Ventilation Principles — Six Objects, Two Controllers

System Overview listed the ventilation skeleton — six objects, two computers. Pack Principles and Pressurisation Principles covered the conditioning and pressure sides. This article opens up ventilation itself: why six objects each need their own airflow, how the two computers split the work, the implementation quirk of each object, and the four ventilation controls the pilot touches.

It is a role-level overview; AEVC internals and per-object engineering are deferred to Avionics Ventilation, Cabin / Pack-Bay / Battery / Lav-Galley Ventilation, and Cargo Ventilation; failure handling to Ventilation & Cargo-Fire Interaction.

1. Ventilation's place among the three subsystems

The mental model: conditioning + pressurisation manage the "whole-aircraft air"; ventilation manages "local air for special locations". They cooperate through the shared main airflow and through independent ventilation branches.

            main airflow (managed by conditioning + pressurisation)
                          │
                    ┌─────┴─────┐
                    │  cabin / cockpit
                    │
                    │  → outflow valves → overboard
                    │    (pressurisation meters the rate)
                    │
       ┌────────────┼──────── dedicated ventilation branches ───────┐
       ▼            ▼                                                ▼
  avionics       battery / lav-galley                       pack bay / cargo
  (AEVC):        each on its own path                       each on its own path
  cabin recirc   (venturi / fan / …)                        (NACA / turbofan /
  + extract                                                  piccolo ducts)
       │
       └→ underfloor extract  or  overboard extract

2. The six objects and the two computers

The ventilation system includes the avionics ground cooling and ventilation for: The avionics, The battery, The lavatory and galley, The pack bay, The cabin, The main deck cargo. Two computers are provided: The Avionic Equipments Ventilation Computer (AEVC), and The Ventilation Controller. — FCOM DSC-21-30-10

Plus two functions that are part of the system but not standalone "objects": avionics ground cooling (extra cooling of the avionics ventilation air on a hot ground, triggered above 27 °C cabin-recirc temperature) and cabin-temperature-sensor ventilation (shares the lav/galley extract duct).

The split between the two computers is not by object — it is by control complexity and criticality:

Why two computers (integrative reasoning): avionics smoke/overheat needs a dedicated fast-responding controller, so all avionics-related valves cluster on the AEVC; pack-bay overheat (a fire risk) is also on the AEVC; simpler comfort functions sit on the Ventilation Controller so the AEVC is not overloaded. For the pilot: an AEVC failure hits avionics ventilation + pack-bay turbofan + ground cooling (serious → always an ECAM warning); a Ventilation Controller failure hits lav/galley fans / recirc control (comfort, not safety → may not be a direct ECAM). See Ventilation & Cargo-Fire Interaction.

3. Avionics ventilation — the most complex object

The avionics ventilation system is fully automatic. It cools the electrical and electronic components, in the electronic bay and flight deck (including the instruments). It uses air recirculated from the cabin, and extracts air from the different panels and equipments racks. — FCOM DSC-21-30-20

Hardware: 2 cabin fans (run continuously when powered; cut by the CAB FAN pb) · recirc valve (normally open; the AEVC partially closes it with both packs OFF + cabin fans ON, to preserve avionics flow) · extract fan · underfloor + overboard extract valves (AEVC or EXTRACT pb) · cooling-effect detector (CED, monitors flow + temperature).

Underfloor vs overboard extract (per FCOM DSC-21-30-80, EXTRACT pb):

• Underfloor extract is the default in flight — avionics heat goes out via the outflow valves, re-using the pressurisation release path.
• Overboard extract is for ground + smoke — on the ground with both engines stopped the outflow valves are not "active", so a separate vent is needed.
• OVRD is a crew action — typically during AVNCS SMOKE handling (see Smoke / Fumes / Avionics Smoke).

The CED monitors flow + temperature; either anomaly triggers an ECAM warning, and on the ground an external horn + warning light — the only ATA 21 warning that sounds an external horn. Detail in Avionics Ventilation.

4. Battery ventilation — passive venturi

A venturi in the aircraft skin draws air from the space around the batteries and vents it overboard. The resulting airflow ventilates the batteries. — FCOM DSC-21-30-40

[!warning]- Counter-intuitive: battery ventilation is fully passive — no powered parts

Battery ventilation has no fan, no valve, no controller — a venturi in the skin uses the slipstream to draw air naturally.

The design logic: a battery is a potential hydrogen source (it off-gasses on charge) → it must be ventilated continuously to prevent an explosive build-up; but any electric fan near the battery is itself an ignition source (motor failure, contactor arcing). So the design choice is a purely passive venturi — most reliable (no moving parts = no failure mode but physical blockage) and no spark risk. There is no pilot control and no ECAM indication — a true "fit and forget" subsystem. The principle is classic A330: no electrical sparks where there is hydrogen.

5. Lavatory & galley ventilation

The lavatory and galley are ventilated with air from main cabin distribution system. Air is discharged outside through a venturi. On ground, or when ΔP < 1 PSI, it is extracted by an electrical fan controlled by the ventilation controller. Ventilation of the cabin temperature sensors is connected to the extraction duct. — FCOM DSC-21-30-50

In flight the cabin is at higher pressure than outside, so the venturi works on the pressure difference — no fan needed; on the ground (ΔP = 0) there is no pressure difference, so the electric fan maintains ventilation. The control is the GALLEY & LAV FAN pb: Normal runs continuously; it is switched OFF (CAUTION) on a lavatory/galley fire or smoke — to deny the fire an air source and contain the smoke. Note that the cabin temperature sensors share this extraction duct.

6. Pack bay — NACA inlet in flight, turbofan on ground

Ventilation of the pack bay ensures air circulation in order to maintain, on ground and in flight, a mean temperature that is compatible with the structure constraints in the relevant area. In flight, air from the outside flows into the pack bay through a NACA air inlet. On ground, a turbofan provides sufficient airflow. The turbofan is driven by air from the bleed system, which is supplied through the turbofan supply valve. Controlled by the AEVC, the fan operates when the aircraft is on ground. An ECAM warning, associated with an external horn on ground, is triggered in case of failure of the turbofan (supply valve failed closed or turbofan jammed). — FCOM DSC-21-30-70

The pack bay needs ventilation because the primary/main heat exchangers reject heat into it and the hot ACM parts radiate — without ventilation the bay structure, nearby wiring, and adjacent equipment would overheat.

[!warning]- Counter-intuitive: the pack-bay turbofan is bleed-driven, not electric, and runs only on the ground

The turbofan is driven by bleed air, not an electric motor — re-using the working bleed pressure is simpler, and it avoids adding electrical equipment to a hot, fire-risk bay. The AEVC controls the supply valve (it is the "switch", not the driver). It runs only on the ground; in flight the NACA inlet's passive flow is enough. There is no pilot control; a turbofan failure gives an ECAM warning + ground external horn. A NACA blockage in flight is a silent failure — likely found indirectly through a pack-overheat indication.

7. Cabin ventilation — pack-2 fresh air, not recirculation

[!info] Source note

The quote below is from the current English FCOM (DSC-21-30-75), the fact source.

4 ceiling air outlets and 2 floor air outlets provide ventilation in the cabin. In normal mode, the cabin is supplied with fresh air from pack 2, without any recirculation air to avoid any smell from the main deck cargo payload (in particular livestock or perishable goods). In case of pack 2 failure, the cabin is supplied with recirculated air coming from the mixer unit. The air is extracted through openings at floor level at the barrier wall, which are integrated in the lining below the cabin seats. — FCOM DSC-21-30-75

[!warning]- Counter-intuitive: this cabin uses pack-2 fresh air, not recirculation

Most cabins recirculate to save bleed and hold humidity. This cabin does the opposite — recirculation is disabled, and the cabin runs on pack-2 fresh air. The reason: the main deck carries livestock or perishable goods, whose odour would reach the cabin (crew rest / supernumerary area) through recirculation. The design trades the energy saving for an odour-free cabin. Only on a pack-2 failure does it switch to recirculation (the mixing unit then runs on pack-1 fresh air + recirc — odour takes second place to "still having cooling"). This reflects the freighter configuration (the passenger configuration may differ — see the differing FCOM effectivity in System Overview §1). The operational cue: a pack-2 failure triggers a cabin-ventilation mode change on this configuration.

The four ceiling outlets feed warm air from above (diffusing down for an even cabin); the two floor outlets help temperature control (extra warmth in cold cruise); extraction is through floor-level openings at the barrier wall, integrated in the lining below the seats.

8. Main deck cargo ventilation — piccolo ducts + MSOV

[!info] Source note

The quote below is from the current English FCOM (DSC-21-30-78), the fact source.

The main deck cargo compartment is ventilated by a series of piccolo ducts. The recirculation fans then recirculate air from the main deck cargo to the mixer unit. Main deck Shut Off Valves (MSOV) enable to isolate the main deck cargo. — FCOM DSC-21-30-78

Piccolo ducts are perforated distribution pipes (drilled with many small holes, like a piccolo) running the length of the hold, spreading air evenly. The key element is the MSOV: normally open (cargo air recirculates to the mixing unit); on smoke detection the MSOV closes and recirculation stops, fully isolating the hold to keep smoke/fire out of the cabin — interlocked with Fire Protection cargo-smoke detection.

Note this is the main deck's accompanying ventilation — distinct from the independent cargo environmental control (21-40 bulk hold + 21-45 forward lower hold), which have their own isolation and inlet/outlet valves, covered in Cargo Environmental Control.

9. Avionics ground cooling — the 27 °C trigger

The avionics ground cooling is fully automatic. On ground, it ensures the cooling of the avionics ventilation air, in case of extremely hot outside air. The cooling system is integrated in the avionics ventilation system, but operates independently. — FCOM DSC-21-30-30

AUTO: The ground cool valve opens. The ground cooling fan and the ground cool unit will automatically start, provided the aircraft is on ground, the engines are stopped, and the temperature of cabin recirculated air is above 27 °C. — FCOM DSC-21-30-80 (GND COOL pb)

Three AND conditions: aircraft on ground (in flight the slipstream + pack cooling suffice); engines stopped (APU or ground power supplying); cabin-recirc temperature > 27 °C (80.6 °F). Why 27 °C (integrative reasoning): avionics operating limits are typically below 70 °C, and the cabin-recirc temperature is the starting temperature of the air entering the avionics — 27 °C is a preventive threshold that keeps a margin. The control is the GND COOL pb (AUTO / OFF / FAULT light); a failure goes amber + ECAM + activates the ground-crew call system (so maintenance steps in).

10. Controls and ECAM

The ventilation ECAM fields are on the CAB PRESS page (not COND) plus the FAULT lights on each ventilation pb:

The four (plus one) ventilation pbs:

In practice the pilot leaves them all in AUTO — except in a smoke emergency (EXTRACT → OVRD, GALLEY & LAV FAN → OFF).

11. Counter-intuitive points

12. The day — ventilation contact points

The system is mostly hands-off: it runs as soon as it has power (cabin fans + extract fan continuous), ground cooling may trigger after APU start if cabin-recirc > 27 °C with engines stopped, the underfloor/overboard extract switches with engine state, the pack-bay turbofan stops once airborne (NACA takes over). The pilot rarely acts — it is the failures (CED / VENT / ground cooling / pack-bay turbofan) that bring the system to attention.

Self-test

[!note]- Q1. What are the six ventilation objects, and why these six?

Avionics, battery, lavatory & galley, pack bay, cabin, main deck cargo (FCOM DSC-21-30-10). Each needs dedicated ventilation for a distinct reason: avionics (electronics heat + fast smoke response), battery (hydrogen build-up), lav/galley (odour, vapour, smoke), pack bay (heat radiated by working packs), cabin (the fresh-air consumer), main-deck cargo (odour + smoke isolation). Plus two functions that are not standalone objects: avionics ground cooling and cabin-temperature-sensor ventilation.

[!note]- Q2. What does each computer manage, and why two?

AEVC: avionics extract valves + pack-bay turbofan (ground) + avionics ground cooling. Ventilation Controller: recirc valve partial-close (both packs OFF) + lav/galley electric fan + assorted logic. The split is by complexity and criticality, not by object: the AEVC handles complex automatic, safety-critical functions (avionics fault / pack-bay overheat affect safety); the Ventilation Controller handles simple state, comfort functions. Two computers because avionics-smoke response needs a dedicated controller, AEVC overload would hit critical functions, and isolating the simple functions contains faults.

[!note]- Q3. Why is battery ventilation a passive venturi?

FCOM DSC-21-30-40: a skin venturi draws air over the batteries using the slipstream — no fan, valve, or controller. The logic: a charging battery off-gasses hydrogen and must be ventilated, but any electric fan nearby is an ignition source (motor failure, contactor arcing). A passive venturi has no failure mode (bar physical blockage) and no spark risk — the classic A330 principle: no electrical sparks where there is hydrogen.

[!note]- Q4. Why does this cabin use pack-2 fresh air rather than recirculation?

FCOM DSC-21-30-75: to avoid odour from the main-deck payload (livestock or perishable goods) reaching the cabin. The main-deck recirc fans send hold air to the mixing unit; if the cabin also used mixing-unit recirculation it would smell the cargo. So cabin ventilation bypasses the mixing unit and takes pack-2 fresh air — odour-isolated from the hold. Only on a pack-2 failure does it switch to recirculation, odour taking second place to retaining cooling. This is the freighter configuration; the passenger configuration may differ.

[!note]- Q5. When does avionics ground cooling trigger, and why 27 °C?

FCOM DSC-21-30-80: three AND conditions — aircraft on ground, engines stopped (APU/ground power), and cabin-recirc temperature > 27 °C (80.6 °F). Why 27 °C: avionics operating limits are typically below 70 °C and the cabin-recirc temperature is the start of the air entering the avionics — 27 °C is a preventive threshold that keeps a margin. Typical scenario: a hot ramp with APU power. On failure, FAULT light amber + ECAM + ground-crew call system activated.

Key takeaways

Common misconceptions

Scope — what this primer covers and defers

References

A330 specifics per FCOM DSC-21-30-10 (general — six objects, two computers), DSC-21-30-20 (avionics ventilation), DSC-21-30-30 (avionics ground cooling), DSC-21-30-40 (battery venturi), DSC-21-30-50 (lavatory & galley), DSC-21-30-65 (GALLEY & LAV FAN pb), DSC-21-30-70 (pack bay — NACA / turbofan), DSC-21-30-75 (cabin — pack-2 fresh air, the fact source being the current English FCOM where the single-tail Chinese FCOM does not carry the section), DSC-21-30-78 (main deck cargo — piccolo ducts, MSOV), and DSC-21-30-80 (controls + ECAM + the 27 °C ground-cooling trigger). The "why these six objects", the AEVC-vs-Ventilation-Controller split rationale, the passive-venturi and bleed-driven-turbofan design logic, and the 27 °C margin reasoning are integrative syntheses; the pack-2-fresh-air anti-odour reason is stated directly in the FCOM.

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.