Hydraulic Generation — Overview

Generation is the set of components that produces and maintains 3000 psi in each of the three independent systems. The general principle is the same on Green, Blue, and Yellow: an engine-driven pump (EDP) as the primary source, an electric pump as a backup or ground source, and an accumulator to absorb transient demand. Beyond that common skeleton, each system carries a different secondary pump configuration and a different set of automatic-control triggers — which is why FCOM dedicates a separate page to each.

This article gives the architecture-level view; the next four articles take each system in turn.

1. The shared skeleton

Every system has the same five generation elements:

                       Reservoir
                  (low-pressure storage,
                   bleed-pressurised)
                          │
                          ▼
              ┌───────────────────────┐
              │  Pump suction line    │
              └───────────┬───────────┘
                          │
        ┌─────────────────┼───────────────┐
        ▼                 ▼               ▼
   Engine-Driven      Electric         Other source
   Pump (EDP)         Pump             (RAT on Green;
                                        hand pump on Yellow)
        │                 │               │
        └────────┬────────┴───────┬───────┘
                 ▼                ▼
            High-pressure    Fire shut-off
            manifold          valve (EDP only)
                 │
                 ▼
            Accumulator (transient buffer)
                 │
                 ▼
            To consumers

The role of each element:

• Reservoir — Low-pressure fluid storage. Bleed-pressurised to keep pump inlet above the fluid's vapour pressure (cavitation prevention).
• EDP — Primary pump, driven off the engine accessory gearbox. Pressurises the system whenever its engine is running. Each system's EDP configuration differs (see below).
• Electric pump — A smaller secondary pump driven by an AC motor. Used for ground operation, for backup in flight when triggered, and for a few specific automatic functions. Per FCOM, electric-pump flow is about 18% of EDP flow capacity — sufficient for surface retraction, not as a sustained EDP substitute.
• Fire shut-off valve — Sits upstream of the EDP, closes when the corresponding ENG FIRE pushbutton is pressed or (on Green only) when the HSMU detects a Green reservoir low level.
• Accumulator — Small nitrogen-pre-charged volume that absorbs the difference between pump output and instantaneous demand, keeping pressure stable during cycling (e.g., spoiler deployment).

2. Pump configuration per system

The systems differ in which pumps they carry and what automatic triggers exist. The table below is the at-a-glance picture; each row is unpacked in its dedicated article.

Two architectural points fall out of this table:

• Green is the "high-capacity" system. It carries two EDPs and an emergency air-driven pump because it serves the largest population of consumers — including primary flight controls, landing gear, normal brakes, and reversers — and because it is the system the RAT extends in the loss-of-both-engines case.
• Blue and Yellow are single-EDP systems. Their loss-of-engine cases are covered by triggering the electric pump automatically, on specific conditions related to flight phase and configuration. The triggers are not symmetric between Blue and Yellow because the systems serve different surfaces and different limiting cases (the rudder on Blue, the slats/flaps on Yellow).

The Blue manual hand pump does not exist; the hand pump is on Yellow and exists for one specific purpose — operating the cargo doors when no electrical power is available on the ground.

3. Automatic electric-pump triggers — consolidated

Each electric pump has its own automatic-trigger logic, managed by the HSMU. The three logics are different because the systems serve different surfaces and different limiting cases. Memorising the consolidated table is more efficient than treating each system's trigger as an isolated rule.

Three observations from the table:

• Each trigger is system-specific. Green's trigger is gear-cycle-bounded (25 seconds), Blue's is a controllability concern (until condition clears), Yellow's is configuration-dependent (until the last engine stops or door cycle ends).
• Triggers are mutually exclusive on the AC bus. The Yellow trigger explicitly checks that the Green ELEC PUMP is not running for gear retraction — preventing simultaneous high-current starts that would stress the AC supply. The architecture serialises the pumps when both might otherwise activate.
• No automatic trigger fires under "normal" conditions. All three triggers require an abnormal state (engine failure, PRIM fault, or ground service operation). In normal flight, the electric pumps stay in AUTO without being commanded on. A HYD ELEC PUMP memo appearing without a known cause should prompt the crew to identify which pump and which condition triggered it.

Each trigger is unpacked in the relevant article: Electric Pumps covers the engineering of the pumps themselves and the protection chain; this article gives the consolidated trigger view.

4. Reservoir pressurisation

All three reservoirs are pressurised by HP bleed air, normally drawn from Engine 1. If Engine 1 bleed pressure is too low, the system automatically takes bleed from the crossbleed duct (per FCOM DSC-29-10-20).

The purpose is pump-inlet pressure, not driving fluid out of the reservoir. Without positive inlet pressure the pump cavitates: vapour bubbles form at the inlet, then collapse violently downstream, eroding the pump internals.

Two consequences for the flight deck:

• LO AIR PRESS is an amber caution, not a comfort note. A reservoir running unpressurised is a pump-survival concern.
• Engine 1 bleed is the common source for all three reservoirs. This is not a single-point failure (the crossbleed duct provides the backup), but it explains a behaviour: in an Engine 1 bleed problem, you may see reservoir-pressure indications affecting all three systems before the crossbleed picks up the load.

The full reservoir-pressurisation logic, including bleed-source switching, is dealt with in the Reservoir Pressurisation article.

5. The HSMU's role in generation

The Hydraulic System Monitoring Unit is the controller behind all the automatic behaviour in generation:

• Decides when the electric pumps run in AUTO mode.
• Decides when the RAT extends automatically.
• Closes the Green fire shut-off valves on a Green reservoir low level (and the conditional logic around re-opening them — see Fire Shut-Off Valves).
• Drives the FAULT light on each PUMP pushbutton.
• Inhibits leak-measurement-valve closure in flight.

The pilot sees the HSMU only indirectly — through the FAULT lights, the AUTO behaviour of the electric pumps, and the procedural notes that say things like "Both engine green hydraulic fire shut-off valves are automatically closed by the HSMU". The unit itself is covered in Hydraulic System Monitoring Unit.

6. Generation indications

What the flight deck shows the pilot about generation is concentrated in three places:

The detailed read of the SD HYD page is in Controls and Indications. The point to retain here is that the pilot's window into generation is structured around system-by-system status, not around individual pumps — the ECAM tells you "Green pressure low", not "Green EDP1 specifically degraded". Diagnosing which pump within a system is a maintenance task.

7. What is not in generation

To frame the rest of the chapter: generation produces and maintains pressure. It does not include:

• Distribution — How the pressurised fluid is routed to flight controls, gear, brakes, and reversers. This is Distribution and Priority Function and includes the priority valve that sheds heavy-load users when pressure drops.
• Consumers — The surfaces and systems that use the pressure. These belong to other ATA chapters (27, 32, etc.) and are referenced from there.
• Monitoring logic — The HSMU is generation-adjacent but is treated separately because the same unit also handles distribution-side functions (leak-measurement valve control).

8. No hydraulic PTU — the A330's distributed-redundancy approach

A specific architectural fact worth stating early in the chapter: the A330 has no hydraulic Power Transfer Unit (PTU). Crews transitioning from other Airbus models (notably the A320, which has a PTU between Green and Yellow) sometimes carry over the mental model — and it does not apply here.

What "no PTU" means concretely

• No fluid cross-feed between systems. A leak on Green does not draw fluid from Blue or Yellow to compensate. Each reservoir lives or dies by its own contents.
• No pressure cross-feed between systems. If Green drops to 0 psi, no mechanism transfers Blue or Yellow pressure across to drive Green consumers.
• No mechanical pump-driving-pump arrangement. There is no motor-pump pair (the PTU's signature: one side's pressure drives a hydraulic motor, which drives a pump on the other side).

Searching the FCOM DSC-29 chapter confirms the absence: the term "Power Transfer" appears only in ATA 24 (electrical), in the context of NBPT — No Break Power Transfer for AC bus switching. That is an electrical-side function with no hydraulic role.

How the architecture achieves redundancy instead

Without a PTU, the A330 distributes redundancy across the three systems by multiplying pump sources:

The total is nine pump sources across three systems — without a single one of them shared or interconnected. A PTU would add fluid coupling between two of the systems (typically Green ↔ Yellow), at the cost of introducing a shared component that itself can fail. The A330 design deliberately trades the PTU's instant cross-supply benefit for the cleaner failure isolation of full independence.

Crew mental-model implication

The failure-handling pattern is "each system is recovered by its own internal resources only":

• Green failure → other Green sources (second EDP, Green electric pump, RAT). Not Blue or Yellow.
• Blue failure → other Blue sources (Blue electric pump). Not Green or Yellow.
• Yellow failure → other Yellow sources (Yellow electric pump, hand pump on ground). Not Green or Blue.

A crew member expecting "borrow pressure from another system to fix this one" will look in the wrong place. The correct mental model is "use the backup that exists on the same system; if there isn't one, accept the loss and use the pre-engineered substitute (alternate brakes for normal brakes, gravity gear for normal gear, etc.)."

The full handling implications are covered in Pump vs System Failure § 12 — including why dispatch and emergency-handling habits from PTU-equipped aircraft do not transfer to A330 hydraulics.

Self-test

[!note]- Q1. A student says "Each system has one engine-driven pump." Correct or incorrect, and why?

Incorrect for Green. The Green system has two EDPs, one driven by Engine 1 and one by Engine 2. Blue and Yellow are single-EDP. The reason is consumer load: Green carries the heaviest combined population of flight-control, gear, brake, and reverser users, and benefits from dual engine sourcing during normal operation.

[!note]- Q2. The Blue electric pump is described as a "backup". What automatic trigger brings it on in flight, and why is that specific trigger chosen?

The Blue electric pump runs automatically in flight on an Engine 1 failure that also involves PRIM 1 or PRIM 3 loss. The trigger is chosen because Blue powers the electrical rudder authority used to counter yaw sideslip from asymmetric thrust. Without sufficient Blue pressure under those conditions, rudder authority for counteracting the sideslip would be reduced. The automatic logic exists specifically to preserve rudder authority in that case.

[!note]- Q3. The Yellow electric pump runs automatically on the ground during cargo-door operation. Why, and what additional valve action accompanies it?

The hand pump is the minimum source for cargo doors (used when no electrical power is available). When electrical power is available on the ground, the Yellow electric pump runs automatically during cargo-door operation to provide normal hydraulic flow. To prevent inadvertent flight-control surface movement during this operation, the Yellow leak-measurement valve closes automatically, and Yellow flap motor operation is inhibited.

[!note]- Q4. All three reservoirs are bleed-pressurised from Engine 1. Why is this not classified as a single-point failure?

Two reasons. First, the crossbleed duct provides an automatic backup source if Engine 1 bleed pressure is insufficient — pressurisation continues from Engine 2. Second, the cavitation-prevention purpose of reservoir pressurisation tolerates a brief loss while the crossbleed picks up. The classification therefore counts the entire pressurisation function as redundant at the source level. It does, however, explain why a single bleed-side issue can show up on all three reservoirs simultaneously before the crossbleed switches in.

[!note]- Q5. Per FCOM, electric-pump flow is about 18% of EDP flow capacity. What operational restriction does this number imply?

Electric pumps can move surfaces — they retract spoilers, complete a slats/flaps cycle, run cargo doors — but they cannot sustain continuous EDP-level demand. A standard guidance is that an electric pump should not be selected ON for long periods to substitute for a failed EDP; doing so produces no useful hydraulic margin and stresses the pump. The "as a general rule, do not manually select a HYD ELEC PUMP ON" note in the abnormal procedures is grounded in this 18% figure.

References

Per FCOM DSC-29-10-10 (General), DSC-29-10-20 (Generation — Green/Blue/Yellow System Pumps, RAT, System Accumulators, Fire Shutoff Valves, Reservoir Pressurization, HSMU), DSC-29-20 (Controls and Indicators).

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, and QRH for operational use.