Cargo Environmental Control — 21-40 / 21-45 + Fire Interlock

Ventilation Principles listed the main-deck cargo as one of the six ventilation objects. But the FCOM pulls cargo environmental control out into its own sub-chapters — DSC-21-40 and DSC-21-45 — separate from 21-30 ventilation, because cargo control is more than ventilation: it adds heating, temperature control, smoke interlock, and ditching handling. It is a small self-contained environmental system.

This article covers that system: the 21-40 / 21-45 configuration split, the common ventilation topology, the four isolation-close conditions (including the cascaded smoke isolation), the per-compartment heating differences, and the key thresholds. Component engineering is deferred to Cargo Ventilation; failure handling to Ventilation & Cargo-Fire Interaction. This is the last foundation article before the component deep-dives begin.

1. Why cargo gets its own chapters

21-10 air conditioning  → pack output to the cabin
21-20 pressurisation    → outflow valves meter the release
21-30 ventilation       → six objects (main-deck cargo is one, but only the
                          piccolo-duct + MSOV top layer)
21-40 cargo             → own chapter = bulk + aft + fwd, full environmental
                          control (ventilation + heating + temperature)
21-45 cargo             → own chapter = LD fwd + LD aft + bulk, full control

The design reason (integrative reasoning): cargo control adds heating, temperature control, smoke interlock, and ditching handling on top of ventilation — a different philosophy from 21-30's "local airflow", so the FCOM separates it.

The two chapters correspond to different aircraft configurations:

[!info] This article covers both 21-40 and 21-45

A given aircraft sees only the sub-chapter for its own configuration — check the "Applicable to: MSN …" effectivity at the top of the FCOM section. This article covers both for cross-configuration understanding. (Note: the single-tail Chinese FCOM reflects a 21-40 configuration and does not carry the DSC-21-45 sections; the current English FCOM carries both and is the fact source throughout.)

2. The common ventilation topology

Whether 21-40 or 21-45, every cargo compartment ventilates the same way:

        cabin  (air already cooled by the packs)
              │
              ▼
   ┌───────────────────────┐
   │ inlet isolation valve  │ ◄── ventilation controller
   └───────────┬───────────┘
               ▼
        ┌──────────────┐
        │   cargo       │ ◄── enters via sidewall / ceiling inlets
        │  compartment  │
        └──────┬────────┘
               ▼
   ┌───────────────────────┐
   │ extract fan            │ ◄── ventilation controller;
   └───────────┬───────────┘     stops on overheat
               ▼
   ┌───────────────────────┐
   │ outlet isolation valve │ ◄── ventilation controller;
   └───────────┬───────────┘     closes on ditching
               ▼
            bilge
               │
               ▼
   fwd outflow valve (LD fwd)  /  aft outflow valve (others)
               │
               ▼
           overboard

Three defining features:

• The air comes from the cabin (not directly from the packs) — cargo air is "used cabin air".
• An extract fan actively draws it (not a passive venturi) — the volume is large and cargo can block flow, so it must be forced.
• It exits via the outflow valves — cargo ventilation re-uses the pressurisation release path.

3. The four isolation-close conditions

The controller closes the isolation valves and stops the extract fan, when: The flight crew selects the BULK ISOL VALVE pb to OFF, or The bulk cargo smoke detection unit detects smoke. The outlet valve closes and the extract fan stops, when the flight crew sets the DITCHING pb to ON. — FCOM DSC-21-40-20

The complete set (using the lower-deck case, which adds the main-deck cascade):

[!warning]- Counter-intuitive: cascade isolation — main-deck smoke closes ALL lower-deck compartments

Per FCOM DSC-21-45-20, each lower-deck compartment's isolation-close condition includes "main-deck smoke detected" — even if that compartment is perfectly normal.

The design driver (integrative reasoning): the main deck sits above the lower deck, so main-deck smoke could spill down through structural gaps; main-deck smoke is a high-priority threat, so the system cascades isolation to all lower compartments to prevent spread and concentrate the main-deck fire-fight. This is why ATA 26 cargo fire is deeply coupled with ATA 21 cargo control.

Operational consequence: on a main-deck smoke warning, several compartments' ISOL VALVES FAULT lights may come on at once (pb at ON, but the valve has auto-closed) — this is not a cascade of failures, it is the designed cascade response.

4. 21-40 compartments

4.1 Bulk — ventilation + heating

VENTILATION: Air from the cabin goes via the inlet isolation valve to the bulk cargo compartment, and is driven by an extraction fan. Air is controlled by the outlet isolation valve, goes to the bilge, then through the aft outflow valve. HEATING: Bulk cargo compartment heating is performed by an electrical fan heater. Air from the cabin, driven by the electrical fan, goes into the compartment through a heating element. The temperature demand is selected from the cockpit. — FCOM DSC-21-40-20

Provided the cargo door is closed, the heater operates when the temperature sensors indicate that the compartment air temperature is less than the selected one. ON GROUND OPERATION: When the bulk cargo door is open, electrical power no longer supplies the heating element of the fan heater. Compartment heating is unavailable, as long as cargo door remains open. — FCOM DSC-21-40-20

Why a door open disables heating (integrative reasoning): a safety interlock — fire risk (ground crew / cargo near a hot element), wasted energy (heat escapes), and protection of the heating element (uncontrolled airflow). The door state controls the heating power.

4.2 Aft — ventilation only (NO heating)

Note: No heating is available for the aft cargo compartment. — FCOM DSC-21-40-20

[!warning]- Counter-intuitive: the 21-40 aft compartment cannot be heated

In the 21-40 configuration the aft compartment ventilates but cannot heat — whereas the 21-45 LD aft compartment is the opposite, it heats continuously (§5). The driver (integrative reasoning): the 21-40 aft compartment likely sits nearer heat sources (tanks, bleed ducts, pack bay) so its natural temperature suffices; the 21-45 LD aft sits further aft against the skin and needs active heating. Different effectivities reflect different physical layouts and expected payloads. Operationally: the bulk compartment has a HOT AIR pb + temperature selector; the 21-40 aft compartment has only an ISOL VALVE pb (no heating control).

4.3 Forward — ventilation + temperature control + cooling

Cargo COOLING selector: OFF: The cold air valve is closed. No cold air is added to the ventilation air. NORM: The cold air valve is partially open for normal operation. MAX: The cold air valve is fully open. Maximum quantity of cold air is supplied to cool the forward cargo compartment. — FCOM DSC-21-40-30

Why the forward compartment has cooling and others do not (integrative reasoning): the forward compartment is the temperature-sensitive hold — it often carries perishables (flowers, seafood, pharmaceuticals, mail) that need active cooling, not just low-temperature ventilation; the aft and bulk compartments only need heating because their payloads tolerate temperature well.

5. 21-45 lower-deck compartments

[!info] Source note

The single-tail Chinese FCOM reflects a 21-40 configuration and does not carry the DSC-21-45 sections; the quotes below are the current English FCOM (the fact source).

5.1 LD forward / aft — temperature control + continuous heating

VENTILATION AND TEMPERATURE CONTROL: Due to the extract fan suction, the cabin air flows through the inlet isolation valves into the forward cargo compartment, via the sidewall and ceiling inlets. Air is extracted through outlets, on the opposite sidewall, and goes via the extract fan and outlet isolation valve to the underfloor bilge area near the forward outflow valve. — FCOM DSC-21-45-20

Note: There is no OFF position for the LD AFT selector, the AFT cargo heating operates continuously. The heating system in the LD AFT cargo compartment stops only in case the LD AFT ISOL VALVES selector is set to OFF. — FCOM DSC-21-45-30

[!warning]- Counter-intuitive: LD aft heating runs continuously and cannot be switched off directly

The LD aft temperature selector has no OFF position — heating runs continuously (modulated over the 5–25 °C band). The only way to stop it is to set the LD aft ISOL VALVES pb OFF, which also stops ventilation.

The driver (integrative reasoning): the LD aft compartment is prone to over-cooling — far aft, away from bleed heat sources, against the skin (−50 °C outside) → naturally cold. Continuous heating prevents freezing of moisture-laden cargo, and the absence of an OFF position prevents the crew from inadvertently disabling it. Operationally: you cannot "heat off, ventilation on" — either the isolation valve is open (ventilation + heating both on) or closed (both off).

5.2 LD bulk — ventilation + heating + cascade

Same ventilation + heating as the 21-40 bulk, with the added cascade trigger:

The controller closes the isolation valves and stops the extract fan, when: The flight crew sets the BULK ISOL VALVE pb to OFF, or The aft cargo smoke detection unit detects smoke, or The MD cargo smoke detection unit detects smoke. — FCOM DSC-21-45-20

Full cascade coverage (21-45 configuration):

Main-deck smoke has the widest cascade — matching the §3 counter-intuitive point.

6. Controls and thresholds

Temperature selector range:

Temperature selector: COLD: 5 °C (41 °F) / HOT: 25 °C (79 °F) / Middle position: 15 °C (60 °F) — FCOM DSC-21-40-30

The selector is a target, not a guarantee — the FCOM notes that actual temperature can vary with flight time, outside temperature, and cabin temperature, so it may be higher than the selector position indicates. The LD selectors use the same 5–25 °C range with the mid-point at 15 °C.

Duct overheat (heater-fault trigger):

FAULT lt: The amber light, and associated ECAM message, come on when the duct overheats (above 88 °C/190 °F). The light goes off, when the temperature drops below 70 °C (158 °F), and OFF is selected. The fan heater stops. — FCOM DSC-21-40-30

[!warning]- Counter-intuitive: clearing the duct-overheat light needs TWO conditions

The duct-overheat fault light clears only when (1) the temperature falls below 70 °C AND (2) OFF is selected by the crew. Temperature falling alone does not clear it — the crew must actively select OFF. This is a deliberate crew-in-the-loop design: a duct overheat is serious (a possible ignition source), so it must not "fix itself" unnoticed. The 88 °C trip / 70 °C clear = an 18 °C hysteresis band, preventing chatter near the threshold (the same hysteresis philosophy as the low-pressure thresholds).

Extract-fan overheat:

OVHT COND FANS RESET pushbutton: FAULT lt: The light comes on amber when any fan overheat occurs. Fan is stopped. To resume fan operation press the OVHT COND FANS RESET pushbutton (FAULT light goes out). — FCOM DSC-21-40-30

An extract fan does not auto-recover after an overheat — the crew must press the OVHT COND FANS RESET pb. A latched design: an overheat is serious enough that it must not be silently cleared.

ECAM COND-page cargo fields: the trim-air (heating) valve shows H/C green (failed amber); the cold-air valve shows in-line / in-transit / cross-line green for open / partly-open / closed (amber on the corresponding failure); the compartment duct-temperature indication is normally green and goes amber when the duct is above 88 °C for 5 s (a single spike above 88 °C does not trip it — prevents false alarms). (Per FCOM DSC-21-40-30.)

7. Controls overview

The controls the crew sees depend on the configuration:

Pre-flight, check which fields and pbs your configuration shows to know whether it is a 21-40 or 21-45 aircraft.

8. The day — cargo-control contact points

Loading: a cargo door open disables that compartment's heating (even with the pb ON) — temperature anomalies during loading are expected, not a fault. Before pushback: doors closed + ISOL VALVES pb ON + sensible temperature selected (heating only works once the door is closed). Cruise: automatic; occasionally scan cargo temperature, especially with perishables. Unloading: door open disables heating again. Throughout, the crew rarely acts unless a fault appears.

9. Counter-intuitive points

10. Cargo-fire response (outline — full detail in ata-21-27 / ata-26)

cargo smoke detector triggers
        │
        ▼
ECAM SMOKE / FIRE caution + ATA-26 cargo extinguisher armed
        │
        ▼
ventilation controller auto-closes that compartment's inlet + outlet
isolation valves + stops the extract fan
        │
        ▼
(if it is main-deck smoke) → cascade-closes LD fwd + LD aft + LD bulk
        │
        ▼
crew ECAM procedure: CARGO SMOKE
        │
        ▼
confirm isolation auto-closed → discharge cargo extinguisher (if fitted)
→ divert to the nearest suitable airport

Full detail in Ventilation & Cargo-Fire Interaction and Fire Protection.

Self-test

[!note]- Q1. The FCOM has both DSC-21-40 and DSC-21-45 cargo chapters — what is the relationship, and why both?

They are different configuration effectivities: 21-40 = bulk + aft + fwd; 21-45 = LD fwd + LD aft + bulk. Both exist because A330 variants have different cargo layouts; the FCOM groups them by "Applicable to: MSN …" effectivity, and a given aircraft sees only its own. The common thread: all cargo ventilation runs cabin → inlet isolation valve → compartment → extract fan → outlet isolation valve → bilge → outflow valve.

[!note]- Q2. Why do different compartments (21-40 aft / 21-45 LD aft / bulk) have different heating capability?

21-40 bulk: heated (HOT AIR pb). 21-40 aft: no heating (FCOM explicit). 21-45 LD fwd: heated + cooled (cold-air valve). 21-45 LD aft: heated, continuous, cannot be switched off (only via ISOL VALVES OFF). 21-45 LD bulk: heated (HEATER pb). Driver (integrative): position determines natural temperature — the 21-40 aft sits near heat sources so it needs none; the LD aft sits aft against the skin (−50 °C cruise) so it needs continuous heating; the forward compartment carries perishables needing active cooling.

[!note]- Q3. Which compartments does main-deck smoke isolate, and why a cascade?

Per FCOM DSC-21-45-20: main-deck smoke cascade-closes LD forward + LD aft + LD bulk — all lower-deck isolation. Why (integrative): the main deck sits above the lower deck, so main-deck smoke can spill down through structural gaps; it is a high-priority threat, so all lower compartments are isolated to prevent spread and concentrate the fire-fight. Operationally: several ISOL VALVES FAULT lights coming on together on a main-deck smoke warning is the designed cascade, not a cascade of failures.

[!note]- Q4. Duct overheat trips at 88 °C — what clears it, and why the 88/70 hysteresis?

Per FCOM DSC-21-40-30: it trips above 88 °C (and must persist 5 s — prevents spikes), lighting the amber fault + ECAM + stopping the fan heater. It clears only on both: temperature below 70 °C AND OFF selected by the crew. The 18 °C hysteresis (88/70) prevents chatter near the threshold; the requirement to actively select OFF forces crew acknowledgement of a serious (possible ignition) fault — the same philosophy as the latched fan-overheat reset.

[!note]- Q5. An open cargo door disables heating — why is this designed, and is it a bug?

Not a bug — a safety interlock (FCOM DSC-21-40-20: "When the bulk cargo door is open, electrical power no longer supplies the heating element"). The reasons: fire prevention (ground crew / cargo near a hot element), energy (heat escapes through the open door), element protection (uncontrolled airflow), and forced isolation during ground work. Operationally: cargo temperature not rising during loading is expected — it recovers once the doors are closed. Same family as the pack flow control valve auto-closing with the maintenance door open (ata-21-01 §2).

Key takeaways

Common misconceptions

Scope — what this primer covers and defers

References

A330 specifics per FCOM DSC-21-40-10 (cargo general — ventilation controller, two channels), DSC-21-40-20 (bulk + aft + fwd ventilation/heating, the four isolation-close conditions, the bulk fan heater, the door-open heating interlock, the "no heating for the aft compartment" note, DITCHING), DSC-21-40-30 (controls — ISOL VALVE / HOT AIR pb, the 5–25 °C / mid-15 °C selector, the cargo COOLING selector, OVHT COND FANS RESET, the 88 °C / 70 °C duct-overheat hysteresis, ECAM fields and the 88 °C-for-5 s amber), and DSC-21-45-10/20/30 (lower-deck fwd/aft/bulk ventilation/heating/temperature, the main-deck-smoke cascade isolation, and the LD-aft continuous heating with no OFF position). All quotes verified against the English knowledge base, which carries both 21-40 and 21-45 (the single-tail Chinese FCOM carries only the 21-40 configuration). The configuration-effectivity rationale, the per-compartment heating-capability reasoning, the cascade and door-interlock design drivers, and the hysteresis philosophy are integrative syntheses.

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.