Engine-Driven Pumps (EDPs)

The engine-driven pump is the primary hydraulic power source on every system. Four EDPs total — two on Green (one driven by each engine), one on Blue (Engine 1), one on Yellow (Engine 2). All four are mechanically identical: nine-piston variable-displacement axial pumps with internal compensators that automatically adjust output to maintain 3000 psi.

This article covers the engineering — how the EDP regulates pressure without crew involvement, the specific drive shaft design that fails safely if the pump seizes, the depressurisation logic that protects the pump on engine shutdown, the dry-run tolerance window (10 minutes without suction, 15 minutes after fire valve closure), and the case-drain flow that lets maintenance detect pump wear.

1. Where the EDP sits, and how it is driven

The EDP is mounted on the accessory gearbox at the bottom of each engine. A splined quill drive connects the gearbox to the pump's input shaft. The drive shaft is engineered with a deliberate weak point — if the pump internals seize, the shaft shears, the pump disconnects from the gearbox, and the engine continues to operate normally.

This is the design's principal fail-safe: pump damage cannot propagate to the engine. The pilot sees a PUMP LO PR ECAM with normal engine indications; the maintenance finding is a sheared quill drive and a seized pump, both replaceable as a unit.

The pump attaches via a flange with keyhole-shaped bolt slots, and the suction line uses a self-sealing coupling. Together these allow a rapid pump change during turnaround — the line does not need to be drained, and the bolts come out without removing them from the airframe.

                   Engine accessory gearbox
                            │
                            │ splined quill drive
                            │ (shears if pump seizes — protects engine)
                            ▼
                       EDP input shaft
                            │
                            │ Rotates 2200 to 4400 rpm
                            ▼
              ┌─────────────────────────────────┐
              │           EDP body              │
              │                                 │
              │  ┌─────────────────────────┐    │
              │  │ Inlet boost impeller    │    │  Raises pressure
              │  │ (raises pump-inlet pres.)│   │  at the pump inlet
              │  └─────────────────────────┘    │  → buffer against
              │              │                  │  cavitation
              │              ▼                  │
              │  ┌─────────────────────────┐    │
              │  │ Nine pistons +          │    │  Variable displacement:
              │  │ movable yoke plate      │    │  yoke angle sets stroke
              │  │ (swash plate)           │    │  → sets flow
              │  └─────────────────────────┘    │
              │              │                  │
              │              ▼                  │
              │  ┌─────────────────────────┐    │
              │  │ Compensator valve       │    │  Closed loop:
              │  │  ──► actuator piston    │    │  pressure ↑ → yoke ↓
              │  │      ──► yoke angle     │    │  → flow ↓ → pressure
              │  └─────────────────────────┘    │  back to 3000 psi
              │              │                  │
              │  ┌─────────────────────────┐    │
              │  │ Solenoid valve          │    │  Cockpit-commanded
              │  │ (depressurisation cmd)  │    │  depressurise mode
              │  └─────────────────────────┘    │
              │              │                  │
              │  ┌─────────────────────────┐    │
              │  │ Blocking valve          │    │  Isolates pump outlet
              │  │ (active in depressurise)│    │  in depressurised mode
              │  └─────────────────────────┘    │
              │                                 │
              └─────┬──────────────┬─────┬──────┘
                    │              │     │
                    ▼              ▼     ▼
              To system        Case      Suction
              (~3000 psi)      drain     from reservoir
                               (7-10     (~1.5 bar abs)
                               L/min)

2. Variable displacement — how the EDP self-regulates

All four EDPs are nine-piston variable-displacement axial pumps. The pistons connect to a movable yoke plate (also called a swash plate); the yoke's tilt angle relative to the drive shaft determines the stroke length of each piston, which sets the pump's output flow.

The regulation loop is entirely mechanical:

This loop is self-contained on the pump. The crew does not control flow; the architecture does not vary EDP delivery from the cockpit. The crew commands ON/OFF (or the depressurise mode); the compensator handles everything in between.

A useful consequence: in cruise, when consumer demand is low (no surfaces cycling), the yoke automatically pulls back toward zero angle, and the EDP consumes very little engine power. The pump still rotates with the engine, but the piston stroke is near zero — almost no fluid is being moved. This is why running two engine-driven pumps continuously is not a fuel penalty under steady-state cruise.

3. The numbers

EDP performance is documented at this level:

A few of these are worth flagging operationally rather than memorising:

• The 175 L/min at 196 bar is the maximum flow point. Most of the time the EDP delivers far less; this is the design ceiling, not the typical operating point.
• 150 ms response time means the EDP catches up to a sudden demand (gear retraction, spoiler deployment) within roughly a tenth of a second. The pressure dip during a transient is small and brief.
• 2200 rpm minimum means flight idle and taxi-idle both leave the EDP above its minimum speed — the pump remains effective at all phases of engine operation.

4. Inlet boost impeller — the cavitation buffer

Each EDP contains an inlet boost impeller that raises the fluid pressure at the pump inlet before it enters the piston bores. This is the pump-internal complement to the reservoir's 4.5 bar gas cushion (covered in Reservoir Pressurisation): the cushion provides the suction-line head; the boost impeller adds another increment of pressure directly at the inlet.

The combined effect is to keep the inlet pressure above the fluid's vapour pressure under all operating conditions — even at high altitude, with hot fluid, and at maximum flow demand simultaneously.

A second function of the boost impeller is dry-run tolerance. If the suction line loses fluid (a leak between the reservoir and the pump, or fire-valve closure on the wing-side suction line), the boost impeller continues to provide some residual fluid contact to the piston chambers from the small volume retained inside the pump body. This is what gives the EDP its 10-minute and 15-minute dry-run windows.

5. The dry-run windows — 10 min vs 15 min

Two distinct tolerance windows are documented:

The two windows reflect the same underlying mechanism — the boost impeller maintains some lubrication and cooling on residual fluid — but the 15-minute case is longer because the valve closure preserves a larger sealed fluid volume inside the pump body (no continued outflow through the cut suction line).

Operational consequence. A crew handling a Green leak or an engine fire has a window of roughly 10 to 15 minutes during which the EDPs can run without permanent damage. This is enough time to complete the ECAM procedure, brief the cabin, and consider the diversion options without panicked action. The pump is not in immediate danger; the architecture has bought the time deliberately.

Outside the window, the pump starts to suffer cavitation and bearing damage. The crew is not expected to time-monitor against this — the abnormal procedures call for pump shutdown (via the ECAM steps) well within the protected window.

6. Depressurisation mode

The EDP has a documented depressurised mode in which the pump continues to rotate (driven by the engine) but does not supply fluid to the system.

The mechanism is a solenoid valve that, when energised, commands the blocking valve to isolate the pump outlet from the system manifold. Simultaneously, the yoke moves back to perpendicular (the zero-stroke position), bringing flow to near zero. The pump body is still lubricated and cooled by its own internal leakage (the case-drain flow).

Three triggers for depressurisation:

The ground-vs-flight distinction is deliberate. On the ground, an engine restart should produce immediate hydraulic pressure; depressurising the EDP would delay availability. In flight, depressurising on shutdown saves the pump from running dry as the engine spins down, and matches the depressurisation that the crew would otherwise perform manually.

Key concept to retain: depressurisation is not the same as stopping the pump. The pump still rotates whenever the engine rotates. The depressurised state means the pump's output is isolated, not that the pump is stopped.

7. Fire shut-off valves

The fire shut-off valve on each EDP sits in the suction line between the reservoir and the pump, in the wing. Per AMM 29-11:

• It is a ball valve rotated 90° by a 28 V DC actuator.
• Full open-to-closed (or closed-to-open) travel takes up to 1.8 seconds.
• Position is indicated by an external visual indicator (visible from inside the wing) and electrically by limit switches that feed the ECAM SD HYD page.
• The valve and actuator are mechanically separated, so a maintenance technician can replace the actuator without draining the suction line.

Closure triggers:

• Cockpit ENG FIRE pushbutton for that engine.
• HSMU automatic command on Green reservoir low level (both Green fire shut-off valves close together — Blue and Yellow do not have this automatic logic).

Once closed by either trigger, the valve cannot be reopened in flight by the crew. The HSMU may reopen the Green valves automatically after 150 seconds (the pump-preservation sequence detailed in Fire Shut-Off Valves), but no crew action can override either the closure or the conditional reopen.

8. EDP pressure switch and PUMP LO PR

Between the pump and the check valve at the engine pylon interface sits a pressure switch that triggers when EDP-output pressure decreases to 120 ± 5 bar (1740 ± 72 psi).

This trigger generates the PUMP LO PR ECAM caution — the EDP-specific low-pressure indication, distinct from the system-wide SYS LO PR that triggers at the system-pressure threshold (approximately 1450 psi with hysteresis).

The two cautions are not interchangeable:

On Green, where two EDPs feed the same manifold, PUMP LO PR on EDP 1 can occur with no SYS LO PR if EDP 2 maintains system pressure. The distinction matters for diagnosing which pump or system has actually failed — covered in detail in Pump Failure vs System Failure.

9. Case-drain relief valve — pump-body protection

The EDP has an internal pressure-relief valve on the case-drain line that routes case-drain fluid back to the pump inlet if case-drain pressure rises above its set value (per AMM 29-11).

Per AMM 29-11:

A pressure-relief valve connects the fluid in the EDP case drain to the inlet of the pump if the pressure of the fluid in the case drain is too high (for example if the case drain pipe is blocked).

The protective purpose:

• The case-drain line normally operates at ~4.5 bar absolute, with steady 7–15 L/min flow back to the reservoir.
• If the case-drain external line blocks (frozen, plugged, mechanically damaged), the case-drain pressure inside the pump body rises rapidly — the pistons keep pushing fluid into a closed volume.
• Unchecked, the rising case-drain pressure could rupture the pump body or damage internal seals.
• The relief valve opens before rupture pressure is reached, routing the trapped fluid back to the pump inlet — creating an internal circulation loop that protects the body until the blockage is cleared.

The relief valve is invisible to the crew — no ECAM caution, no SD indication. It is an EDP-internal safety mechanism that operates automatically. Maintenance becomes aware of it only through case-drain anomalies (rising case-drain flow alongside reduced external case-drain delivery) detected during scheduled inspection.

10. EDP-related ECAM cautions — complete map

Three ECAM cautions originate from EDP-side conditions:

The first two relate to the specific pump; the third relates to the system as a whole. On Green, the architecture allows pump-level conditions to occur without system-level conditions, because the second Green EDP can maintain the manifold.

11. Crew actions on EDP failure

A simplified procedural flow (per ECAM/QRH for any single-EDP loss):

[PUMP LO PR caution triggers]
        │
        ▼
[Crew selects affected ENG PUMP pb OFF (per ECAM)]
        │  → solenoid energised → blocking valve isolates pump outlet
        ▼
[Pump enters depressurised mode: still spinning with the engine,
 but yoke at zero, no output, internal case-drain lubrication]
        │
        ▼
[ECAM gives degraded-systems checklist based on the failure scenario]
        │  • Single pump failure on Green: other EDP carries the system → normal operation continues
        │  • Two-pump failure on Green: system pressure drops → SYS LO PR cascade
        │  • Single-EDP system (Blue/Yellow) failure: system pressure drops → SYS LO PR
        ▼
[Continue with the relevant single-system or multi-system loss procedure]

The pump can be commanded OFF (entering depressurised mode) without stopping the engine. Engine operation is unaffected by EDP shutdown. The pump remains driven by the accessory gearbox throughout; only its output is isolated.

Detailed procedural handling for each loss combination is in Single-System Loss and Dual-System Loss. The distinction between pump-level and system-level conditions is detailed in Pump vs System Failure.

12. Maintenance interfaces and task references

EDP-related maintenance tasks documented in AMM 29-11:

The quick-release pump fittings (keyhole-slot mounting + self-sealing coupling) support rapid turnaround for EDP replacement. A line-maintenance pump change is typically a matter of hours rather than days.

Two architectural facts the maintenance side relies on:

• 12,000 flight-hour case-drain flow trend as the wear signature (rising from 7–10 L/min new to ~15 L/min by 12,000 hours). Above 12,000 hours, the wear acceleration may justify pump removal even without explicit failure.
• Case-drain filter element inspection for metallic particles — the direct read on pump internal wear. Particle type (steel vs bronze) hints at which pump component is degrading.

13. Case drain — the wear-monitoring window

A small flow continuously bypasses each EDP through its case drain line. This flow:

• Lubricates and cools the pump's internal bearings and seals.
• Is nominally 7 to 10 L/min on a new pump.
• Rises to roughly 15 L/min by 12,000 flight hours, as internal clearances increase due to wear.
• Carries any metallic wear particles produced inside the pump body to the case-drain filter.

The case-drain filter (one per EDP, installed on the engine fan case) is:

• A non-bypassing type — when it clogs, it does not divert flow back to the system. Instead, a clogging indicator activates at 6 bar (87 psi) ± 10% differential pressure.
• Filtration capability: 15 microns absolute.
• The filter element is non-cleanable — replaced as a unit at scheduled intervals or on indicator activation.
• The bowl includes an automatic shut-off to prevent fluid leakage and air ingress during filter removal.

The wear-monitoring function is what makes the case-drain filter interesting beyond its filtration role. Maintenance inspects the filter element for metallic particles as a direct read on pump internal wear. A pump that is shedding more metal than expected is a candidate for early replacement, before it becomes a pressure-drop or seizure event.

14. Summary — the EDP in five points

1. Variable-displacement, nine-piston, axial. Yoke angle adjusts piston stroke; compensator valve closes the pressure regulation loop. Output stays at 3000 psi without crew involvement.
2. Drives mechanically protected by a shear-design quill drive. Pump seizure does not affect the engine.
3. Dry-run tolerant for 10 minutes (no suction) or 15 minutes (after fire-valve closure). Crew has time to work the procedure without rushing.
4. Depressurise vs stop. Depressurised mode rotates the pump with zero output. Stopping requires engine shutdown.
5. Wear-monitored via case-drain flow and the case-drain filter. The 7→15 L/min trend over flight hours is the pump's age signature.

The pilot's window into all of this is narrower: the ENG PUMP pushbutton, its FAULT light, and the ECAM HYD page. Everything above happens in the background, with maintenance documentation as the reference for the underlying logic.

Self-test

[!note]- Q1. The Captain sees HYD G PUMP LO PR (1) on ECAM during cruise, but Green system pressure is steady at 3000 psi on the SD HYD page. Is the pump failure real?

The pump-specific low-pressure caution is real — EDP 1 has dropped below its 120 ± 5 bar output trigger. System pressure remains at 3000 psi because EDP 2 is carrying the system on its own. This is the normal architecture of Green: dual-EDP redundancy means a single-pump failure does not produce a system-level loss. The crew handles PUMP LO PR per ECAM (typically OFF for the affected pump) and continues operation on the remaining EDP. The distinction between pump-level and system-level cautions is the key conceptual point.

[!note]- Q2. An engine fire forces fire-shut-off-valve closure on Engine 2. The Yellow EDP, driven by Engine 2, is being cut off from suction. How long before the EDP suffers damage?

Approximately 15 minutes from the moment the fire shut-off valve closes. The boost impeller inside the EDP retains fluid contact on the pump-side of the closed valve, providing lubrication and cooling for that window. The 15-minute figure is well above the typical procedure-completion time, so the crew has ample margin to execute the ECAM (which will command Yellow ENG PUMP OFF, ending pump rotation against zero suction). The window is engineered as a buffer, not as a permission to delay.

[!note]- Q3. The crew presses the ENG 1 PUMP (Green) pushbutton to OFF. The EDP is still being driven by Engine 1's accessory gearbox. What happens inside the pump?

The pushbutton energises the solenoid valve inside the EDP. The solenoid commands the blocking valve to isolate the pump outlet from the Green manifold. Simultaneously, the yoke moves to its perpendicular (zero-stroke) position, bringing flow to near zero. The pump body continues to rotate with the engine, lubricated by case-drain leakage. Outwardly, the pump is "off"; mechanically, it is still spinning but in depressurised mode with negligible output. This is the normal commanded behaviour, not a failure.

[!note]- Q4. A maintenance technician reports rising case-drain flow on Green EDP 1 over the last 1000 flight hours — from 8 L/min to 12 L/min. Is this a failure?

Not a failure — but a wear signature. The case-drain flow naturally rises as internal pump clearances increase with use. The documented range allows up to ~15 L/min at 12,000 flight hours. A trend from 8 to 12 L/min is within the expected progression but is now a data point that maintenance will track. The case-drain filter element will be inspected for metallic particles, which is the direct measure of pump wear. The pump is not removed for the flow trend alone; it is removed if particles indicate accelerated wear or if performance drops below specification.

[!note]- Q5. Engine 2 shuts down in flight (engine failure, not commanded). What happens to the Green EDP on Engine 2?

The Green EDP on Engine 2 is automatically depressurised by the HSMU. The solenoid is energised, the blocking valve isolates the outlet, the yoke moves to zero. The pump continues to spin down with the engine until both stop. The depressurisation prevents the pump from running dry as the engine winds down and fluid demand drops. Note that this is automatic and specific to in-flight engine shutdown on Green — Blue and Yellow EDPs do not have this automatic logic, and on the ground no EDP is depressurised on engine shutdown (so that ground restarts produce immediate pressure).

References

Per FCOM DSC-29-10-20 (Green/Blue/Yellow System Pumps, HSMU); AMM 29-11 (EDP location and drive, variable-displacement description, performance parameters, inlet boost impeller, dry-run windows, depressurisation mode, fire shut-off valves 2JG1/2JG2, pressure switch settings, case drain and case-drain filter); maintenance documentation for pump replacement and case-drain filter element inspection.

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