Physical Layout — Where the Parts Live

Cross-System Interfaces gave the connection view. This article gives the location view — gathering the physical positions scattered through stages A and B into a three-dimensional map of parts + ducting. The detail a pilot should know but the FCOM does not draw.

1. Three concentration zones

   [A330 longitudinal section]
   ┌─ cockpit overhead ─────────────────────────────┐
   │   20+ controls: AIR panel, OVHD 225VU, 212VU    │
   └─────────────────────────────────────────────────┘
            cabin
   ┌─ AVIONICS BAY (rack 800VU) ─────────────────────┐
   │   CPC 311/312HL · pack 531/532HH · zone 630HK   │
   │   · VC 280HN · AEVC 2HQ · RPCU 314HL (standalone)│
   └─────────────────────────────────────────────────┘
            underfloor
   ┌─ UNDERFLOOR ────────────────────────────────────┐
   │   8 recirc filters · 2 housings · 2 recirc fans │
   │   · 2 recirc valves · mixer (no FIN) · 3 cargo  │
   └─────────────────────────────────────────────────┘
   ┌─ BELLY FAIRING (aft of FR 40) ──────────────────┐
   │   2 packs 521/522HH + ACM/heat exch/water loop  │
   │   · PFCV 511/512HB · 2 outflow valves (below FL)│
   │   · negative-relief valve 5319HL (fwd, above FL)│
   └─────────────────────────────────────────────────┘
   ┌─ AFT PRESSURE BULKHEAD ─────────────────────────┐
   │   3 safety valves 316/317/318HL (above FL)      │
   └─────────────────────────────────────────────────┘

2. The flotation line and FR 40

The flotation line is the waterline when the aircraft floats after a ditching — about a metre below the cabin floor in cross-section, sloping slightly up from nose to tail. Above it: safety valves + negative-relief valve (not closed by DITCHING). Below it: outflow valves + the avionics overboard valve (closed by DITCHING to keep water out).

FR 40 is fuselage frame 40 — mid-fuselage (the cabin-middle / cargo-forward junction). Many ATA 21 parts are referenced to it.

[!note]- Many ATA 21 parts reference FR 40 — the mid-fuselage is the air-con concentration

Not because FR 40 itself matters, but because the mid-fuselage is the optimum location: best airflow distribution (reaches the cabin fore and aft), shortest ducting (bleed → packs → mixer → distribution), and closest to the wing root (bleed from the wing/engines). If maintenance says "work near FR 40", it almost certainly involves ATA 21.

3. Avionics bay — the five computers

The RPCU (314HL) is in the avionics bay's right side but not on a rack (standalone, permanently on the 28 V DC essential bus 301PP). The forward avionics access panel is 811.

[!warning]- All five ATA 21 computers are in rack 800VU — one cabinet

The CPC, pack, zone, VC and AEVC are all in avionics rack 800VU (different boards/slots but physically adjacent). The design: physical isolation from belly-fairing anomalies (a belly-fairing fire / heat / leak does not directly hit the avionics), the avionics power topology, maintenance access from panel 811, and the bay's own ventilation for cooling. The cost: a single avionics-bay fire could hit five ATA 21 computers at once — but the bay has SDCU fire detection + independent ventilation.

4. Belly fairing aft of FR 40 — the pack zone

Inside each pack (aft of FR 40): ACM (521/522HH12), primary + main heat exchangers (…18/…19), condenser (…13), water extractor (…14), reheater (…15), compact unit (…17), anti-ice valve (…HH1), TCV (…HH3) — see ata-21-07.

5. Aft pressure bulkhead — the safety valves

The three safety valves (316/317/318HL, each with a filter …HL1) are all on the aft pressure bulkhead, above the flotation line.

[!note]- All three safety valves are at one location — not dispersed

A pneumatic mechanical valve needs no dispersed redundancy; one location eases maintenance (one access panel, one test procedure); the three work in parallel — one open is enough.

6. Forward lower fuselage — the negative-relief valve

The negative-relief valve (5319HL) is in the forward lower fuselage, above the flotation line, aft of door 1.

[!note]- The negative-relief valve is forward, not aft like the safety valves

Large-flow negative-pressure relief needs a large opening; the forward lower fuselage has the structural space. Forward vs aft also gives two-location redundancy, and above the flotation line + aft of door 1 keeps it clear of the door.

7. Underfloor — recirculation + lower-deck cargo

The underfloor (between the cabin floor and the cargo ceiling) holds: 8 recirculation filters (5250–5257HG), 2 forward + 2 aft housings (5210–5213HG), 2 recirculation fans (210/211HG), 2 recirculation valves (212/213HG), 2 clogging indicators (214/215HG), 2 check valves (5214/5215HG) — see ata-21-10 — plus the three lower-deck cargo systems' fans (282/285/291HN), isolation valves and the forward cold-air valve (298HN).

8. Avionics ventilation — distributed

9. Cockpit controls — 20+ parts, by panel

[!note]- Panel 212VU is the A330's cargo-control concentration point

On the pilot's right overhead, panel 212VU carries the three ISOL pbs + the bulk HOT AIR pb + the bulk/forward temperature selectors + the forward cooling selector. Handling a cargo issue: look at 212VU and read every cargo control at a glance. (On a freighter variant, panel 205VU adds the aft-cargo temperature selector.)

10. Ram-air inlet, mixer, smoke paths

The emergency ram-air inlet is on the upper fuselage aft of the cockpit (not below the flotation line — dry in a ditching), its valve controlled by the RAM AIR pb (2HZ).

The mixer has no independent FIN — it is a manifold structure (an aluminium plenum) near FR 40 with six ports (pack 1 + pack 2 + left/right recirculation + emergency ram air + LP ground), and on a freighter variant the MORV (260HG) + the main-deck shutoff-valve outputs.

Smoke exit paths (four areas, independent): cabin → mixer recirculation upstream → overboard (outflow/safety valves); cockpit → fresh air → aft + windows (some aircraft); cargo → isolation valves close + fan stops (the area is isolated); avionics → extract fan → underfloor or overboard valve.

11. Pilot / maintenance application

Self-test

[!note]- Q1. Where are the five ATA 21 computers, and why concentrated?

All in avionics rack 800VU — CPC 311/312HL + pack 531/532HH + zone 630HK + VC 280HN + AEVC 2HQ. The RPCU 314HL is in the bay's right side but not on a rack (standalone, permanently on 28 V DC essential bus 301PP). Concentrated for: isolation from belly-fairing anomalies, the avionics power topology, access from panel 811, and the bay's own ventilation. The cost: a single bay fire could hit five computers — but the bay has SDCU fire detection + independent ventilation.

[!note]- Q2. What is FR 40, and why do many ATA 21 parts reference it?

Fuselage frame 40 — mid-fuselage (cabin-middle / cargo-forward junction). The packs (aft of FR 40), the mixer (near it), the trim-air check valves (at it) and the pressure-reg valves (near it) reference it. Why concentrate there: best airflow distribution (reaches the cabin fore and aft), shortest ducting, closest to the wing root (bleed source). "Work near FR 40" almost certainly involves ATA 21.

[!note]- Q3. What is the above/below flotation-line classification, and why?

The flotation line is the ditching waterline (~a metre below the cabin floor; sloping up nose-to-tail). Above: the 3 safety valves (aft bulkhead) + the negative-relief valve (forward) → not closed by DITCHING (they stay dry). Below: the 2 outflow valves + the avionics overboard valve → closed by DITCHING (keep water out). The logic: above-line parts keep working in a ditching; below-line parts close. The DITCHING pb is the pilot's one-press "close the below-the-line openings".

[!note]- Q4. Why are all three safety valves at one location, not dispersed?

They are all on the aft pressure bulkhead, above the flotation line. A pneumatic mechanical valve (poppet + spring + diaphragm + control valve) needs no dispersed redundancy; one location eases maintenance (one access panel, one test, AMM 21-31-00-710-805-A); the three work in parallel — one open is enough, and even two failed leaves the third. (The negative-relief valve is forward instead, because large-flow relief needs a large opening and the forward lower fuselage has the space.)

[!note]- Q5. Where is panel 212VU and what ATA 21 controls are on it?

The pilot's right overhead — the cargo-control concentration point: the BULK/AFT/FWD ISOL pbs (7/10/14HN), the bulk HOT AIR pb (3HC), the bulk/forward temperature selectors (430HC/437HC), and the forward cooling selector (297HN). Other ATA 21 controls are mainly on the AIR panel (temp selectors + heaters + PACK FLOW), OVHD 225VU (PACK/HOT AIR/RAM AIR/CPC manual/DITCHING/CAB FANS/OVHT RESET/EXTRACT), reset panel 262VU (zone reset 3HK), and the FAP. About 20+ controls in total.

Key takeaways

Common misconceptions

Scope — what this article covers and defers

References

This article integrates the physical positions first cited in the stage-A and stage-B deep-dives, drawing the FIN-location data from the AMM §2 FIN tables: AMM 21-26-00 §2 (avionics-ventilation zones), AMM 21-28-00 §2 (cargo-ventilation zones), AMM 21-21-00 §2 (mixer + recirculation), AMM 21-31-00 §2 (CPC + outflow valves + safety valves + negative-relief valve + RPCU), AMM 21-52-00 §2 (pack parts aft of FR 40), AMM 21-53-00 §2 (pack control), and AMM 21-63-00 §2 (zone controller + trim air). Each position was first cited and sourced in its stage-A/B chapter. The flotation-line cross-section estimate, the FR 40 mid-fuselage significance, the five-computers-in-one-cabinet rationale, and the panel-212VU concentration are integrative syntheses. All engineering detail is from the A330 knowledge base; no cross-type comparison is made, and no fleet tail numbers appear.

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.