Reservoir Pressurisation

Each hydraulic reservoir is pressurised at its top by a regulated gas cushion. The purpose is not to drive fluid out of the reservoir — that is the pump's job — but to keep the pump-inlet pressure above the fluid's vapour pressure at operating temperature. Without that margin, the pump cavitates and self-destructs within minutes.

This article covers the full pressurisation chain: the bleed source, the in-line components, the regulated value (4.5 bar absolute), the relief and low-pressure thresholds, the backup source hierarchy, and the 12-hour static seal that allows the aircraft to sit overnight without losing reservoir pressure.

1. The pressurisation chain

Engine 1 HP compressor               Backup sources (any one is sufficient)
    │                                              │
    │ HP bleed up to ~40 bar                       ├── Engine 2 IP compressor (via crossbleed duct)
    │                                              ├── APU (via crossbleed duct)
    ▼                                              ├── Ground source — connector 5241JM (B/Y)
[Restrictor + filter 5242JM                        │   and 5241JM2 (G), on the blue
 in engine pylon — limits                          │   ground service panel
 mass flow if the line                             │
 ruptures downstream]                              │
    │                                              │
    └────────────────────► [Air pressurisation manifold 5239JM] ◄──────────────┘
                                          │
                              ┌───────────┴───────────┐
                              ▼                       ▼
                       [Unit 5240JM]            [Unit 5240JM2]
                       For BLUE and             For GREEN
                       YELLOW reservoirs        reservoir
                              │                       │
                              ▼                       ▼
                  Regulated to 4.5 bar ± 0.1 absolute  
                  (≈ 65 psi abs, ≈ 3.5 bar relative at sea level)
                              │                       │
                              ▼                       ▼
                        Reservoir top manifold (each reservoir):
                          • Check valve (12-hour static seal)
                          • Pressure-relief valve (5.3 bar ± 0.2 relative)
                          • Direct-reading pressure gauge
                          • Low-pressure switch (triggers LO AIR PRESS at ≤ 1.5 bar relative)
                          • Port for the manual depressurisation valve

Three architectural facts fall out of this chain:

• Single primary source — but with three backup paths. Engine 1 HP bleed is the normal supply. Engine 2 IP, APU, and ground source all feed the same downstream manifold (5239JM) through the air-conditioning crossbleed duct. The selection is automatic; the crew does not switch sources.
• Two air pressurisation units, not three. Blue and Yellow share unit 5240JM; Green has its own unit 5240JM2. The split reflects maintenance-side isolation — a unit failure on 5240JM affects two systems; a failure on 5240JM2 affects only Green.
• Restrictor at the engine pylon. The 5242JM restrictor limits the bleed mass flow into the system. If the air line ruptures between the pylon and the reservoirs (in the wing leading edge or fuselage), the restrictor caps the leak rate so the rest of the bleed system does not bleed dry into the breach.

2. The regulated value: 4.5 bar absolute

The air pressurisation units reduce and regulate the bleed pressure to 4.5 bar ± 0.1 absolute (per AMM 29-14). The choice is set by two constraints:

• High enough to keep pump-inlet pressure above the fluid's vapour pressure across the full operating temperature range.
• Low enough that the reservoir structure does not see excessive pressure under any combination of altitude, temperature, and demand transient.

Translating into pilot-relative figures:

• Absolute at sea level: 4.5 bar abs ≈ 65 psi abs
• Relative at sea level: ≈ 3.5 bar rel ≈ 51 psi gauge
• Relative at cruise altitude: higher (lower ambient), but the relief valve setting and the absolute regulation keep this within design limits

A common pitfall when reading the maintenance documentation is to treat the 4.5 bar figure as gauge pressure. It is absolute. Treating it as gauge would overestimate the reservoir pressure by ~30%.

3. The components on top of each reservoir

Each reservoir's top manifold (per AMM 29-11) hosts five components:

The set is intentionally simple. There is no electrically-modulated regulator on top of the reservoir; the regulation happens upstream at the air pressurisation unit. The reservoir-top components are protective (relief, check) and indicative (gauge, switch) — not control elements.

4. The 12-hour static seal

Per AMM 29-11, the reservoirs are designed to remain pressurised for at least 12 hours at ISO temperature without any active bleed supply. The mechanism is the check valve isolating the regulated cushion from the supply line; once the bleed source stops, the cushion remains trapped above the fluid.

Two operational consequences:

• Overnight aircraft do not need ground-source pressurisation to start. A normally turned-around aircraft sitting for fewer than 12 hours retains reservoir pressure throughout. Engine start proceeds with the cushion already in place.
• Multi-day parked aircraft may show LO AIR PRESS at startup. Beyond the 12-hour design window, the cushion can decay below the LO AIR PRESS threshold (1.5 bar relative). The maintenance action is to use a ground source to repressurise before engine start.

The 12-hour figure is at ISO temperature. Cold-soaked overnight stops in extreme cold may shorten the effective hold; the principle holds, but absolute timing varies. For the pilot, the practical rule is: if LO AIR PRESS shows on a freshly cold-and-dark aircraft, it is more likely a sit-time issue than a system failure, and ground-source repressurisation is the routine response.

5. Backup source hierarchy

The primary supply is Engine 1 HP bleed. The architecture switches to backup paths automatically when primary pressure is insufficient.

Per AMM 29-14 and FCOM DSC-29-10-20:

Three things to retain:

• The backup paths share the same downstream manifold (5239JM). Switching between Engine 2, APU, and ground source happens through the air-conditioning side and is invisible to the crew. The pilot does not "select reservoir pressurisation source"; the architecture selects it.
• Engine 2 alone is not a routine backup. If Engine 1 is operating normally, Engine 2 bleed does not feed reservoir pressurisation. The backup activates only when Engine 1 supply is insufficient — engine off, bleed valve closed, or pressure dropped.
• The ground-source connector for Green (5241JM2) is located on the blue ground service panel, not on a separate "green" panel. The naming convention follows the panel, not the system fed. Maintenance personnel know this; the pilot's awareness is limited to recognising that ground servicing of Green reservoir pressurisation happens through the same physical panel as Blue and Yellow.

6. The LO AIR PRESS caution — trigger and interpretation

The LO AIR PRESS caution triggers when the low-pressure switch on the affected reservoir detects ≤ 1.5 bar relative.

The threshold sits well below the regulated 4.5 bar absolute (≈ 3.5 bar relative). Detection at 1.5 bar relative means the cushion has already lost roughly two-thirds of its normal pressure by the time the caution appears. This is intentional — it filters out transient fluctuations and triggers only on a genuinely depleted cushion.

Causes and their behaviours

A LO AIR PRESS that clears within seconds is consistent with the crossbleed pickup. A LO AIR PRESS that persists points either to a complete source loss or to a reservoir-side leak.

True versus false fault — the cross-check

A LO AIR PRESS in isolation, with the SD HYD page showing stable normal system pressure and no other anomalies, may be a sensor transient rather than a genuine condition. The pragmatic discrimination is:

• Stable 3000 psi on SD HYD page during the LO AIR PRESS → unlikely to be a true cavitation-imminent condition; monitor.
• System pressure fluctuating or LO PR annunciating during LO AIR PRESS → genuine pump-inlet starvation; act per ECAM.

The full discrimination logic, including how it interacts with the per-pump FAULT light, is in Pump Failure vs System Failure.

7. Why the cushion exists at all — cavitation, briefly

The pump inlet sees a pressure that is the sum of:

• The reservoir top pressure (4.5 bar abs),
• Minus the static head losses through the suction line,
• Minus the dynamic losses at the pump inlet.

If the resulting inlet pressure drops to the vapour pressure of the fluid at operating temperature, the fluid boils at the inlet — vapour bubbles form, then collapse violently as they enter the high-pressure side downstream. The collapse produces shock waves on the pump internals; sustained operation produces erosion pitting and rapid failure.

Phosphate-ester fluid at typical operating temperature has vapour pressure well below atmospheric, but the margin is closed under three conditions:

• High altitude (lower ambient suppresses the relative pressure contribution).
• High fluid temperature (raises vapour pressure).
• High flow demand at the pump (drops dynamic inlet pressure).

The 4.5 bar absolute cushion is sized to keep the pump-inlet pressure above vapour pressure across the full envelope of these three variables. It is the architectural protection against cavitation; without it, the pump would be operating at the edge of cavitation in normal flight.

What cavitation actually does to the pump

The damage mechanism is worth understanding because it informs why the LO AIR PRESS caution is taken so seriously even when the system pressure still reads normal:

1. Vapour bubbles form at the pump inlet. When inlet pressure drops to vapour pressure, the fluid boils locally — small vapour bubbles entrained in the suction flow.
2. The bubbles enter the high-pressure side. Inside the pump, the swash plate and piston action pressurise the fluid abruptly. The vapour bubbles cannot exist at high pressure and collapse almost instantaneously.
3. The collapse is violent. Liquid rushes in to fill the void left by each collapsing bubble. The micro-jet velocities and the local pressure spike produce an instantaneous condition reaching hundreds of bar and high local temperature within a volume of microns.
4. Repeated collapses erode the metal. Each bubble collapse picks at the cylinder wall, valve plate, and piston surfaces. Sustained operation produces a characteristic pitted finish on these components — "cavitation pitting" — and the pump's volumetric efficiency drops as the clearances grow.

A pump operating in mild cavitation can continue producing output pressure for hours, but the internal damage accumulates monotonically. By the time the damage manifests as a measurable pressure drop, the pump has typically reached a state requiring replacement rather than overhaul. The LO AIR PRESS caution is an upstream warning that comes well before any output-pressure symptom — heeding it preserves the pumps; ignoring it shortens their service life dramatically.

This is why the architectural protection is the cushion, not the diagnostic: the goal is to never let cavitation begin, because once it does the damage is already being inflicted.

8. Maintenance interfaces — what the pilot may see

Beyond the cockpit, the pressurisation system is serviced and tested through several maintenance tasks. The pilot's reading of these is awareness, not action:

• External-bench pressurisation restores the cushion on aircraft sitting beyond 12 hours.
• APU bleed test of the pressurisation path verifies the supply chain after engine-side maintenance.
• Three-reservoir gauge comparison is the maintenance cross-check when an indication anomaly is suspected.
• Engine-1-driven path test confirms the primary supply after engine work.

If the crew sees a maintenance person at the blue ground service panel applying compressed air to fill a reservoir cushion, that is the standard external-bench task, not an indication of a serious failure.

9. The full numerical reference

Memorising the absolute numbers is not necessary in flight, but understanding the ratio — regulated cushion roughly 3.5 bar relative, LO AIR PRESS at 1.5 bar relative — gives the cross-check that the caution implies a significant cushion loss, not a transient dip.

Self-test

[!note]- Q1. After engine start on a cold morning, HYD G RSVR LO AIR PRESS triggers. The SD HYD page shows Green system pressure stable at 3000 psi. Is this a real fault?

Probably not, in the sense that the pump is not yet at cavitation risk. A stable 3000 psi on the SD page means the EDP is being fed adequately. The LO AIR PRESS may be a transient — the crossbleed has not yet equalised, the cushion is recovering from overnight settling, or the sensor has a momentary anomaly. The correct action is to monitor: if system pressure remains stable and the caution clears, no further action is needed. If the caution persists or system pressure begins to fluctuate, treat as a genuine pressurisation problem and follow the abnormal procedure.

[!note]- Q2. The aircraft has been parked for 36 hours. After power-up, all three RSVR LO AIR PRESS cautions are active. The crew has not started engines yet. Is this a fault?

No, this is an expected condition. The 12-hour design seal has been exceeded by a wide margin, and the cushion in all three reservoirs has decayed below the 1.5 bar relative trigger. The standard maintenance action is to use a ground source (external bench, via the blue ground service panel connectors 5241JM and 5241JM2) to repressurise the reservoirs before engine start. Once pressurised, the cautions clear. The HSMU is reporting an accurate, expected condition; the architecture's recovery path is ground-source supply.

[!note]- Q3. The reservoir top cushion is regulated to "4.5 bar". An instructor asks: 4.5 bar of what — gauge or absolute? What is the practical difference?

Absolute. The regulated value is 4.5 bar absolute, equivalent to roughly 3.5 bar relative (gauge) at sea level, equivalent to roughly 51 psi gauge. The difference matters because treating 4.5 bar as gauge would over-state the cushion by about 30%, leading to incorrect intuitions about how close the LO AIR PRESS threshold (1.5 bar relative) is to normal. The absolute regulation also means the cushion does not vary significantly with altitude — important because the pump inlet operating margin needs to hold from sea level to FL410.

[!note]- Q4. The primary supply is Engine 1 HP bleed. If Engine 1's bleed valve is selected OFF for an engine bleed-off departure, does reservoir pressurisation continue?

Yes, via the crossbleed duct. With Engine 1 bleed off, the air-conditioning system shifts to Engine 2 (or APU) bleed, and reservoir pressurisation draws from the same crossbleed. The switching is automatic and invisible from the cockpit. The pilot does not need to verify reservoir pressurisation source independently; the LO AIR PRESS caution would appear if any path failed. The architecture is designed so that engine bleed configuration choices on the crew's part do not break reservoir pressurisation.

[!note]- Q5. The pressure-relief valve on each reservoir opens at 5.3 bar relative on the ground. Why is the trigger specified "on the ground" rather than as a single absolute number?

The relief setting is specified relative to ambient because the structural concern is the differential pressure across the reservoir wall, not the absolute. On the ground, ambient is roughly 1 bar absolute; the relief opens at 5.3 bar relative = 6.3 bar absolute reservoir pressure. At cruise altitude with ambient roughly 0.3 bar absolute, the relief setting expressed in absolute terms would be different but the same in relative terms — and the relative figure is the one that matches the structural design margin. AMM specifies "on the ground" to anchor the reference but the underlying parameter is the differential. The pilot's takeaway is that the relief opens at a pressure well above the 4.5 bar absolute regulated value, providing structural headroom.

References

Per FCOM DSC-29-10-20 (Reservoir Pressurization); AMM 29-14 (Air Pressurization Unit, restrictor 5242JM, units 5240JM/5240JM2, manifold 5239JM, ground connectors 5241JM/5241JM2, LP switch setting, relief valve setting); AMM 29-11 (top manifold components, 12-hour static seal design).

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