A320 captain in IMC - dual hydraulic failure, degraded authority, elevated workload
Quick Answer
The A320 Power Transfer Unit (PTU) transfers hydraulic power - not fluid - between the GREEN and YELLOW systems. It operates automatically when the differential pressure between GREEN and YELLOW exceeds 500 PSI. In a dual hydraulic failure where both GREEN and YELLOW are lost, there is no pressure source to create that differential, so the PTU cannot run. It will not save your flight controls. The remaining hydraulic system is BLUE - normally pressurized by the BLUE electric pump when AC power is available. If both AC BUS 1 and AC BUS 2 are lost, the RAT deploys and can pressurize BLUE in an emergency.
Bottom line: If GREEN and YELLOW are both gone, stop waiting for the PTU. Confirm what BLUE gives you on the F/CTL synoptic, complete your ECAM actions before the approach, and fly the airplane.
The Scenario
IMC. Heavy turbulence. The descent is rough, and the workload is already elevated when ECAM indicates a double hydraulic failure - HYD G+Y SYS LO PR - with red LAND ASAP displayed. Both GREEN and YELLOW systems are gone.
The PM starts the ECAM actions. The PF is flying with degraded authority and unfamiliar handling qualities. And somewhere in the back of both minds, a thought forms: the PTU will kick in. It transfers pressure between GREEN and YELLOW. It will recover something.
It will not.
The PTU Mirage
This is the trap Airbus crews fall into more than almost any other hydraulic scenario. The Power Transfer Unit is a brilliant piece of engineering - a bidirectional hydraulic motor/pump that can pressurize one system using the pressure from the other. It runs automatically, it requires no crew input, and in a single-system failure it is genuinely protective.
But it has one absolute requirement: one of the two systems - GREEN or YELLOW - must have pressure available.
The PTU runs automatically when the differential pressure between GREEN and YELLOW exceeds 500 PSI. That is the trigger. Not a threshold. Not an inhibition logic. A simple differential. If GREEN has 3,000 PSI and YELLOW has zero, the PTU activates and pressurizes YELLOW using GREEN's power. If YELLOW has pressure and GREEN does not, it works in reverse.
But in a dual failure of GREEN and YELLOW, there is no remaining source to create that differential. Zero minus zero is zero. The PTU will not run. It cannot run. There is nothing to transfer.
The word mirage is deliberate. Crews who have internalized "PTU = automatic backup" see protection where none exists. They delay their transition to degraded-mode thinking because they are subconsciously waiting for a recovery that is mechanically impossible.
The PTU couples GREEN and YELLOW hydraulic systems - but requires pressure in at least one system to operate
What the PTU Actually Does - And Does Not Do
Power Transfer, Not Fluid Transfer
The PTU transfers hydraulic power between GREEN and YELLOW only. It does not transfer hydraulic fluid. The two systems remain hydraulically isolated - separate reservoirs, separate fluid, separate return circuits. The PTU is a mechanical coupling: one side is a motor, the other a pump. Pressure in one system drives the motor, which drives the pump, which pressurizes the other system.
This distinction matters operationally. In certain failure cases - reservoir low level, reservoir overheat, reservoir low air pressure - ECAM will direct the crew to select PTU OFF. Specifically: HYD G(Y) RSVR LO LVL requires PTU OFF. HYD G(Y) RSVR LO AIR PR requires PTU OFF only if pressure fluctuates. HYD G(Y) RSVR OVHT requires PTU OFF. These are the ECAM-driven triggers. There is no generic "ground operations" case for PTU OFF.
The Only Automatic Inhibit
The published automatic inhibit of the PTU is during the first engine start - a noise-suppression function, not a safety logic. Beyond that, the PTU is always available to activate whenever the 500 PSI differential criterion is met. It does not "inhibit" itself when both systems are depressurized. The correct framing is simpler: with no pressure in either GREEN or YELLOW, there is no differential, and no energy source to drive the unit. It is not inhibited. It simply has nothing to work with.
What Remains: The BLUE System
With GREEN and YELLOW both lost, the remaining hydraulic system is BLUE. This is not a typically situation - for a G+Y dual failure, it is definitively BLUE. Other dual-failure combinations (G+B, Y+B) leave different remaining systems with different degradations. Do not generalize.
The BLUE system is normally pressurized by the BLUE electric pump, which requires AC electrical power. If AC BUS 1 and AC BUS 2 are both available, the electric pump operates and BLUE has full pressure. If both AC buses are lost, the Ram Air Turbine (RAT) deploys automatically and can pressurize BLUE in an emergency. The RAT is not a "mode choice" within the dual-hydraulic scenario - its deployment is tied to electrical bus loss conditions.
HYD G+Y SYS LO PR: What You Actually Have
For a GREEN + YELLOW dual failure in cruise, the remaining capability on BLUE is specific and limited:
Ailerons: AVAIL. Elevator: AVAIL. Spoilers: 1 per wing. Yaw damper: INOP. Autopilot: INOP. Control law: ALTERNATE, degrading to DIRECT LAW when the landing gear is extended.
That last point is the real trap within the trap. When you lower the gear, the flight control law changes from ALTERNATE to DIRECT. In DIRECT law, there is no autotrim. The sidestick-neutral reference shifts when the gear comes down. If the crew does not wait until stabilized at VAPP before extending the landing gear, the trim reference will be set at the wrong speed, and the flare and any go-around will require manual pitch compensation that the crew may not be expecting.
Confirm remaining surfaces on the F/CTL SD page. Cross-reference with QRH OPS DATA. Do not guess.
Priority Valves: The Silent Protector
When hydraulic pressure in a system gets low, priority valves automatically cut off hydraulic power to heavy load users. This preserves remaining pressure for essential flight controls at the expense of ground maneuvering capability - normal brakes, nose wheel steering, and landing gear extension may be lost while the flight control actuators retain pressure for as long as physically possible.
This is automatic. No crew action is required. But the crew needs to understand what the priority valves have done so they can anticipate what is and is not available during the approach and landing.
The Professional Response
Immediate Action: Maintain control first. The PF flies the airplane. Once stabilized, the PM carries out ECAM actions. Avoid non-ECAM troubleshooting. For a double hydraulic failure, ECAM actions should be completed before the approach.
Handling and Landing: With HYD G+Y SYS LO PR, handling and landing are degraded. The procedure requires waiting for stabilization at VAPP before landing gear extension to ensure the proper centered sidestick reference when the control law transitions from ALTERNATE to DIRECT. This is not optional - it is a required technique tied to law degradation and trim behavior that directly affects flare and go-around pitch control.
Declare MAYDAY. Request priority handling. Brief the approach and go-around considerations with the specific degradations confirmed on the synoptic pages.
Test Your Knowledge
1. In a dual hydraulic failure (GREEN + YELLOW lost), will the PTU restore any hydraulic capability?
No. The PTU requires at least one of the two systems (GREEN or YELLOW) to have pressure available. With both lost, there is no differential pressure, and the PTU cannot operate.
2. What is the PTU trigger criterion?
Differential pressure between GREEN and YELLOW exceeding 500 PSI. It is not an absolute pressure threshold - it is a differential.
3. Does the PTU transfer fluid between systems?
No. The PTU transfers hydraulic power only. The GREEN and YELLOW systems remain hydraulically isolated with separate reservoirs and fluid.
4. After a G+Y dual failure, what flight control law applies during approach with gear down?
DIRECT law. The aircraft transitions from ALTERNATE law to DIRECT law when the landing gear is extended. This means no autotrim, manual pitch compensation required, and the sidestick neutral reference set at the moment of gear extension.
5. When should the crew select PTU OFF?
Only when ECAM directs it. Specifically: HYD G(Y) RSVR LO LVL, HYD G(Y) RSVR LO AIR PR (if pressure fluctuates), or HYD G(Y) RSVR OVHT. Leave PTU in AUTO otherwise.
6. How does your handling strategy change if BLUE hydraulic pressure is available via the BLUE electric pump versus via the RAT pump?
The answer depends on whether the electric pump provides full BLUE system capability versus the reduced flow rate of the RAT-driven pump, and whether additional electrical degradations affect other systems (flight instruments, communications, automation). The RAT scenario compounds the hydraulic failure with an electrical emergency.
7. Why must the crew wait until stabilized at VAPP before extending the landing gear in a G+Y failure?
Because gear extension triggers the transition from ALTERNATE law to DIRECT law. In DIRECT law, the sidestick neutral reference (trim) is captured at the speed existing at the moment of transition. Extending gear early sets the trim reference at the wrong speed, creating unexpected pitch forces during the flare and any go-around.
Our A320 Oral Exam Prepper drills exactly these scenarios - PTU logic, dual hydraulic failure procedures, control law degradation, and the VAPP-before-gear technique that examiners test and real operations demand.
Explore the A320 Oral Exam Prepper →Manual References
A320 FCOM DSC-29-10-20 (17 JUN 2019): Generation - Power Transfer Unit (PTU), Priority Valves, Blue System Pumps / RAT.
A320 FCOM DSC-29-20 (17 JUN 2019): Controls and Indicators - PTU pb-sw, BLUE ELEC PUMP pb, RAT MAN ON pb.
A320 FCTM AO-029 (18 MAR 2015): Hydraulic Generation Particularities - PTU Principle, PTU OFF Recommendations.
A320 FCTM AO-029 (18 MAR 2015): Dual Hydraulic Failures - General Guidelines, ECAM completion prior to approach.
A320 FCTM AO-029 (18 MAR 2015): Remaining Systems - HYD G+Y SYS LO PR surface/system effects, VAPP-before-gear technique.
