Fire Protection

Fire and overheat detection, extinguishing systems and the shut-down/isolate-before-discharge logic.

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Main ideas

Engine and (where fitted) APU bays are protected by fire DETECTION — continuous-loop or spot thermal detectors — and fixed fire EXTINGUISHER bottles of a gaseous agent discharged into the isolated zone.
The engine fire control isolates the zone — shutting off fuel and, as applicable, hydraulics/bleed to the bay — and arms/discharges the bottle; the engine is shut down before the agent is discharged.
Overheat detection warns of a hot condition (e.g. a bleed leak) before a fire; the cabin carries portable extinguishers.
A test facility confirms detector and circuit integrity; agents are Halon or a modern replacement.
For the B1 technician the detectors, loops, warning circuits and bottle squibs are the key items.
⚠ Exam trap: a continuous-loop detector senses temperature along its whole length; on a fire the engine is shut down and the zone isolated (fuel off) BEFORE the extinguisher is discharged.

Solved examples

What must happen before the fire-extinguisher bottle is discharged into an engine bay?
The engine must be shut down and the supplies to the bay isolated — the fuel shut-off closed (and any hydraulic/bleed feeds cut) — so the fire is no longer being fed. Only then is the bottle discharged, allowing the gaseous agent to smother the fire in the sealed zone rather than being blown through by continuing fuel/airflow.

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