Priority Function and Fire Shut-Off Valves

Two valve assemblies sit between the pump output and the distribution side, both critical to how the hydraulic system handles abnormal conditions. The priority valve automatically sheds heavy-load consumers when system pressure drops, preserving pressure for flight controls and normal brakes. The fire shut-off valves isolate the pump's suction line on command, stopping fluid feed to a possible fire source or — on Green only — stopping the loss of fluid through a leak.

Both valves are passive in normal operation; both come into play during specific failure scenarios. This article covers their engineering: where they sit, what triggers them, and what the architecture does with them.

1. The priority valve — 5121JM on Green

The Green priority valve is installed on the brake manifold 5105JM. The valve is pilot-operated and spring-biased closed.

                  Pump output
                       │
                       ▼
              [HP filter manifold 5102JM]
                       │
                       │
            ┌──────────┴───────────┐
            ▼                       ▼
   [Brake manifold 5105JM]   [HP manifold 5103JM]
            │                       │
   ┌────────┴────────┐               │
   │                 │               │
   ▼ Direct          ▼               ▼
   Normal           [Priority         Green leakage measurement
   brakes           valve              manifold 5109JM1
                    5121JM]            │
                       │                ▼
                       │              Flight controls
                       │              (servocontrols)
            Secondary  ▼
            circuit:
            • Landing gear
            • Nose wheel steering
            • CSM/G emergency
              generator drive

Pilot-operated means hydraulically controlled

"Pilot-operated" in valve terminology refers to control by hydraulic pressure, not by the crew. The valve responds passively to system pressure:

• System pressure high (normal) → valve held open against the spring → secondary circuit supplied normally.
• System pressure drops → spring overpowers pressure → valve closes → secondary circuit isolated.

There is no electrical signal, no HSMU command, no crew action. The valve operates by feedback on the same pressure it is protecting.

The thresholds

Per AMM 29-11:

The 18 bar hysteresis between close (127) and re-open (140-145) prevents oscillation when system pressure hovers near the threshold. The 250 L/min flow rating handles the maximum demand of the secondary circuit consumers (gear retraction or extension, full nose wheel steering authority, CSM/G drive).

Primary versus secondary — what is preserved, what is shed

The architectural logic: flight controls and braking are the must-haves under any condition; landing gear extension and steering can be managed by alternate means (gravity gear extension, differential braking for ground manoeuvring) if pressure is unavailable. Shedding the heavy consumers preserves the must-haves.

Pressure-off brakes — the alternative shedding mechanism

Slats, flaps, and the THS (trimmable horizontal stabiliser) actuator use a different shedding mechanism — pressure-off brakes that lock the surface in place when pressure to that actuator drops. The effect is similar (the consumer stops drawing flow) but the surface stays in its current position rather than backdriving.

This is the architectural reason flap motion stops cleanly at the current detent rather than retracting toward 0 when Green pressure is lost mid-transit.

2. The priority function in action

When Green is RAT-supplied (2500 psi, reduced flow), the priority valve sheds heavy consumers automatically. The abnormal-procedure restrictions that result:

The crew accepts these restrictions and follows the abnormal procedure for landing on accumulator brakes, with manual gear extension, and with steering managed by differential braking.

The full set of restrictions is also referenced in the relevant abnormal procedures (Reservoir Low Level Procedures, Pump Failure vs System Failure).

3. The fire shut-off valves — 2JG1 and 2JG2

The Green fire shut-off valves sit in the wing, on the suction line between the reservoir and each engine-driven pump.

           Green reservoir
                  │
                  │ Suction line
                  │
        ┌─────────┴─────────┐
        │                   │
        ▼                   ▼
  [Fire shut-off       [Fire shut-off
   valve 2JG1]          valve 2JG2]
   (wing)               (wing)
        │                   │
        ▼                   ▼
   Green EDP 1         Green EDP 2
   (Engine 1)          (Engine 2)

The same architecture applies to Blue and Yellow (each system's fire shut-off valves sit upstream of its respective engine-driven pump), but the Green-specific HSMU automatic logic is what makes the Green pair unique. Blue and Yellow fire shut-off valves close only on ENG FIRE pushbutton press; Green fire shut-off valves close on the pushbutton press or on the HSMU's automatic command on Green reservoir low level.

Engineering specifications

Per AMM 29-11:

The 1.8-second travel time is the worst-case figure. Combined with the rapid ENG FIRE pushbutton command path, a fire shut-off valve responds within roughly 2 seconds of the crew action — fast enough to limit fire feeding without being so fast that pressure transients become an issue downstream.

4. Closure triggers

The pushbutton trigger applies to all three systems' fire shut-off valves. The HSMU automatic command applies only to the Green pair, for the architectural reason that Green is the only system with multi-pump and multi-source recovery paths that justify the additional decision logic.

The crew cannot reopen in flight

Once closed by either trigger, the fire shut-off valves cannot be reopened in flight by the crew. This is a deliberate safety choice: reopening risks re-establishing fluid feed to a fire or to a still-leaking path, and the in-flight environment does not provide the diagnostic visibility to assess whether the underlying condition is cleared.

On the ground, with maintenance access, the valves can be reset and reopened for the next dispatch.

The HSMU may reopen Green automatically — under conditions

Per the documented logic:

Green reservoir LO LEVEL → HSMU closes both Green fire shut-off valves
                          │
                          ▼
                  Wait 150 seconds
                          │
                          ▼
         Blue and Yellow reservoirs normal?
              │              │
             YES             NO
              │              │
              ▼              ▼
         Depressurise   Keep valves closed
         Green EDPs +   (Green recoverable only
         REOPEN valves  via RAT path, which is
         (lubricate     independent of these
          pumps)        fire shut-off valves)

The 150-second reopen is pump preservation, not recovery. The Green EDPs are still depressurised when the valves reopen; the fluid contact serves only to keep the pump internals from running dry. The system pressure is not restored by this action.

This sequence appears in real time on the SD HYD page as an apparent "valves close, then valves reopen" cycle. The crew should recognise it as normal automatic behaviour, not a second failure.

5. The valve and actuator separation

A useful maintenance detail: the fire shut-off valve and its actuator are mechanically separable. The valve body is plumbed into the suction line; the actuator (motor + position indicator) bolts onto the valve body externally. A failed actuator can be replaced without draining the suction line — the valve stays in place and continues to seal the line during the swap.

This is part of why the architecture documents specific actuator failure behaviours separately from valve failure behaviours. An actuator failure may produce an ECAM symbol of valve position inconsistency (commanded versus actual), while the valve itself remains in its last commanded position. The actuator is replaceable as a unit; the valve replacement is a larger task that does require line draining.

6. Indications and crew interaction

On the SD HYD page, the fire shut-off valve indication for each EDP shows:

The crew interacts with these only via:

• The ENG FIRE pushbutton (during a fire procedure).
• Monitoring the SD HYD page during a Green RSVR LO LVL event (observing the automatic close and possible 150-second reopen).

No direct crew control exists in flight to operate the fire shut-off valves outside of an ENG FIRE procedure. The HSMU manages the automatic Green logic without further crew input.

7. Priority valve operating characteristics

The priority valve carries a documented maximum flow rating of 250 L/min. This figure deserves explanation because it shapes what the valve can and cannot handle without itself becoming a limit:

The 250 L/min flow rating means the priority valve can pass essentially the entire secondary-circuit demand during normal operation. Under heavy simultaneous demand (e.g., a high-rate gear cycle combined with sharp nose wheel steering), the valve approaches its flow ceiling but does not become a bottleneck.

When the priority valve closes (pressure drops below 127 bar), the consumers downstream of the valve are isolated entirely. The protected primary consumers (flight controls, normal brakes, slats/flaps) can then receive the available flow without competing with the heavy users.

8. The Pressure-Off Brake — pressure-loss locking

Per FCOM DSC-29-10-30:

A Pressure-Off Brake system (installed on the flaps, slats, and THS actuator) ensures the same function.

The Pressure-Off Brake (POB) is a mechanical brake integrated into specific actuators that engages automatically when hydraulic pressure to that actuator drops below a threshold. Its purpose is to lock the surface (slat, flap, or THS) in its current position rather than allow it to backdrive when hydraulic supply is lost.

Where each POB applies:

The architectural reasoning: a surface that locks in place is safer than one that backdrives. An unconstrained flap or slat during transit could shift unpredictably and produce asymmetric or hard-to-control configurations. An unconstrained THS could trim unexpectedly. The POB ensures these surfaces remain in their last-commanded position regardless of system state.

The POB is parallel to the priority valve, not redundant with it. The priority valve handles heavy-flow consumers (gear, NWS, CSM/G); the POB handles position-preservation consumers (slats, flaps, THS). Both mechanisms operate automatically and require no crew action.

9. Fire shut-off valve — three trigger scenarios

The fire shut-off valves close in three documented scenarios:

The Green automatic HSMU closure (second trigger) is the most architecturally distinctive — Blue and Yellow do not have an equivalent automatic closure on reservoir low level. The reason is Green's heavier consumer load and the architectural pump-preservation logic (150-second reopen after evaluating Blue + Yellow state).

For Blue and Yellow, the fire shut-off valves close only on ENG FIRE pushbutton press. There is no automatic reservoir-low-level closure for these systems — their abnormal procedures call for manual pump shutdown rather than valve closure.

10. Crew indications during priority valve action

The priority valve operates silently. There is no dedicated ECAM indication showing the valve's state — closed or open. The crew detects the valve's action only through its consequences:

The crew interprets these consequences as evidence that the priority valve has shed the heavy consumers. There is no need (and no mechanism) to directly read the valve state — the procedural cues are sufficient.

11. Crew indications after ENG FIRE pushbutton press

When the crew presses ENG FIRE on either engine, the sequence visible on the SD HYD page:

[ENG FIRE pushbutton press]
        │
        ▼ Up to 1.8 seconds for valve travel
[Fire shut-off valve indicator changes:
 In-line green → in-line green (partial) → cross-line amber]
        │
        ▼ Within seconds
[EDP downstream of closed valve loses suction (boost impeller buffers briefly)]
        │
        ▼
[EDP indicator changes: green → `LO` amber → cross-line amber if crew commands pump OFF]
        │
        ▼
[System pressure (numeric value) may briefly dip then stabilise on the remaining pump
 (if the affected system has another pump source) or drop further to SYS LO PR
 threshold if no other pump available]

The crew's mental model: ENG FIRE pushbutton press triggers a cascade in the affected system that, depending on architecture (single or dual EDP), either:

• Green case: System pressure remains normal because the other engine's Green EDP continues running. The closed FSOV affects only the engine of the pressed button.
• Blue case: The single Blue EDP is on Engine 1. Pressing ENG 1 FIRE closes the Blue FSOV; Blue depressurises (unless Blue ELEC PUMP is selected ON).
• Yellow case: The single Yellow EDP is on Engine 2. Pressing ENG 2 FIRE closes the Yellow FSOV; Yellow depressurises (unless Yellow ELEC PUMP is selected ON).

12. Maintenance tasks

Priority valve and fire shut-off valve maintenance tasks:

The actuator can be replaced separately from the valve body for the fire shut-off valve, allowing actuator-only fault isolation without disturbing the hydraulic line. This is the maintenance benefit of the separable valve-and-actuator design.

13. How priority valve and fire shut-off valves interact

The two valve assemblies serve different purposes and rarely overlap, but one scenario brings them together:

Dual engine failure → RAT extends → Green on 2500 psi. The priority valve has shed nose wheel steering, normal gear, and CSM/G. If a Green reservoir low level also occurs (perhaps from a leak that triggered the dual-engine condition), the HSMU closes both Green fire shut-off valves automatically.

At this point:

• The RAT is supplying Green at 2500 psi, but Green is rapidly losing fluid through the leak.
• The fire shut-off valves close to limit further fluid loss.
• 150 seconds later: if Blue and Yellow are also showing reservoir issues, the valves stay closed.

The combined scenario is rare but covered architecturally. The two valve systems operate independently and in parallel; neither blocks the other.

Self-test

[!note]- Q1. The priority valve closes at 127 bar and reopens at 140-145 bar. Why is the reopen value higher than the close value?

Standard hysteresis design. If the valve closed and reopened at the same pressure, it would oscillate at the boundary — closing, fluid stops flowing to secondary consumers (relieving demand), pressure rises, valve reopens, fluid resumes flowing, pressure drops, valve closes again. The cycle would repeat at high frequency. The 18 bar hysteresis (145 − 127) ensures that pressure must recover meaningfully before the secondary circuit is restored, providing a clear "shed" or "supplied" state. The threshold is chosen well below 196 bar (full-flow EDP output) so normal operation does not approach it; secondary shedding only happens during genuine pressure loss.

[!note]- Q2. The priority valve is "pilot-operated." Does this mean the pilot controls it from the cockpit?

No. In valve terminology, "pilot-operated" means the valve is controlled by hydraulic pressure, specifically by the system pressure it is also protecting. The "pilot" in this context is the hydraulic pilot signal — a small fluid pressure path that drives the main valve element. There is no electrical signal and no cockpit control. The valve operates passively: when system pressure is high, the pilot signal holds the valve open; when system pressure drops below 127 bar, the spring closes the valve. The naming is potentially confusing but is standard mechanical-engineering terminology.

[!note]- Q3. The Green fire shut-off valves close in 1.8 seconds (worst case) after the ENG FIRE pushbutton press. Does the EDP suffer damage during this 1.8 seconds while still drawing from the (about-to-be-closed) suction line?

No. The EDP is running normally with fluid supply during the 1.8 seconds. After the valve closes, the EDP loses suction. The boost impeller inside the EDP retains internal fluid contact for approximately 15 minutes (the documented run time after fire shut-off valve closure). The 1.8-second figure refers to valve travel time only; the 15-minute figure refers to how long the pump can run after the valve closes. The two numbers cover different phases of the timeline.

[!note]- Q4. After a Green reservoir low-level event, the HSMU closes the Green fire shut-off valves. 150 seconds later, with Blue and Yellow showing normal reservoirs, the valves automatically reopen. Is the Green system pressurised again?

No. The valves reopen, but the EDPs are depressurised in the same automatic action. The reopen is for pump lubrication — letting residual fluid touch the EDP internals to prevent dry-running damage — not for system pressure restoration. The pumps are commanded off; the system is not pressurised by EDPs after this sequence. If Green is needed during this state, the recovery path is the RAT (which has its own fluid path independent of the EDPs' suction lines).

[!note]- Q5. The priority valve sheds nose wheel steering, normal gear, and CSM/G when pressure drops. Flight controls and normal brakes are preserved. What is the architectural reasoning for this specific division?

The classification matches each consumer's flow demand profile and operational substitutability. Heavy-flow, brief-use consumers (gear cycle, NWS during taxi turns, CSM/G start-up) can be shed because alternatives exist: gravity gear extension, differential braking for ground steering, and CSM/G off-line being a degraded electrical configuration the aircraft can accept. Light-flow, continuous-need consumers (flight controls, brakes) cannot be shed because no equivalent alternative exists in flight. The priority valve enforces this hierarchy automatically. The architecture's design philosophy: preserve what cannot be substituted; shed what can be.

References

Per FCOM DSC-29-10-30 (Priority Function, Fire Shut-Off Valves); AMM 29-11 (priority valve 5121JM, fire shut-off valves 2JG1/2JG2 — ball valve type, travel time, separable actuator, ECAM SD HYD page indications); AMM 29-10-20 (HSMU automatic logic for Green fire shut-off valve closure on RSVR LO LVL with 150-second reopen behaviour).

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