General Description

The aircraft has three independent, continuously-operating hydraulic systems: GREEN, BLUE, and YELLOW. Each system is supplied by its own reservoir. There is no fluid transfer between systems under any circumstance. The whole architecture is monitored by a single computer — the Hydraulic System Monitoring Unit (HSMU).

This article fixes those four facts, the operating pressures, the low-pressure thresholds with their hysteresis bands, and the reservoir pressurisation source. Every later article in the chapter assumes the reader already takes them for granted.

1. Three independent systems

        ┌──────── GREEN ────────┐    ┌──────── BLUE ────────┐    ┌──────── YELLOW ────────┐
        │  Reservoir            │    │  Reservoir           │    │  Reservoir             │
        │  ENG 1 + ENG 2 EDP    │    │  ENG 1 EDP           │    │  ENG 2 EDP             │
        │  Electric pump        │    │  Electric pump       │    │  Electric pump         │
        │  RAT pump (emerg.)    │    │                      │    │  Hand pump (cargo)     │
        │  Accumulator          │    │  Accumulator         │    │  Accumulator           │
        └───────────┬───────────┘    └──────────┬───────────┘    └────────────┬───────────┘
                    │                            │                              │
                    ▼                            ▼                              ▼
                 USERS                        USERS                          USERS
                 (flight controls, gear, brakes, slats/flaps, reversers, etc.)
                    ▲                            ▲                              ▲
                    └────────────────────────────┴──────────────────────────────┘
                                                 │
                                      No fluid cross-feed

Two consequences of the independence rule:

• Loss of one system cannot be recovered from the others. Any procedure assuming "switch to the other side" exists in electrical and pneumatic architecture, not here.
• Each loop must be self-sufficient. Green and Yellow each carry a redundant electric pump for the cases where the engine-driven pump (EDP) is unavailable. Blue carries one electric pump for the same reason but with a narrower set of triggers (see Electric Pumps).

The HSMU sits across all three systems. It controls electric pump logic, RAT extension, the green fire shut-off valves, leak-measurement valve operation, reservoir-temperature correction of the quantity indication, and the FAULT light logic for the overhead panel pushbuttons. The HSMU does not carry hydraulic fluid — it is a controller, not a transfer path.

Physical placement — what the pilot should know but the FCOM does not draw

The FCOM treats the three systems as a logical block diagram. For a working mental model, the pilot benefits from knowing where each system physically sits and which engine drives it. The summary below is derived from the AMM 29-00 topology (specific component P/Ns and detailed routing are unpacked in Power Distribution Map):

The physical separation between the three reservoirs is itself part of the independence design. Locating Green at the centre, Blue on one side of the gear bay, and Yellow on the other prevents a single localised event (fire, impact damage, leak) from compromising more than one reservoir at a time. The "no fluid transfer" rule has a hardware-level complement: no shared physical neighbourhood for the three reservoirs.

Detailed pipe routing, materials by zone (titanium in non-fire HP, stainless steel in fire-zone HP, light alloy in LP), manifold organisation, and seal-drain collection are covered in Power Distribution Map § 6 Physical layout.

2. Normal operating pressure

Normal system operating pressure is 3000 psi (≈ 206 bar). When the Green system is supplied by the RAT pump only — that is, in the loss-of-all-engines case — Green regulates to 2500 psi (≈ 172 bar).

The reduced figure under RAT is not a fault. It reflects the limited extraction power of an air-driven turbine against the same downstream consumers. When the RAT pressurises Green, the aileron, elevator, and spoiler servo control operating speeds are reduced; the surfaces still work, they move more slowly.

3. Low-pressure thresholds and hysteresis

The architecture monitors pressure at multiple points, each with a documented trigger and recovery threshold. The hysteresis between trigger and recovery is deliberate: a single threshold value with no hysteresis would oscillate ("flutter") at the boundary, producing nuisance ECAM cautions during normal demand transients. The hysteresis bands also tell the crew something important — that a triggered caution means real pressure loss, not a momentary dip.

System pressure — SYS LO PR

The 1450 psi trigger is well below normal operating pressure (3000 psi) — a SYS LO PR caution implies that system pressure has dropped by more than half, indicating a genuine loss rather than a load transient. The 1750 psi recovery means pressure must climb a clear 300 psi above the trigger before the system is considered restored.

Engine-driven pump — PUMP LO PR

The EDP pressure switch sits between the pump and the system check valve. It senses what this pump is delivering, not what the system as a whole is at. On Green with two EDPs, PUMP LO PR (1) can occur with no SYS LO PR — the other EDP carries the system.

Electric pump — ELEC PUMP LO PR

The electric pump's lower trigger (100 bar vs the EDP's 120 bar) reflects its lower full-flow output pressure (150 bar full flow vs the EDP's 196 bar). Setting the trigger 50 bar below full-flow operation prevents spurious triggers during normal heavy demand.

Reservoir air pressure — LO AIR PRESS

The 1.5 bar relative threshold means the reservoir cushion has decayed substantially — roughly two-thirds of the normal pressure has been lost — before the caution appears. Transient bleed-side events typically don't reach this threshold; a persistent LO AIR PRESS indicates either a complete bleed source loss or a reservoir-side leak.

Reservoir level and temperature

The 95 °C trigger is well above normal operating temperature — in cruise, fluid temperature is typically in the 30–50 °C range. A real OVHT shows gradual temperature rise; an abrupt jump to 150 °C is consistent with sensor failure (which the HSMU treats as a fail-safe OVHT default — see Hydraulic Fluid).

The LO PR vs LO AIR PRESS semantic trap

A high-frequency mix-up among crews learning the architecture. Both cautions contain "PR" or "PRESS" in the name, and both relate to "pressure being low" — but the underlying physics is completely different, and so is the correct crew response.

The distinguishing word is "AIR". A LO PR caution refers to fluid-output pressure (the working hydraulic side); a LO AIR PRESS caution refers to the air cushion at the top of the reservoir (the supply-side conditioning that keeps the pump inlet above vapour pressure).

The diagnostic discipline is to cross-check the SD HYD page system-pressure value:

• LO AIR PRESS with system pressure still stable at 3000 psi → cushion is depleted but pumps are still being fed adequately; likely a transient or bleed-source issue; do not switch off pumps, monitor.
• LO AIR PRESS with system pressure fluctuating or dropping → pump-inlet cavitation in progress; this is a genuine pump-survival concern; follow ECAM.

Treating LO AIR PRESS as if it were SYS LO PR and shutting down pumps unnecessarily is the classic over-reaction. The two cautions look alike on the EWD but live in different physical layers of the architecture. Full discrimination logic is in Reservoir Pressurisation and Pump vs System Failure.

4. Reservoir pressurisation

Each reservoir is pressurised by HP bleed air, normally drawn from Engine 1. If Engine 1 bleed pressure is insufficient, the system draws from the crossbleed duct.

The purpose is not to drive fluid out of the reservoir — that is the pump's job. The pressurisation keeps the pump inlet above the vapour pressure of the fluid at operating temperature. Without it, the fluid would cavitate at the pump inlet, forming vapour bubbles that collapse violently against the pump internals and destroy the unit in minutes.

This is why a reservoir-air-pressure low caution appears as an amber LO AIR PRESS annunciation on the ECAM HYD page rather than a passive note: low reservoir pressure is a pump-survival item, not a comfort item. The full Reservoir Pressurisation logic, including the bleed-source selection, is covered in Reservoir Pressurisation.

5. Why no fluid transfer between systems

The independence is not a redundancy preference; it is a certification requirement. Transport-category rules (CS-25.1309 / FAR 25.1309) require that no single failure produces a catastrophic condition and that catastrophic failure probability remain "extremely improbable". Two interconnected systems share common-mode failure paths — a single contaminated batch of fluid, one ruptured cross-connection, one shared bleed source — and cannot meet the target.

Three fully independent systems break the common mode. The Ram Air Turbine, by extending Green via an entirely separate power source (ram air), extends independence into the loss-of-all-engines case. The fluid in the three systems is the same chemistry, but the loops never touch.

For the pilot, this means every hydraulic ECAM event reduces to two questions:

1. Which system is degraded?
2. What is lost on that system, and what remains?

Never "switch to the other side." There is no other side.

6. Pilot interface — at a glance

The architecture surfaces in three places in the flight deck. Each gets its own article in this chapter, but the picture should already be familiar:

Memo strip messages (HYD ELEC PUMP, RAT OUT) provide quiet confirmation that an action is in effect, without raising a caution. RAT OUT becomes amber during takeoff phases 1 and 2 — that case is treated in the abnormal-procedure articles.

7. Reading order for the rest of the chapter

The remaining articles unpack each block on the diagram above:

• Hydraulic Generation Overview — pump configurations across the three systems.
• Hydraulic Reservoirs — physical layout, quantity bands, low-level cascade behaviour.
• Reservoir Pressurisation — bleed source, 4.5 bar regulation, 12-hour static seal.
• Hydraulic Fluid — chemistry, temperature monitoring, overheat protocol.
• Engine-Driven Pumps — EDP engineering and depressurise mode.
• Electric Pumps — three-segment assembly with electrical protection chain.
• Ram Air Turbine — deployment logic, stowing constraint.
• Manual Pump — Yellow hand pump for cargo doors.
• System Accumulators — what they store, when they cover.
• Priority Function and Fire Shut-Off Valves — automatic shedding under low pressure.
• Filters and Leak Measurement Valves — filtration architecture, maintenance interfaces.
• HSMU — the controller behind every automatic behaviour.
• Power Distribution Map — which system feeds which consumers.
• Gravity Gear Extension — when Green normal extension is unavailable.
• ECAM HYD Page Reading — symbol-by-symbol interpretation.
• Hydraulic Warnings Reference — consolidated warning lookup.
• Single-System Loss, Dual-System Loss, Pump vs System Failure — abnormal scenarios.
• MEL — Dispatch and Operational Limits, Typical Day Operations, Pilot Maintenance View — operational context.

Self-test

[!note]- Q1. The Green and Yellow reservoir quantity indications drop together in cruise. Without looking at anything else, what does this tell you about the architecture?

It tells you that whatever you are looking at is not a hydraulic-fluid cross-feed, because that does not exist. Two reservoirs dropping together points to a common pressurisation cause (reservoir air pressure issue affecting both, since both are bleed-pressurised from the same source) or to a coincidence of two unrelated events. The architecture rules out fluid transfer as an explanation.

[!note]- Q2. With the RAT pressurising Green, the SD HYD page shows Green at 2500 psi. The trainee asks if this triggers G SYS LO PR. Does it?

No. 2500 psi is the regulated RAT output, not a low-pressure condition. The LO PR logic compares pump-output pressure against a fixed low-pressure switch threshold (1450 psi for SYS LO PR with hysteresis to 1750 psi); the RAT regulator sits well above that threshold. What does change under RAT supply is flow — the RAT delivers 15% to 45% of an EDP, depending on airspeed — and as a result aileron, elevator, and spoiler servo speeds are reduced.

[!note]- Q3. The SYS LO PR caution trigger is 1450 psi but the recovery is 1750 psi. Why the 300 psi gap?

The 300 psi hysteresis prevents the caution from oscillating on/off around the threshold during normal demand transients. A single-threshold design would produce frequent nuisance cautions during heavy hydraulic activity (gear cycling, simultaneous spoiler and brake activity). The 300 psi gap ensures that once triggered, the caution remains active until pressure has demonstrably recovered, and once cleared, the caution does not re-trigger until pressure drops significantly again. The hysteresis also tells the crew that a triggered SYS LO PR is a real condition, not a transient — pressure has dropped by half (from 3000 to 1450) and has not yet recovered to a safe margin.

[!note]- Q4. The reservoir air pressurisation source is HP bleed from Engine 1. What happens if Engine 1 is the failed engine, and why is this not a hydraulic problem in itself?

Reservoir pressurisation automatically switches to the crossbleed duct, drawing from Engine 2's bleed. The architecture treats reservoir pressurisation as a function with its own redundancy, independent of which engine drives each system's EDP. This is why a single-engine failure does not, by itself, raise a reservoir-air-pressure caution.

[!note]- Q5. A trainee says: "If Green fails, just open a cross-feed from Yellow." How would you correct them?

There is no fluid cross-feed. The three systems are physically isolated for certification reasons. Recovery of a failed system uses its own redundant pump path — the Green electric pump, or in the emergency case the RAT-driven pump for Green — never fluid from another system. The misconception usually comes from electrical or fuel architecture, where cross-feed exists.

[!note]- Q6. The HSMU fails. Walk through what kind of degradation that produces, conceptually.

The HSMU is the controller, not a fluid path. Loss of it removes the automatic logic for: electric pump control, RAT extension, automatic closure of the green fire shut-off valves on low Green reservoir level, leak-measurement valve inhibition, temperature correction of the quantity indication, and the FAULT light logic. Hydraulic fluid continues to flow under the engine-driven pumps as long as the EDPs are running. The pilot's workload increases — automatic protections are gone — but the system is not pressureless. The associated ECAM procedure spells out what the crew assumes manually.

References

Per FCOM DSC-29-10-10 (General), DSC-29-10-20 (Generation), DSC-29-20 (Controls and Indicators), DSC-29-10-30 (Distribution and Priority); FCOM PRO-ABN-HYD (SYS LO PR hysteresis 1450/1750 psi); AMM 29-11 (EDP pressure switch 120 ± 5 bar), AMM 29-14 (reservoir LO AIR PRESS at 1.5 bar relative, regulated cushion 4.5 bar absolute), AMM 29-21 (electric pump pressure switch 5JV at 100/120 bar), AMM 29-31 (reservoir overheat threshold 95 °C ± 2).

This article is independent study material. It is not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.