A320 Hydraulic System — Complete Guide: Systems, Services, Failures and Effects

Three independent 3,000 PSI circuits. No single failure removes all hydraulic power. But the effects of each individual failure cascade through flight controls, braking, landing gear, flaps, slats, and the spoilers — and understanding exactly what is lost and what remains is fundamental knowledge for any A320 type rating and ATPL oral.

1. 1. Hydraulic System — complete guide
2. 2. Autoflight System — AP, FD, ATHR, FCU and FMA logic
3. 3. Flight Controls — Normal law, Alternate law and Direct law
4. 4. Electrical System
5. 5. Pneumatics — Air conditioning, Pressurisation and Ventilation
6. 6. Engines
7. 7. APU
8. 8. Fire Fighting
9. 9. Landing Gear and Brakes
10. 10. Ice and Rain Protection

System overview

The A320 has three independent hydraulic systems — Green Blue Yellow — each operating at a nominal pressure of 3,000 PSI (206 bar). The systems are completely independent: they use separate reservoirs, separate fluid circuits, and separate pressurisation sources. No fluid is shared between circuits under any normal condition. A leak in one system cannot drain another.

Each system consists of:

• A reservoir (pressurised to prevent pump cavitation)
• One or more pumps (engine-driven and/or electric)
• Pressure regulation and filtration components
• Actuators at each powered surface or system
• A leak measurement system (LMS) for reservoir quantity monitoring

The working fluid is a phosphate ester hydraulic fluid (Skydrol or equivalent), which is fire-resistant but highly corrosive to skin and eye tissue. Ground engineers handle it with full PPE. The colour coding — green, blue, yellow — exists only in training diagrams; the fluid itself is not coloured differently.

All services — complete breakdown by system

The PTU — power transfer without fluid transfer

The Power Transfer Unit connects Green and Yellow hydraulically via a motor-pump assembly. When a pressure differential of approximately 500 PSI ⚑ verify develops between the two systems — most commonly when ENG 1 is shut down but ENG 2 remains running — the PTU activates automatically. Yellow pressure drives the PTU motor shaft, which drives a pump on the Green side, restoring Green system pressure.

The RAT — Ram Air Turbine

The Ram Air Turbine is a small variable-pitch propeller that deploys automatically into the airstream on dual generator failure, or can be manually deployed by the crew via a pushbutton on the overhead panel.

When deployed, the RAT drives a hydraulic pump that pressurises the Blue system to approximately 2,500 PSI ⚑ verify — lower than the normal 3,000 PSI. Blue pressure then powers:

• Flight controls (Blue circuit — ailerons and elevators)
• The CSM/U (Constant Speed Motor/Generator), which converts Blue hydraulic power into AC electrical power for essential avionics

The RAT provides sufficient power to fly the aircraft to a landing. It is not sufficient for normal operations — some Blue-powered functions may be degraded at reduced pressure, and systems that require Green or Yellow remain unpowered.

Minimum effective RAT deployment speed: approximately 140 knots ⚑ verify. Below this airspeed, turbine output is insufficient to maintain Blue system pressure. This is operationally relevant for approach planning in a dual-generator scenario — if airspeed drops below approximately 140 knots, CSM/U output and Blue system pressure will degrade.

Failure effects — complete analysis

Effects on flight controls

The A320 flight control surfaces draw hydraulic power from multiple systems to ensure that no single hydraulic failure removes control of any axis. The distribution is as follows:

Effects on braking

The A320 braking system has three modes, each drawing from a different hydraulic source:

The brake accumulator is pre-charged from the Yellow system and maintains pressure for parking brake application and emergency braking independent of any hydraulic system availability. This is why brake pressure is available even with all hydraulic pumps off and on a cold-and-dark aircraft.

Green system failure — braking effect: Normal braking is lost. Alternate braking (Yellow) activates automatically. Anti-skid protection remains available through the alternate system. No change to stopping distance in normal conditions — alternate braking is essentially equivalent to normal braking in terms of deceleration capability.

Yellow system failure — braking effect: Alternate braking is lost. Normal braking (Green) remains fully available. The accumulator continues to hold pressure for parking brake and emergency use. No change to normal braking.

Green + Yellow dual failure — braking effect: Both normal and alternate braking are lost. Only the pre-charged accumulator remains for braking. No anti-skid protection. The accumulator provides a limited number of brake applications — approximately 7 full brake applications ⚑ verify — before pressure is depleted. Landing distance will increase significantly. This scenario requires immediate ECAM action and careful energy management on approach.

Effects on landing gear

Landing gear extension and retraction is powered exclusively by the Green system. Landing gear doors are also Green-powered.

Green system failure — landing gear effect:

• Normal (hydraulic) gear extension is unavailable
• Gravity extension via the manual free-fall handle is always available as a backup
• Gravity extension procedure: flight crew manually trip the uplocks, allowing gear to free-fall to the down-and-locked position under gravity and aerodynamic drag
• After gravity extension, the gear cannot be retracted — it is a one-way operation
• Gear doors may not close fully after gravity extension, increasing drag for the approach
• Nose wheel steering (also Green-powered) is lost. Ground directional control after landing is via differential braking and rudder.

Effects on flaps and slats

Flaps and slats have separate hydraulic supplies, which means they can be affected differently by hydraulic failures.

Flaps are powered by Green and Yellow through a shared Power Control Unit (PCU). The PCU protects against asymmetric flap extension. If one system fails, the other continues to drive both flap surfaces, though at reduced speed. If both Green and Yellow fail, flaps are hydraulically locked in their current position.

Slats are primarily powered by Blue, with Green as backup. If Blue fails, slat extension continues via Green. If both Blue and Green fail, slats are hydraulically locked.

Complete failure mode summary

ECAM monitoring and hydraulic indications

The ECAM System Display HYD page provides real-time status of all three hydraulic systems. In normal operations, the HYD page is not continuously displayed — it appears automatically when an abnormality is detected. The crew can also call it manually via the SD selector.

Key indications on the HYD page:

• System pressure: Each system shows current pressure. Low pressure is indicated by amber colouring when pressure drops below the low-pressure threshold.
• Pump status: Each pump shows ON/OFF/FAULT. A pump running with no pressure output indicates a pump fault, not necessarily a system leak.
• Reservoir quantity: Shown as a percentage. Decreasing quantity indicates either normal consumption (small amount) or a leak (significant, progressive decrease). A leak decreasing quantity toward zero is a system emergency.
• Reservoir temperature: Shown for each system. High temperature indicates excessive pump cycling or restriction in return lines.
• PTU status: AUTO in normal operations. AUTO inhibited on ground with both engines running and parking brake released.

Key numbers

⚑ = Verify against your operator's FCOM before use in training or operations. Values vary by aircraft variant and modification state.