Every control surface movement, every brake application, every landing gear extension in the A320/A321 depends on hydraulic power. But what happens when one system fails? How does the aircraft ensure you maintain control when hydraulic pressure drops? The hydraulic system solves this through three independent circuits working together, each capable of backing up the others when needed.

How the System Works

Your aircraft operates with three completely separate hydraulic systems—green, blue, and yellow—each maintaining 3000 PSI under normal conditions. Think of these as three independent power plants, each with its own reservoir and specific responsibilities, but designed to work together seamlessly.

The Green System connects to engine 1, using an engine-driven pump to generate hydraulic pressure. This system powers the left elevator, normal braking, and shares responsibility for various flight controls. When engine 1 runs, the green system maintains full pressure automatically.

The Blue System operates differently. During normal operations, it uses an electric pump, making it independent of engine operation. This system becomes critical during emergencies because it can be powered by the Ram Air Turbine (RAT) when both engines fail. The blue system handles the left and right elevators and provides backup for many flight controls.

The Yellow System connects to engine 2 through its engine-driven pump, but also features an electric pump. This system manages the right elevator, alternate braking, and cargo door operations. The electric pump allows ground crews to operate cargo doors even when engines are shut down.

Managing Power Distribution

The Power Transfer Unit (PTU) acts like an intelligent power-sharing device between the green and yellow systems. When the pressure difference between these systems exceeds 500 PSI, the PTU automatically activates, allowing the stronger system to help power the weaker one. This bidirectional capability means that if you lose engine 1, the yellow system can help maintain the green system's functions through the PTU.

Understanding this becomes crucial during single-engine operations. The PTU's distinctive barking sound during engine start is actually the system testing itself—it's inhibited during the first engine start but automatically tested during the second engine start. This explains why you hear that characteristic noise pattern during normal startup procedures.

The hydraulic system's priority logic ensures critical flight controls always receive power first. Priority valves automatically restrict flow to less critical systems when pressure drops, maintaining elevator and rudder control even under degraded conditions. This means your primary flight controls remain functional even when secondary systems lose power.

When Things Go Wrong

Single hydraulic failures rarely create emergencies because of the system's redundancy. However, understanding which systems are affected helps you anticipate handling changes and plan accordingly.

Green System Failure means you lose normal braking and some flight control authority. The aircraft switches to alternate braking using the yellow system, and you'll notice the PTU working harder to maintain system pressure. Flight controls remain available through the blue and yellow systems, though you may experience some degraded response.

Blue System Failure primarily affects backup systems since the blue system serves as the emergency power source. Normal operations continue with green and yellow systems, but you lose the critical RAT backup capability. This failure becomes significant only if you subsequently lose engine-driven power sources.

Yellow System Failure eliminates engine 2's hydraulic contribution and affects alternate braking. The PTU can help maintain some yellow system functions using green system pressure, but you'll lose certain backup capabilities and may experience different braking characteristics.

Dual Hydraulic Failures create the most challenging scenarios. These rare events significantly increase the workload and require immediate attention. The autopilot becomes unavailable, flight control laws may degrade to alternate or direct law, and the landing configuration becomes abnormal. The key is recognizing that even if two systems fail, the remaining system usually provides enough control authority to land safely.

What This Means for You

During normal operations, the hydraulic system works transparently—you simply move controls and the system responds. However, understanding the system helps you recognize abnormal indications and respond appropriately.

Reservoir levels displayed on the HYD system page should remain in the normal range. Slightly high levels in hot weather are acceptable, but low levels indicate leaks requiring immediate attention. The system's leak measurement valves can isolate problems, but this may affect flight control availability.

System pressures normally maintain 3000 PSI, dropping to 2500 PSI when powered by the RAT. These pressure differences affect control response and system capability. Understanding this helps explain why emergency procedures specify minimum speeds—the RAT needs adequate airflow to maintain pressure.

PTU operation creates distinctive sounds and vibrations that are normal during pressure transfers. However, ECAM may direct you to turn off the PTU during certain failures to prevent overheating or further system damage. This decision balances maintaining system capability against preventing additional failures.

Brake accumulator pressure becomes critical during hydraulic failures. The yellow system's accumulator stores enough pressure for at least seven brake applications, but this finite resource requires careful management during emergency landings. Understanding this limitation helps explain why procedures emphasize smooth, controlled braking rather than aggressive applications.

The hydraulic system's design philosophy prioritizes flight safety through redundancy and intelligent power management. Even with multiple failures, the system typically provides enough control authority to complete a safe landing. Your role is recognizing system status, following appropriate procedures, and adapting your flying technique to accommodate any limitations.

Remember that hydraulic failures often cascade—losing one system increases workload on the others, potentially leading to overheating or additional failures. Quick recognition and appropriate action prevent minor problems from becoming major emergencies. The system's sophisticated design gives you multiple backup options, but only if you understand how to use them effectively.

The A320’s three independent hydraulic systems ensure control even after failures—understanding them turns ECAM messages into clear action.