Passagiersauerstoffsystem

Überblick

The passenger oxygen system provides supplemental breathing oxygen to all cabin occupants in the event of a depressurisation that raises the cabin altitude above a safe threshold. Unlike the gaseous crew system, the passenger system uses sodium chlorate chemical oxygen generators — compact solid-fuel devices that produce oxygen through an exothermic chemical reaction when a lanyard activates a firing pin. Each generator serves a small group of passengers (typically three or four masks per generator) through a shared manifold. The system is entirely automatic: when cabin altitude exceeds approximately 14,000 feet, the overhead Passenger Service Unit (PSU) panels drop open and masks fall into reach. Passengers must pull the mask toward them to activate the generator before donning the mask.

Funktionsweise

Chemical oxygen generators contain a mixture of sodium chlorate (NaClO3) and iron powder. When heated by a firing pin striking a percussion cap, the mixture decomposes: NaClO3 → NaCl + 1.5 O2. The reaction is exothermic, sustaining itself once initiated, and produces oxygen at a steady rate for approximately 12 to 22 minutes depending on generator size. The outer canister becomes hot during operation — a safety consideration relevant to generator storage and handling after use. Each mask position has an individual lanyard that arms the firing pin; pulling the mask pulls the lanyard and fires the generator, regardless of whether adjacent masks have been pulled. This allows partial group activation if some passengers are absent from their seats.

Oxygen flows from the generator through a distribution manifold and short individual tubes to each mask. The masks are continuous-flow type — oxygen flows at a constant rate regardless of whether the wearer is inhaling — and incorporate a small reservoir bag that accumulates oxygen between breaths, ensuring an adequate supply even at the continuous flow rate. Masks include an adjustable elastic strap and a sealing ring around the face. The system provides no regulation based on altitude; the same flow rate is delivered from activation until generator exhaustion.

Hauptkomponenten

Chemical Oxygen Generator: Cylindrical canister containing the sodium chlorate/iron mixture with a percussion cap primer and pull-pin arming device. Certified service life is typically 12 years from manufacture date.

PSU Door Latch: Electrically or mechanically released latch holding the PSU panel closed. The primary release signal comes from the cabin altitude sensor at the depressurisation warning unit; the crew can also manually drop panels from the cockpit or local panel.

Continuous-Flow Masks: Clear plastic facepieces with elastic headbands and a reservoir bag (rebreather bag) that accumulates oxygen between breaths. The mask does not create a pressure seal — it is a simple continuous-flow device providing oxygen enrichment rather than a sealed pressure-demand system.

Cabin Altitude Sensor: Aneroid pressure switch or electronic pressure transducer that triggers automatic panel release at the regulatory threshold, typically 14,000 ft cabin altitude.

Supplemental Galley & Lavatory Masks: Additional generator/mask sets at galley crew stations and within lavatories, ensuring coverage for all aircraft occupants regardless of location.

Anwendungen bei Flugzeugen

All commercial transport aircraft operating above 25,000 feet are required by FAR 25.1443 and CS-25 to carry passenger oxygen capable of supplying all occupants simultaneously. The Boeing 737-800 carries generators in PSU boxes above each row of seats, with additional units at galleys. The Airbus A320 uses a similar layout. Wide-body aircraft with premium cabin configurations space generators to account for wider seat pitch and varied seat counts per PSU. The Airbus A380-800 requires extensive generator networks across both decks. Generator counts must account for 10% spare capacity above the maximum certified passenger count, ensuring coverage even with full passenger manifests.

Advantages & Limitations

Chemical oxygen generators are remarkably compact, lightweight, and reliable — requiring no electrical power, no pressurised plumbing, and no active maintenance beyond periodic generator replacement at service life expiry. They provide sufficient oxygen for passengers to remain conscious during an emergency descent to a breathable altitude, which is the system's sole intended purpose. Key limitations are the fixed 12–22 minute duration (adequate for descent from cruise altitude but with no margin for extended emergencies at altitude), the continuous-flow design which provides oxygen enrichment rather than the sealed pressure-demand supply that crew masks offer, and the heat produced during operation. The generators cannot be reused or refilled, making inadvertent activations — which occasionally occur during maintenance — costly. From a passenger safety perspective, the critical vulnerability is passenger non-compliance: failure to pull the mask to activate the generator (not obvious to untrained passengers) can result in unconsciousness despite masks being deployed.