Fire Protection Interface — Dual-Loop Detection, AND/OR Logic, ENG FIRE pb

This article takes fire from the engine-interface side: the detection hardware (dual loops + FDU + sensing elements), the AND/OR trigger logic, and the eight actions the ENG FIRE pb performs on the engine — gathering the ENG FIRE links scattered through earlier articles into one ledger. The complete ENG FIRE ECAM procedure (agent discharge timing, confirmation, both-engine fire) belongs to ATA 26; here only the engine-side detection and pb interface.

1. Detection — dual loops + FDU

The engines and the APU each have a fire and overheat detection system consisting of : ‐ Two identical gas detection loops (A and B) mounted in parallel. ‐ A Fire Detection Unit (FDU). The gas detection loops consist of : ‐ Five sensing elements for each engine, located in the pylon nacelle, in the engine core, compressor and fan sections. When a sensing element is subjected to heat, it sends a signal to the fire detection unit. As soon as loops A and B detect temperature at a preset level, it triggers the fire warning system.

Per DSC-26-20-10: two identical gas-detection loops (A and B) in parallel + an FDU, with five sensing elements per engine in the pylon nacelle, engine core, compressor and fan sections.

2. AND / OR trigger logic

Per the engine-fire-detection diagram:

 SENSING ELEMENTS (along the engine)
   │
 LOOP A ┐
 LOOP B ┴─► FDU ─[AND*/OR]─► CRC (continuous repetitive chime)
                           ► ECAM
                           ► MASTER WARNING
                           ► ENG FIRE PUSH light (red)
                           ► FIRE / FAULT light
   * AND when both loops operative; OR when either loop inoperative

[!warning]- The fire logic adapts — AND with both loops, OR with one failed Per the diagram's note: AND logic when both loops are operative (both must detect → no single-loop false warning); OR logic when either loop is inoperative (the remaining loop alone can trigger → no loss of sensitivity). A classic reliability-vs-sensitivity balance — a loop fault does not paralyse detection.

3. ENG FIRE pb — eight actions

ENG FIRE PB-SW — The PB-SW normal position is in, and guarded. When the flight crew pushes it, the PB-SW is released and sends an electrical signal that performs the following for the corresponding engine: ‐ Silences the aural fire warning ‐ Arms the fire extinguisher squibs ‐ Closes the low-pressure fuel valve ‐ Closes the hydraulic fire shut off valve ‐ Closes the engine bleed valve ‐ Closes the pack flow control valve ‐ Cuts off the FADEC power supply ‐ Deactivates the IDG.

Per DSC-26-20-20, pushing the guarded ENG FIRE pb does eight things for that engine:

[!note]- The essence: one button cuts every energy / hazard source on the engine (integrative synthesis) Fuel (③), hydraulics (④), bleed (⑤⑥), electrics (⑦⑧) — the ENG FIRE pb cuts every energy in and out of the engine, plus arms the bottles (②). This is why 11's FADEC GND PWR condition includes "ENG FIRE not pressed" (pressed, the FADEC power is cut, so it cannot be supplied). A reading of the eight verbatim actions.

4. Counterintuitive points

[!warning]- The fire logic is adaptive — AND with both loops, OR with one failed Not fixed AND or OR — both loops good → AND (anti-false-warning), one failed → OR automatically (keep sensitivity). A loop fault does not paralyse detection.

[!warning]- The ENG FIRE pb cuts more than fuel One push cuts fuel + hydraulics + bleed + generation (IDG) + FADEC power — eight actions (DSC-26-20-20). It is "cut every source," not just fuel.

Self-test

[!note]- Q1. What makes up engine fire detection, and how many sensing elements per engine? Two identical gas-detection loops (A/B) in parallel + an FDU; five sensing elements per engine (pylon nacelle / core / compressor / fan).

[!note]- Q2. When is the logic AND, when OR, and why? Both loops operative → AND (anti-false-warning); either loop inoperative → OR (keep sensitivity). A loop fault does not paralyse detection.

[!note]- Q3. The eight ENG FIRE pb actions? silence / arm squibs / close LP fuel valve / close hydraulic fire shut-off / close engine bleed / close PFCV / cut FADEC power / deactivate IDG.

[!note]- Q4. Which energy sources does the ENG FIRE pb cut? Fuel (LP valve) + hydraulics (fire shut-off) + bleed (bleed + PFCV) + electrics (FADEC power + IDG) — every source.

[!note]- Q5. Where is the complete ENG FIRE procedure? ATA 26 (this article covers only the engine-side detection + pb interface; agent discharge timing / confirmation / both-engine fire → ATA 26).

Key takeaways

References

• FCOM DSC-26-20-10 — fire detection: two loops A/B in parallel + FDU, five sensing elements per engine.
• FCOM DSC-26-20-20 — ENG FIRE pb eight actions (silence / arm squibs / close LP fuel / hydraulic fire shut-off / engine bleed / PFCV / cut FADEC power / deactivate IDG).
• Engine-fire-detection diagram — sensing elements → loops A/B → FDU (AND both good / OR one failed) → CRC / ECAM / MASTER WARNING / ENG FIRE light.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.