Pump Failure vs System Failure

The ECAM produces two distinct categories of caution for low hydraulic pressure: PUMP LO PR (a specific pump has dropped offline) and SYS LO PR (the entire system manifold has lost pressure). Confusing the two leads to incorrect interpretation of system state — and to incorrect procedural choices.

This article isolates the distinction. The two cautions have different trigger sources, different operational implications, and different procedural responses. Understanding the difference is foundational to interpreting any hydraulic anomaly.

1. The two triggers

The EDP switch sits upstream of the system check valve, on the pump's individual delivery line. It senses what this pump is delivering. The system switch sits on the manifold, after the check valves have combined the outputs from multiple pumps. It senses what the system as a whole is at.

The electric-pump pressure switch (5JV on Green and equivalents) triggers at 100 ± 5 bar with recovery at 120 bar — a different setting from the EDP, reflecting the electric pump's lower full-flow pressure (150 bar vs the EDP's 196 bar). But it serves the same role: pump-output-side monitoring.

2. What each tells the crew

PUMP LO PR

This specific pump has stopped maintaining pressure.

Causes:

• The pump itself has failed internally.
• The drive shaft (quill drive on EDP) has sheared.
• The engine has stopped (EDP).
• The pump was commanded OFF (FAULT light extinguishes when OFF except for OVHT).
• The fire shut-off valve upstream has closed.
• The pressure compensator inside the pump is malfunctioning.

What it does not tell the crew:

• Whether the system as a whole is unpressurised.
• Whether other pumps on the same system are still working.
• Whether consumers are affected.

SYS LO PR

The whole system manifold has dropped to 1450 psi or below.

Causes:

• All pumps on the system have failed simultaneously, or
• A leak downstream of the pumps has drained pressure faster than the pumps can supply, or
• Both EDPs are intentionally off (e.g., during a reservoir low-level procedure), or
• Reservoir is empty (no fluid to pump).

What it tells the crew:

• The system is effectively lost.
• Consumers are unpowered.
• The priority valve has likely closed (at 127 bar threshold).
• Flight control law on this system may degrade.
• Abnormal procedure for this specific system must be executed.

3. The Green system — where the distinction matters most

Green has two EDPs feeding the same manifold. This makes the pump-vs-system distinction most operationally significant on Green.

                Engine 1 EDP            Engine 2 EDP
                    │                        │
        ┌───────────┴───┐          ┌───────┴───────┐
        │  Pressure     │          │  Pressure     │
        │  switch (1)   │          │  switch (2)   │
        │  120 ± 5 bar  │          │  120 ± 5 bar  │
        └───────────┬───┘          └───────┬───────┘
                    │                        │
                    ▼                        ▼
                  Check valve            Check valve
                    │                        │
                    └────────┬───────────────┘
                             │
                             ▼
                  HP filter manifold 5102JM
                             │
                             ▼
                  System manifold
                             │
                             ▼
                    System pressure switch
                    1450/1750 psi
                             │
                             ▼
                       Consumers

The two pump pressure switches are independent. A single pump's failure produces PUMP LO PR (1) or PUMP LO PR (2) without affecting the system as a whole — the other pump carries the manifold pressure. Only if both pumps fail simultaneously (or both are commanded off, or the reservoir is empty) does the system pressure drop below 1450 psi and SYS LO PR trigger.

Typical Green scenarios

In each case, the pattern of pump and system cautions tells the diagnosis.

4. Blue and Yellow — simpler distinctions

Blue and Yellow each have only one EDP. The pump-vs-system distinction is less meaningful — if the single EDP fails, the system itself loses pressure soon after (the electric pump's 18% flow cannot sustain the system).

For Blue specifically, the architectural quirk: the Blue electric pump only triggers automatically on Engine 1 failure + PRIM 1 or PRIM 3 loss. A pure Blue EDP internal failure with no engine or PRIM issue does not trigger the electric pump automatically; the crew may need to select it manually if Blue pressure recovery is needed.

5. The procedural difference

A pilot who treats PUMP LO PR as a system loss (executing the full system-loss procedure including manual pump shutdowns and consumer transitions) is over-reacting. The remaining pump is still maintaining the system, and the procedure should be limited to switching off the affected pump only.

Conversely, a pilot who sees SYS LO PR and assumes it is just a pump issue (selecting one pump OFF and continuing) is under-reacting. The system is gone; the consumers must be transitioned.

6. The diagnostic checklist

For any low-pressure caution on the SD HYD page:

1. Read the ECAM text. Is it PUMP LO PR or SYS LO PR?
2. Cross-check the SD HYD page. Does the system pressure indication show a value at the normal band (≥ 2500 or 3000 psi) or below the threshold?
3. Identify the pumps. For Green, are both pumps showing in-line green, or is one LO and one normal?
4. Decide the response. If pump-level only with system normal: pump off, continue. If system-level: execute system-loss procedure.

The cross-check between the ECAM text and the SD HYD page is the diagnostic discipline. Treating them in isolation produces errors; treating them together produces the right diagnosis.

7. The "PUMP LO PR with no associated SYS LO PR" pattern

A specific recognition pattern: PUMP LO PR (X) appears, system pressure on SD HYD page remains at the normal band, no other cautions. Diagnosis: single pump dropped offline, redundancy carrying.

The right action:

• Select the affected pump OFF (extinguishes the FAULT light unless an overheat trigger persists).
• Continue normal operation; the remaining pump(s) provide system supply.
• Note the failure for post-flight reporting; no operational action beyond the pump-off selection.

This pattern is the most common hydraulic anomaly — far more frequent than full system losses. It is also the easiest to mishandle: a crew that escalates the procedure to "system loss" wastes time and may inadvertently introduce additional system-level concerns (manual pump shutdowns can cascade if applied incorrectly).

8. Three-tier pressure monitoring — the complete picture

The pump-vs-system distinction sits inside a broader three-tier pressure-monitoring architecture. Understanding all three tiers is what lets the crew distinguish which layer has reported a fault, and therefore what the underlying condition actually is.

                Pump outputs (EDP or electric pump)
                            │
                            │  Each pump has its own pressure switch
                            ▼
              ┌────────────────────────────────────────┐
              │ Tier 1 — Pump-output pressure switch     │
              │   • EDP: 120 ± 5 bar (decreasing)        │ → triggers PUMP LO PR
              │   • ELEC pump: 100 ± 5 / 120 bar         │
              └────────────────────────────────────────┘
                            │ Signals go to HSMU
                            ▼
                       System manifold (HP)
                            │
                            │  System pressure switch + transducer
                            ▼
              ┌────────────────────────────────────────┐
              │ Tier 2 — Manifold pressure (HSMU view)   │
              │   • Switch 7JS: 100 / 120.5 bar          │
              │   • Transducer 6JS: 1 V (0 bar) → 5 V    │
              │                              (200 bar)   │
              └────────────────────────────────────────┘
                            │ Signals to HSMU and SDACs
                            ▼
              ┌────────────────────────────────────────┐
              │ Tier 3 — System-level ECAM threshold     │
              │   • SYS LO PR triggers at 1450 psi       │ → ECAM SYS LO PR
              │     (decreasing), recovers above 1750 psi│
              └────────────────────────────────────────┘

                    Reservoir cushion
                            │
                            │  Reservoir-pressure switch 1JS
                            ▼
              ┌────────────────────────────────────────┐
              │ Reservoir air-pressure switch            │
              │   • 1.5 / 1.7 bar relative hysteresis    │ → triggers LO AIR PRESS
              └────────────────────────────────────────┘

The three pressure tiers (pump output / manifold / system threshold) plus the reservoir-side air pressure are independent signal chains. Each tier produces a different ECAM caution; mixing them up is the most common single source of incorrect diagnosis.

9. Single-pump failure does not equal system failure — the patterns

The architectural premise — redundancy keeps the system pressurised when a single pump fails — produces several recognisable scenarios:

Scenario A — Green EDP 1 fails in flight

• Green EDP 1 output drops below 120 bar trigger → HYD G ENG 1 PUMP LO PR triggers.
• Green EDP 2 continues running (both engines normal) → Green system pressure stays at 3000 psi.
• HYD G SYS LO PR does NOT trigger.
• Crew action: select ENG 1 PUMP OFF per ECAM; the remaining EDP carries the system.
• Flow impact: Green flow capacity halved (175 L/min from one pump instead of 350 from two), but steady-state pressure is maintained because under typical demand a single EDP suffices.

Scenario B — Blue EDP fails in flight, both engines running

• Blue EDP fails (internal fault, depressurise mode error) → HYD B ENG 1 PUMP LO PR triggers.
• Blue electric pump automatic trigger requires "Engine 1 failure + PRIM 1/3 fault" — neither condition is true → ELEC PUMP does not auto-activate.
• Blue system has no pump supplying it → pressure drops → HYD B SYS LO PR triggers.
• Both PUMP LO PR and SYS LO PR appear simultaneously.
• Crew action: per ECAM, switch Blue ENG PUMP OFF; may manually select Blue ELEC PUMP ON temporarily (e.g., to retract spoilers), but not as a sustained substitute.

Key insight: whether PUMP LO PR escalates to SYS LO PR depends on whether a redundant backup activates for that specific system. Green's dual-EDP architecture often absorbs a single-pump loss; Blue's single-EDP architecture with conditional electric pump usually escalates.

10. Real-fault versus spurious-fault discrimination

A330 hydraulic ECAM cautions can occasionally trigger spuriously (sensor noise, transient transponder pickup, signal-conditioning anomalies, transient startup conditions). The discrimination uses the SD HYD page indications as the cross-reference:

The diagnostic anchor: whether the SD HYD page system pressure is fluctuating. A real hydraulic fault produces visible pressure instability; a sensor or wiring fault often leaves the system pressure indication unchanged.

For the OVHT sensor failure case, the 150 °C jump pattern is documented in Hydraulic Fluid — the HSMU treats a sensor loss as 150 °C input (a fail-safe), so a 150 °C reading is the signature of a transmitter or wiring fault rather than a real temperature event.

11. When to switch off a pump versus when not to

The procedural rule is straightforward: always follow ECAM. Beyond that, the recognisable patterns:

General principle: never switch off a pump without ECAM direction. The ECAM procedure is the architecture's tested response. The exceptions to "always follow ECAM" are limited cases where the ECAM itself is failed (HSMU loss, display failure, etc.) — in those rare scenarios the QRH provides backup procedures.

12. The no-hydraulic-PTU reminder

A330 does not have a hydraulic Power Transfer Unit (PTU). This is significant because it differentiates A330 architecture from other Airbus types and from common mental models that operators may bring from earlier aircraft.

What "no PTU" means for fault handling:

• Fluid cross-feed does not exist between systems. A Green leak is contained on the Green side; no fluid is moved to or from Blue or Yellow to compensate.
• Pressure cross-feed does not exist. If Green is at 0 psi, no mechanism transfers Blue or Yellow pressure to Green.
• System recovery uses only that system's own resources. Green's recovery options are: redundant EDP, electric pump, RAT pump. Blue's are: EDP, electric pump. Yellow's are: EDP, electric pump, hand pump (ground only).
• The RAT is not a PTU. It is an independent ram-air-driven pump on Green specifically; it does not transfer power between hydraulic systems.

For pilots transitioning from aircraft with hydraulic PTU, the A330 mental model shift is essential: expect each system to live and die by its own pumps. The architectural redundancy is distributed across the three systems (2 EDPs on Green + 1 electric pump per system + RAT) rather than concentrated in a transfer mechanism.

13. LO AIR PRESS — complete real-vs-spurious flowchart

A reservoir LO AIR PRESS caution is one of the more commonly spurious of the architecture's reservoir-side warnings. The complete discrimination flow:

[HYD G(B/Y) RSVR LO AIR PRESS triggers]
       │
       ▼
[Check the SD HYD page system pressure for the affected system]
       │
       ├─ Pressure still close to 3000 psi, stable?
       │   │ YES
       │   ▼
       │ Likely SPURIOUS (or recovering transient)
       │   • Monitor SD pressure for changes
       │   • Do NOT switch off pumps
       │   • Continue normal operation
       │   • Report to maintenance after flight (sensor or bleed-source investigation)
       │
       └─ Pressure fluctuating or dropping below normal band?
           │ YES
           ▼
         Likely REAL
           • Follow ECAM procedure
           • Affected pump may need to be switched off
           • Monitor reservoir quantity (may indicate ongoing leak)
           • Prepare for single-system loss procedure (see ATA 29-18) if condition develops

The 1.5 bar relative trigger has been hit — the cushion has dropped from the regulated ~3.5 bar relative to below 1.5 bar relative. That is a real loss in cushion pressure regardless of the cause. The question is whether the pump is yet experiencing cavitation:

• Stable system pressure → pump still being fed adequately → no cavitation yet → may be transient bleed-side issue.
• Fluctuating system pressure → pump inlet pressure is varying → cavitation in progress → real pump-survival concern → ECAM action.

14. OVHT — real vs sensor fault discrimination

The OVHT caution has a similar but distinct discrimination logic, anchored in the SD temperature reading rather than pressure:

[HYD G(B/Y) RSVR OVHT triggers]
       │
       ▼
[Check SD HYD page reservoir temperature reading]
       │
       ├─ Temperature shown near 95 °C with rising trend?
       │   │ YES
       │   ▼
       │ Likely REAL OVHT
       │   • Follow ECAM (typically both pumps off)
       │   • Post-flight fluid sampling per maintenance procedure
       │   • Aircraft can continue to fly during sampling (per AMM 29-00)
       │
       ├─ Temperature jumps abruptly to 150 °C?
       │   │ YES
       │   ▼
       │ Likely SENSOR FAULT
       │   • The 150 °C value is the HSMU fail-safe default when temperature input is lost
       │   • Still follow ECAM (architecture treats it as overheat for safety)
       │   • Cross-check: any other temperature-related indications (white quantity, etc.)?
       │   • Report maintenance for temperature transmitter or wiring investigation
       │
       └─ Temperature reading not displayed (XX or amber)?
           │ YES
           ▼
         Sensor or display fault — follow ECAM, expect maintenance investigation post-flight

The 150 °C default is a critical recognition pattern. A real fluid overheat from a normal operating temperature shows a gradual rise; a sensor failure shows a step change to the fail-safe value.

15. The "SYS LO PR with PUMP LO PR" pattern

Another pattern: SYS LO PR appears together with PUMP LO PR indications on both (Green) or the single (Blue/Yellow) pump.

Diagnosis: the system is genuinely lost. Both Green pumps have dropped, or Blue/Yellow's EDP and electric pump are both off, or the reservoir is empty.

The procedural response is the system-loss procedure, not pump-level handling. The crew accepts the loss, transitions consumers, and configures for landing.

Self-test

[!note]- Q1. HYD G PUMP LO PR (1) appears during cruise. The SD HYD page shows Green at 3000 psi, both EDP indications, but EDP 1 is showing LO (amber) while EDP 2 is in-line green. The Captain considers executing the full Green system-loss procedure (transition to alternate brakes, plan gravity gear). Is this the right call?

No. The Green system is at 3000 psi, indicating EDP 2 is carrying the system fully. Executing the system-loss procedure now would unnecessarily transition the brake source and gear preparation, treating a manageable single-pump failure as a critical event. The correct response is to select ENG PUMP 1 OFF (which extinguishes the FAULT light, assuming no overheat is involved), accept that Green is now operating on EDP 2 alone, and continue normal flight. The system-loss procedure becomes relevant only if a second event occurs (EDP 2 also failing, reservoir issue, etc.).

[!note]- Q2. HYD B SYS LO PR appears in cruise. The SD HYD page shows Blue pressure at 0 psi. The crew sees no HYD B PUMP LO PR. Why is the pump-level caution missing?

Likely because the Blue EDP has been switched off (intentionally or automatically) — the pump-level caution does not trigger when the pump is commanded off; the indication on the SD page is cross-line amber rather than LO. With the pump off, the system manifold pressure has decayed below the 1450 psi threshold, triggering SYS LO PR. The diagnosis: Blue is intentionally off (maintenance, abnormal procedure execution earlier, etc.) and the system is now depressurised. The crew should verify why the pump is off and proceed with the appropriate procedural response.

[!note]- Q3. EDP 1 has been switched OFF; EDP 2 is healthy. Green system pressure stays at 3000 psi. The ENG PUMP 1 FAULT light is illuminated. Why is the FAULT light on if the pump is intentionally off?

The FAULT light extinguishes when the pump is selected OFF, except if an overheat condition is involved. If the original trigger to switch off was a reservoir overheat (which the FCOM specifically calls out — the light goes off when the crew selects OFF, except during an overheat), the FAULT stays illuminated until the fluid temperature returns to normal. The pump is off; the system is normal (EDP 2 carrying); the FAULT light is the architectural notification that the underlying thermal condition has not cleared.

[!note]- Q4. The Yellow EDP has failed (HYD Y PUMP LO PR). The Yellow ELEC PUMP has triggered automatically and is producing pressure. Yellow system pressure shows 3000 psi (or close to it). Has the system actually recovered?

The system is being supplied by the electric pump at ~18% of EDP flow capacity. Under low demand (cruise), the electric pump can maintain the manifold at near-3000 psi. Under heavy demand (gear cycling, simultaneous large surface motion), the pump may fall short, and pressure could dip into the PUMP LO PR or even SYS LO PR territory. The "recovery" is conditional: yes, the system is pressurised, but the flow margin is much reduced. The general rule against running an electric pump for sustained substitution applies — the architecture is in a degraded resilience state, not back to normal. The Yellow EDP failure should still be addressed by maintenance after landing.

[!note]- Q5. The Captain sees HYD G PUMP LO PR (1) and HYD G PUMP LO PR (2) appearing within seconds of each other, followed by HYD G SYS LO PR. What diagnosis fits this sequence?

Likely a common-cause event affecting both EDPs simultaneously. The most common causes: dual engine failure (both engines stop, both EDPs lose their drive, both pump pressure switches trigger nearly simultaneously); or a major fluid loss (Green reservoir empty, both pumps cavitate and lose output). The dual PUMP LO PR pattern is the signature; the subsequent SYS LO PR confirms the system has dropped below the manifold threshold. The crew checks for engine indications (RPM, EGT) — if engines are off, dual-engine-failure scenario; if engines are normal, fluid-side issue. Either way, the system loss is real and the appropriate abnormal procedure applies.

References

Per FCOM DSC-29-10-20 (Pump descriptions and pressure switch settings); FCOM DSC-29-20 (ECAM indications and FAULT light logic); FCOM PRO-ABN-HYD (PUMP LO PR and SYS LO PR procedures); AMM 29-11 (EDP pressure switch 120 ± 5 bar trigger); AMM 29-21 (electric pump pressure switch 5JV, 100/120 bar hysteresis).

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