# Aircraft Oxygen Systems: Flight Crew, Passenger, and Protective Equipment

> Learn aircraft oxygen system architecture, including crew gaseous systems, passenger chemical generators, and protective breathing equipment.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-13 — Oxygen Systems
- **Topic code:** 021-13-01
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/oxygen-systems/cockpit-portable-and-chemical-oxygen-systems/

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## General Architecture
Modern transport aircraft typically employ **two independent oxygen systems**:
1.  **Flight Crew**: Gaseous System (High pressure cylinders).
2.  **Passengers**: Chemical Oxygen Generators (Solid state) or Gaseous (less common, e.g., some Boeings).

## Flight Crew System (Gaseous)
*   **Source**: Oxygen cylinder (usually green).
*   **Masks**: "Quick Donning" type with integrated microphone and regulator.
*   **Regulator Modes**:
    1.  **Normal / Diluter Demand (N)**:
        *   Delivers a mixture of **Cabin Air + Oxygen**.
        *   Ratio increases with cabin altitude.
        *   **Flow**: Only on demand (inhalation).
        *   *Auto-100%*: Typically switches to 100% automatically above ~35,000 ft.
    2.  **100%**:
        *   Delivers pure oxygen on demand.
        *   Used if smoke/fumes are suspected or at high altitude.
    3.  **Emergency**:
        *   Delivers **100% Oxygen at Positive Pressure**.
        *   **Purpose**: Prevents ambient smoke/fumes from entering the mask (overpressure pushes out).
        *   **Usage**: Fire, Smoke, or Depressurization at very high altitudes.
*   **Safety**:
    *   **Grease/Oil Hazard**: Oxygen accelerates combustion violently. Contact with grease/oil can cause **spontaneous combustion/explosion**. (Crew must avoid grease-based cosmetics).

## Passenger System (Chemical Generators)
*   **Source**: "Chemical Candle" (e.g., Sodium Chlorate).
*   **Location**: PSU (Passenger Service Unit) above seats, Lavatories, Galleys, Crew Stations.
*   **Deployment**:
    *   **Automatic**: When cabin altitude exceeds **14,000 - 15,000 ft** (CS 25.1447).
    *   **Manual**: Switch in cockpit or manual release tool (pin).
    *   **Quantity**: Masks must exceed number of seats by at least **10%**.
*   **Operation**:
    *   **Activation**: Pulling a mask pulls a lanyard/pin, igniting the chemical reaction.
    *   **Flow**: **Continuous Flow** (Constant rate, not demand).
    *   **Duration**: **12 to 20 minutes** (Typically ~15 min).
    *   **Stop**: **Cannot be stopped** once started.
    *   **Heat**: Reaction is exothermic. The canister gets very hot (smell of burning/dust is possible).
*   **Limitation**: Does **not** protect against smoke/fumes (mixes with generated oxygen).

## Protective Breathing Equipment (PBE / Smoke Hood)
*   **User**: Crew members fighting a fire or during evacuation with smoke.
*   **Function**: Protects eyes and respiratory system from toxic gases/smoke.
*   **Duration**: ~15 minutes (Closed circuit chemical regeneration).

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.