A320 hydraulic system explained: Green, Blue and Yellow circuits, pumps, PTU, RAT, ECAM checks, failures and simulator troubleshooting.
The Airbus A320 uses three independent hydraulic circuits—Green, Blue and Yellow—normally pressurised to about 3,000 psi. Engine-driven and electric pumps supply hydraulic power to actuators for flight controls, landing gear, brakes, steering and other services; the PTU and ram-air turbine provide redundancy without mixing the circuits.
For Aviation & Real-World Flying readers, the practical distinction is between a system that is unpressurised because its pump has no power—which is normal during parts of a cold-and-dark start—and one that has lost pressure or fluid after it should be operating. A320 add-ons vary widely in how faithfully they reproduce that distinction.
What are the Green, Blue and Yellow hydraulic systems?
Green, Blue and Yellow are separate circuits, each with its own reservoir, pumps, pipework and assigned actuators.
| System | Normal pressure source | Typical services |
|---|---|---|
| Green | Engine 1-driven pump, with PTU assistance available | Landing gear, nose-wheel steering, normal brakes, engine 1 thrust reverser and selected flight-control actuators |
| Blue | Electric pump during normal operation; RAT in an emergency | Selected aileron, elevator, rudder, spoiler and slat actuators |
| Yellow | Engine 2-driven pump, Yellow electric pump and PTU assistance | Alternate and parking brakes, cargo doors, engine 2 thrust reverser and selected flight-control actuators |
The allocation prevents one failure from removing every actuator for a primary control surface. For example, the elevators, ailerons and rudder have hydraulic redundancy across different systems, while utility functions such as the landing gear or cargo doors can be assigned to one circuit with an alternative procedure available.
This architecture applies broadly across the A320 family, including ceo and neo aircraft, but exact actuator allocation and automatic logic can vary with subtype and modification standard. The table gives the core services rather than an exhaustive dispatch reference.
How do the pumps and actuators create movement?
The pumps produce hydraulic flow; pressure builds when that flow meets resistance, and electro-hydraulic valves direct it to an actuator.
- Engine-driven pumps are the main sources for Green and Yellow. Engine 1 normally powers Green, while engine 2 normally powers Yellow.
- The Blue electric pump normally starts automatically when at least one engine is operating, provided the electrical supply and system configuration are valid.
- The Yellow electric pump can pressurise Yellow without engine 2. It is useful for ground services, and cargo-door operation can command it through the aircraft’s automatic logic.
- Accumulators smooth pressure fluctuations and store limited hydraulic energy. The brake accumulator can preserve some braking or parking-brake capability, but it does not keep the entire Yellow system alive indefinitely.
Each reservoir is also air-pressurised to help feed its pumps and prevent cavitation, especially at altitude. This reservoir pressure is not the 3,000 psi system pressure shown on the hydraulic synoptic.
The A320 sidestick does not directly open a hydraulic valve. Flight-control computers interpret the pilot’s input and command servo valves, which use hydraulic pressure to move the control surfaces. Our A320 cockpit panel and display explanation identifies the relevant overhead controls and system indications.
How does the A320 PTU work?
The power transfer unit transfers mechanical power between the Green and Yellow systems without transferring hydraulic fluid.
The PTU is bidirectional. When the pressure difference between Green and Yellow reaches roughly 500 psi and the automatic logic permits operation, pressure from the stronger system drives a hydraulic motor which powers a pump on the weaker side. The two reservoirs and their fluid remain separate.
This distinction matters during a leak. The PTU can compensate for a failed pump while enough fluid remains, but it cannot refill an empty reservoir, repair a leak or assist the Blue system. A mistake we see often is treating it as a crossfeed valve.
The characteristic barking or sawing noise heard during some engine-start, shutdown and one-engine ground configurations is usually the PTU cycling under changing load. It does not run in every apparent pressure imbalance because Airbus logic can inhibit it during particular start, cargo-door and ground configurations.
What does the ram-air turbine do?
The ram-air turbine, or RAT, is an emergency source which can pressurise the Blue hydraulic system when normal power has been lost.
It deploys automatically after a qualifying loss of the main AC electrical supplies in flight and can also be commanded manually. Blue hydraulic power then supports essential flight controls and emergency electrical generation. The RAT does not pressurise Green or Yellow, and it cannot restore Blue pressure if the Blue circuit has lost its fluid.
A manually deployed RAT is not a normal retractable cockpit control. In a detailed simulator it may remain extended until the aircraft is reset or serviced, reflecting the real requirement for ground attention.
What happens when one hydraulic system fails?
A single hydraulic-system failure usually removes specific services and some actuator redundancy rather than making the A320 uncontrollable.
| Failure | Typical consequences | Available protection or alternative |
|---|---|---|
| Green pressure lost | Normal landing-gear operation, nose-wheel steering, normal brakes, thrust reverser 1 and some control actuators are affected | Alternate braking uses Yellow; the landing gear has gravity extension; other hydraulic systems retain primary-control authority |
| Yellow pressure lost | Cargo doors, thrust reverser 2, alternate or parking-brake pressure and some control actuators are affected | Normal braking remains on Green; stored brake-accumulator pressure may remain temporarily |
| Blue pressure lost | Part of the flight-control actuator and spoiler redundancy is lost | Green and Yellow retain other actuators; the RAT is available only when deployed and cannot replace lost fluid |
| Two systems lost | Substantial loss of control redundancy and utility functions, depending on the pair | Follow the specific ECAM procedure; the remaining capability differs for each combination |
Two indications tell different stories: low pressure with normal quantity often points to a pump, power-source or configuration problem, while an abnormally low or persistently falling reservoir quantity suggests fluid loss. Simulator aircraft may not model this distinction.
How do you check A320 hydraulics in a flight simulator?
Check the ECAM HYD page against the aircraft’s phase of operation rather than expecting all three systems to be pressurised at all times.
- Start with the aircraft state. In a cold-and-dark A320, low hydraulic pressure is expected because the pumps have no valid power source. Do not treat every amber indication as a failure.
- Display the HYD page. Check Green, Blue and Yellow pressure, reservoir quantity, pump indications and the displayed PTU or RAT status. Our guide to reading A320 ECAM pages and cautions explains how to interpret the colours and system messages.
- Observe the first engine start. The system associated with that engine should gain pressure, and the Blue electric pump should normally come online. The opposite engine-driven system may remain low if PTU operation is inhibited by the ground logic.
- Check after both engines stabilise. Green, Blue and Yellow should normally indicate approximately 3,000 psi with no inappropriate pump low-pressure warnings.
- Confirm the required services. Before taxi, verify that braking, steering and flight-control indications are available. Correct-looking pushbuttons alone do not prove the actuators are powered.
If the problem begins during power-up, follow a consistent cold-and-dark aircraft start sequence rather than switching pumps randomly. This makes it much easier to identify the missing electrical, engine or hydraulic source.
Why do hydraulic switches sometimes seem to do nothing?
Hydraulic controls may appear ineffective because the selected pump lacks electrical or engine power, automatic inhibition is active, another system is already supplying pressure, or the simulator uses simplified system modelling.
| Symptom | What to check |
|---|---|
| No Green pressure after both engines start | Confirm engine 1 is genuinely running and stabilised, the Green engine pump is not selected off, and no hydraulic failure is active |
| No Yellow pressure after both engines start | Check engine 2, its engine-driven pump and any saved failure state; use the Yellow electric pump only when the procedure requires it |
| Blue pressure low before engine start | This can be normal because the Blue electric pump’s automatic conditions have not yet been met |
| PTU does not run during a pressure imbalance | Check its overhead selection and remember that start, cargo-door and ground inhibition logic may be active |
| Pressure is normal but controls do not move | Check electrical power and the flight-control computers; hydraulic pressure alone cannot command a fly-by-wire actuator |
| Switches animate but gear, brakes and controls remain unaffected | The aircraft may model the cockpit indication without modelling the real hydraulic consequences |
| A pump repeatedly changes state by itself | Look for duplicate keyboard, controller or hardware assignments overriding the cockpit selection |
Basic aircraft may provide a convincing HYD page while using generic braking, gear and control-surface logic underneath. Systems-focused add-ons are more likely to model PTU operation, leaks, RAT deployment and actuator degradation, but they also require the correct electrical and engine configuration.
Why do A320 control surfaces droop after shutdown?
Aileron and elevator droop after shutdown can be a normal result of hydraulic pressure bleeding away.
Once the engine-driven and electric pumps stop, residual pressure gradually decays and gravity can move unpowered surfaces against their actuators. The surfaces do not necessarily settle together or at the same rate because actuator geometry, damping and remaining pressure differ.
Some simulator aircraft reproduce this only as a visual animation, while detailed models tie it to actual pressure and reservoir logic. Residual brake-accumulator pressure can also leave the parking brake effective after the other hydraulic indications have fallen, but that stored energy is finite.