Main Airflow — Single-Line Walk-Through

System Overview put up the skeleton single-line: bleed in, conditioned, through the cabin, metered overboard. This article walks that line node by node — what each node is, what it does, the key control logic, and how it hands off to the next. It is a role-level tour, not a part-level teardown; the component deep-dives (PFCV, ACM, trim air …) come later in the chapter.

By the end you should be able to locate every ECAM COND field on the airflow line, know which node a given control (PACK FLOW selector, HOT AIR pb, RAM AIR pb) acts on, and reason about which segment a failure (zone-controller loss, pack-controller loss, ACM loss) affects.

1. The single-line schematic, annotated

The same path as the overview, now annotated with the temperature / pressure / flow references at each segment:

        ATA-36 BLEED MANIFOLD  (HP/IP/LP bleed · APU · ground cart)
        ~30–50 psi  ·  ~200 °C normal / ~150 °C reduced demand
                                  │
                ┌─────────────────┴─────────────────┐
                ▼                                   ▼
        ┌───────────────┐                   ┌───────────────┐
   ①    │  PFCV 1       │                   │  PFCV 2       │   ①
        │ pneumatic +   │                   │ pneumatic +   │
        │ electric ctrl │                   │ electric ctrl │
        │ no power→open │                   │ no power→open │
        │ (≈120% / HI)  │                   │ (≈120% / HI)  │
        └───────┬───────┘                   └───────┬───────┘
                ▼                                   ▼
        ┌───────────────┐                   ┌───────────────┐
   ②    │  PACK 1       │                   │  PACK 2       │   ②
        │ primary HX    │                   │ primary HX    │
        │ → ACM compr.  │                   │ → ACM compr.  │
        │ → main HX     │                   │ → main HX     │
        │ → ACM turbine │                   │ → ACM turbine │
        │ → water extr. │                   │ → water extr. │
        │ → reheater    │                   │ → reheater    │
        │ out ≈5–30 °C  │                   │ out ≈5–30 °C  │
        └───────┬───────┘                   └───────┬───────┘
                └─────────────────┬─────────────────┘
                                  ▼
                    ┌─────────────────────────┐
   ③               │       MIXING UNIT       │ ◄── recirc fans
                   │  pack air + recirc air  │     + filters
                   └────────────┬────────────┘     (cabin / cargo)
                                ▼
                    ┌─────────────────────────┐
   ④               │     HOT-AIR MANIFOLD    │ ◄── HOT AIR pb
                   │  trim-air valves +      │     governs the
                   │  HAPRVs (per zone)      │     HAPRVs
                   └────────────┬────────────┘
                                ▼
          ┌──────────┬──────────┴──────────┬──────────┐
          ▼          ▼                     ▼          ▼
   ⑤  cockpit   fwd cabin             mid cabin   aft cabin
       zone      ±3 °C PIM             ±3 °C PIM   ±3 °C PIM
          └──────────┴──────────┬──────────┴──────────┘
                                ▼
                    ┌─────────────────────────┐
   ⑥               │   cabin air splits:     │
                   │  → recirc fans (return) │  ≈40–50%
                   │  → outflow (overboard)  │  ≈50–60%
                   └────────────┬────────────┘
                                ▼
                    ┌─────────────────────────┐
   ⑦               │   2 × OUTFLOW VALVES    │
                   │   fwd + aft · 6 motors  │
                   │   CPC modulates rate    │
                   └────────────┬────────────┘
                                ▼
                            OVERBOARD

   ⑧  EMERGENCY RAM AIR  (dual-pack failure only)
       ambient air → mixing unit · opens only if ΔP < 1 PSI
       · outflow valves auto-open ≈50% (AUTO + ΔP<1) · inhibited by ditching

Grouped by function, the nine nodes fall into four segments:

The emergency ram-air path (⑧) is independent of the main loop and is used only in a dual-pack failure.

2. Pack Flow Control Valve (PFCV) — node ①

This valve is pneumatically-operated and electrically-controlled. It regulates the air flow in accordance with signals received from the pack controller. In the absence of air pressure, a spring keeps the valve closed. In the absence of electrical supply, the valve is open in a position equivalent to the HIGH selection, provided air supply is available. — FCOM DSC-21-10-20

Three facts define the PFCV's behaviour:

• Pneumatic + electric — air pressure drives it, an electrical signal sets the position. A classic pneumo-electric valve.
• No electrical supply → open at HIGH — the fail-safe is ≈120 % flow: lose power and the design would rather over-supply than under-supply.
• No air pressure → closed (mechanical spring) — if upstream bleed is gone, the PFCV shuts, protecting the pack from running dry.

The valve closes automatically under six conditions:

The cockpit control is the PACK 1 / PACK 2 pb on the AIR panel — On / OFF / FAULT light. Flow-detent timing and the 30-second engine-start closure sequence are in PFCV Deep-Dive.

3. The pack (air-cycle machine) — node ②

The two packs operate automatically and independently of each other. Pack operation is controlled by the pack controller. Warm pre-conditioned bleed air enters the cooling path via the pack flow control valve and is ducted to the primary heat exchanger. Then, the cooled bleed air enters the compressor section of the aircycle machine and is compressed to a higher pressure and temperature. It is again cooled in the main heat exchanger, and enters the turbine section where it expands. In expanding, it generates power to drive the compressor and cooling air fan. The removal of energy during this process reduces the air temperature, resulting in a very low air temperature at turbine discharge. The temperature control valve can modify the pack outlet temperature by adding uncooled air to the turbine outlet flow. — FCOM DSC-21-10-20

The pack is the heart of the main airflow — it takes ~200 °C bleed down to a cabin-usable ~5–30 °C. The key ideas:

• The air-cycle machine (ACM) is turbine + compressor + cooling fan on one shaft: the turbine's output power equals the compressor draw plus the fan draw. The machine balances its own energy — no external drive.
• Cooling comes from expansion through the turbine, not from a refrigerant. Removing energy as shaft work drops the temperature; turbine discharge is very cold.
• The temperature control valve sets pack outlet temperature by bleeding uncooled bypass air into the cold turbine outlet.
• ACM-failed degradation: flow is cooled by the heat exchangers only (and the pack must be off on the ground, where there is no ram air to cool it).

Full thermodynamic cycle, heat-exchanger geometry, water extractor, and reheater are in Pack Principles and ACM Deep-Dive.

4. Mixing unit + recirculation + filters — node ③

The mixing unit is the confluence of the main airflow: pack 1 and pack 2 cold outputs meet here, and the recirculation fans push cabin return air back into it, through filters that remove particulates and odour.

On : The two cabin fans are active, one is running and the other is in standby. Air from the MD cargo is blown to the avionics compartment and also to the mixer unit of the conditioning systems. — FCOM DSC-21-10-50 (CAB FANS pb)

Why recirculate, rather than condition 100 % fresh bleed:

Typical recirculation fraction is ≈40–50 % (exact value per AMM 21-22; see Mixing & Recirculation).

5. Hot-air manifold + trim air — node ④

The mixing-unit output is a single temperature (the average of both pack outputs plus recirculation). But the cabin needs different target temperatures for the cockpit and the fwd / mid / aft zones. The mechanism: a small stream of uncooled hot air is tapped upstream of the packs → through the hot-air pressure-regulating valves (governed by the HOT AIR pb) → into the hot-air manifold → through a trim-air valve per zone → adding heat to each zone individually.

Temperature regulation is automatic and controlled by one zone controller and two pack controllers. The cabin is divided into 3 temperature control zones: FWD, MID and AFT. ... The cockpit cabin temperature selector is used to select prior to flight a common cabin master temperature for all zones. The common master temperature can be individually adapted for the three cabin zones at any time via the cabin temperature page of the Programming and Indication Modul (PIM) on the forward attendant panel. An individual zone correction up to +/– 3 °C (5.4 °F) can be selected for the FWD, MID or AFT cabin zones in steps of 0.5 °C (0.9 °F). — FCOM DSC-21-10-30

Two layers of temperature control: the cockpit selector sets a common master temperature pre-flight; the cabin crew trims each zone via the FAP/PIM within ± 3 °C (5.4 °F) in 0.5 °C (0.9 °F) steps. With the cockpit as its own zone, there are effectively four controlled objects (cockpit + fwd/mid/aft). Selecting HOT AIR pb OFF disables trim air — every zone reverts to the pack-outlet temperature and per-zone trimming is lost. Detail in Zone Controller / Trim Air.

6. Distribution and the cabin air split — nodes ⑤⑥

The conditioned air from the hot-air manifold is distributed to four zones (cockpit, fwd / mid / aft cabin) through dedicated ducts and outlets. Distribution itself is passive (set by duct cross-section and outlet design); the active regulation happened at node ④.

Inside the cabin the air is not single-pass — it splits:

                cabin air
                    │
        ┌───────────┴───────────┐
        ▼                       ▼
   recirc fans              outflow valves
   (back to mixing unit)    (overboard)
   ≈40–50%                  ≈50–60%

The cabin must have continuous fresh-air renewal (the bleed-sourced pack air coming in) balanced by continuous release (through the outflow valves). When inflow and metered outflow balance, cabin altitude is stable. Detail in Mixing & Recirculation and Outflow Valve.

7. Outflow valves — node ⑦

The last link of the main airflow: two outflow valves (fwd + aft), six motors total (CPC controls two automatic + one manual per valve), modulating the release rate to control cabin pressure.

The outflow valve is a modulating release valve, not an on/off switch — see counterintuitive point 2 in System Overview.

Architecture and motor redundancy in System Overview §5 and Outflow Valve.

8. Emergency ram air — node ⑧

An emergency ram air inlet ventilates the cockpit and cabin, if both packs fail. The emergency ram air inlet valve is controlled by the RAM AIR pb on the AIR COND panel. This pushbutton opens the ram air valve, provided that ditching is not selected. The outflow valves open about 50 %, provided that they are under automatic control and ΔP is less than 1 PSI. They do not automatically open if they are under manual control, even if the ΔP is less than 1 PSI. If ΔP is greater than 1 PSI, the check valve, located downstream the ram air door, will not open. No airflow will then be supplied. — FCOM DSC-21-10-20

[!warning]- Counter-intuitive: ram air has three preconditions, not "push and flow"

Pressing RAM AIR pb does not guarantee airflow. All three must hold:

1. Ditching not selected — the ditching override force-closes every valve below the flotation line, including ram air.
2. ΔP < 1 PSI — the check valve downstream of the ram-air door will not open against a higher cabin pressure; above 1 PSI there is no airflow.
3. Outflow valves in AUTO + ΔP < 1 PSI — they auto-open ≈50 % to give the ram air an exit; in manual control the crew must open them by hand.

Operational consequence: at high altitude with a large ΔP, pressing RAM AIR pb produces no immediate flow. The cabin must first de-pressurise to ΔP < 1 PSI (ambient pressure rises in the descent, cabin leaks down) before ram air does anything. This is why the emergency descent and the ram-air selection go together — RAM AIR pb alone, without descending, achieves nothing. See Dual-Pack Failure.

9. Pack ram-air flaps (NOT the emergency ram air) — node ⑨

This is the most common naming confusion in ATA 21. Two different things are both called "ram air":

The ram air inlet and outlet flaps close during takeoff and landing to avoid ingestion of foreign objects. During takeoff, the ram air inlet and outlet flaps close when the thrust lever is at or above CL detent. During landing, they close as soon as the landing gear is compressed, when the speed is at or above 70 kt. They open, when the speed is below 70 kt, with a 10 s delay. — FCOM DSC-21-10-30

The trap is that both carry the word "ram air", but physically they are entirely separate — the pack flaps are the pack's own cooling-air shutters (nothing to do with cabin ventilation), the emergency ram air is a pack-bypass that ventilates the cabin in an emergency.

10. PACK FLOW selector logic

The flight crew can use the PACK FLOW selector to adjust the pack flow for the number of passengers and the external conditions. When the selector is in the AUTO position, the air flow is automatically managed by the pack controller, depending on the number of passengers entered in the MCDU. Whatever the flight crew selects, the system receives a high flow demand for any of the following circumstances: Single pack operation, or When the APU is supplying bleed air. — FCOM DSC-21-10-30

Two forced-HI cases (regardless of selector): single-pack operation (the remaining pack must run at full flow to supply the whole aircraft) and APU supplying bleed (APU flow limit + ventilation demand). LO auto-upgrades to NORM when the temperature demand cannot be met, and as an ACM anti-stall protection when LO is selected with heavy heating demand — with an ECAM advisory to the crew.

11. Zone controller — three external demands

The zone controller does more than set temperature: when a zone's demand cannot be met, it signals three other systems.

ENGINE PRESSURE DEMAND: When the cooling demand in one zone cannot be satisfied, if the bleed pressure is too low, the zone controller sends a pressure demand signal to both engines' Engine Interface Units (EIU), in order to increase the minimum idle and to raise the bleed pressure. APU FLOW DEMAND: When the APU bleed valve is open, the zone controller signals the APU Electronic Control Box (ECB) to increase the APU flow output when any zone temperature demand cannot be satisfied. BLEED TEMPERATURE DEMAND: If the cooling demand cannot be satisfied, the zone controller signals the Bleed Monitoring Computer (BMC) to decrease the bleed temperature from normal (200 °C) to reduced setting (150 °C). This reduction is inhibited, if the wing-anti ice is ON. — FCOM DSC-21-10-30

The takeaways for the pilot:

• A cabin over-temperature is not handled by the pack alone — the zone controller reaches out to engine idle / APU / BMC.
• The 200 °C → 150 °C bleed-temperature reduction is inhibited when wing anti-ice is ON — anti-ice has priority over cabin cooling (dropping bleed temperature would defeat the anti-ice). This is the ATA 21 ↔ ATA 36 / ATA 49 / ATA 30 cross-chapter control loop.

12. Failure degradation (dual-channel controllers)

Each Controller has a Channel 1 and a Channel 2. One channel is active and the other is in standby. After each touchdown, the active channel changes. — FCOM DSC-21-10-40

CHANNELS 1 AND 2 FAILURE (Zone Controller): Optimized and backup temperature regulation are lost. The packs deliver a fixed pack outlet temperature of 20 °C (68 °F). A Channel 1 and 2 failure removes all information from the COND SD page, which then displays "PACK REG". Flow selection from the PACK FLOW selector is lost. — FCOM DSC-21-10-40

What this means for the crew:

• Dual-channel zone-controller failure = "PACK REG" on the COND page — a distinctive visual cue that the system has dropped to fixed-20 °C delivery.
• 20 °C (68 °F) is a slightly cool "safe" temperature — no more zone trimming; passengers may feel cool but never overheat.
• Active channel swaps every landing to age both channels evenly — the same philosophy as the CPC's 80-second post-touchdown swap and the ATA 29 HSMU dual channel.

Detail in ECAM COND/BLEED and Single-Pack Failure.

13. Pilot scan along the airflow

Pre-flight ECAM COND scan, read in airflow order:

1. PACK 1/2 = green                        (node ②)
2. pack outlet temperature = value, green  (node ②)
3. flow field = NORM / LO / HI             (node ① PFCV)
4. zone temperatures (fwd/mid/aft/cockpit) = sensible  (node ⑤)
5. trim-air status = normal                (node ④)
6. HOT AIR = green                         (node ④, HOT AIR pb)
7. cargo ventilation status = normal       (cross-chapter 21-40/-45)

Any field turning amber / boxed / "PACK REG" → trace back along the airflow to find which segment failed.

Self-test

[!note]- Q1. From the ATA 36 bleed to overboard, what are the main nodes and the core role of each?

Nine nodes plus one emergency path: ① PFCV ×2 (flow control + pack inlet valve), ② pack ×2 (air-cycle cooling ~200 °C → ~5–30 °C), ③ mixing unit + recirc fans + filters (pack air + cabin recirculated air), ④ hot-air manifold + trim air (per-zone heating to target), ⑤ distribution (four zones: cockpit + fwd/mid/aft), ⑥ cabin air split (part recirculated, part released), ⑦ outflow valves ×2 (modulate release rate for cabin pressure), ⑧ emergency ram air (dual-pack failure, opens only if ΔP < 1 PSI), ⑨ pack ram-air flaps (the pack's own cooling shutters — not the emergency ram air).

[!note]- Q2. PACK FLOW selector AUTO / LO / NORM / HI — what is the logic, and when is HI forced?

AUTO lets the pack controller manage flow by the MCDU passenger number (default). LO / NORM / HI are manual. HI is forced regardless of the selector in two cases: single-pack operation (the remaining pack must supply the whole aircraft) and APU supplying bleed. LO auto-upgrades to NORM when the temperature demand cannot be met or as an ACM anti-stall protection, with an ECAM advisory. Source: FCOM DSC-21-10-30.

[!note]- Q3. After pressing RAM AIR pb the cabin gets no immediate airflow. Why, and what conditions make it work?

Three conditions must all hold: ditching not selected; ΔP < 1 PSI (the check valve downstream of the ram-air door will not open above 1 PSI); and outflow valves in AUTO + ΔP < 1 PSI (they auto-open ≈50 % to give an exit; in manual the crew opens them). At high altitude with a large ΔP there is no flow until the cabin de-pressurises below 1 PSI — which is why the emergency descent and the ram-air selection are done together, never RAM AIR pb alone. Source: FCOM DSC-21-10-20.

[!note]- Q4. The pack ram-air flaps close on takeoff and landing — why? Is this the same as the RAM AIR-pb emergency ram air?

Not the same — this is the most common ATA 21 naming confusion. The pack ram-air flaps cool the pack (ambient air through the pack heat exchangers); they close on takeoff (thrust lever at/above CL) and landing (gear compressed, speed ≥ 70 kt; reopen with a 10 s delay below 70 kt) to avoid foreign-object ingestion. The emergency ram air bypasses the packs to ventilate the cabin in a dual-pack failure, controlled by RAM AIR pb. Physically separate. Source: FCOM DSC-21-10-30 + DSC-21-10-20.

[!note]- Q5. With both channels of the zone controller / pack controller failed, what does pack outlet temperature become?

Zone controller channels 1+2 failed → pack outlet fixed at 20 °C (68 °F), COND page shows "PACK REG", PACK FLOW selector lost. Pack controller channels 1+2 failed → the anti-icing valve holds pack outlet at 9–15 °C (48–59 °F), PFCV regulates pneumatically to ≈120 % NORM. Both degradations keep the outlet bounded — fixed-20 °C or the 9–15 °C anti-ice band — never letting the cabin overheat or over-cool. Source: FCOM DSC-21-10-40.

Key takeaways

Common misconceptions

Scope — what this walk-through covers and defers

References

Per FCOM DSC-21-10-20 (pack and PFCV — air-cycle cooling chain, pneumatic/electric control, fail-open HIGH, six auto-close conditions, emergency ram air with the ΔP < 1 PSI check-valve logic), DSC-21-10-30 (temperature and flow regulation — one zone controller + two pack controllers, three temperature zones with PIM ± 3 °C / 0.5 °C, pack ram-air flap closure on takeoff/landing, PACK FLOW selector logic with forced-HI cases, the three zone-controller external demands and the wing-anti-ice inhibit), DSC-21-10-40 (dual-channel controllers, post-touchdown active-channel swap, channels-1-and-2 degradation to fixed 20 °C / "PACK REG" / 9–15 °C anti-ice band), and DSC-21-10-50 (PACK pb and CAB FANS pb). The "why recirculate" rationale and the PFCV fail-safe design philosophy are integrative syntheses, consistent with standard pneumatic-control practice.

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.