Airbus A320, Hydraulic System
Airbus A320 Hydraulic System Explained – Green, Yellow & Blue Circuits
Nov
The Airbus A320 hydraulic system delivers the hydraulic power to heavy load users such as flight controls, landing gear, brakes, and slats/flaps. According to the Airbus A320 FCOM, the aircraft has 3 hydraulic systems — Green, Yellow, and Blue — each operating around 3000 psi of hydraulic pressure.
Each system has its own hydraulic reservoir, pumps, accumulator, and protection logic. The aircraft relies on this triple-redundant architecture to maintain controllability even if pressure in a system gets low or lost entirely.
In normal operation, the engine driven pump on Engine 1 powers the Green system, while Engine 2 supplies the Yellow system. The Blue system is powered electrically and serves essential flight controls and selected spoilers.
Pilots routinely monitor these circuits because hydraulic fluid cannot be transferred between systems — hydraulic power is transferred from one system to another mechanically via the PTU, but the fluid cannot be transferred from one circuit to another.
Overview of the Airbus A320 Hydraulic System
The A320 hydraulics provide actuation for:
- Primary flight controls (ailerons — including the left aileron, elevators, rudder, yaw control through the yaw damper)
- Secondary controls (slats, flaps, spoilers)
- Landing gear: extend, retract, and raise the landing gear
- Nose wheel steering and braking
- Thrust reversers and to operate the cargo doors
Each hydraulic circuit is hydraulically independent. The reservoirs are pressurized by bleed air from engine sources, ensuring reliable pump suction even if aircraft attitude or temperature changes.
A320 Hydraulic System Components
Green, Yellow & Blue Circuits
| System | Power Source | Main Functions | Backup / Notes |
|---|---|---|---|
| Green System | Engine 1 engine driven pump | Flight controls, spoilers, normal brakes, landing gear | The Yellow system can pressurize the Green system via the PTU when pressure between the Green and Yellow circuits differs significantly |
| Yellow System | Engine 2 EDP + electric pump | Slats, flaps, alternate brakes, cargo doors | The Green system can power the Yellow hydraulic system through the PTU; the electric pump enables the Yellow system on ground when one engine is running or off |
| Blue System | Electric pump + RAT | Essential controls, some spoilers | During dual engine failure, RAT supplies Blue; interaction with blue and yellow systems depends on pump availability |
These three different systems ensure that primary flight controls always receive adequate power, even when system gets low on pressure or fluid.
Engine-Driven & Electric Pumps
During normal flight, the Green and Yellow systems rely on engine driven pump output. The Yellow system’s electric pump allows maintenance and cargo door operation on the ground. When the electric pump is active, the system is powered even if only one engine is running.
If an EDP fails or pressure is too low, hydraulic power can be transferred from one system to another via the PTU. The Blue electric pump runs continuously in flight when electrical power is available.
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The Power Transfer Unit (PTU)
The PTU is a bidirectional power transfer unit that provides hydraulic power to heavy load users by transferring mechanical energy between the green and the yellow systems. It does not move hydraulic fluid — energy only — so fluid contamination is impossible.
The PTU comes into action automatically when the differential pressure between the Green and Yellow systems becomes greater than 500 psi. In Airbus terminology, it engages automatically when the differential pressure between the two systems is greater than 500.
- Supports landing gear operation and high-demand actuators
- Provides backup if the Yellow hydraulic EDP or Green EDP fails
- Ensures the yellow system to pressurize the Green system and vice versa (green system and vice versa)
- Activates frequently during single-engine taxi
Because hydraulic fluid cannot be transferred between systems, the PTU simply equalizes power delivery to heavy load users if hydraulic pressure drops in one circuit.
Hydraulic Pressure, Fluid & System Operation
The A320 uses a phosphate-ester hydraulic fluid (Skydrol type), supplying all three hydraulic circuits at hydraulic pressure in a system of ~3000 psi. Each reservoir is pressurized by bleed air from engine sources through a pressure regulator.
Reservoir Pressurization
- Maintains pump suction and stable system flow
- Supports pumps during aggressive maneuvering or low temperatures
- System is pressurized pneumatically
Common Hydraulic Components
- Pump driven actuators and EDPs
- Accumulators for peak demand
- Filters and NRVs
- Priority valves cut flow to nonessential loads when pressure drops
- Emergency RAT-driven pump
- Pressure relief valves
- A manual hand pump exists only in related ground equipment, not in normal aircraft operation
Emergency & Backup Operation (RAT & PTU)
In a dual engine failure or severe electrical loss, the RAT deploys automatically. Because the RAT is pump driven by ram airflow, it restores the Blue system, supporting green and blue flight control demands.
Essential control surfaces powered during RAT operation include:
- Ailerons (including the left aileron)
- Elevators
- Rudder and yaw control via the yaw damper
When the Blue system is restored by the RAT, PTU assistance depends on whether the Yellow electric pump is running. If Yellow is active, hydraulic power can be redistributed to the Green system to allow landing gear extension using the PTU’s system to pressurize the Green function.
Common Failure Scenarios & Maintenance Notes
| Scenario | Effect | System Response |
|---|---|---|
| Green system low pressure | Loss of gear and normal brakes | Yellow PTU assistance comes into action automatically |
| Yellow system failure | No cargo door power; alternate brakes affected | Green can power yellow systems through PTU when Yellow pressure is too low |
| Blue system failure | Essential controls rely on Green/Yellow redundancy | RAT will restore Blue if both engines fail |
| Hydraulic leak in one system | Reservoir level drops; system gets low pressure | Priority valves cut flow to protect critical loads |
A320 Hydraulic Flow Summary
- Green system: EDP → primary controls, landing gear
- Yellow system: EDP + electric pump → slats, flaps, alternate brakes, cargo doors
- Blue system: Electric → essentials; RAT when required
- PTU enables power exchange between green and blue systems via Yellow when necessary
Frequently Asked Questions
What are the hydraulic systems on the A320?
The aircraft has three: Green, Yellow, and Blue. The aircraft has three fully independent hydraulic circuits powering all major systems.
When does the PTU activate?
It activates automatically when the differential pressure between the Green and Yellow circuits becomes greater than 500 psi.
What fluid and pressure does the A320 use?
The A320 uses phosphate-ester hydraulic fluid (Skydrol) at approximately 3000 psi.
Closing Insight
By understanding how hydraulic power is distributed among Green, Yellow, and Blue systems — and how the PTU ensures redundancy when pressure in a system changes — pilots gain a clearer picture of why ECAM logic behaves the way it does. This knowledge reinforces safe, predictable operation whenever abnormal hydraulic conditions occur.
Read more about A320 Systems.