Hydraulic Power Distribution Map

The three hydraulic systems do not all drive the same consumers. Each system has a distinct consumer list — the surfaces, actuators, and subsystems that it powers — and the operational consequence of losing one system depends entirely on which consumers go with it. This article maps each consumer to its source(s) so that the question "what is lost if Green fails?" has a precise answer.

This map is the reference behind every abnormal procedure in the rest of the chapter. The remaining-systems table in the FCTM is built on this distribution; the flight-control law degradation logic depends on this distribution; the priority-shed and post-failure profile changes all originate here.

1. The high-level picture

                                  ╔═══════════════╗
                                  ║  GREEN        ║
                                  ║  (heaviest    ║
                                  ║   consumer    ║
                                  ║   load)       ║
                                  ╚═══════╤═══════╝
                                          │
        ┌─────────────────────────────────┼─────────────────────────────────┐
        ▼                                 ▼                                 ▼
   Flight controls               Heavy/cycle consumers           Continuous consumers
   (servocontrols on             • Landing gear (normal)          • Normal brakes
    aileron, elevator,           • Nose wheel steering (normal)   • One slat motor
    spoilers, rudder)            • CSM/G drive (emergency gen)    • One flap motor
                                                                   • Slat + flap wing-tip brakes


                                  ╔═══════════════╗
                                  ║  BLUE         ║
                                  ║  (light load) ║
                                  ╚═══════╤═══════╝
                                          │
        ┌─────────────────────────────────┼─────────────────────────────────┐
        ▼                                 ▼                                 ▼
   Flight controls               Brake & rudder                   Slat-side
   (servocontrols, esp.          • Alternate brakes               • One slat motor
    rudder, parts of             • Engine 1 reverser              • Slat wing-tip brake
    elev/ail)                    • Electrical rudder authority
                                 • Brake accumulator
                                   (parking brake via accu)


                                  ╔═══════════════╗
                                  ║  YELLOW       ║
                                  ║  (mid load)   ║
                                  ╚═══════╤═══════╝
                                          │
        ┌─────────────────────────────────┼─────────────────────────────────┐
        ▼                                 ▼                                 ▼
   Flight controls               Special consumers                Flap-side
   (servocontrols on             • Cargo doors (ground)           • One flap motor
    aileron, elevator,           • Engine 2 reverser              • Flap wing-tip brake
    spoilers — different
    actuators than Green)

This is approximate. The exact mapping varies by build, but the architectural rule holds: every flight-control surface has actuators on at least two of the three systems, so loss of any single system never disables flight controls entirely.

2. Flight-control redundancy

The most critical architectural fact: every primary control surface has actuators on more than one hydraulic system. The detailed mapping is in ATA 27, but the summary:

A loss of any one system reduces but does not eliminate any control surface. The flight-control law (Normal, Alternate, Direct) degrades step-by-step based on which combinations are lost; this is ATA 27 territory.

3. Consumers unique to each system

Green-only consumers (per AMM 29-00 authoritative list)

Several consumers are only on Green:

• Normal brakes (primary brake circuit)
• Landing gear (normal extension and retraction)
• Nose wheel steering (normal)
• CSM/G drive (Constant-Speed Motor/Generator — the emergency electrical generator)
• One slat motor channel + slat wing-tip brake (Green side)
• One flap motor channel + flap wing-tip brake (Green side)
• Green-supplied flight-control servocontrols on primary and secondary surfaces

Loss of Green mitigations:

• Normal brakes lost → switch to alternate brakes (Blue).
• Landing gear normal extension lost → use gravity extension.
• Nose wheel steering lost → use differential braking.
• CSM/G drive off → degraded emergency generator availability.
• Slats / flaps operate on the remaining channel only (Blue for slats, Yellow for flaps), at reduced motor count.

Blue-only consumers (per AMM 29-00 authoritative list)

Blue's unique consumers:

• Alternate brakes (the backup brake circuit, used when Green normal brakes are unavailable)
• Engine 1 reverser
• Brake accumulator (the Blue system charges the brake accumulator; this is what allows parking-brake hold after pump shutdown)
• Parking brake (supplied by the Blue brake accumulator)
• Electrical rudder authority (specific actuators on the rudder driven only by Blue)
• One slat motor channel + slat wing-tip brake (Blue side)
• Blue-supplied flight-control servocontrols on primary and secondary surfaces

Loss of Blue mitigations:

• Alternate brakes lost → rely on normal brakes (Green) for landing; if Green is also lost, the residual accumulator provides limited braking only.
• Engine 1 reverser lost → asymmetric reverser on landing (only Engine 2 reverser available).
• Brake accumulator no longer recharged → parking brake will deplete over hours after shutdown; chocks needed.
• Electrical rudder authority reduced → conventional rudder paths still operate.

Yellow-only consumers (per AMM 29-00 authoritative list)

Yellow's unique consumers:

• Engine 2 reverser
• Cargo doors (ground operation)
• One flap motor channel + flap wing-tip brake (Yellow side)
• Yellow-supplied flight-control servocontrols on primary and secondary surfaces

Loss of Yellow mitigations:

• Engine 2 reverser lost → asymmetric reverser on landing (only Engine 1 reverser available).
• Cargo doors → hand pump required for ground operation when no electrical power.
• Flap motor count reduces by one channel; flaps extend more slowly on the remaining Green channel.

(The parking brake is supplied via the Blue brake accumulator — see the Blue-only consumers section above, not Yellow.)

Loss of Yellow summary → Engine 2 reverser lost (asymmetric reverser at landing); cargo doors require hand pump on ground; one flap motor channel down (Green flap channel still drives flaps at reduced speed). Normal brakes (Green) and alternate brakes (Blue) are both unaffected by Yellow loss.

4. The brake circuit — a special case

Braking is the most safety-critical consumer of the hydraulic system, and the architecture reflects that with dual independent brake paths:

The architecture provides three independent brake-pressure sources, with progressive degradation:

• Both Green and Yellow available → normal braking (Green primary).
• Green unavailable, Yellow normal → alternate braking (Yellow).
• Both Green and Yellow unavailable in pump-supplied form → accumulator braking on Blue brake accumulator (limited applications).

The BRK B ACCU PR ONLY indication appears when the system has degraded to the third tier.

5. Consumer impact tables — what is lost when each system is lost

Loss of Green

Loss of Blue

Loss of Yellow

6. Physical layout — pipes, manifolds, and drains

The consumer-mapping above is the functional layer of the architecture. Sitting underneath it is a physical layer — what the pipes are made of, where the manifolds sit, why no fluid runs through the cabin or cockpit, and how seal leakage is collected. The physical choices are documented in AMM 29-00 §3.A and matter for diagnosing leaks, understanding fire-zone routing, and recognising what maintenance is doing in each bay.

6.1 Pipe materials by zone

Per AMM 29-00 §3.A (1), pipe material is selected by the zone the pipe runs through, not by the system (Green/Blue/Yellow are all the same):

Three reasons drive the selection:

• Stainless steel in fire zones. Titanium burns at sustained high temperature; stainless steel does not. Pipes routed through the engine and APU bays must survive a localised fire long enough for crew action (fire-shut-off-valve closure) to take effect. Stainless steel is the fire-survival material.
• Titanium in non-fire HP runs. High-pressure pipes in safer zones use titanium for its strength-to-weight ratio. Lighter than steel, strong enough for 206 bar working pressure.
• Light alloy in standard LP runs. Return-line pressures are low (a few bar at most). Strength is not the constraint; weight is. Aluminium-alloy pipes painted for corrosion are the standard low-stress choice.
• Steel or titanium in landing-gear wells. The well environment combines mechanical stress (gear cycling, vibration), exposure (rain, slush, ice, debris ingestion), and proximity to high-impact failure modes. LP pipes in this zone use the higher-strength materials despite the low fluid pressure.

6.2 Electrical bonding

Stainless-steel and titanium-alloy pipes are electrically bonded to the airframe with dedicated leads and clips. Light-alloy pipes are not bonded.

The reason is conductivity. Stainless steel and titanium are poor conductors compared with aluminium; without bonding, they could accumulate static charge or carry an unequal potential during a lightning strike. Bonding ensures equipotential. Aluminium alloy is naturally conductive enough that the airframe structure to which the pipe is clamped already provides the path.

6.3 Flexible pipes — where vibration or movement exists

Rigid pipes cannot tolerate continuous flex; rigid joints crack over time at vibration interfaces. The architecture uses flexible pipes at every interface where one end moves relative to the other:

• Engine pylon ↔ wing — the engine vibrates relative to the airframe.
• Landing-gear strut ↔ fuselage — the strut moves during retraction/extension.
• EDP ↔ accessory gearbox — high vibration, high temperature, frequent maintenance access.

Rigid runs are used everywhere else, with proper anti-vibration clamps. The transition between flexible and rigid is itself a maintenance-significant interface (and a common leak inspection point).

6.4 No hydraulic pipes in the passenger cabin or cockpit

A specific design decision: the cabin and cockpit are hydraulic-pipe-free. This is not coincidence; it is mandatory routing.

Three reasons:

• Phosphate-ester fluid is corrosive to skin and harmful on contact. Routing it through occupied areas would create a hazard during a line breach.
• Skydrol-family fluid degrades many cabin materials on contact (interior plastics, fabrics, electronics). A small leak inside the cabin would produce extensive collateral damage.
• Maintenance access without cabin disturbance. Hydraulic work on lines does not require pulling cabin floor panels or disturbing furnishings; everything is in the lower fuselage, the wing, the landing-gear bays, or the engine pylons.

The lower-fuselage routing (under the floor, in the centre wing box, in the gear bays) is the architectural choice that supports both safety and maintainability.

6.5 Green system — safety valves on rudder and outboard ailerons

The Green system carries dedicated safety valves in the supply to the rudder servocontrol and the outboard aileron servocontrols (per AMM 29-00 §3.A). The valves close automatically if the downstream line is damaged.

The architectural reasoning:

• Green is the last hydraulic system standing in a dual-loss case (B+Y both lost), per the Power Distribution Map consumer list. Protecting Green from rapid fluid loss is what preserves the flight-control authority that the architecture relies on for that scenario.
• The rudder, vertical stabiliser, and wing tips are the most exposure-prone surfaces — most vulnerable to bird strike, lightning, hail, or in-flight collision. A line breach at one of these locations could otherwise drain the Green reservoir.
• Closing the supply at the safety valve isolates the damaged section without losing the rest of Green. The aircraft loses partial authority on the affected surface but retains Green pressure to all the other consumers — flight controls on other surfaces, normal brakes, landing gear, the CSM/G drive.

This is damage containment, not damage prevention. The safety valve does not stop the surface from being struck; it stops the strike from cascading into a system-wide Green failure.

Blue and Yellow do not have equivalent safety valves on their flight-control supply lines, because their loss does not threaten the dual-loss recovery configuration in the same way.

6.6 Manifolds — components attached to a single block

Per AMM 29-00 §3.A, manifolds are used wherever possible in both HP and LP sections. The maximum number of components are mounted on the manifold itself rather than on separate brackets connected by pipes.

The maintenance advantage is direct: replacing a component on a manifold does not require cutting the hydraulic pipes — the component unbolts from the manifold face, leaves a sealed port behind, and the replacement bolts on. No pipe drainage, no re-bleeding, no risk of contamination during the swap. The line-side architecture is undisturbed.

Selected manifolds in the Green system, as a reference (per AMM 29-11):

Blue and Yellow have analogous manifolds, documented in AMM 29-12 / 29-13. The pattern is the same: clustered components on a single block, fewer pipe connections, faster line-maintenance turnaround.

6.7 Seal drain system — controlled collection of design leakage

Some components (emergency generator drive, hydraulic motors) and the reservoir pressure-relief valves have designed leakage — a small fluid bypass that is part of normal operation, not a fault. Per AMM 29-00 §3.A, this leakage is routed into a seal drain system that collects it in dedicated tanks.

The collected fluid is drained on the ground during routine maintenance. The architecture treats the collected leakage as a known quantity managed by scheduled service rather than a fault to be eliminated.

Two design benefits:

• Environmental containment. Phosphate-ester fluid does not drip onto the airframe surfaces or escape into the slipstream. The collection prevents both airframe damage (paint, electrical connectors) and external contamination.
• Maintenance observability. The drain tanks themselves become a diagnostic surface: an unexpectedly full tank between scheduled drains indicates a component shedding more fluid than its design allowance — a developing fault detected before it becomes a system-level leak.

For the pilot, the seal drain system is largely invisible. The crew interface is recognising that a maintenance technician working at a drain port in the lower fuselage is performing routine service, not investigating a leak.

7. Why this distribution

The architecture chose not to symmetrically distribute consumers across the three systems. Green carries more than its share; Blue is intentionally light. The reasoning:

• Green's heavy load matches its EDP redundancy. Green has two EDPs (one on each engine) plus the electric pump plus the RAT. The architecture rewards Green's robust supply with the heaviest consumer responsibility — flight controls, normal brakes, gear, nose-wheel steering, and the CSM/G drive that brings the emergency generator online.
• Blue is the "single-engine resilient" system. Blue has only one EDP (Engine 1), but its consumer set is light enough that even with Blue at minimal output, the loss is operationally manageable (rudder authority reduced, one slat motor channel down, but no critical-path function disabled).
• Yellow has a more limited consumer set; the brake accumulator architecture sits on Blue. Yellow has one EDP (Engine 2), one electric pump, and a hand pump for cargo doors. The Blue brake accumulator is what allows the architecture to support parking brake and emergency braking after pump shutdown — a function that no other system would handle.

The asymmetric load is itself a design feature, not a compromise.

8. The pilot's mental sequence in an abnormal

Looking at this distribution map, the pilot's mental sequence in any hydraulic abnormal becomes:

1. Which system has degraded? Identify from the ECAM HYD page and the cautions displayed.
2. What is lost from this system's consumer list? Cross-reference against the distribution map (above tables).
3. What is the substitute, if any? Alternate brakes for Green normal brakes; gravity gear for normal gear; manual differential braking for steering, etc.
4. What is the degraded profile? Approach speed adjustment, flap-detent restriction, brake-channel choice, reverser availability, etc.
5. What is the dispatch consequence on the ground after landing? What MEL items apply, and what crew action is required for the affected systems.

Each abnormal procedure walks the crew through this sequence specifically for the relevant case. The distribution map is the lookup table.

Self-test

[!note]- Q1. The Yellow hydraulic system is lost (low level + procedure executed, both pumps off). What is lost from a braking perspective?

Alternate brakes are lost (alternate brakes are Blue-supplied). Normal braking (Green-supplied) is still available, assuming Green is healthy. The Blue brake accumulator is separately charged from the Blue system and so is not directly affected by the Yellow loss — but with the alternate-brake path (Yellow) gone, the architectural fallback in case Green normal brakes also fail would be accumulator braking only. The architectural posture: rely on Green normal brakes for landing; if Blue is also intact, the Blue brake accumulator continues to provide parking-brake pressure on the ground.

[!note]- Q2. On landing, only one thrust reverser is available — Engine 2's. Which hydraulic system has been lost, and what was the architectural mapping that put it there?

Blue has been lost. Engine 1 thrust reverser is the unique consumer of Blue listed in AMM 29-00 §3.A (5)(b); Engine 2 thrust reverser is the corresponding entry on Yellow under §3.A (5)(c). The architectural choice puts each engine's reverser on the system that the same engine does not drive: Engine 1 drives Blue (and one Green EDP), but its reverser is on... wait, this is worth checking carefully. In fact the published mapping is simpler than a clever rule: Engine 1 reverser is on Blue, Engine 2 reverser is on Yellow, full stop, as listed in the AMM. The asymmetric reverser case after a Blue loss leaves only Engine 2's reverser available; the crew expects shorter reverser deceleration on landing and accounts for that in runway selection.

[!note]- Q3. The Blue hydraulic system is lost. The flight-control law remains in Normal Law. Is this correct?

Possibly correct, depending on which combined fault is present. Blue alone supports specific actuators on the rudder, ailerons, elevators, and one slat motor channel. Loss of Blue reduces actuator count on each control surface but does not eliminate any surface. The flight-control law transitions to Alternate (or further degraded) only when combined losses cross the law-degradation thresholds defined in ATA 27. A pure Blue loss might leave the law in Normal — this is the architecture's intent: Blue is the system whose loss is most operationally manageable.

[!note]- Q4. After a RAT extension (Green on 2500 psi), the priority valve sheds the CSM/G drive. What is the immediate electrical consequence?

The CSM/G drive is the hydraulic-driven path that brings the emergency generator online. With the priority valve having shed this drive, the architecture relies on the RAT's own emergency generator (mechanically connected to the RAT turbine, separately from the hydraulic pump) to provide AC EMER bus power. The CSM/G being off-line is therefore consistent with the RAT-deployed configuration; the architecture has another electrical path. The crew sees the emergency generator status via electrical indications, not hydraulic indications.

[!note]- Q5. Cargo doors are listed as a Yellow consumer. They are powered by Yellow pressure but only operate on the ground. Why?

Cargo door operation requires high-flow Yellow pressure for the door actuators. In flight, the system architecture does not provide enough excess Yellow capacity to drive cargo doors (which would compete with flight control demand). On the ground, however, the cargo doors are a primary consumer of Yellow when the electric pump is running. The Yellow leak measurement valve and flap motor inhibits during cargo-door operation enforce the ground-only nature of this consumer: while cargo doors are moving, the system is configured for door operation, with flight controls temporarily isolated to prevent inadvertent motion.

References

Per FCOM DSC-29-10-30 (Distribution, Priority Function); FCTM PR-AEP-HYD (remaining-systems tables for various hydraulic loss scenarios); ATA 27 (Flight Controls — detailed actuator assignment to hydraulic systems); ATA 32 (Landing Gear and Brakes — detailed brake circuit architecture); ATA 24 (Electrical — CSM/G drive details); ATA 78 (Engine Reverser — hydraulic source for each reverser).

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