Fire Detection Interface

Fire protection belongs to ATA-26, but its eyes live on the engine: ten thermal detectors stationed along the fire zones drawn in articles 02/03, two continuous loops translating "hot or not" into resistance, and a Fire Detection Unit (FDU) whose three comparators translate resistance into three very different messages — fire (FIRE), blind eye (LOOP FAULT), and dirt in the eye (CONTAMINATION). This article walks that chain from engine to cockpit, and closes out what the ENG FIRE pushbutton severs within this chapter's systems (the final destination of the EIVMU's hardwired R1/R2 lines from article 06).

1. The sentry map

Five per loop: loop A takes one detector from each zone, loop B takes the other — so both loops keep a pair of eyes in every zone, the spatial precondition for AND logic to be meaningful everywhere (synthesis). One FDU per engine sits in the avionics bay; the cockpit end is the fire panel: one guarded ENG FIRE pushbutton per engine, AGENT 1/2 (SQUIB/DISCH) controls, and a shared TEST button.

The detection chain

"On each engine, there are two continuous loops for the fire detection. The loops are connected in parallel to a Fire Detection Unit (FDU). The connection is made through an AND logic to avoid spurious FIRE warnings. In case of failure of one loop, the AND logic becomes an OR logic. The aircraft can be released in this configuration."

That last half-sentence is the MEL philosophy in preview (article 35): a single failed loop does not ground the aircraft, because the logic automatically degrades to OR — detection coverage is not lost; what is lost is the anti-spurious redundancy. Even the warning light is redundantly wired: the FIRE pushbutton contains eight bulbs, four lit by each loop — if one loop loses even its lamp power, the other still turns the button red.

2. Resistance speaks three languages

Each detector is a thermistor element plus a responder containing an ALARM switch and a MONITOR switch. The FDU's input stage per channel is a bridge circuit: a reference voltage against the loop's variable voltage (five detectors in parallel — 7.5 kΩ each, 1.5 kΩ loop equivalent). Any change of state in the monitored bay changes the equivalent resistance, the voltage follows — and one voltage line carries three entirely different messages, sorted by three comparators:

"The reference voltage is sent to the three comparators (FIRE, INTEGRITY and CONTAMINATION) which form the thresholds."

This is the electrical basis of "the system can tell a fire from a failure": fire, breakage and dirt land in three distinct intervals on the resistance axis — every loop carries its own self-diagnosis.

3. The FDU's rules of judgment: four combinations, 20 seconds, 5 seconds

The four FIRE combinations:

"There is a FIRE warning signal if any of the following conditions occurs: ‐ FIRE A and FIRE B ‐ FIRE A and FAULT B ‐ FAULT A and FIRE B ‐ FAULT A and FAULT B in less than 5 seconds."

This matches the FCOM's alert triggering conditions exactly:

"This alert triggers when: ‐ Fire is detected by both loops, or ‐ Fire is detected by one loop when the other loop is failed, or ‐ A rupture occurs in both loops within 5 s."

The third condition is the cleverest line in the whole logic (synthesis): two loops breaking within five seconds of each other is not how coincidental failures behave — it is how a fire burning through the wiring behaves, so it is treated as a fire. Two loop failures separated by more than five seconds are judged instead as a detection-system failure — the ENG FIRE DET FAULT alert (MASTER CAUTION, single chime): a crew-awareness message meaning "if a fire starts now, there will be no warning." And in the opposite direction stands an anti-spurious gate:

"…the detection by a single fire detection loop for a time of more than 20 sec while the other loop is in normal condition (→ LOOP A(B) FAULT)."

A real fire does not roast only one loop for twenty seconds. An uncorroborated witness is disbelieved — the loop that cried fire alone is itself declared faulty. Adaptive AND/OR, four combinations, the 20-second lone-witness rule and the 5-second double-rupture verdict: four rules that together answer neither miss a fire nor cry wolf.

4. The ENG FIRE alert: inhibition, and the engine-side mechanics of the procedure

The inhibition window: the alert is inhibited from V1 until lift-off + 15 seconds — in the most critical twenty-odd seconds of flight, even a fire warning waits its turn (a fire in the window is announced when the window closes). The in-flight procedure's engine-side annotations each land on ground prepared earlier:

"ENG MASTER (AFFECTED) OFF — Associated LP and HP valves close. / ENG FIRE P/B (AFFECTED) PUSH — When pushed: ‐ Aural warning stops ‐ The light remains on, until the fire is extinguished, regardless of the position of the ENG FIRE pb-sw ‐ FADEC is no longer supplied. / AGENT 1 AFT 10 S DISCH — The 10 s delay allows N1 to decrease, reducing nacelle ventilation, and thereby increasing the effect of the agent."

MASTER OFF closes both fuel valves (article 09). The FIRE pushbutton silences the aural, cuts the FADEC's power (the hardwired R1 of article 06) — and the light stays on until the fire is actually out, regardless of the button's position. The ten-second delay before AGENT 1 has a ventilation logic behind it: zone ventilation, the first line of fire prevention in article 03, becomes the enemy at extinguishing time — letting N1 spool down weakens the through-flow so the agent concentration can hold. The procedure then isolates every air source on the affected side (engine bleed, APU bleed where applicable, crossbleed, wing anti-ice); if the fire persists 30 seconds, AGENT 2; and no relight is ever attempted. The ground version differs tellingly: stop the aircraft (full reverse remains available), discharge both agents together — on the ground there is nothing to hold the second bottle for — shut down and fire-push the other engine too, then decide on evacuation. One sentence deserves separate memory: the red light is the live status of the fire, not of the button — a lit light after the push means the fire still burns; that is the criterion for the second bottle.

5. TEST: one button, two examinations

"The TEST pushbutton switch checks simultaneously for the two engines the condition of the: ‐ fire detectors (Loops A and B), Fire Detection Unit (FDU), indications, warnings and associated wirings (loop test) ‐ percussion cartridge filaments of the fire extinguisher bottles and associated wiring (squib test)."

The daily-check blaze of red lights examines two different things at once: the eyes (detector loops, FDU, indications, wiring) and the fists (the extinguisher bottles' percussion-cartridge filaments — the SQUIB light only comes on if both filaments are correctly supplied). Hold the button until the test completes; faults found are annunciated as LOOP INOP-class messages.

6. Where the fire interface meets the chapters

Self-test

[!note]- Q1. How are the five detectors of each loop distributed? One per zone — Zone 1 left, Zone 1 right, Zone 2, Zone 3 and the pylon — so that both loops keep a pair of eyes in every zone, which is what makes the AND logic meaningful everywhere.

[!note]- Q2. One loop has failed. How does the system work now — and can the aircraft be dispatched? The AND becomes an OR (the surviving loop alone can trigger the warning), and the aircraft can be released in this configuration — coverage is intact; only the anti-spurious redundancy is gone.

[!note]- Q3. Both loops fail. When is it DET FAULT, and when is it FIRE? Failures separated by more than 5 seconds = ENG FIRE DET FAULT (the detection system has died — crew awareness). Failures within 5 seconds of each other = treated as FIRE: two wires almost never break together by coincidence, but a fire burns through both.

[!note]- Q4. Why does AGENT 1 wait 10 seconds in flight — and why no wait on the ground? The delay lets N1 spool down and nacelle ventilation weaken, so the agent concentration holds. On the ground, with the aircraft stopped, both agents are discharged together — there is no later phase to save the second bottle for.

[!note]- Q5. The FIRE pushbutton has been pressed and the light is still on. Meaning? The fire is not out. The light follows the fire, not the button — and a light still burning around the 30-second mark is the criterion for discharging AGENT 2. (The FADEC, meanwhile, lost its power the moment the button went in.)

Key takeaways

References

• AMM 26-12 (engine fire detection, D/O) — detector construction and zone allocation, AND/OR adaptation and releasability, eight-bulb wiring, bridge values, three comparators, the four combinations, the 20-second and 5-second rules, TEST coverage.
• FCOM PRO (ENG FIRE in flight) — trigger conditions, V1 inhibition, engine-side procedure annotations (valve closures, light behaviour, FADEC cut, 10-second rationale, AGENT 2 at 30 s, no relight).
• FCOM PRO (ENG FIRE on ground) — both-agents-at-once and both-engines differences, evacuation branch.
• FCOM PRO (FIRE DET FAULT / LOOP FAULT) — alert meanings and INOP listings.
• AMM equipment-location figure (read directly) — FDU bay position and fire-panel layout.
• Integrative synthesis (marked in text): the pair-of-eyes-per-zone observation; the fire-burns-wires reading of the 5-second rule; the ventilation-turned-enemy logic; the light-is-the-fire formulation.

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.