Oxygen Systems

Understanding Aircraft Oxygen Systems

Oxygen systems are a critical component of pressurized aircraft, ensuring crew and passenger safety during decompression events and high-altitude operations. In modern transport-category aeroplanes, aviation regulations and ATPL-level knowledge emphasize that there are typically two independent oxygen systems: one dedicated to the flight deck and one for the cabin. Flight deck systems usually store oxygen as a high-pressure gas and supply it through demand or pressure-demand regulators, enabling 100% oxygen and positive-pressure breathing when required. Cabin systems, by contrast, are designed primarily for emergencies and typically use continuous-flow delivery, optimized to keep passengers conscious and safe until the aircraft descends to a breathable cabin altitude.

Passenger oxygen is most often supplied via chemical oxygen generators that activate when a mask is pulled sharply down from its stowage. This action ignites the generator and starts a continuous flow of oxygen. Importantly, passenger masks deliver a mixture of oxygen and cabin air rather than pure oxygen; this is adequate for short-term use following a decompression. Masks deploy automatically when cabin altitude rises to around 14,000 ft, ensuring rapid access in a sudden decompression at cruise levels (e.g., FL370). Because these masks do not filter smoke, they are not intended for smoke or fume events. For such scenarios, smoke hoods or PBE (protective breathing equipment) cover the entire head and provide a separate, continuous oxygen source to maintain a protective environment.

Oxygen system handling and maintenance are governed by strict procedures. Cylinders are pressure vessels; pilots determine onboard oxygen quantity by reading system pressure, noting that indicated pressure varies with temperature. Overpressure protection is provided by relief devices that discharge oxygen overboard if maximum operating pressure is exceeded. Equally critical is contamination control: oxygen equipment must be kept absolutely free of oil or grease, as these substances can ignite spontaneously in an oxygen-rich environment. Typical flight deck procedures include preflight checks of cylinder pressure, regulator settings, flow indicators, and mask microphone operation, in accordance with the AFM/FCOM and company SOPs.

Understanding delivery modes is key at ATPL level. Continuous-flow masks (primarily for passengers) deliver a steady oxygen supply regardless of the user’s breathing cycle. Demand masks supply oxygen only on inhalation, conserving gas, while pressure-demand masks add positive pressure to protect against hypoxia at very high cabin altitudes and to counteract smoke ingress. Knowing when and how these systems activate, how to don masks promptly, and the required descent procedures following decompression are core elements of aircraft systems knowledge and emergency procedures training.

What This Question Bank Covers

• System architecture: independent cockpit and cabin oxygen systems; storage as compressed gas on the flight deck.
• Passenger oxygen operation: automatic mask presentation, chemical oxygen generators, continuous-flow delivery, and oxygen/cabin-air mixing.
• Crew oxygen delivery modes: demand and pressure-demand functions, 100%/emergency selections, and positive-pressure breathing.
• Regulatory triggers and procedures: mask deployment thresholds, immediate use during decompression, and emergency descent considerations.
• Inspection and safety: reading cylinder pressure for quantity, temperature effects, overpressure relief (safety plugs), and strict no oil/grease contamination.
• Smoke/fume protection: limitations of passenger masks and the role of smoke hoods/PBE for effective head and airway protection.