エンジン火災検知・消火システム

概要

The engine fire detection and suppression system provides early warning of fire or overheat conditions in the engine nacelle, thrust reverser bay, and APU compartment, and equips the flight crew with the means to extinguish any detected fire. Engine nacelle fires are among the most serious inflight emergency scenarios: an uncontrolled nacelle fire can rapidly damage fuel lines, hydraulic tubes, and structural components, potentially leading to engine separation or wing fuel tank involvement. Rapid detection and effective suppression are therefore essential safety requirements for all certified commercial aircraft.

Modern fire detection systems use dual-loop continuous element detection rather than discrete point detectors. This architecture provides both redundancy (a single loop failure does not disable the system) and the ability to distinguish between a genuine fire (detected by both loops) and a single-loop fault — avoiding nuisance warnings that might prompt unnecessary engine shutdowns.

動作原理

Continuous element fire detectors consist of a corrugated metal outer tube containing a centre conductor, with a thermally sensitive resistive material filling the annular space. At normal temperatures the material has high resistance. When temperature rises above the detection threshold — due to fire or severe overheat — the resistance drops dramatically, completing a circuit that triggers the fire warning. Two loops (Loop A and Loop B) are routed independently through each fire zone; an AND gate logic requires both loops to detect simultaneously for a fire warning, while a single-loop signal produces an advisory fault message.

Upon fire warning, the crew follows the Engine Fire drill: thrust lever to idle (if not already), then the Engine Fire Handle is pulled, which simultaneously closes the engine bleed shutoff valve, hydraulic shutoff valve, fuel shutoff valve, and arms the fire extinguisher bottles. The handle is then rotated to discharge one of two extinguisher bottles into the nacelle. If fire persists, a second rotation discharges the remaining bottle. FAA regulations require that the onboard agent be capable of controlling a simulated fire for at least 30 minutes after the second bottle is discharged — a standard established by empirical fire testing.

主要コンポーネント

• Dual-Loop Continuous Element Detectors: Helium-filled or resistance-wire tubes routed through the nacelle core zone, fan case zone, and thrust reverser cavity. Replaced at scheduled intervals or on fault indication.
• Fire Detection Control Unit (FDCU): Electronic module that interprets detector loop resistance signals, applies dual-loop logic, and outputs fire and fault alerts to the crew warning system.
• Engine Fire Handle: Red T-handle on the flight deck (or overhead panel on Airbus aircraft) that arms the system for suppression when pulled and discharges extinguishant when rotated.
• Fire Extinguisher Bottles: High-pressure spherical or cylindrical containers charged with Halon 1301 (on older aircraft) or HFC-227ea (on newer designs as Halon phase-out progresses). Typically two bottles per engine, with cross-plumbing allowing either bottle to serve either engine.
• Squib (Explosive Discharge Valve): Electrically fired detonator that ruptures the bottle seal and releases extinguishant. Non-resettable; bottles must be recharged after any discharge.
• APU Fire Detection: Separate continuous element loop around the APU bay, with a dedicated APU fire handle on the overhead panel. APU fire shuts down the APU automatically on most modern aircraft.

航空機への適用

• Boeing 737-800 — dual-loop detection in engine core and thrust reverser zones; two bottles per engine cross-plumbed
• Airbus A320-200 — fire detection managed through ECAM; automatic APU shutdown on ground fire detection
• Boeing 777-300ER — three-zone detection per engine (inlet, core, thrust reverser); HFC-227ea bottles
• Boeing 787-9 — updated detection architecture coordinated with the 787's distributed avionics system

利点と制限

Dual-loop continuous element detection provides an excellent combination of sensitivity and fault discrimination. The system is capable of detecting early-stage overheats before open flame develops, enabling crew action before structural damage occurs. The cross-feed capability between extinguisher bottles maximises the agent available per event without the weight and cost of dedicating four separate bottles.

The reliance on Halon 1301 — a highly effective halon gas that is also an ozone-depleting substance — creates a long-term regulatory challenge. The Montreal Protocol restricts new Halon production, and aviation is one of the "critical uses" exemptions. HFC-based alternatives are being qualified on new aircraft types, though they are not fully equivalent in all fire suppression scenarios. Detector element degradation over time can produce nuisance warnings (hot spots, chafed loops); a comprehensive maintenance programme including periodic loop resistance checks is essential for system reliability. False fire warnings, while rare, can prompt unnecessary single-engine operations or diversion, with significant operational consequences.