Smoke Detection — Avionics, Lavatory and Crew-Rest Areas

From here the chapter leaves the temperature-sensing engine and APU and enters the smoke-sensing world ("cool zones sense smoke", overview). Smoke detection covers a wide band of cool zones — the avionics bay, lavatories, crew-rest areas, video and broadband centres, stowage. This article settles those "detect-but-do-not-extinguish, or auto-extinguish" zones and takes apart the smoke processing chain (SDCU/CIDS-SDF) that the cargo article reuses.

1. Optical detection depends on the ventilation flow

The detectors are optical (they see combustion particles), but they do not go looking for smoke — the ventilation flow must bring it to them.

"The smoke detectors are installed at the air extraction-duct. Piccolo tubes, which are installed in the air extraction-duct, connect the smoke detectors with the air extraction-duct. When smoke appears in the avionics-compartment the avionics-compartment ventilation system removes it through the air extraction-duct. The piccolo tubes pass the air particles into the smoke detectors."

[!warning]- The detector sits on the extraction duct, not the ceiling It samples the smoke the ventilation system is drawing away, through a piccolo tube (a multi-hole sampling pipe) acting as a straw. This carries a system consequence: stop the ventilation and you stop delivering smoke, so detection can go blind. Avionics and cargo smoke detection are therefore tied to the ventilation system — the seed of why a cargo warning drives the isolation valves (cargo article). This is the opposite philosophy to the engine heat loop, which is heated directly and needs no airflow.

2. The avionics bay — detect only, fight with ventilation and power

The avionics bay is the only zone that is detect-only (overview task list):

"The system has two smoke detectors. They are of the optical type... They are installed with dual loop wiring to prevent incorrect warnings."

The FCOM gives the trigger and annunciation:

"The avionics smoke warning triggers, if: - Both smoke detectors detect smoke for more than 5 s , or - One smoke detector detects smoke for more than 5 s , and the other smoke detector is inoperative."

"- A Repetitive Chime (CRC) sounds - The MASTER WARNING lights flash - The ECAM SMOKE AVNCS VENT SMOKE alert triggers - The SMOKE light, on the VENTILATION panel, comes on."

[!warning]- Why no Halon in the avionics bay An avionics-bay fire is typically electrical — the source is a still-powered box, so without cutting power no agent holds; the response is power isolation plus ventilation to clear the smoke, not agent. And the avionics bay is accessible in flight (behind the cockpit), unlike a sealed floodable cargo hold — Halon total-flooding suits sealed holds, not an accessible bay. The AMM assigns the response to the crew: > "When a warning is triggered, the crew members must initiate the smoke procedure." The procedure is developed in the smoke/fumes article.

3. The lavatory — a detector plus an automatic waste-bin bottle

The lavatory is doubly protected — it both detects (alarm) and auto-extinguishes (waste bin):

"One smoke detector, in the air extraction duct of each lavatory... If smoke is detected in the lavatory, the smoke detector sends a signal to the CIDS-SDF that triggers a warning in the cockpit (on the ECAM) and in the cabin (on the FAP)."

Extinguishing is automatic, independent and needs no crew:

"A fire extinguisher is installed above the waste bin in each lavatory. If a fire occurs in the waste bin, the fire extinguisher extinguishes the fire automatically. The fire extinguisher operates when the temperature in the waste bin increases to a given level. The lavatory fire extinguishers operate independently of other systems."

[!warning]- The only fire suppression the crew never sees The waste-bin bottle is temperature-triggered by a thermal fuse: reach the set temperature and the fuse melts, the bottle discharges into the bin. No network, no power, no button — a physical answer to a recurring hazard (a discarded cigarette in the waste paper). Note the two separate items in the lavatory: the smoke detector (alarms the crew/cabin, handles smoke) and the waste-bin bottle (self-extinguishes a bin fire) — do not conflate them.

4. SDCU versus CIDS-SDF — the smoke processing chain, two build standards

The smoke side has its own "FDU". The AMM states both build standards:

"A safety bus system connects the detectors to the Smoke Detection Control-Unit (SDCU)." / "A CAN bus system connects the detectors to the Smoke Detection Function of the Cabin Intercommunication Data System (CIDS-SDF)."

The "one detector reports, the other silent" resolution:

"When the SDCU receives a warning signal only from one detector the SDCU does a test of the associated second detector: - if the associated detector has a normal function the smoke signal from the first detector is ignored, - if the associated detector does not have a normal function the SDCU sends the smoke signal to the aircraft warning system."

[!warning]- A lone report is resolved by asking "is the other one healthy?" If the other detector is healthy yet silent, the first is probably spurious (real smoke would reach both) — ignored; if the other is failed, the first is the only working detector — believed. This is the smoke-side version of the engine's AND→OR fail-conservative logic: healthy needs two, degraded accepts one. The later-build passenger configuration uses the CIDS-SDF; the logic is identical, the bus differs.

5. Other smoke zones — crew rest, video, broadband, stowage

These are configuration-dependent, driven by the same SDCU/CIDS-SDF → cockpit ECAM + cabin warning. Two carry a memorable specific.

The broadband centre self-isolates on a three-second timer:

"If the pushbutton-switch is not pressed approximately 3 s following the smoke warning activation, the ventilation extraction fan will automatically stop and electrical power to the Broadband System is shut down."

[!warning]- The broadband three seconds is the same philosophy as the APU ground auto A three-second manual window; act, or the system cuts power and stops the fan itself. Another three-second hand-over (APU article).

The video centre notes what does not raise a cockpit smoke indication:

"Note: ISPSS, seat actuators, and PED do not generate smoke indication in the cockpit."

Crew-rest smoke additionally raises local alarms inside the compartment — a continuous chime and "LEAVE MCR" / "DO NOT OPEN HATCH" signs — to wake and evacuate crew resting inside.

6. Operations — what the crew does with a smoke detection

AVNCS SMOKE (full procedure in the smoke/fumes article): the response is a SMOKE / FUMES procedure — oxygen masks (avoid inhalation) + ventilation control (clear/re-route the smoke) + isolating equipment power to find the source. There is no AGENT to press in the avionics bay; every action is ventilation, power and diagnosis.

Lavatory smoke: cockpit ECAM and cabin FAP alarm together, and cabin crew reach the scene first with a hand-held Halon 1211 extinguisher (overview) for open flame; the bin fire self-extinguishes on its fuse.

Detection fault (SMOKE … DET FAULT): a contaminated or failed detector demotes that zone's smoke detection; dispatch is per the MEL article.

Self-test

[!note]- Q1. Why is the smoke detector on the extraction duct, not the ceiling? What is the piccolo tube, and what system coupling does this create? It samples the smoke the ventilation is drawing away; the piccolo tube is a straw feeding particles to the detector. Coupling: stop ventilation and detection can go blind — smoke detection is tied to the ventilation system.

[!note]- Q2. Why is the avionics bay detect-only? What are the three core crew actions? An electrical fire needs power cut, not agent, and the bay is accessible (not a sealed floodable hold). Core actions: cut power, ventilate/clear smoke, diagnose the source.

[!note]- Q3. What two fire items does a lavatory have? How does the waste-bin bottle trigger? A smoke detector (alarms crew/cabin) and an automatic waste-bin bottle. The bottle triggers on a thermal fuse when bin temperature reaches a set level — no crew action.

[!note]- Q4. With one detector reporting and the other silent, how does the system decide? It tests the other: healthy-but-silent → ignore the first (spurious); failed → believe the first (only working detector).

[!note]- Q5. What differs between SDCU and CIDS-SDF, and which does the later-build configuration use? Functionally equivalent; the bus/carrier differs (safety bus vs CAN, standalone box vs integrated into the cabin data system). Later-build uses the CIDS-SDF.

Key takeaways

References

• FCOM DSC-26-30-10 — avionics: two detectors, > 5 s trigger, VENTILATION SMOKE light.
• FCOM DSC-26-40-10 — lavatory smoke detector + cockpit/cabin warning; waste-bin extinguishing.
• FCOM DSC-26-56/57/59-20 — video-centre note, crew-rest local alarms, broadband 3 s auto.
• AMM 26-15-00 — optical detectors, piccolo tubes, ventilation-fed sampling; SDCU/CIDS-SDF; lone-report resolution.
• AMM 26-10-00 — crew must initiate the smoke procedure.
• AMM 26-25-00 — waste-bin thermal-fuse automatic extinguishing, independent of other systems.

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.