Dual-System Loss

Loss of two hydraulic systems simultaneously is a serious abnormal that materially degrades the aircraft. With only one system remaining, the flight-control law definitely degrades, the brake source narrows, gear extension requires gravity, and the abnormal procedures call for LAND ASAP in most combinations.

This article covers each two-system combination: what is lost, what remains, and the operational profile 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 summarised in Power Distribution Map. Key reminders: Alternate brakes = Blue; Engine 1 reverser = Blue; Engine 2 reverser = Yellow; Normal brakes = Green; Brake accumulator = Blue (charges parking brake).

1. The combinations

Three combinations of two-system loss:

2. Green + Blue Loss

Trigger sequence

What is retained

• Yellow flight controls (ailerons, elevator, spoilers, Yellow rudder paths)
• Yellow flap motor channel
• Engine 2 reverser (Yellow-supplied)
• Cargo doors (Yellow)
• Green at 2500 psi via RAT → flight controls on Green at reduced surface speeds, limited brake authority (1000 psi)

What is lost

• Normal brakes (Green)
• Alternate brakes (Blue) — both brake paths lost simultaneously
• Engine 1 reverser (Blue)
• Brake accumulator charge (Blue) → parking-brake hold depletes
• CSM/G drive (Green)
• Landing gear normal extension (Green) → use gravity
• Electrical rudder authority (Blue)
• Both slat motors lost (Green + Blue slat motors are the only two; both are now down) — per FCTM T1, only FLAPS slow only is available on this combination
• One flap motor channel (Green) — flaps on Yellow channel only, extending slowly
• Nose wheel steering normal (Green)

This combination is the most braking-limited dual loss. With both Green normal and Blue alternate brakes lost, the only brake source is the Blue brake accumulator residual pressure — finite, depleting with each application.

Profile

Pilot's mental script

"G and B lost, RAT out. Yellow + Green-on-RAT only. Brake accumulator only — count applications. Gravity gear. Eng 2 reverser only. Land ASAP."

3. Green + Yellow Loss

Trigger sequence

What is retained

• Blue flight controls (electrical rudder, one slat motor, Blue actuators on each surface)
• Blue at 3000 psi via EDP (Engine 1)
• Alternate brakes (Blue) — primary brake source for this configuration
• Engine 1 reverser (Blue)
• Brake accumulator (Blue) → parking brake retained
• Green at 2500 psi via RAT (limited authority surfaces, restricted braking)

What is lost

• Normal brakes (Green) — switch to alternate (Blue)
• Engine 2 reverser (Yellow) — asymmetric reverser
• Cargo doors (Yellow) → hand pump required on ground
• Both flap motors lost (Green + Yellow flap motors are the only two; both are now down — Green's normal supply is gone, RAT cannot sustain flap motor flow) — per FCTM T1, only SLATS slow only is available on this combination
• CSM/G drive (Green)
• Landing gear normal extension (Green) → use gravity
• Nose wheel steering normal (Green)

Profile

This combination preserves alternate brakes — the architecture's deepest brake redundancy is on Blue, so this dual-loss case is less braking-critical than G+B.

Pilot's mental script

"G and Y lost, RAT out. Blue only normal. Alternate brakes (Blue). Eng 1 reverser only. Gravity gear. NWS off. Land ASAP."

4. Blue + Yellow Loss

Trigger sequence

What is retained

• Green flight controls (full Green authority on each surface)
• Green at 3000 psi via both EDPs (engines running)
• Normal brakes (Green)
• Normal gear extension and nose wheel steering (Green) — but procedure calls for gravity gear to preserve Green for flight controls (see FCTM table)
• CSM/G drive

What is lost

• Blue: Alternate brakes, Engine 1 reverser, electrical rudder, one slat motor channel, brake accumulator charge
• Yellow: Engine 2 reverser, cargo doors, one flap motor channel
• Both reversers lost — no reverse thrust on landing
• Slats and flaps operate at half motor count each
• Parking brake: limited (Blue brake accumulator no longer charged)

Profile

This combination is the no-reverser case: Engine 1 reverser (Blue-supplied) and Engine 2 reverser (Yellow-supplied) are both unavailable. Landing relies on normal brakes (Green) and spoilers only for deceleration. Stopping distance is materially increased; runway selection accounts for this.

Pilot's mental script

"B and Y lost. Green only. Normal brakes only. No reversers — brakes and spoilers only. Gravity gear (preserve Green). Slats and flaps slow. Land ASAP."

5. The RAT-extension behaviour by combination

The asymmetry reflects that Green is the system the RAT supplies. RAT extension is triggered by conditions that compromise Green's normal supply (dual engine failure, or dual reservoir losses involving Green). Blue + Yellow loss does not affect Green's supply path, so the RAT stays stowed.

6. The flight-control law in dual-system loss

The law transitions are governed by ATA 27 logic, but in summary:

The actual transition depends on the combined fault set, not just on which hydraulic systems are lost. Other contributing conditions (FCU faults, ADIRU faults, IRS issues) may push the law further down even with the same hydraulic loss. The pilot consults the law indication on PFD (which displays the current law) rather than deducing it from the hydraulic configuration alone.

7. The procedural decision sequence

For any dual-system loss:

1. Identify both losses. Cross-reference ECAM cautions and SD HYD page indications. Confirm both systems are at 0 psi (or, for Green-involved cases, RAT-supplied).
2. Verify the law. Check the PFD law indication. Manage flying within the law's limits.
3. Plan for LAND ASAP. Identify the nearest suitable runway. Brief the cabin and ATC.
4. Configure for landing. Approach speed, flap detent, brake source, gear extension method, reverser availability all follow from the specific dual-loss case.
5. Execute the approach. Manual flying may be required if autopilot is unavailable. The remaining system carries the workload.

The procedural detail is in the FCOM/FCTM. This article provides the architectural framework for recognising the situation and anticipating what the procedure will require.

8. The handling framework — FCTM guidance for dual-loss flight

FCTM PR-AEP-HYD includes specific guidance for crew handling during dual-system loss. Four points cut across all three combinations (G+B, G+Y, B+Y) and shape how the flight is flown from ECAM completion through to touchdown.

8.1 AP is lost — FD and A/THR remain

A specific consequence of dual-system loss is loss of the autopilot. The FCTM lists this explicitly alongside flight-control law degradation, abnormal-configuration landing, and the extensive ECAM workload. But the flight directors and autothrust remain available in nearly all dual-loss configurations.

The operational implication:

• The approach is hand-flown from ECAM completion to touchdown. The PF cannot re-engage AP at any point; attempts to do so will not succeed.
• FDs continue to provide steering bars for the active flight modes (LOC/G/S, heading, altitude). The PF uses them as a visual aid for the manual inputs.
• A/THR continues to manage thrust to the selected speed. This is significant — the PF is freed from continuous throttle adjustment, which would otherwise compete with manual flight-control inputs in a degraded configuration.

The combined "hand-fly with FD steering, A/THR for speed" arrangement is the standard configuration for the entire remainder of the flight. There is no path back to AP control.

8.2 PF must maneuver with care

A second FCTM principle: the PF must maneuver with care to avoid high hydraulic demand on the remaining system. With only one or two pump-driven systems available (the others lost or, for Green, RAT-supplied at reduced flow), demand peaks must be smoothed:

• No sharp control inputs. Aileron, elevator, and rudder inputs are progressive rather than abrupt. Peak demand from sudden surface deflections may exceed what the remaining system can deliver, particularly under RAT supply where flow is airspeed-limited.
• No simultaneous large configuration changes. Gear extension (by gravity) and flap extension are sequenced rather than overlapped. The remaining system handles one heavy demand at a time.
• No autopilot — by definition (see §8.1).

The "maneuver with care" principle is the pilot-side complement to the architectural decision to use gravity gear extension in the B+Y case (covered in Gravity Gear Extension) — both reduce the load on the remaining system at critical phases.

8.3 Complex procedure — QRH summary mandatory

FCTM categorises dual-system loss as a complex procedure. After the ECAM procedure is complete (which may run several minutes given the cascade of cautions and inserted sub-procedures), the FCTM mandates a follow-up step: consult the QRH summary (referenced as AOP-30-60 Use of Summaries).

Two reasons:

• ECAM completeness check. Long ECAM procedures may include steps that the crew read as "noted" rather than "actioned" under workload pressure. The QRH summary lists the must-be-done items concisely; reviewing it confirms nothing was missed during the live procedure.
• Approach-configuration framing. The QRH summary lists the approach configuration items (flap detent, brake source, gear extension method, approach speed, go-around configuration) in a single condensed reference. This becomes the briefing structure (see §8.4).

A crew that completes the ECAM but skips the QRH summary may arrive at the approach without having consciously integrated all the configuration constraints — landing under sub-optimal settings. The summary discipline is what prevents this.

8.4 The 6-item approach briefing

The FCTM-prescribed briefing for a dual-loss approach concentrates on six safety-related items, because the approach is hand-flown and full of non-standard operating restrictions:

The briefing structure exists because the dual-loss approach has no default profile. Every parameter is altered from normal; the briefing constructs the specific profile for this specific dual-loss scenario.

The briefing is conducted by the PF, with PM reading from the QRH summary and confirming each item. It happens after ECAM completion and before approach configuration — typically with the aircraft at a comfortable holding altitude or in a long final-approach segment, giving the crew time to integrate the procedural items without time pressure overlapping with manual flying demands.

9. The pilot's situational awareness during dual-system loss

Two cognitive habits matter:

• Know which system is left. The remaining system's identity drives every operational decision. The crew should be able to state the remaining system without referring to the SD HYD page after a dual-system event.
• Know what the remaining system feeds uniquely. This is the Power Distribution Map reference. Quick recall of what each system uniquely supplies lets the crew rapidly assess what the aircraft has left:
  • Green alone (B+Y lost): normal brakes, gear (but procedure uses gravity to preserve Green), CSM/G, NWS — but no reversers, no alternate brakes.
  • Blue alone (G+Y lost): alternate brakes, Engine 1 reverser, parking brake (accumulator), electrical rudder.
  • Yellow alone (G+B lost): Engine 2 reverser, cargo doors, one flap motor — but no brakes other than the depleting Blue accumulator residual.

The dual-system loss is rare but architecturally addressed. The procedural responses are tested in simulator training; the architectural understanding is what makes the procedures intuitive rather than mechanical.

Self-test

[!note]- Q1. Green + Blue reservoir losses are reported simultaneously. The RAT extends automatically. The crew now has Yellow at 3000 psi via Engine 2 EDP and Green at 2500 psi via RAT. What is the braking situation?

Both normal brakes (Green) and alternate brakes (Blue) are lost. The Blue brake accumulator no longer recharges (Blue lost), so braking is limited to whatever pressure remains in the accumulator. The ECAM displays BRK B ACCU PR ONLY. The crew uses discrete, deliberate brake applications rather than continuous pressure, accepting that each application depletes the accumulator further. Yellow is healthy and provides Engine 2 reverser; Green-on-RAT provides flight controls at reduced speeds. The crew plans the landing for the longest available runway and uses reverse thrust on Engine 2 (the only available reverser) plus spoilers to bear as much of the deceleration as possible.

[!note]- Q2. In a Green + Yellow loss with RAT extension, the crew sees Blue at 3000 psi. What brake source do they have, and how does it compare with the Green+Blue case?

The crew has Blue alternate brakes as the primary brake source — Blue is healthy and pumping, so alternate brakes (which are Blue-supplied) operate at full authority. This contrasts with Green+Blue loss, where both normal and alternate paths are gone and only accumulator residual remains. The G+Y combination is therefore less braking-limited than G+B, even though both are critical dual-loss situations. The crew uses Blue alternate brakes as they would for any Green loss, and benefits from the Blue brake accumulator continuing to support the parking brake on the ground.

[!note]- Q3. A Blue + Yellow simultaneous loss occurs. The crew expects the RAT to extend automatically — it does not. Why?

The RAT's automatic extension triggers are dual engine failure (N2 < 50% on both) or low reservoir level on Green + Blue or Green + Yellow. The combination Blue + Yellow is not in the automatic trigger list because Green's EDPs are still being driven by the engines, so Green has normal supply. The RAT's role is specifically Green recovery; Blue + Yellow loss does not affect Green's supply path. The crew has Green fully available and does not need the RAT.

[!note]- Q4. In a Blue + Yellow loss, both Engine 1 reverser (Blue) and Engine 2 reverser (Yellow) are lost. How does the crew compensate on the landing rollout?

The crew relies on normal brakes (Green) + spoilers for deceleration. With no reverse thrust available on either engine, the stopping distance increases — runway selection accounts for this in the diversion decision. Spoilers deploy on touchdown and provide significant aerodynamic braking; normal brakes (Green-supplied) provide wheel braking. The pilot uses firm, steady brake application rather than the modulated braking that reverse thrust would otherwise assist. Landing on a long runway, planned in advance, is the architectural response to the no-reverser configuration.

[!note]- Q5. A Blue + Yellow loss occurs with Green healthy. The procedure calls for gravity gear extension even though Green is normal. Why?

Per the FCTM Remaining Systems table footnote, gravity gear extension is used in this case to preserve Green system integrity for flight controls. With Blue and Yellow both lost, Green is the only remaining hydraulic system, and flight controls depend on Green pressure for surface actuation. Normal gear extension would consume significant Green flow at a critical moment (approach and landing), competing with flight controls. The architecture deliberately mandates gravity extension to dedicate Green to flight controls and braking, with gravity handling the gear. This is a procedural commitment, not a forced choice — Green could in principle extend the gear normally, but the architecture optimises for flight-control authority over normal-gear-extension convenience.

References

Per FCOM PRO-ABN-HYD (combined reservoir low-level procedures, RAT extension conditions); FCTM PR-AEP-HYD (remaining-systems analysis for dual-system combinations); AMM 29-00 §3.A (5) — authoritative consumer-to-system mapping; ATA 27 (flight-control law degradation logic); Power Distribution Map; Ram Air Turbine.

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