Aircraft Fire Protection Systems: Detection, Extinguishing, and Procedures

021-12-02Pablo Asensio Martínez2026-03-272 min

Fire Detection Systems

Engine and APU compartments use thermal/heat detection (not smoke detection due to high airflow).

1. Gaseous Loop (Pneumatic)

  • Principle: Gas Laws (Pressure $\propto$ Temperature).
  • Component: Stainless steel tube filled with inert gas and a gas-absorbing material.
  • Operation:
    • Fire conditions heat the tube.
    • Gas expands, pressure increases.
    • At a predetermined pressure, a pressure switch closes to trigger the alarm.
  • Fault Monitoring:
    • If the tube leaks, pressure drops.
    • A low-pressure switch detects this and triggers a Fault alert (inhibits false fire warnings).

2. Continuous Loop (Electrical)

  • Principle: Thermistor material properties (Resistance/Capacitance change with heat).
  • Component: A central wire embedded in thermistor material inside a conductive tube.
  • Operation:
    • As temperature rises, the resistance decreases (Negative Temperature Coefficient) or capacitance increases.
    • Current flows between the center wire and the tube (ground).
    • When current exceeds a threshold, the alarm triggers.
  • Reset: The system resets automatically when the temperature drops (fire extinguished).
  • Redundancy (Dual Loop):
    • Modern aircraft use two parallel loops (A and B).
    • AND Logic: Both loops must detect fire to trigger the warning (minimizes false alarms).
    • Fault: If one loop is faulty (e.g., short to ground), the system may revert to "Single Loop" operation or indicate a fault. A condition of low resistance AND low capacitance typically indicates a fault/short.

Fire Extinguishing

  • Mechanism: High-rate discharge bottles (spheres) containing extinguishing agent (Halon replacement).
  • Activation: Electrically fired cartridges (Squibs) break a frangible disc to release the agent.
  • Regulations (CS 25.1195):
    • Engines must have two separate discharges available.
    • Configuration: Typically 2 bottles shared between engines, or 2 dedicated bottles per engine.
      • Cross-feed: Bottle 1 can be discharged into Engine 1 OR Engine 2.

Operational Procedure

  1. Detection:
    • Aural: Continuous bell/chime or voice ("Engine Fire").
    • Visual: Master Warning (Red) + Fire Handle Light (Red).
  2. Isolation (Pulling the Fire Handle):
    • Arms the squibs (explosive cartridges).
    • Closes Fuel Shut-off Valve (LP Valve).
    • Closes Hydraulic Shut-off Valve.
    • Closes Bleed Air Valve.
    • Trips Generator (Field Relay/Breaker).
    • Disables Thrust Reverser.
  3. Extinguishing (Rotating the Handle):
    • Pilot rotates handle Left (Bottle 1) or Right (Bottle 2).
    • Agent is discharged into the compartment.

Fire Test

  • Simulates a fire condition (e.g., by heating a test element or electrically simulating low resistance).
  • Success: All lights illuminate + Aural warning.
  • Failure: No indication or specific "Loop Fault" light (if checks continuity).