Fire Protection

Fire and overheat detection, extinguishing systems and agents, and the engine fire handle logic.

Notes

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Main ideas

DETECTION methods suit the zone: engine/APU zones use CONTINUOUS-LOOP (thermal) or spot thermal detectors sensing overheat/fire along their length; cargo and lavatories use SMOKE detectors (optical/ionisation); overheat detection warns before a fire.
EXTINGUISHING: engine and APU fires are fought with fixed bottles of a gaseous agent (Halon 1301 or a modern replacement) discharged into the zone; cargo holds have knock-down and metered discharge; the cabin has portable extinguishers (water-glycol, BCF/Halon, CO₂).
The FIRE HANDLE/switch for an engine both ISOLATES the zone — shutting off fuel, hydraulics and bleed air and tripping the generator — AND arms the extinguisher bottles; the engine is shut down before the agent is discharged.
Systems distinguish a FIRE warning from a lesser OVERHEAT warning, and include a test facility to confirm detector and circuit integrity.
Halon is a very effective agent because it chemically interrupts combustion, but is being replaced for environmental reasons.
⚠ Exam trap: a continuous-loop detector senses temperature along its WHOLE length (not one point); pulling the engine fire handle isolates fuel/hydraulics/bleed air to the zone and arms the bottle — always shut the engine down BEFORE discharging the extinguisher.

Solved examples

What sequence of things happens when the flight crew pull an engine fire handle?
Pulling the fire handle shuts down the engine's supplies to the fire zone — it closes the fuel shut-off valve, closes the hydraulic and bleed-air valves and trips the generator — cutting off what feeds the fire, and simultaneously arms the fire-extinguisher bottles so that rotating/pressing the handle discharges the agent into the now-isolated zone.

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