Three tiers of oxygen, sized around failure

Airliner oxygen on the 747 is delivered in layers: high-pressure demand masks for the two pilots, protective breathing equipment for the cabin crew, and chemical-generator masks that deploy from the ceiling for passengers.

Key facts
Flight crew	Quick-donning demand masks, mandatory above FL250
Cabin crew	Protective breathing equipment (PBE) with its own generator
Passengers	Drop-down masks fed by chemical oxygen generators
Sizing case	Rapid descent to 10,000 ft plus continued supply to land

Regulations require each flight-crew member on duty above FL250 to have a mask that can be donned quickly from its ready position and that supplies oxygen on demand; the pilot flying must wear oxygen at all times above FL250 unless quick-donning masks are fitted.

Passenger oxygen covers at least 10 percent of the seats with about a 30-minute supply, engineered to support a 10-minute descent to 10,000 ft followed by continued supply. Both crew and passenger systems are sized around survivable pressurisation failures.

Three tiers, three purposes

The 747 carries oxygen for three distinct populations, and the systems are sized differently for each.

Flight crew use a pressurised, demand-flow system delivered through quick-donning masks. The masks sit at the crew stations ready to be donned in seconds, and each crew member has their own independent supply. Because the flight deck is pressurised separately from the cabin, a flight-deck depressurisation is handled without reference to the passenger supply.

Cabin crew carry portable protective breathing equipment - a hood or facepiece with its own chemical oxygen generator, sized to protect one person for several minutes. It exists for the specific case where the cabin is smoke-filled or the crew must enter a hostile atmosphere to complete an evacuation, and it is deliberately independent of the aircraft's fixed systems.

Passengers are served by drop-down masks fed by chemical oxygen generators mounted in the overhead panels. Pulling a mask pulls a generator pin, starting an exothermic reaction that makes oxygen without needing any aircraft power or plumbing. That autonomy is the point: the system has to keep working after the exact failure it exists for.

Sizing around the worst credible case

Passenger oxygen is not sized for comfort; it is sized for a failure. The regulations and the type's certification basis assume a rapid descent from cruise altitude to 10,000 ft after a decompression, followed by enough continued supply to reach an airport. The generator count reflects that, which is why the mask rows are spread through the cabin rather than concentrated near the exits.

This is a different philosophy from crew oxygen, which is sized for immediate, indefinite supply because the flight deck has to keep flying.

Related

• Pressurisation
• Cabin systems
• Fire detection and extinguishing
• 747-8 systems differences