The Airbus A320 hydraulic system provides power for flight controls, landing gear, brakes, and many other systems. Three independent hydraulic circuits (green, blue, yellow) ensure redundancy and system availability even during failures. Understanding hydraulic system operation, PTU logic, and RAT deployment is essential for A320 type rating exams.
Hydraulic System Overview
The A320 uses three separate hydraulic systems operating at 3000 PSI (207 bar). Each system has its own reservoir, pumps, and dedicated consumers. This triple redundancy ensures flight control authority even with multiple hydraulic failures.
💚 Green System
Engine 1 driven pump only, powers most flight controls and critical systems
💙 Blue System
Electric pump only, RAT emergency backup, powers essential flight controls and systems
💛 Yellow System
Engine 2 driven pump, electric pump for ground operations, powers landing gear, cargo doors, and flight controls
🔄 PTU
Power Transfer Unit transfers power between green and yellow (not fluid), distinctive barking sound
Green Hydraulic System
The green system is powered by an engine-driven pump on engine 1. Unlike blue and yellow systems, green has no electric pump.
Green System Pumps
- Engine 1 Pump: Only source, runs whenever engine 1 operating
Green System Powers:
- Normal brakes
- Spoilers 1 & 5
- PTU (can receive or provide power)
🔧 Green System Limitation
The green system has NO electric pump. If engine 1 fails or is shut down, green hydraulic pressure is lost. The PTU can transfer power from yellow to green if yellow pressure is available, but green itself has no backup pump.
Blue Hydraulic System
The blue system is unique - it has only an electric pump for normal operations and the RAT (Ram Air Turbine) for emergency backup. No engine-driven pump.
Blue System Pumps
- Electric Pump: Runs automatically when either electrical system powered
- RAT Pump: Emergency source, deployed when AC BUS 1 & 2 both lose power
Blue System Powers:
- Slats
- Emergency generator (via RAT)
- Alternate brakes (if green fails)
🚨 RAT Deployment
The Ram Air Turbine deploys automatically when:
• AC BUS 1 AND AC BUS 2 both unpowered, AND
• Airspeed > 100 knots
Once deployed, RAT cannot be retracted in flight. RAT drives blue hydraulic pump and emergency generator, providing essential hydraulic power and electrical power for emergency electrical configuration.
Yellow Hydraulic System
The yellow system is powered by an engine-driven pump on engine 2 and has an electric pump that can be used on ground with engines off.
Yellow System Pumps
- Engine 2 Pump: Primary source, runs whenever engine 2 operating
- Electric Pump: Allows yellow system to be used on ground with both engines off
- Hand Pump: Manual pump for cargo door operation without electrical power
Yellow System Powers:
- Alternate brakes (if green fails)
- Cargo door operation
- Nosewheel steering
- Flaps
- PTU (can receive or provide power)
⚠️ Yellow System Critical for Landing Gear
Yellow hydraulic is essential for landing gear operation. If yellow pressure lost, landing gear can still be extended via gravity (free fall) using emergency extension handle, but retraction requires yellow hydraulic pressure.
Power Transfer Unit (PTU)
The PTU is a hydraulic motor/pump that transfers mechanical power between the green and yellow systems. It does NOT transfer fluid - reservoirs remain separate.
How PTU Works
When a pressure differential exists between green and yellow (typically > 500 PSI difference), the PTU automatically activates:
- High-pressure system drives the motor side of PTU
- Motor drives the pump side, pressurizing the low-pressure system
- Makes distinctive barking/grinding sound (normal operation)
- Allows one engine to pressurize both green and yellow systems
PTU Operating Logic
- PTU Runs When: Differential pressure exceeds 500 PSI automatically
- PTU Inhibited When: During first engine start, cargo door operation, or PTU pb OFF
- Most Common: Engine start (after first) - one engine running creates pressure differential, PTU barks
🔊 PTU Barking Sound
The PTU's characteristic barking/knocking sound is normal and occurs frequently during engine start, single-engine taxi, or when only one engine-driven pump is running. Passengers may be alarmed by this sound, but it indicates normal hydraulic system operation. The sound is caused by the PTU cycling as it transfers power between systems.
Hydraulic System Priorities
When hydraulic quantity decreases (leak), the system automatically prioritizes critical consumers. Non-essential systems lose hydraulic power first.
| Priority Level | Systems Powered | When Shed |
|---|---|---|
| 1 (Highest) | Flight controls | Never (until complete hydraulic loss) |
| 2 | Landing gear, brakes | Low hydraulic quantity |
| 3 (Lowest) | Slats/flaps, cargo doors, nose steering | Very low hydraulic quantity |
Common Hydraulic Failures
Single Hydraulic System Loss
Loss of one hydraulic system (green, blue, or yellow):
- Aircraft remains fully controllable
- Redundant systems take over critical functions
- Some non-critical systems may be unavailable
- Follow ECAM procedures for specific system loss
Dual Hydraulic Loss
Loss of two hydraulic systems is more serious but aircraft still controllable:
- Green + Blue: Yellow provides some flight controls, landing gear, brakes
- Green + Yellow: Blue provides flight controls, PTU inoperative
- Blue + Yellow: Green provides flight controls, may lose some systems
Triple Hydraulic Loss
Extremely rare, but in triple hydraulic failure:
- Pitch control via mechanical backup (THS only)
- Very limited roll control via differential thrust
- Extreme emergency - declare MAYDAY
- Landing extremely challenging, consider ditching preparation
Frequently Asked Questions
Master A320 Hydraulics with Practice Questions
Test your knowledge with 50+ hydraulic system questions in the A320 Edge quiz app. Covers green/blue/yellow systems, PTU logic, RAT deployment, priorities, and failure scenarios.
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