This chapter covers aircraft fire protection systems, including detection and extinguishing components. The material addresses system design principles, troubleshooting methodologies, maintenance requirements, and airworthiness considerations. Understanding these systems is critical for ensuring aircraft safety, as fire protection equipment must remain fully functional at all times when the aircraft is operated.
Key Concepts Explained
Fire Detection Systems
Continuous-Loop Detectors (Fenwal Type)
Continuous-loop detectors consist of a sensing element containing a temperature-sensitive core material surrounded by a conductive outer sheath. When heated, the core material's resistance changes, triggering the control unit to activate warnings. These detectors respond to temperature increases along any portion of the loop.
Failure modes:
Short circuit: Caused by moisture ingress or physical damage to the element. Results in low resistance readings below specified minimums. A shorted element may cause continuous false warnings that persist after the heat source is removed, as the core material may be permanently damaged.
Open circuit: Results in infinite resistance, preventing the system from detecting fires. The test switch will fail to illuminate the warning light.
Contamination: Foreign material on the detector element can increase sensitivity to normal temperature variations, causing false warnings during flight conditions where airflow and temperature changes occur. Ground testing may appear normal.
Thermocouple-Type Detectors
Thermocouple systems operate on the principle of rate-of-temperature-rise. They generate a voltage differential when one junction experiences a rapid temperature increase relative to a reference junction. This design makes them sensitive to rapid temperature changes characteristic of actual fires.
Critical characteristic: These systems do not respond to slow, steady application of heat. When testing with a heat gun, the heat must be applied rapidly to simulate fire conditions. A slow temperature rise may not generate sufficient voltage to trigger the warning, even if the heat source is hot enough.
False warnings: During engine start, transient heat from the starter or exhaust can cause brief false alarms because the system detects the rapid temperature change.
Smoke Detectors
Smoke detectors in cargo compartments and lavatories use optical or ionization principles to detect combustion particles. Testing requires introducing actual smoke or using manufacturer-approved test methods. If a detector fails to alarm, troubleshooting must follow manufacturer procedures, which may include checking power supply, cleaning, recalibration, or replacement.
Fire Extinguishing Systems
Extinguisher Bottle Components
Pressure gauge: Indicates agent charge state. Green arc indicates serviceable range. Red zones indicate discharged (low pressure) or overcharged (high pressure) conditions.
Discharge indicator (green disc): Blown out when the bottle discharges. Missing disc indicates the bottle has been used.
Discharge cartridge (squib): Electrically initiated device that releases the extinguishing agent. Contains a bridge wire that must have specific resistance within manufacturer-specified limits.
Discharge nozzle: Distributes extinguishing agent to protected areas. Must be approved part conforming to type design.
High-Rate Discharge (HRD) Bottles
Used in cargo compartment systems, HRD bottles deliver extinguishing agent rapidly. The discharge pressure gauge provides direct indication of agent charge state and container integrity. Pressure below the green arc indicates loss of agent or pressurizing gas, rendering the bottle unserviceable.
Two-Shot Systems
Some installations use bottles with separate discharge cartridges for first and second shots. Each shot has its own green discharge indicator disk. A missing disk on the first shot indicates that shot has been used, while the second shot remains available.
System Testing and Troubleshooting
Test Switch Operation
The test switch simulates a fire condition by applying voltage to the detector loop. When activated:
Both visual (warning light) and aural warnings should activate
If the light illuminates but the aural does not, the fault is likely in the aural module
If neither activates, the loop may be open, requiring continuity checks
Troubleshooting Sequence
42.Check power supply first: A tripped circuit breaker or loss of power prevents system operation. This is the first step when a detector fails to respond to test inputs.
43.Verify detector loop integrity: Measure resistance and check for shorts or opens.
44.Inspect for contamination or moisture: Moisture in detector loops can cause temporary false warnings as it heats and evaporates.
45.Check control unit: Only after eliminating more common causes.
False Warning Investigation
A burning smell without visible evidence of fire requires thorough investigation. Electrical overheating can occur without visible damage. The AME must inspect wiring, connectors, and components for signs of overheating per maintenance standards.
Important Regulations and Procedures
Airworthiness Requirements
Standard 571.02: Fire protection systems must be maintained per manufacturer's instructions. Defects must be investigated and corrected. Mandatory overhaul dates require the extinguisher to be overhauled (disassembly, inspection, seal replacement).
Standard 571.07: Parts must be approved for installation and conform to type design. Different part numbers require verification through the Illustrated Parts Catalog (IPC) or approved data.
Maintenance Actions
Extinguisher Bottle Replacement Criteria:
Pressure gauge in red discharged range (without use) → Replace or recharge per manufacturer's instructions
Pressure gauge in red overcharged zone → Remove from service, recharge or replace
