Filters and Leak Measurement Valves

The hydraulic system carries a layered filtration architecture — high-pressure filters on the pump output, low-pressure filters on the return path, dedicated filters on case drains, reservoir fill, and brake circuits. The system also carries leak measurement valves upstream of the primary flight controls, used during maintenance to isolate the flight-control side for leak-rate testing.

This article covers what each filter does, where it sits, and the specific behaviours of the leak measurement valves — including the HSMU inhibit that prevents them from closing in flight, and the automatic Yellow closure during cargo-door operation.

1. The filtration layers

Per FCOM DSC-29-10-20 (Filters):

The hydraulic fluid is maintained clean by filters:

Two HP filters on green system

One HP filter on blue system and one on yellow system

One on the reservoir filling system

One on the braking system

One return line filter on each system (LP filters)

One case drain filter on engine pump permits the monitoring of wear by detection of metallic particles in the filters.

                        Reservoir
                            │
                            ▼
                ┌───────────────────────┐
                │  Reservoir fill filter│  Filters fluid being added
                │  (one per system)     │  during servicing
                └───────────────────────┘
                            │
                            ▼
                       (Pump intake)
                            │
                            ▼
                   ┌────────────────┐
                   │     Pump       │
                   └────────┬───────┘
                            │
                            │ Output pressure (3000 psi nominal)
                            ▼
                ┌───────────────────────┐
                │  HP filter manifold   │  Removes contaminants from
                │  (2 on Green / 1 on  │  pump output before they reach
                │   each Blue, Yellow) │  consumers; 15 µm absolute
                │  Clogging indicator  │
                │  at 6 bar ± 10% ΔP   │
                └───────────────────────┘
                            │
                            ▼
                ┌───────────────────────┐
                │  System manifold      │
                │  (distribution)       │
                └───────────────────────┘
                            │
                ┌───────────┼───────────┬─────────────────┐
                ▼           ▼           ▼                 ▼
            Brake        Flight       Other            Case drain
            manifold     controls     consumers        (per EDP)
                │            │            │                 │
                ▼            │            │                 │
        ┌───────────┐        │            │                 ▼
        │ Brake     │        │            │        ┌──────────────┐
        │ filter    │        │            │        │ Case drain   │
        └───────────┘        │            │        │ filter       │
                │            │            │        │ (per EDP)    │
                ▼            ▼            ▼        │ 15 µm abs    │
              Brake      Servos       Consumers   │ Wear monitor │
              actuators                            └──────┬───────┘
                                                          │
                ▼            ▼            ▼               │
            ────────────────────────────────────────       │
                                                          │
                                                          ▼
                                              Case drain return
                                                  to reservoir
                            │
                            │ Return path
                            ▼
                ┌───────────────────────┐
                │  LP (return line)     │  Removes contaminants on
                │  filter               │  return to reservoir
                │  (one per system)     │
                └───────────────────────┘
                            │
                            ▼
                       Reservoir

2. The HP filter — clogging protection

Each system carries one or more high-pressure filters on the pump output. Green has two (because Green carries two EDPs, and each EDP gets its own HP filter); Blue and Yellow each have one.

The HP filter specification:

Parameter	Value
Filtration	15 microns absolute
Type	Non-bypassing
Clogging indicator	Activates at 6 bar (87 psi) ± 10% ΔP
Element	Non-cleanable (replaced as a unit)
Bowl	Includes automatic shut-off — prevents fluid leak and air ingress during element removal

Three details worth noting:

Non-bypassing. When the filter clogs, the architecture does not allow fluid to bypass the filter element. Instead, the clogging indicator activates and the maintenance protocol replaces the element. Allowing bypass would let contaminants reach downstream consumers — defeating the filter's purpose.
6 bar ΔP clogging trigger. The 6 bar differential between the filter inlet and outlet indicates the element is restricting flow. The indicator is visual (a mechanical pop-up); maintenance personnel see it during routine inspection.
Automatic shut-off during change. When the filter bowl is removed for element replacement, an internal mechanism seals off the line, preventing fluid spillage and air ingress. This allows quick element changes without depressurising the entire system.

3. Case-drain filter — the wear monitoring window

Each engine-driven pump has its own case-drain filter mounted on the engine fan case. This filter sits on the pump's internal-leakage return line, and serves two purposes:

Filtration: same 15-micron absolute capability as the HP filter, removing fine particles before the case-drain fluid returns to the reservoir.
Wear monitoring: the filter element traps metallic particles generated by internal pump wear. Periodic element inspection lets maintenance assess pump condition.

The flow through the case-drain filter is documented at 40 L/min, with the clogging indicator at the same 6 bar ± 10% ΔP as the HP filter.

The wear-monitoring role makes the case-drain filter a particularly valuable diagnostic tool. A pump that is shedding more metallic particles than expected is approaching the end of its service life; maintenance can plan a replacement before the pump fails outright. The filter element is non-cleanable, so each inspection produces a single data point on the pump's wear state.

A pump's case-drain flow increases gradually over its service life — from 7-10 L/min when new to approximately 15 L/min by 12,000 flight hours. Rising flow correlates with internal clearance growth (wear); falling flow can indicate a failing case-drain path. Both trends are tracked by maintenance against published norms.

4. LP filter — return-line protection

Each system carries one low-pressure filter on the return line, between the consumers and the reservoir, adjacent to the reservoir itself. Its purpose: capture wear particles and contaminants generated by the consumers (actuator wear, dynamic-seal degradation) before they re-enter the reservoir.

Per AMM 29-11 §5.L:

Parameter	Value
Filtration	3 microns absolute (five times finer than HP)
Type	With bypass at 5 ± 0.5 bar ΔP
Clogging indicator	Activates at 3 +0 / -0.5 bar ΔP
Element	Non-cleanable

The architectural pattern is the opposite of the HP filter, and the asymmetry is deliberate:

Why 3 µm — finer than HP. The return-line fluid has just passed through every consumer: servocontrols, landing-gear actuators, brake pistons, slat/flap drives. Each of these contributes microscopic wear products. A 3-micron LP filter ensures the fluid arriving back at the reservoir is cleaner than the fluid the pump originally delivered, breaking the contamination cascade. Without this, particles generated downstream would recirculate to the pump inlet on the next stroke.
Why bypass — opposite of HP. A clogged LP filter that bypasses sends dirty fluid back to the reservoir — undesirable, but not destructive. A clogged HP filter that bypassed would send dirty fluid straight to the servocontrols — destructive. The bypass on LP preserves return-flow continuity (preventing pump-inlet starvation if the filter is severely clogged); the lack of bypass on HP preserves consumer protection.
Why the clogging indicator (3 bar) trips before the bypass (5 bar). The indicator gives maintenance a warning at 3 bar ΔP, well before the bypass activates at 5 bar. The two-stage design means a clogged LP filter is flagged for replacement before it actually starts shunting unfiltered fluid back to the reservoir.

5. Reservoir fill filter and brake filter

Two additional specialised filters listed in FCOM DSC-29-10-20:

Filter	Purpose
Reservoir fill filter	Filters fluid added during ground servicing (hydraulic service cart or hand pump). Prevents contamination introduced during fill from entering the system.
Brake filter	Additional filtration in the brake circuit. The brake actuators have tight tolerances and benefit from cleaner fluid than the general system.

The reservoir fill filter is documented in AMM 29-16 maintenance tasks. Specific filtration ratings for the brake filter are not given in the ATA 29 reservoir/fill section; consult ATA 32 (Brakes) for the brake-circuit filtration details.

6. Leak measurement valves — the maintenance interface

A leak measurement valve sits on each system, upstream of the primary flight controls. Its purpose is purely maintenance-side: closing the valve isolates the flight-control side from the pump-side pressure source, allowing measurement of internal leakage in the flight-control segment.

The measurement procedure:

With the system pressurised by the pump and the leak measurement valve open, the entire system is at 3000 psi.
Close the leak measurement valve.
The pump-side now pressurises only the section between the pump and the closed valve (manifolds, lines downstream of the pump, but not the flight controls).
Measure pressure decay or leakage rate in the isolated flight-control section.

This produces a quantitative leak-rate value, comparable across maintenance intervals, used to identify deteriorating seals or developing leaks before they produce in-flight symptoms.

How the valve itself works

Per AMM 29-19-00 §5.A, the three leak measurement solenoid valves are identical, and the design has one detail worth understanding:

Property	Description
Type	Two-position, three-way, solenoid-operated
Default state	De-energized → normally open (port A connected to C — fluid flows)
Energized state	Closed (port B connected to C — fluid path to consumers cut off)
Leakage when closed	Approximately 2 – 2.5 L/min from port A to port B

The "leakage when closed" looks counter-intuitive — why would a valve designed to close still allow 2–2.5 L/min of flow? The AMM gives two explicit engineering reasons:

Quick depressurisation of the upstream line (A). When the valve closes and the consumers downstream stop drawing fluid, the section between the valve and the system manifold would otherwise stay pressurised. The deliberate leak path drains that residual pressure cleanly.
Prevention of pressure peaks above 25 bar in line C. When the valve transitions back from energized (closed) to de-energized (open) — i.e., when fluid is re-admitted to the consumers — the abrupt repressurisation could otherwise spike line C above 25 bar. The 2–2.5 L/min path acts as a calibrated pressure-equalisation bypass, smoothing the transition.

The AMM specifically calls out Yellow ground safety: Yellow drives the primary flight controls, the cargo doors, and the engine 2 thrust reverser. A sudden Yellow re-pressurisation with no equalisation could move flight-control surfaces while ground personnel are nearby — the 25 bar peak limit is a safety design value.

So the leak measurement valve is not a simple shut-off — it's a controlled-flow device with built-in pressure-management behaviour. The "leak" is not a defect; it's by design.

The pushbutton — 285VU maintenance panel

The LEAK MEASUREMENT VALVES pushbutton lives on the maintenance panel 285VU, not the main overhead 29 panel. Per AMM 29-19-00 §3, "the pushbutton switch on the maintenance panel 285VU controls the solenoid valve."

The panel placement is itself a usability statement: the pushbutton is for maintenance personnel, and flight crew interaction is the exception case. The pushbutton is guarded (lift-cover protected) and labelled "to be used on ground only" per FCOM DSC-29-20.

The HSMU inhibit in flight

The leak measurement valves are NEVER allowed to close in flight. Per FCOM DSC-29-10-30:

The HSMU inhibits the closure of the green, blue and yellow hydraulic leak measurement valves in flight.

The architectural reason is straightforward: closing the valve in flight would isolate the primary flight controls from their hydraulic supply, removing flight-control authority on that system. The inhibit prevents this from happening for any reason — manual command, electrical noise on the control line, anything.

The pushbutton OFF light behaviour

Per FCOM DSC-29-20:

Even if the function is inhibited in flight: If the flight crew inadvertently presses this pushbutton, the OFF light will still come on to advise the crew that the electrohydraulic valves will close at landing, when the aircraft speed is less than 100 kt.

This is a thoughtful detail. The pushbutton does not silently refuse the command. The OFF light illuminates to confirm that the command is armed for execution at landing rollout (when speed drops below 100 kt — i.e., when the aircraft is no longer "in flight" by the inhibit's definition).

If the crew did not intend to arm the leak measurement valves for closure at landing, they need to deselect the pushbutton before the landing rollout falls below 100 kt. Failing to do so will close the valves on landing, with the consequence of losing flight-control authority during the rollout — a serious operational issue.

The pushbutton is located on the maintenance panel, not the main overhead 29 panel. The maintenance-panel placement reflects that the pushbutton is intended for maintenance personnel, not for normal flight crew interaction. The inadvertent-press case is the exception, and the OFF light is the warning.

The Yellow cargo-door automatic closure

The Yellow leak measurement valve has an additional automatic behaviour: it closes when the cargo-door manual selector valve is set to OPEN or CLOSE on the ground.

The architectural purpose: prevent inadvertent flight-control surface movement during cargo-door operation. With cargo doors moving, ground personnel may be near control surfaces. Closing the Yellow leak measurement valve isolates Yellow primary flight controls from the Yellow pressure source during the door cycle, ensuring no surface motion can be driven by Yellow during that time.

The Yellow flap motor is also inhibited during cargo-door operation by the same logic, preventing slat/flap motion. The combined interlock (closed leak measurement valve + inhibited flap motor) makes cargo-door operation safe from a flight-control-movement perspective.

7. The clogging indicator philosophy

A recurring theme across the filtration architecture is the clogging indicator at 6 bar ± 10% ΔP. The number is consistent across HP filters and case-drain filters because it reflects a common engineering compromise:

Low enough that the indicator activates well before the filter element is fully blocked.
High enough that normal operating pressure variations do not produce false indicator activations.
Mechanical — no electrical signal, no monitoring computer — so the indicator works regardless of aircraft electrical state.

The indicator is a visual pop-up that maintenance personnel inspect during routine checks. There is no ECAM display of filter clogging in flight; the crew has no indication that a filter is approaching its replacement threshold. The architecture treats filter clogging as a maintenance-side issue, addressed during ground servicing rather than monitored continuously.

8. What the crew sees, what they do

The filtration architecture is essentially invisible from the cockpit:

No filter-clogging ECAM cautions.
No filter-state indications on the SD HYD page.
No crew action related to filtration in any normal or abnormal procedure.

The leak measurement valves are slightly more visible:

The maintenance-panel pushbutton can be inadvertently pressed; the OFF light then warns of arming at landing.
The Yellow leak measurement valve indication on the SD HYD page changes state during cargo-door operation on the ground (visible if the crew is in the cockpit during ground servicing).

For the crew, the primary takeaway is knowledge that the filters and leak measurement valves exist and what they do — useful for interpreting why specific maintenance actions are scheduled and what a ground technician is doing when accessing the maintenance panel.

Self-test

[!note]- Q1. The HP filter is "non-bypassing." What does this mean, and why is it the architectural choice?

Non-bypassing means that when the filter element clogs, the fluid does not bypass the filter and continue downstream. Instead, the clogging indicator activates, and maintenance replaces the element before fluid flow is restored to normal. The alternative (bypassing filter) would let contaminants reach downstream consumers when the filter is clogged — defeating the filter's protective purpose. Non-bypassing is the safer choice because the architecture prefers a temporary loss of system performance (during element replacement) over chronic contamination of downstream components.

[!note]- Q2. The crew inadvertently presses the LEAK MEASUREMENT VALVES pushbutton during cruise. The HSMU inhibits the closure in flight. What does the pushbutton's OFF light do?

The OFF light illuminates, advising the crew that the closure command has been armed for execution at landing rollout (below 100 kt). The HSMU inhibit prevents the valve from actually closing in flight, so flight controls remain pressurised throughout the flight. If the crew does not deselect the pushbutton before landing, the leak measurement valves will close as the aircraft decelerates below 100 kt — at which point flight-control hydraulic supply is interrupted, potentially during the rollout. The crew needs to deselect the pushbutton during cruise or approach, returning the OFF light to extinguished.

[!note]- Q3. The case-drain filter on Green EDP 1 is found with elevated metallic particle counts during scheduled maintenance. What does this indicate, and what is the expected maintenance response?

Elevated metallic particles in the case-drain filter element indicate internal pump wear above the expected rate. The fluid passes through the pump's internal-leakage path on its way to the case drain, picking up wear products from the piston-cylinder interfaces, the swash plate, and the bearings. A higher-than-expected particle count means the pump is shedding more material than its operating-hour count would predict. The maintenance response is to plan an earlier-than-scheduled pump replacement, before the wear progresses to a performance loss or seizure. The particle inspection is the architecture's direct read on pump health, available with each filter inspection.

[!note]- Q4. Why does the Yellow leak measurement valve close automatically during cargo-door operation, and why doesn't the same logic apply to Green and Blue?

Cargo-door operation uses the Yellow hydraulic system to drive the door actuators. Closing the Yellow leak measurement valve isolates the Yellow primary flight controls during the door operation, preventing inadvertent surface movement driven by Yellow pressure while ground personnel are working near control surfaces. Green and Blue have no operational use case where their systems are pressurised during ground operations involving personnel near control surfaces, so they do not need the equivalent automatic closure. The architecture matches each interlock to a specific operational case rather than applying a uniform rule across all three systems.

[!note]- Q5. Each EDP has a case-drain filter mounted on the engine fan case. A new pump shows 8 L/min case-drain flow; the same pump at 12,000 flight hours shows 14 L/min. Is the pump healthy?

The rising case-drain flow is the expected wear signature. The documented range allows up to ~15 L/min at 12,000 flight hours. A trend from 8 to 14 L/min is within the expected progression — not a failure, but a data point that maintenance tracks. The element inspection looks for metallic particles to confirm that the wear is mechanical wear of the expected type, not a structural component shedding. If particles are within expected size and quantity for the pump's flight hours, the pump continues in service. If particles indicate accelerated wear or unusual failure modes, the pump is removed earlier than the rising flow alone would justify.

References

Per FCOM DSC-29-10-20 (Filters); FCOM DSC-29-10-30 (Leak Measurement Valves, Priority Function); FCOM DSC-29-20 (LEAK MEASUREMENT VALVES pushbutton with OFF light behaviour, leak-measurement-valve indication on SD HYD page); AMM 29-11 (HP filter specifications, case-drain filter, LP return-line filter, reservoir fill filter, brake filter, all at 15-micron absolute filtration with 6 bar ± 10% clogging indicators); AMM 29-31 (leak measurement valve electrical-hydraulic actuator design and HSMU inhibit logic).

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