Single-System Loss

Loss of a single hydraulic system is the most common hydraulic abnormal scenario. The aircraft retains two of three systems, all flight-control surfaces are still supplied (each by more than one system), and the abnormal procedure manages the consequences specific to the failed system.

This article walks through each single-system loss case (Green, Blue, Yellow), the cascade of indications that announce it, the procedural response, and the degraded profile the aircraft adopts for the rest of the flight.

Note on system attribution. This article uses the consumer-to-system mapping documented in the authoritative AMM 29-00 table (the "Services supplied by the hydraulic systems" list), summarised in Power Distribution Map. Verify against the operator FCOM if in doubt.

1. The architectural premise

Every primary flight-control surface has actuators on at least two of the three systems. A single-system loss therefore never disables a complete control axis; it reduces the number of actuators on each affected surface. The flight-control law assesses the surviving combinations and may transition to Alternate Law if the loss combined with other conditions crosses the law-degradation threshold (an ATA 27 question).

For the hydraulic perspective, what matters is what each system feeds uniquely, and what the aircraft loses when that system is gone. Per AMM 29-00:

• Green system uniquely supplies: Normal brakes, landing gear, nose wheel steering (normal), CSM/G drive, one slat motor channel, one flap motor channel, slat and flap wing-tip brakes (Green side).
• Blue system uniquely supplies: Alternate brakes, Engine 1 reverser, one slat motor channel, slat wing-tip brake (Blue side), electrical rudder authority, and (via the brake accumulator) parking-brake hold.
• Yellow system uniquely supplies: Engine 2 reverser, cargo doors (ground operation), one flap motor channel, flap wing-tip brake (Yellow side).

The detailed map is in Power Distribution Map. The losses below follow directly from that map.

2. FCTM key statements and universal handling rules

Before walking through each system's loss case, four FCTM-level statements apply to every single-system loss and shape the crew's approach. They are stated at the head of the article because they cut across all three system-loss scenarios.

Single-system loss is manageable — but landing capability degrades

The headline FCTM PR-AEP-HYD statement for any single hydraulic failure:

• Handling effect is "very little". Flight controls retain authority on all primary surfaces. The autopilot, flight directors, and autothrust all remain available. The flight-control law typically stays in Normal Law (Blue loss) or degrades only mildly depending on the specific surface count remaining.
• Landing capability degrades to CAT 3 SINGLE. From the full CAT 3 DUAL (fail-passive auto-land), the architecture drops to CAT 3 SINGLE (single-AP auto-land with human monitoring required). Minimum DH and visibility increase; low-visibility destinations may become unreachable.

The framing matters: single-system loss is not an emergency. The aircraft can fly the planned route, the AP can hold the cruise, and the abnormal procedure is procedural rather than urgent. The dispatch decision afterwards is governed by MEL (MEL — Dispatch and Operational Limits) and operator considerations. Dual loss — covered in Dual-System Loss — is the qualitative step into emergency territory.

ELEC PUMP — do not manually select ON

A universal FCTM rule: do not manually select a HYD ELEC PUMP ON, except in one narrow case — temporarily, to retract the spoilers if they remain out after a hydraulic failure.

The reasoning is the flow-capacity mismatch:

• Electric-pump flow is ~18% of EDP flow (32 L/min vs 175 L/min). It cannot sustain steady-state flight-control demand.
• The electric pump cannot match the EDP's response to sharp demand peaks. Running it as a substitute under demand can produce flight-control surface "jerk" — small in single loss, more pronounced in dual loss.
• Sustained operation against high demand accelerates wear and may trigger the 193 °C (or 200 °C) overheat protection — converting a manageable backup into a latched fault that ground reset cannot recover in flight.

The single permitted use case is the spoiler retraction: after a hydraulic failure leaves the spoilers in the deployed position with no automatic retraction (the supplying system has failed), a brief manual ELEC PUMP ON provides enough flow to drive them back. Once retracted, OFF immediately.

Outside this case, the architectural expectation is: leave the electric pump in AUTO and let the HSMU's automatic triggers decide.

RAT — do not use in Green hydraulic overheat

A specific FCTM caution: the RAT must not be used in case of Green hydraulic overheat. The reasoning is direct — the RAT draws fluid from the same Green reservoir that is reporting overheat. Forcing the RAT into service would circulate already-elevated-temperature fluid through the RAT pump itself, accelerating thermal damage to a component the architecture relies on as a last resort.

The procedural scope is narrow (RAT supply on Green is a dual-engine-out scenario, and a concurrent Green OVHT during dual-engine-out is rare), but the rule is unambiguous.

RAT — anti-stall limit at 125 kt; anticipation required

At very low airspeed, the RAT turbine stalls despite the anti-stall valve protection. The documented anti-stall limit is 125 kt — below this airspeed, the turbine cannot sustain itself against the pump load.

The operational implication is "anticipation required": the crew must plan an approach speed schedule that keeps the airspeed above 125 kt through the critical phases. The risk is reduced rudder authority on a crosswind landing if airspeed dips toward the threshold during the flare. Approach planning under any RAT-supplied scenario builds an explicit airspeed margin above 125 kt.

The 125 kt threshold is engineering-specific, not a regulatory minimum. It marks the point below which the architecture's last-resort hydraulic source cannot be assumed available — and flight-control authority on the RAT-supplied side degrades with the source.

3. Green System Loss

Trigger sequence

Procedural intent

The Green abnormal procedure has three goals:

• Stop fluid loss. If a leak is the cause, the HSMU has already closed the fire shut-off valves on a reservoir low level. The crew may need to manually select the pump pushbuttons OFF.
• Transition consumers. Switch from normal brakes (Green) to alternate brakes (Blue). Plan for gravity gear extension. Accept the loss of CSM/G drive (the emergency generator's hydraulic-driven path).
• Configure for landing. Approach speed may need adjustment; available flap detents may be restricted; brake-channel selection is alternate brakes only.

Degraded profile

After Green loss with engines running:

Pilot's mental script

"Green lost. Brake to alternate (Blue). Plan gravity gear. NWS off — differential braking. CSM/G off. Reverser symmetric (both Eng 1 + 2 still available)."

4. Blue System Loss

Trigger sequence

Procedural intent

• Switch off both Blue pumps. No HSMU automatic action; the crew action is the primary mitigation.
• Accept consequences. Alternate brakes lost (Green normal brakes still available). Engine 1 reverser lost (Engine 2 still available — asymmetric reverser). Rudder authority reduced (electrical rudder lost); one slat motor channel is down. The brake accumulator charge path is lost — parking brake will decay over extended ground time.
• Flight-control law usually remains in Normal Law unless combined with other conditions.

Degraded profile

After Blue loss with engines running:

The crew accepts the configuration and continues. Slat extension takes longer (one motor instead of two); rudder authority for sideslip is reduced but generally manageable; reverser usage on landing is asymmetric.

The LAND ASAP consideration

A Blue reservoir low level alone does not trigger LAND ASAP. The advisory triggers only if Blue is lost and Green is supplied by the RAT — the combined condition that significantly reduces remaining capability.

Pilot's mental script

"Blue lost. Pumps off. Alternate brakes lost — Green normal only. Reverser asymmetric (Eng 1 lost). Reduced rudder. Slats one motor. Parking brake will deplete after shutdown."

5. Yellow System Loss

Trigger sequence

Procedural intent

Yellow loss removes Engine 2 reverser, cargo doors (ground operation), and one flap motor channel. With Green still healthy, the aircraft retains normal brakes for landing. With Blue still healthy, alternate brakes remain available, and the parking brake (via Blue brake accumulator) is unaffected.

• Switch off both Yellow pumps.
• Transition consumers. Brakes are normal (Green) with alternate (Blue) backup still available. Accept loss of Engine 2 reverser → asymmetric reverser.
• Accept asymmetric reverser. Engine 2 reverser is lost; reverser usage on landing is asymmetric.

Degraded profile

After Yellow loss with engines running:

The LAND ASAP consideration

Same as Blue: LAND ASAP only if Yellow is lost AND Green is supplied by the RAT.

Pilot's mental script

"Yellow lost. Pumps off. Normal and alternate brakes both available. Reverser asymmetric (Eng 2 lost). Flaps one motor."

6. The procedural decision tree

For any single-system loss, the procedural decision sequence:

Single-system loss detected
        │
        ▼
What system?
        │
   ┌────┼────┐
   ▼    ▼    ▼
 Green Blue Yellow
   │    │    │
   ▼    ▼    ▼
Per ECAM:                Per ECAM:                 Per ECAM:
 • Pumps OFF (if not     • Both pumps OFF          • Both pumps OFF
   automatic)            • Accept rudder           • Accept reverser
 • Brake to alternate      reduction                 asymmetric (Eng 2)
   (Blue)                • Accept slat motor       • Accept flap motor
 • Plan gravity gear       reduction                 reduction
 • CSM/G off             • Accept alternate        • Cargo doors via
 • NWS off → diff brake    brakes lost               hand pump (gnd)
 • Reverser symmetric    • Accept reverser
                           asymmetric (Eng 1)
        │                    │                          │
        └────────────┬───────┴──────────────────┬──────┘
                     │                           │
                     ▼                           ▼
            Approach planning            Approach planning
            (per QRH/STATUS)             (per QRH/STATUS)

The QRH or the STATUS page after the ECAM procedure provides the specific approach speed adjustments, flap restriction, and any other items unique to the loss.

7. The combined "Green + something" pattern

A single-system loss becomes more serious when combined with another hydraulic condition:

The dual-system cases are covered in Dual-System Loss.

8. The case of Green with engines still running

A subtle case: Green can be lost (reservoir level low, both pumps shut down) while both engines continue running normally. The engines drive their EDPs, but if both Green EDPs are commanded off (by the abnormal procedure or by automatic depressurisation), Green has no pump output.

This differs from the dual-engine-failure case, where Green is supplied by the RAT (independent of EDPs). In the engines-running case, Green stays at 0 psi for the remainder of the flight (no RAT deployment because the engines are not failed); the aircraft lands without Green, using alternate brakes (Blue) and gravity gear.

The pilot's awareness: a Green loss is operationally a Green loss whether or not the engines are running. The difference is whether the RAT is deployed and whether dual-system constraints apply.

Self-test

[!note]- Q1. Green reservoir low level triggers in cruise. The HSMU closes the fire shut-off valves. The crew completes the ECAM. After landing, the brakes feel different from normal. Which brake circuit is now active?

The alternate brake circuit, supplied by Blue, is now the primary brake source. With Green lost, normal brakes (Green-supplied) are unavailable, and the architecture transitions to alternate brakes. The alternate brake circuit has different characteristics from Green normal brakes — pressure response, modulation, and anti-skid behaviour all match the Blue-supplied architecture. The difference is documented but is not unsafe; landing distance per the QRH or STATUS-page guidance accounts for the alternate-brake characteristics.

[!note]- Q2. The ECAM shows HYD B RSVR LO LVL. After both Blue pumps OFF, the crew sees no LAND ASAP advisory. The Captain considers continuing to the destination. Is this acceptable?

Yes — Blue loss alone is operationally manageable, though it removes several functions: alternate brakes (Green normal brakes remain), Engine 1 reverser (Engine 2 reverser remains — landing with asymmetric reverser is acceptable), electrical rudder authority (conventional rudder paths still operate), and one slat motor channel (Yellow channel takes over, slats extend more slowly). The parking brake will deplete over time after shutdown because the Blue brake accumulator is no longer being recharged. Flight controls retain authority on Green and Yellow; rudder is reduced but generally manageable. The Captain's decision considers the broader picture (weather, fuel, alternates), but the absence of LAND ASAP is the architecture confirming that the single Blue loss does not require immediate diversion.

[!note]- Q3. After Green loss, the crew has switched to alternate brakes. On landing, alternate brakes feel less aggressive than expected. Why?

The alternate brake circuit is on Blue, and its braking characteristics are slightly different from Green normal brakes — pressure response, modulation, anti-skid behaviour all match the Blue architecture rather than Green. The difference is noticeable but not unsafe; landing distance per the QRH or STATUS-page guidance accounts for the alternate-brake characteristics. The crew applies steady pressure rather than the modulation patterns from normal brakes, and relies on the spoilers and any available reverser to assist deceleration. The slightly different feel is expected behaviour for alternate-brake operation.

[!note]- Q4. Blue loss occurs in flight with Green and Yellow both healthy. The crew completes the procedure, accepts the loss, and continues. After landing and several hours on the ramp, the crew finds the parking brake will not hold pressure. Why?

The parking brake is supplied by the Blue brake accumulator. With Blue lost (both pumps off, system depressurised), the brake accumulator has no recharge source. Whatever pressure was in the accumulator at the time of Blue's failure held the parking brake initially, but with no pump replenishment the pressure gradually depletes over time. After several hours, the accumulator is exhausted and no pressure remains for the parking brake. The aircraft requires alternative restraint — typically chocks placed under the wheels by ground personnel before the parking brake's lack-of-pressure becomes a concern. The crew's procedural responsibility is to communicate the Blue loss to ground personnel so chocks are placed promptly after arrival.

[!note]- Q5. The architecture allows a single-system loss to occur "with engines running" or "with engines failed." How does the procedural handling differ between these two cases?

With engines running, a single-system loss (e.g., Green leak depleting the reservoir) is managed by the per-system abnormal procedure, with the failed system at 0 psi for the remainder of the flight. The other two systems carry the aircraft. With engines failed (dual engine failure), Green becomes RAT-supplied at 2500 psi — not the same as "Green at 0 psi" — and Green still provides flight controls (at reduced surface speeds) and some limited functions. The architectural difference: engines running + single system loss = system at 0 psi, no RAT involvement. Engines failed + Green = RAT-supplied at 2500 psi, RAT involvement central. The procedural responses are different in each case, even though both involve "Green not at normal."

References

Per FCOM DSC-29-10-20 (system-pump descriptions); FCOM PRO-ABN-HYD (procedural responses for reservoir LO LVL on each system); FCTM PR-AEP-HYD (remaining-systems tables for each single-system loss); AMM 29-00 §3.A (5) "Services supplied by the hydraulic systems" — the authoritative consumer-to-system mapping; Power Distribution Map (consumer mapping for what each system uniquely feeds); ECAM HYD Page Reading (the indication pattern for each loss).

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