Fire Protection

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

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Main ideas

DETECTION suits the zone: engine/APU zones use CONTINUOUS-LOOP (thermal) or spot thermal detectors sensing overheat/fire along their length; cargo holds 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 isolated zone; cargo holds have metered discharge; the cabin has portable extinguishers.
The engine FIRE HANDLE 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.
For the B2 technician the detection loops, warning circuits, indication and test logic are the focus.
⚠ Exam trap: a continuous-loop detector senses temperature along its WHOLE length; pulling the engine fire handle isolates fuel/hydraulics/bleed air and arms the bottle — always shut the engine down BEFORE discharging the extinguisher.

Solved examples

How does the system tell the difference between an overheat and a fire, and why does it matter?
Detection elements/loops are set to trigger an OVERHEAT caution at a lower temperature threshold and a FIRE warning at a higher one (or via separate loops). The overheat alert lets the crew act — e.g. shut a leaking bleed duct — before a fire develops, whereas a fire warning demands the full engine fire drill. Distinguishing them avoids an unnecessary engine shutdown for a mere overheat.

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