Hydraulic Reservoirs

The reservoir is the working stock of fluid that feeds the pump. On the A330 it is also the host for the pressurisation gas cushion, the temperature sensor, the low-level switch, and the cascading logic that closes the Green fire shut-off valves when the level drops. None of these belong elsewhere in the architecture — they all sit on, around, or inside the reservoir housing.

This article covers the reservoir itself: the three independent units, what they store, how their condition is monitored, and the cascade behaviours triggered by their condition signals. The reservoir's pressurisation supply is covered in Reservoir Pressurisation. The fluid chemistry and thermal limits are in Hydraulic Fluid.

1. Three independent reservoirs

       GREEN reservoir            BLUE reservoir              YELLOW reservoir
   ┌────────────────────┐    ┌────────────────────┐      ┌────────────────────┐
   │  ╔═════════════╗   │    │  ╔═════════════╗   │      │  ╔═════════════╗   │
   │  ║   air gap   ║   │    │  ║   air gap   ║   │      │  ║   air gap   ║   │
   │  ║   ~4.5 bar  ║   │    │  ║   ~4.5 bar  ║   │      │  ║   ~4.5 bar  ║   │
   │  ╠═════════════╣   │    │  ╠═════════════╣   │      │  ╠═════════════╣   │
   │  ║             ║   │    │  ║             ║   │      │  ║             ║   │
   │  ║   fluid     ║   │    │  ║   fluid     ║   │      │  ║   fluid     ║   │
   │  ║  (largest)  ║   │    │  ║  (smallest) ║   │      │  ║  (medium)   ║   │
   │  ║             ║   │    │  ║             ║   │      │  ║             ║   │
   │  ╚══█══════█═══╝   │    │  ╚══█══════█═══╝   │      │  ╚══█══════█═══╝   │
   │     ▲      ▲       │    │     ▲      ▲       │      │     ▲      ▲       │
   │  return  supply    │    │  return  supply    │      │  return  supply    │
   │  port    port      │    │  port    port      │      │  port    port      │
   └─────────┬──────────┘    └─────────┬──────────┘      └─────────┬──────────┘
             │                          │                            │
             ▼                          ▼                            ▼
       Green pumps               Blue pumps                    Yellow pumps
       
       NO FLUID PATH BETWEEN RESERVOIRS — by design and by certification.

Each system has its own reservoir, sized to its consumer population:

Green is largest (heaviest consumer load — flight controls, gear, normal brakes, reversers).
Blue is smallest (narrower consumer set, single-EDP system).
Yellow is intermediate.

The physical reservoirs are located in the lower-fuselage area in the vicinity of the centre wing box and main landing gear bay. Exact volumes, low-level thresholds, and part numbers are documented in maintenance documentation; the pilot's reference is the SD HYD page graduation, not absolute litres or gallons.

2. What the reservoir does — five functions in one housing

The reservoir is not a passive tank. It serves five functions, each with its own component or interface:

Function	Implementation
Fluid storage	Bulk tank at the system's low-pressure side
Pump-inlet conditioning	Bleed-pressurised gas cushion above the fluid
Quantity monitoring	Level sensor → HSMU → SD HYD page (temperature-corrected)
Temperature monitoring	Return-line temperature sensor → HSMU → OVHT caution
Air-pressure monitoring	Reservoir-pressure sensor → HSMU → LO AIR PRESS caution

The HSMU is the controller behind every monitored signal. Sensors do not connect directly to the ECAM; they connect to the HSMU, which processes the signals (apply temperature correction, evaluate thresholds, manage FAULT-light logic) and then forwards the displayed values to the SD page.

3. Temperature correction of the quantity indication

The HSMU corrects the displayed reservoir quantity for fluid temperature (per FCOM DSC-29-10-20). Hydraulic fluid expands with temperature; the same mass occupies more volume hot than cold. Without correction:

Cold-soaked aircraft at sub-zero ambient → indication would read low → false low-level alert.
Hot-soaked aircraft after a long cruise → indication would read high → real low-level masked.

The correction normalises the displayed value to a reference temperature. The SD HYD quantity indication is green when temperature data is available and the correction is applied; white when temperature data is unavailable and the value shown is the raw (uncorrected) measurement.

Practical consequence for the pilot. A cold-morning preflight may show quantities at the lower end of the normal band without indicating a fault — the correction is already applied; the pilot is looking at a normalised value, not a "low reading needing servicing". Conversely, a white indication means the value should be cross-checked against expected ranges with the knowledge that compensation has been lost.

4. Low-level cascade — the Green-specific automatic logic

The most architecturally interesting reservoir behaviour is what happens on a Green reservoir low level.

Per FCOM DSC-29-10-20:

Both engine green hydraulic fire shut-off valves are automatically closed by the HSMU, in the event of a green reservoir low level. If the blue and yellow reservoir levels are normal, 150 s after the initial closure of the fire shut-off valves, the green engine-driven pumps are automatically depressurized and the fire shut-off valves are reopened to lubricate and avoid damage to the engine-driven pumps.

The full cascade:

                   Green reservoir LO LEVEL detected
                                │
                                ▼
                ┌──────────────────────────────┐
                │ HSMU closes BOTH Green        │  Step 1: stop the leak from
                │ fire shut-off valves          │  draining the reservoir further.
                │ (one upstream of each EDP)    │
                └────────────────┬─────────────┘
                                 │
                                 ▼
                ┌──────────────────────────────┐
                │ Wait 150 seconds              │  Engineering compromise:
                │ Monitor Blue and Yellow       │  long enough for the leak rate
                │ reservoir levels              │  to stabilise; short enough that
                └────────────────┬─────────────┘  EDPs do not seize dry.
                                 │
                  Blue + Yellow normal? ─── YES ─── ► ┌──────────────────────────┐
                                 │                    │ Depressurise both Green   │
                                 │                    │ EDPs (command off) +      │
                                 │                    │ REOPEN fire shut-off      │
                                 │                    │ valves → fluid touches    │
                                 │                    │ EDP internals for         │
                                 │                    │ lubrication only.         │
                                 │                    └──────────────────────────┘
                                NO
                                 │
                                 ▼
                ┌──────────────────────────────┐
                │ KEEP fire shut-off valves     │  Multi-system loss in progress.
                │ CLOSED for remainder of       │  Green can be recovered via the
                │ flight. Crew cannot reopen.   │  RAT path (independent of EDPs).
                └──────────────────────────────┘

Two things are worth understanding clearly:

The "close → reopen" sequence is not contradictory. It is two separate protective actions: first containment (stop fluid loss), then pump preservation (a depressurised EDP with fluid contact survives; a depressurised EDP run dry does not).
The 150-second window is the time during which the HSMU assesses whether the failure is isolated to Green. If Blue or Yellow also drops within that window, the architecture concludes the situation has escalated beyond simple Green leakage, and pump preservation is no longer the priority — the valves stay shut.

This logic is Green-only. Blue and Yellow reservoirs trigger no automatic fire-shut-off-valve closure on low level. Their abnormal procedures call for the crew to manually switch off both pumps on the affected system.

5. Overheat — the parallel monitor

Each reservoir carries a temperature sensor at the return port — the inlet through which fluid coming back from the consumers re-enters the reservoir. Per maintenance documentation, the return-port position is chosen because it represents the hottest point in the cycle, reflecting the cumulative thermal load on the system.

Threshold: the HSMU generates an overheat signal when reservoir temperature reaches 95 °C ± 2 °C on a rising trend (per AMM 29-31). The full thermal architecture, including the electric-pump overheat at a much higher threshold and the cooling paths, is covered in Hydraulic Fluid.

When HYD G (B/Y) RSVR OVHT triggers:

ECAM caution appears with the affected system labelled.
The ENG PUMP FAULT light illuminates on the corresponding pushbutton.
The procedural response is to switch off both pumps on the affected system (EDP and ELEC PUMP).
The FAULT light stays on as long as the overheat condition persists, even after the pumps are off — distinct from the level-triggered FAULT light, which extinguishes when the pump is switched off.

The overheat condition does not produce automatic fire-shut-off-valve closure. It is a "pump damage prevention" caution, not a "fluid loss containment" caution.

6. Low air pressure

The reservoir pressure sensor monitors the gas cushion above the fluid. Below a threshold (approximately 1.5 bar relative per maintenance documentation), the HSMU generates LO AIR PRESS.

The detection is at the reservoir itself, not at the bleed supply. Three failure paths produce the caution:

Cause	Recovery
Engine 1 bleed off; crossbleed not yet picking up	Self-recovers as crossbleed takes load
Both bleed sources unavailable	Reservoir slowly depressurises; cavitation risk on continued pumping
Leak in the reservoir's own pressurisation interface	Continuous loss; bleed supply cannot keep up

The full pressurisation source architecture (Engine 1 HP bleed, crossbleed, APU, ground source) and the air pressurisation units (5240JM, 5240JM2) are detailed in Reservoir Pressurisation.

7. The pushbutton FAULT light — four triggers, one indicator

The ENG PUMP and ELEC PUMP pushbuttons on the overhead 29 panel each carry a FAULT light. Per FCOM DSC-29-20, four conditions illuminate it:

Reservoir level low.
Reservoir overheat.
Reservoir low air pressure.
Pump low pressure (inhibited on ground when engine is stopped).

Three of the four triggers are reservoir-side conditions, not pump-side. The FAULT light therefore does not tell the crew what has failed — only that something has failed on a circuit that includes this pump. The SD HYD page provides the disambiguation: quantity (level), OVHT, LO AIR PRESS, or pump LO PR.

The FAULT light extinguishes when the pump is switched off, except for the overheat case — that exception is the FCOM's way of telling the crew that switching off the pump does not clear the overheat; the underlying condition is in the fluid, not in the pump operation.

8. Servicing — how the reservoir is filled

Fluid replenishment uses one of two ground methods:

Method	Equipment	When used
Hydraulic service cart	Powered cart with pressure fitting	Routine post-flight servicing — large refills
Hand pump (manual)	Mechanical hand pump	Service cart unavailable; emergency or remote-station servicing

Both methods fill through the top of the reservoir. The bulk fluid is at low pressure inside the reservoir (the high pressure is downstream of the pump, not at the storage). Maintenance documentation lists six fill tasks — three reservoirs × two methods — each with safety preconditions (control surfaces clear, system depressurised, etc.).

The pilot's awareness here is limited: a manual-pump fill at a remote station is not unusual and does not indicate a maintenance issue, only that the service cart was not available.

9. Reservoir-related cautions summarised

ECAM caution	Source	Crew action
`HYD G RSVR LO LVL`	Green level switch	Per ECAM. HSMU manages fire-shut-off valves automatically.
`HYD B RSVR LO LVL`	Blue level switch	Per ECAM. Both Blue pumps off. No automatic valve action.
`HYD Y RSVR LO LVL`	Yellow level switch	Per ECAM. Both Yellow pumps off. No automatic valve action.
`HYD G (B/Y) RSVR OVHT`	Return-port temperature sensor	Both pumps off on affected system. FAULT light persists.
`HYD G (B/Y) RSVR LO AIR PRESS`	Reservoir pressure sensor	Per ECAM. May self-recover if bleed source switches.

Detailed procedures and remaining-systems cascades are in the Q&A articles (Reservoir Low Level Procedures, Reservoir Overheat Procedures).

10. What this article ends, what the next two pick up

This article ended at the level of "what is in the reservoir, what monitors it, what its condition cascades into." Two adjacent topics complete the picture:

Reservoir Pressurisation — the bleed-air supply that maintains the gas cushion, including the air pressurisation units and the backup source hierarchy.
Hydraulic Fluid — the chemistry, thermal limits, sensor locations, and the cooling paths that determine whether the OVHT threshold is reached during normal operation.

Self-test

[!note]- Q1. A cold-morning preflight shows the Green reservoir quantity in the lower part of the green band. The Captain asks if maintenance should be called for servicing. What is the correct interpretation?

The displayed value is already temperature-corrected by the HSMU. A reading in the lower part of the normal band is not, by itself, a low-fluid indication — fluid quantity follows the corrected scale, not the raw uncorrected volume. The pilot's check is whether the indication is within the green normal band; if so, no servicing is required. The architecture's expectation is that the SD HYD page is read at face value. (Maintenance follows separate servicing schedules independent of cockpit observation, and would only be called if the indication is below the normal band or trending downward in flight.)

[!note]- Q2. A Green reservoir low level triggers. The crew sees the Green fire shut-off valve indications change to cross-line amber on the SD HYD page. About two and a half minutes later, both valve indications return to in-line green. Is this a second failure?

No. It is the HSMU's automatic 150-second sequence behaving exactly as designed. The valves were closed initially to stop fluid loss; after 150 seconds, with Blue and Yellow normal, the EDPs were depressurised (commanded off) and the valves reopened to allow fluid contact with the EDPs for lubrication — preventing pump damage from running dry. The pumps are not pressurising the system, but they are protected from internal damage. This is a known and documented cascade.

[!note]- Q3. The Yellow reservoir low-level caution triggers in flight. The crew correctly switches off both Yellow pumps. Will the HSMU close the Yellow fire shut-off valve automatically, as it does for Green?

No. The 150-second cascade with automatic fire-shut-off-valve closure is Green-specific. Blue and Yellow rely on the crew's procedural action (both pumps off) to limit damage from a low-level condition. Architecturally, this is because Green has the multi-system decision tree (RAT path, dual EDPs) that justifies the automatic logic; Blue and Yellow have simpler single-EDP architectures where the crew action suffices and additional automation would not improve the outcome.

[!note]- Q4. The Captain notices the Green reservoir quantity displayed in white rather than green. Quantity is in the normal band. Is action required?

The white colour indicates that the HSMU has lost the temperature signal from the reservoir temperature transmitter. The quantity displayed is the raw, uncorrected value. The numerical reading is still being shown, and if it remains in the normal range, no immediate action is required — but the pilot is now reading an uncompensated value and should watch for actual quantity changes (which would still produce the correct cautions if a threshold is crossed). The temperature-signal loss is also a precursor to potential spurious FAULT light illumination, per maintenance-related notes in the abnormal procedures.

[!note]- Q5. The HYD G RSVR OVHT caution triggers. The crew switches off both Green pumps as the procedure directs. The FAULT lights on the Green pump pushbuttons do not extinguish. Is this a second failure?

No. The FAULT light extinguishes on a switched-off pump except for the overheat case. The overheat condition is in the fluid (and persists until the fluid temperature drops below the threshold with appropriate hysteresis), not in the pump operation. The lights staying on confirm that the underlying thermal condition is still active. The pumps are protected from operating in over-temperature fluid; the lights are the correct indication of the persistent fluid-side condition.

References

Per FCOM DSC-29-10-20 (Hydraulic System Monitoring Unit, Fire Shutoff Valves, Reservoir Pressurization), DSC-29-20 (FAULT light logic, SD HYD page indications). Reservoir thermal architecture and sensor positions per AMM 29-31. Reservoir-fill methods per maintenance documentation (AMM 12-12-29 series).

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