Cargo Compartment Smoke Detection and Extinguishing

The cargo hold is the most dangerous place on the aircraft to have a fire: you cannot see it, cannot reach it in flight, and it is full of flammable cargo. So its strategy differs from the detect-only avionics bay and the self-extinguishing lavatory: detect smoke and immediately flood with Halon, then race a diversion clock. This article covers the lower-deck holds (FWD/AFT/BULK) of the passenger configuration; the freighter main deck is in the next article.

1. Detection — heated optical detectors, three-condition trigger

The detectors sit in paired cavities in the ceiling, and — unusually — they are heated:

"The detectors are of the optical type. They are installed in each cargo compartment in cavities in the ceiling... The detectors are heated to increase the temperature inside the smoke detectors, so that the measurement is optimized especially in humidity areas."

[!warning]- The smoke detector is heated — not to sense fire, but to reject moisture Cargo holds are humid, and water vapour or condensation in the optical chamber scatters light and reads as smoke. Heating keeps the detector interior slightly warm and drives off condensation, avoiding humidity nuisance warnings — which is why the detectors have their own left/right heating supplies.

The FCOM gives the layout and the three-condition trigger of the later-build configuration:

"Smoke detectors, that are housed in pairs in cavities within the cargo compartment ceiling panels. There are: - Two cavities in the forward cargo compartment - Two cavities in the aft cargo compartment - One cavity in the bulk cargo compartment."

"The cargo smoke warning triggers, if: - Both smoke detectors detect smoke, or - One smoke detector detects smoke, and the other smoke detector is inoperative - Two smoke detectors detect smoke in different cavities."

[!warning]- "Two detectors in different cavities" also triggers The third condition is specific to the later-build configuration (earlier-build has only the first two). The logic: two detectors in the same cavity form an anti-spurious AND, but if one detector in cavity 1 and one in cavity 2 both report, the odds that this is real smoke that has spread across cavities are high — so it triggers. The AMM states it plainly: > "When the SDCU receives the smoke signal from two of the detectors (their location within the relevant cargo compartment is not important)" — any two detectors reporting is a warning, regardless of position. A lone report still runs the "test the other detector" resolution (article 05).

2. First response — isolate before extinguishing

Unlike the avionics/lavatory "detect and only alarm", a cargo warning drives immediate automatic isolation:

"When the cargo smoke warning triggers, the isolation valves of the cargo ventilation system of the affected compartment close automatically, and the extraction fan stops."

[!warning]- The system's first act is to contain, not extinguish Automatically closing the isolation valves and stopping the extraction fan (the SDCU commands the Ventilation Controller, coupling into ATA-21) serves three ends: cut the oxygen (no fresh air to the fire); hold the Halon (the agent about to be released will not be sucked out and can build concentration in the sealed hold, recalling "ventilation carries smoke — and Halon" from article 05); contain the smoke. Seal the room, then flood it — this is exactly why a cargo hold can be Halon total-flooded while an avionics bay cannot: the hold can be sealed into a floodable vessel.

3. Two bottles, two jobs — knockdown (~60 s) plus maintenance (260 min)

The two bottles have completely different roles:

"Two fire bottles with two discharge heads, one for each compartment, supply four nozzles... Pressing the AGENT pb, associated with the FWD (AFT/BULK) compartment, ignites the squib of the two bottles. Bottle 1 discharges its extinguishing agent into that compartment, and then bottle 2. Bottle 1 discharges in approximately 60 s . A flow metering system controls the discharge of bottle 2, to ensure sufficient agent concentration for 260 min ."

The AMM adds the mechanism and the concentration targets:

"The extinguishing agent from bottle 1 flows directly to the cargo compartments. The extinguishing agent from bottle 2 flows through a flow metering system which makes sure that the agent is released slowly... The flow metering system makes sure that the minimum halon concentration is: - 5 % volume in flight, - 3 % volume when the aircraft lands. ... All of the agent from bottle 1 is released in approximately 1 minute."

[!warning]- The two bottles are a fast/slow pair — not two equal shots, and not "60–75 minutes" Bottle 1 = knockdown: direct, ~1 minute, dumps its charge to slam Halon concentration to the extinguishing level and beat down the flame. Bottle 2 = maintenance: flow-metered (Halon filter + pressure switch + reducer + restrictor + check valve) to hold ≥ 5 % (in flight) / ≥ 3 % (on landing) for 260 minutes — because a cargo fire is invisible and cannot be confirmed out, so it must be held down until landing. 260 minutes is the diversion ruler: the diversion must complete in under 260 minutes, or the agent runs out first (abnormal article).

Compartment selection — one bottle or both:

"FWD AGENT pushbutton switch fires the forward cartridge in each bottle, AFT AGENT pushbutton switch fires the aft cartridge in each bottle."

[!warning]- Every bottle has both a FWD and an AFT outlet Pressing FWD AGENT fires both bottles toward the forward hold (bottle 1 knockdown + bottle 2 maintenance); AFT AGENT fires both toward the aft. It is not "one bottle per compartment" — it is two shared bottles routed to the compartment you select. So the cargo agent is a single-hold, one-shot resource: fire FWD and there is nothing left if AFT then ignites. Confirm the compartment before pressing.

4. The CARGO SMOKE panel

"FWD(AFT) AGENT PB-SW: This pushbutton-switch is guarded. When pressed, the associated squib is ignited... SMOKE light: Comes on red... when smoke is detected. SQUIB light: Comes on white, in case of a positive test. DISCH LIGHT: The BTL1(2) light comes on white, when the associated bottle has discharged."

The TEST pushbutton (held ≥ 3 s) sequences the FWD/AFT/BULK and avionics detectors, closes the isolation valves, and lights BTL/SQUIB white — with roughly 25 s per channel and 30 s between; developed in the test article. BTL logic mirrors the engine: bottle pressure falls → pressure switch → SDCU/CIDS-SDF → BTL white.

5. Operations — discharge, then LAND ASAP

  (FWD/AFT/BULK) CRG SMOKE (red) + CRC + MASTER WARN + COND SD page + 212VU SMOKE red
   -> already automatic: isolation valve closed + extraction fan stopped
   1  CARGO SMOKE ...... (confirm compartment)
   2  AGENT (compartment) DISCH   both bottles to that hold (bottle 1 knockdown + bottle 2 260 min)
   3  LAND ASAP                    diversion must complete in under 260 min

[!warning]- Why a cargo fire almost always means LAND ASAP The hold is invisible and cannot be confirmed extinguished — Halon only suppresses, and bottle 2's concentration decays to zero at 260 minutes, after which the fire may reignite. The only safe assumption is "the fire is still there, on a countdown", so a cargo fire's handling is the diversion decision — developed in the cargo-smoke abnormal article.

The "seal-and-flood" model of this article fails on the freighter main deck, which is too large to flood and carries no extinguishing system — the next article.

Self-test

[!note]- Q1. Why are cargo detectors heated? What happens without it? Cargo holds are humid; condensation in the optical chamber reads as smoke. Heating rejects moisture, preventing humidity nuisance warnings.

[!note]- Q2. What are the three trigger conditions, and why does "two detectors in different cavities" count? Both detect; one detects + other inoperative; two detect in different cavities. The third counts because smoke reaching detectors in two cavities is likely a real, spreading fire.

[!note]- Q3. How do bottles 1 and 2 differ? How long does each act? Bottle 1 discharges directly, ~1 min, to knock the fire down. Bottle 2 is flow-metered to hold ≥ 5 %/3 % concentration for 260 min.

[!note]- Q4. Pressing FWD AGENT fires one bottle or two? How many times can the cargo agent be used? Both bottles (knockdown + maintenance), routed to the selected compartment. It is a single-hold, one-shot resource — spent on the compartment you select.

[!note]- Q5. Why does a cargo fire almost always mean LAND ASAP? It is invisible and unconfirmable; Halon only suppresses and decays at 260 min, so assume the fire persists and land within the window.

Key takeaways

References

• FCOM DSC-26-50-10 — cavity layout, three-condition logic, auto isolation + fan stop, two-bottle 60 s/260 min.
• FCOM DSC-26-50-20 — CARGO SMOKE panel AGENT/SMOKE/SQUIB/BTL/TEST.
• AMM 26-16-00 — heated optical detectors (anti-humidity), paired AND, any-two trigger, Ventilation Controller isolation, split supplies.
• AMM 26-23-00 — two bottles with dual outlets, bottle 1 direct ~1 min / bottle 2 flow-metered, 5 %/3 % concentration, FWD/AFT AGENT firing.

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.