Aircraft Fire Protection Systems: Detection, Extinguishing, and Procedures

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).