, preventing freezing and the formation of corrosive hydrochloric acid. A vacuum pump lowers the boiling point of water, allowing it to be removed as vapor. Purging should be slow to avoid losing refrigerant oil. Oil seepage indicates a Freon leak. Special refrigeration oil must be used. If the low-pressure gauge fails to come out of vacuum during charging, there's a blockage.
Combustion heaters, found in various aircraft, heat cabins and sometimes provide anti-icing. Their operation is controlled by a thermostat cycling the fuel on and off. Two types of air are used: combustion air (for burning fuel) and ventilating air (warmed by a heat exchanger and distributed for heating). Combustion air relief valves or differential pressure regulators prevent too much air from entering the heater. After fuel system filter maintenance, pressurize the system and check for leaks to prevent fire hazards. In smaller aircraft, engine exhaust heat exchangers provide cabin heat. Regular inspections and carbon monoxide detection tests are crucial due to the risk of carbon monoxide entering the cabin if the heat exchanger leaks.