飞机供氧系统

概述

The aircraft oxygen system provides supplemental breathing oxygen to passengers and crew in the event of a cabin pressurization failure that causes cabin altitude to rise above safe breathable limits. Because useful consciousness time at 35,000 feet without supplemental oxygen is as little as 15–30 seconds, rapid automatic deployment of passenger oxygen masks is a life-critical function. The system is designed to deploy within seconds of a depressurization event and to supply oxygen for the duration of an emergency descent to 10,000 feet — typically 10–15 minutes.

Commercial aircraft use two fundamentally different oxygen supply technologies: chemical oxygen generators for passengers, and compressed gaseous (or liquid) oxygen cylinders for flight crew and cabin crew. This separation reflects the different reliability, duration, and operational requirements of each user group.

工作原理

Passenger oxygen is delivered through Chemical Oxygen Generators (COGs) housed in the Passenger Service Units (PSUs) above each seat, as well as in lavatories and at crew stations. Each COG contains a canister of sodium chlorate (NaClO₃) mixed with iron powder. When the mask is pulled down by the passenger — pulling a lanyard that initiates an igniter — a percussive cap fires, heating the sodium chlorate mixture to approximately 450–600°C. At this temperature the sodium chlorate decomposes exothermically, releasing pure oxygen gas at a rate of approximately 2.5–5 liters per minute at 97+ percent purity. The canister surface becomes hot during this process (a normal characteristic that does not represent a fire hazard under normal conditions). Each COG runs for approximately 12–22 minutes depending on model.

The cockpit oxygen system is separate and uses high-pressure cylinders (typically 1,800 psi) of gaseous oxygen stored in the avionics bay or forward cargo area. Pilot and co-pilot quick-donning masks at each station can be deployed in under five seconds by releasing a clip, and the masks deliver oxygen in pressure demand mode — positive pressure is applied to the face seal, ensuring oxygen flows even if ambient pressure is very low. This is critical because pilots must remain conscious and functional to execute the emergency descent.

关键组件

• Chemical Oxygen Generator (COG): Solid-state exothermic canister producing oxygen on demand; non-rechargeable, replaced after activation or at scheduled maintenance intervals (typically 10–12 years).
• Passenger Oxygen Mask: Continuous-flow oro-nasal mask with a reservoir bag that fills between breaths, improving oxygen efficiency; yellow color coding worldwide for ease of recognition.
• Automatic Deployment Controller: Electrically or pneumatically triggered mechanism that opens PSU oxygen compartment doors when cabin altitude exceeds approximately 14,000 feet (configurable threshold), or manually activated by crew from the cockpit panel.
• Crew Oxygen Cylinder: High-pressure steel or composite cylinder providing gaseous oxygen to flight deck quick-donning masks and portable crew masks; pressure-gauged and inspected at each heavy maintenance check.
• Pressure Demand Regulator: Cockpit mask regulator that switches between dilution (normal), 100% oxygen, and pressure demand (emergency) modes; ensures positive pressure seal in severe depressurization.
• Portable Oxygen Bottles (PBE): Portable emergency oxygen bottles or Protective Breathing Equipment (PBE — smoke hoods with 15–20 min supply) distributed at crew stations for smoke/fume events.

飞机应用

On the Boeing 737-800 and Airbus A320-200, passenger COGs are located in every PSU row with a minimum of one mask per occupant plus 10 percent spares (regulatory requirement). The flight deck uses a gaseous cylinder system with two quick-donning masks. Both aircraft have a third crew oxygen outlet at the observer/jump seat position.

The Boeing 787-9 and Boeing 777-300ER carry additional oxygen capacity commensurate with their long-haul operations, including supplemental portable oxygen for in-flight medical events — a more common concern on ultra-long flights. Some operators of the 777 opt for liquid oxygen systems in the lower lobe, which provide a greater supply duration for very long sectors, though regulatory and ground handling complexities make gaseous systems more prevalent.

优势与局限

The chemical oxygen generator system is highly reliable — it has no moving parts, requires no power, and cannot fail to activate if the lanyard is pulled. Its self-contained nature means it works even in a complete electrical failure scenario. The system is also lightweight and compact compared with the cylinder-based alternatives needed to supply the same duration of oxygen.

Limitations include the fixed-duration supply (12–22 min) that, while adequate for an emergency descent, provides no reserve if the aircraft cannot descend promptly. COGs are also a single-use item — once activated they cannot be reset, requiring replacement of the entire unit at significant cost. The exothermic activation process and the nature of the oxidizer chemistry require careful handling during maintenance and cargo loading: activated COGs have been implicated in cargo fires when improperly stowed in past incidents, leading to strict International Air Transport Association regulations on their carriage as dangerous goods.