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:

Flight Crew: Gaseous System (High pressure cylinders).
Passengers: Chemical Oxygen Generators (Solid state) or Gaseous (less common, e.g., some Boeings).

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).

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).