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

021-13-01Pablo Asensio Martínez2026-03-272 min

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).
Written and reviewed by

Pablo Asensio · 2026-03-27

Aeronautical engineer. Passed the thirteen EASA ATPL(A) theoretical knowledge exams studying from these very notes. Writes and reviews each topic against the Part-FCL Learning Objectives.

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