Fire Detection and Extinguishing Systems

Fire Detection Types in Aircraft

Aircraft fire detection systems employ several technologies to monitor for abnormal heat or fire:

• Bimetallic detectors use two metals with different expansion rates; when heated, they bend to close an electrical circuit and trigger a warning.
• Continuous loop detectors (such as resistance or capacitance-type firewires) run along protected areas and sense temperature increases over their length. They are especially common for engine fire protection and overheat detection.
• Gas-filled (gaseous loop) detectors rely on pressure changes inside a sealed tube when exposed to heat, activating a warning if a threshold is exceeded.

Principles of Detection

• Resistance and capacitance: In continuous loop systems, temperature changes alter the electrical resistance or capacitance, which is monitored by the detection circuit.
• Gas pressure: Gas-filled loops detect fires by monitoring pressure increases caused by heating the gas inside the tube.

Double-Loop Systems

Modern aircraft typically use double-loop (dual-channel) fire detection systems. Two independent loops must both sense a fire condition before a warning is triggered. This redundancy greatly reduces false alarms and increases reliability. If one loop fails, the system can operate in single-loop mode, but with reduced redundancy.

Fire Warning and Indication

When a fire is detected, the cockpit receives clear warnings: red lights, aural alarms, and, on many aircraft, dedicated fire handles. These warnings persist until the fire is extinguished or the system is reset. Function tests are available to verify system integrity, often illuminating warning lights and sounding alarms during pre-flight checks.

Fire Extinguishing Systems

Built-in fire extinguishing systems consist of:

• Pressurized extinguishing agent bottles (commonly Halon or similar agents)
• Discharge lines and nozzles targeting the protected area
• Electrically activated squibs (explosive charges) to release the agent
• Controls and indicators in the cockpit (fire handles, discharge switches)

Regulations require at least two separate discharges for each engine, ensuring a backup if the first attempt does not fully suppress the fire. The system can be armed and discharged from the cockpit, typically by pulling and turning the fire handle.

Typical Fire Extinguishing Agents

Halon remains the most common agent, valued for its effectiveness against fuel and electrical fires. Other agents may be used depending on environmental and regulatory considerations.