Pneumatic Architecture and Distribution

Electrics, hydraulics and fuel get all the attention, but the A330 runs on a fourth energy network too: a web of hot, high-pressure air. ATA-36 is a compact chapter, yet it sits upstream of three big ones — air conditioning and pressurisation (ATA-21), ice protection (ATA-30) and engine starting (ATA-70/80). If bleed dies, everything downstream starves. This opening article builds the map: who consumes the air, where it comes from, what shape the network is, and which computers run it. Stage selection and pressure regulation follow in article 02, temperature control in article 03, the APU and crossbleed in article 04, monitoring and indications in article 05, and leak detection in article 06.

1. Eight consumers — the family the bleed system feeds

The FCOM opens the chapter with a census. Per FCOM DSC-36-10-10:

The pneumatic system supplies high pressure air for : ‐ Air conditioning ‐ Engine starting ‐ Wing anti-icing ‐ Water pressurization ‐ Hydraulic reservoir pressurization ‐ Pack bay ventilation turbofan actuation ‐ Cargo heating ‐ Fuel Tank Inerting System (FTIS)

Eight consumers, and the last two carry option marks in the FCOM (cargo heating and FTIS depend on airframe configuration). Ask a crew to list the bleed users and most stop at three — air conditioning, starting, wing anti-ice. The ones that slip through are the small households: water-system pressurisation, hydraulic-reservoir pressurisation, the pack-bay ventilation turbofan, cargo heating, and fuel-tank inerting.

[!warning]- Nacelle anti-ice is NOT on this list Engine (nacelle) anti-ice takes its air from the engine's own independent bleed port and never enters the ATA-36 network. The AMM interface list routes it to chapter 30-21 through an independent bleed-air port. Wing anti-ice, by contrast, is a full ATA-36 customer. This is a recurring exam trap: losing an engine bleed system kills that side's wing anti-ice supply chain, not the engine's own cowl anti-ice.

The AMM version of the list hides a detail that matters for hydraulics. Per AMM 36-00-00:

the hydraulic-reservoir pressurizing system (high pressure from engine 1 regulated low pressure from the crossfeed duct)

Reservoir pressurisation takes its high-pressure tap from engine 1, with regulated low pressure drawn from the crossfeed duct. That single line of plumbing is why the number-1 engine's bleed system carries slightly more weight than number 2 at dispatch time — a thread picked up again in article 04 and article 11.

One more boundary note from the distribution section, per AMM 36-10-00: the network is to supply the wing anti-ice system (only during flight, test on ground is possible) — on the ground the wing anti-ice valves open only for a brief test sequence, which pays off when we read the BLEED-page arrow logic in article 05.

2. Three sources and the H-shaped network

Per FCOM DSC-36-10-10:

High pressure air is supplied from three sources : ‐ Engine bleed systems ‐ APU load compressor ‐ Two HP ground connections.

Engine bleed systems are interconnected by a crossbleed duct, to which the APU and ground sources are connected. A valve, mounted on the crossbleed duct, allows the left side (ENG 1) and the right side (ENG 2) to be interconnected.

Draw it once and it stays drawn — the network is an H: two vertical strokes are the engine bleed ducts running out into each wing; the crossbar is the crossbleed duct; the APU and the ground connectors both plug into that crossbar; and the crossbleed valve guards the middle of it.

        ENG 1 (IP/HP offtake)                 ENG 2 (IP/HP offtake)
             │ HPV+IPC ─► PRV ─► precooler         │ HPV+IPC ─► PRV ─► precooler
             ▼                                     ▼
   LEFT wing network ◄──┬── crossbleed duct ──┬──► RIGHT wing network
    │ WAI-L             │   [X-BLEED valve]   │             WAI-R │
    │ ENG 1 start       │         │           │        ENG 2 start│
    ▼                   │         │           │                   ▼
   PACK 1     HP ground conn. x2  │      APU check valve          PACK 2
   hyd rsvr   (LEFT of the valve) │           ▲
   press.                         │      APU bleed valve ◄── APU load compressor
   water / cargo heat / FTIS      │      (ECB-controlled)

The AMM assigns the duty roster. Per AMM 36-00-00:

The aircraft engines are the primary source of compressed air in flight.

The Auxiliary Power Unit (APU) is the primary source of compressed air on the ground.

The APU can supply bleed air: - during the climb, from the ground until the aircraft reaches 25000 ft. (7620 m), - during the descent, from 23000 ft. (7010 m).

Note the asymmetric pair — closes at 25 000 ft climbing, reopens at 23 000 ft descending. That is deliberate hysteresis, so the valve does not chatter open-closed while cruising near the boundary. The same two numbers appear on the APU BLEED pushbutton conditions and inside the APU ECB's own logic (article 04).

The ground source is a guest who caters for itself. Per AMM 36-10-00:

The ground air source controls the pressure and temperature of the compressed air supplied.

The aircraft does no conditioning of ground air at all — which is why a marginal air-start unit is a stated reason to expect a start abort and brief for a manual start (article 07).

[!warning]- The ground connectors feed only the LEFT half of the network Per AMM 36-13-00: A short duct connects the HP ground connectors to the crossbleed duct, to the left of the crossbleed valve. With the crossbleed valve closed, a ground cart pressurises only the left half. That is why the FCOM's entire HP GROUND AIR SUPPLY description is a single operational sentence — The crossbleed valve has to be opened manually to provide air for both sides — and why starting engine 2 from a ground cart requires the crew to open the crossbleed by hand (article 04).

The crossbar itself, for spatial awareness — per AMM 36-12-00: The crossbleed duct, which is 6.0 in. (152.40 mm) in dia., is installed in the unpressurized area between FR39.1 and FR39.2. "Unpressurised area" is the phrase to hold onto: the hot ducting runs where no crew member could ever smell a leak, which is exactly why a dedicated overheat-detection loop system exists (article 06).

3. Four brains behind three buttons

The FCOM gives the crew-facing control chain. Per FCOM DSC-36-10-10:

Pneumatic system operation is controlled and monitored by two Bleed Monitoring Computers (BMC 1 and 2), the overhead control panel and the ECAM. The APU bleed supply is controlled by the APU Electronic Control Box (ECB).

The AMM completes the picture — the automation is a partnership. Per AMM 36-00-00:

The control of the pneumatic system is usually automatic. Bleed-Air Monitoring Computers (BMCs) and the Full Authority Digital Engine Control (FADEC) system control the automatic function.

There is one BMC and one FADEC for each engine.

A serviceable division of labour: the BMCs own the wing side — pressure-regulating-valve closure, crossbleed opening, temperature-setting selection, leak detection, data to ECAM. The FADEC (EEC) owns the engine side — closure and monitoring of the HP valve, with HP-valve position relayed to the BMC through the engine interface unit (article 05). And the APU's ECB owns the tail — the APU bleed valve and the load-compressor inlet guide vanes. Count them and one air network answers to five controllers (2 BMC + 2 FADEC + 1 ECB); yet the overhead panel shows the crew only two ENG BLEED pushbuttons, one APU BLEED pushbutton and one X-BLEED rotary selector. The automation swallows the complexity — and announces it with darkness. Per AMM 36-00-00:

The pushbutton switches on the overhead panel 225VU have no indication when the system operates correctly.

Textbook dark-cockpit design: a lit bleed pushbutton always means something — white OFF means you switched it off; amber FAULT means the system did (article 05). The only steady light in normal use is the blue ON of the APU BLEED pushbutton, which is a state annunciation, not a fault.

4. "8th and 14th stage" — offtake anatomy on a three-shaft engine

The AMM defines the two offtake ports with a note it considers important enough to print twice:

The 8th compression stage refers to the IP PORT at the end of the IP compressor. The 14th compression stage (8th from intermediate pressure + 6th from high pressure) refers to the HP PORT at the end of the HP compressor.

On a three-shaft Rolls-Royce engine the compressor string is LP fan, then an 8-stage IP compressor, then a 6-stage HP compressor. So "8th stage" is the IP compressor exit and "14th stage" is the HP compressor exit — 14 is a running total (8 + 6), not the fourteenth blade row of one spool. Why the system prefers the 8th and when it is forced onto the 14th is the whole story of article 02.

[!warning]- Two different things are both called "bleed valves" on this engine Per FCOM DSC-70-60: Two air bleed systems provide greater compressor stability in different flight phases. The volume of airflow through the intermediate pressure and low pressure compressors is regulated by four intermediate pressure stage 8, and three high pressure stage 3, bleed valves controlled by the FADEC. At low engine speed, the bleed valves are open to prevent engine stall.

Those seven valves are handling bleed valves — they dump compressor air overboard/into the bypass to protect the engine from surge. They are not part of ATA-36. The ATA-36 offtakes export air to feed the aircraft. Same word, opposite purpose: handling bleeds protect the engine; offtake bleeds feed the aircraft. The engine's own air system also cools the engine compartment, turbine and oil — the pneumatic chapter is only one of its customers.

5. When a duct bursts: the passive protection layer

Leak-detection loops (article 06) are the active layer — they alarm and isolate. ATA-36 also carries a purely structural layer, "environment protection", for the case nothing catches. Per AMM 36-14-00:

The protection systems of the wing leading edge, the pylon and the nacelles keep the pressure in these areas to a limit. The protection systems operate if a pneumatic duct or a wing anti-ice duct bursts or has a large leak. This prevents damage to the structure and the systems installed.

Three zones, three devices:

Wing leading edge — thirteen blow-out panels per wing. Per AMM 36-14-00: The pressure relief access panels are of the 'blow out' type. and There are thirteen panels installed on each wing. The release mechanism is a set of shear rivets: To shear the rivets, a pressure of approximately 7 psi (0.48 bar) is necessary. The adjacent skin structure is made to hold pressure of 15 psi (1.03 bar). A burst duct pressurises the leading-edge bay; at roughly 7 psi the rivets shear and the panel's trailing edge blows open (a lanyard restrains it), venting the bay — while the surrounding skin is built for 15 psi. A factor-of-two margin: sacrifice a panel, save the leading edge.

Pylon — spring-loaded doors at 2.9 psi. Per AMM 36-14-00: The overpressure in this area is limited to 2.9 psi (0.20 bar) through the pressure relief doors. Above this value, the pressure relief doors open and stay open. Two titanium spring-loaded doors in each pylon leading edge plus one latch-type door at the pylon/wing interface. "Open and stay open" is deliberate — the door does not reseat, so the evidence survives to the walkaround.

Nacelle — the telltale you can see. Each nacelle has two pressure-relief doors, and per AMM 36-14-00: When the aircraft is on the ground you can see the open pressure relief doors. This is an indication that there has been an increase in pressure in the nacelle. Walkaround teaching point: a popped panel on the leading edge, a hanging pylon door or an open nacelle relief door all say the same thing — hot high-pressure air went somewhere it should not have. Do not push back; call maintenance.

Finally, the starting system is this network's biggest single gulp of air, and its source options mirror section 2 exactly. Per AMM 80-11-00:

The air used to operate the pneumatic starter comes from: - a ground pneumatic connection - the APU (Auxiliary Power Unit) - an adjacent engine which has started.

The third option — an adjacent running engine — is the legal basis of the crossbleed start, developed with its numbers in article 07.

Self-test

[!note]- Q1. Name the eight bleed consumers. Which are most often forgotten, and why is nacelle anti-ice not among them?

Air conditioning, engine starting, wing anti-icing, water pressurisation, hydraulic-reservoir pressurisation, pack-bay ventilation turbofan actuation, cargo heating, FTIS. The commonly forgotten ones are the last five. Nacelle anti-ice draws from the engine's own independent bleed port and never enters the ATA-36 network.

[!note]- Q2. Which source is primary in flight, which on the ground, and what are the APU bleed altitude numbers — why two different ones?

Engines in flight; APU on the ground. APU bleed is available up to 25 000 ft climbing and again from 23 000 ft descending — a hysteresis band that stops the valve cycling at the boundary.

[!note]- Q3. Where do the HP ground connectors join the network, and what does that force the crew to do when starting engine 2 from a ground cart?

Via a short duct into the crossbleed duct left of the crossbleed valve. Ground air therefore reaches only the left half until the crossbleed valve is opened manually — which is exactly what the FCOM requires to supply both sides.

[!note]- Q4. Who controls what: BMC, FADEC, ECB?

BMCs: wing-side functions — PRV closure, crossbleed auto opening, temperature setting, leak detection, ECAM data. FADEC/EEC: the HP valve (closure and monitoring, position relayed via the engine interface unit). ECB: the APU bleed valve and the load-compressor IGVs.

[!note]- Q5. What do "8th stage" and "14th stage" mean on a three-shaft engine, and how do the seven FADEC-controlled stability bleed valves relate to them?

8th = IP compressor exit (IP port); 14th = HP compressor exit (8 IP + 6 HP stages). The four IP-stage-8 and three HP-stage-3 handling bleed valves are engine-surge protection that dumps air at low speed — a different system entirely from the ATA-36 offtakes.

[!note]- Q6. Quote the three environment-protection numbers and their meaning.

Wing leading edge: shear rivets release at ~7 psi, skin designed for 15 psi (thirteen blow-out panels per wing). Pylon: relief doors limit overpressure to 2.9 psi and stay open once released. Nacelle: two relief doors whose open position on the ground is the visible sign that an overpressure occurred.

Key takeaways

References

System definition, consumer list, sources and crossbleed topology per FCOM DSC-36-10-10; ground-supply sentence per FCOM DSC-36-10-40; engine airbleed system purposes and stability bleed valves per FCOM DSC-70-60. Automatic control by BMC + FADEC, one of each per engine, dark-cockpit note, offtake stage definitions, APU altitude envelope and the consumer interfaces (including the engine-1 hydraulic-reservoir tap and the nacelle anti-ice independent port) per AMM 36-00-00 and AMM 36-10-00. Crossbleed duct dimension and location per AMM 36-12-00; HP ground connector arrangement per AMM 36-13-00; environment protection (wing blow-out panels, pylon and nacelle relief doors with their release values) per AMM 36-14-00; starter air sources per AMM 80-11-00. The H-network diagram and the "four brains" framing are integrative syntheses of the referenced text. Maintenance-layer detail (FIN codes, access panels, BITE menus) is intentionally excluded.

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.