The aircraft features three hydraulic systems: green, yellow, and blue. The green and yellow systems are powered by engine-driven pumps, with the green system connected to Engine 1 and the yellow system to Engine 2. Both systems normally operate at a pressure of 3,000 PSI. The blue system, on the other hand, is powered by an electric pump and also operates at 3,000 PSI. In emergency situations, the blue system can be powered by the Ram Air Turbine (RAT), which reduces the operating pressure to 2,500 PSI. It’s important to note that hydraulic fluid remains isolated within each system, preventing any transfer between them.
All three hydraulic reservoirs are pressurized by high-pressure (HP) bleed air sourced from Engine 1. This pressurization ensures that a positive pressure is maintained within the reservoirs, preventing pump cavitation regardless of the aircraft’s load factor. In cases where the bleed air pressure from Engine 1 is insufficient, the system automatically draws bleed air pressure from the cross bleed duct.
Components of Hydraulic System
Yellow System Pumps:
- ENG 2 Pump for normal operations
- Hand Pump to pressurize yellow system for cargo doors operation (when no electrical power is available)
- Electric Pump to pressurize yellow system which allows yellow hydraulics to be used on the ground (when both engines are stopped)
Power Transfer Unit (PTU)
A PTU, or Power Transfer Unit, is a device used in aircraft hydraulic systems to transfer power between two hydraulic systems without transferring fluid. This ensures that essential systems and controls continue to operate even if one hydraulic system loses pressure. PTU is of no use if there is loss of hydraulic fluid in any system.
PTU Working
The PTU uses the hydraulic pressure from the working system to drive a hydraulic motor, which in turn drives a hydraulic pump. This mechanism allows the pump to generate hydraulic pressure for the system that is experiencing a pressure loss, effectively “borrowing” power from the functioning system. Importantly, the PTU transfers hydraulic power, not hydraulic fluid, between the systems, ensuring that the hydraulic fluids remain isolated and uncontaminated.
PTU comes into action automatically when the differential pressure between green and the yellow systems is greater than 500 PSI.
Ram Air Turbine (RAT)
The RAT is a small, wind-powered turbine that provides emergency hydraulic and electrical power to the aircraft in the event of a failure of the main power sources. RAT deploys automatically or can be manually activated when both engines fail or if there is a significant loss of hydraulic and electrical power. It can be stowed only when the aircraft is on ground.
System Accumulators
Accumulators store hydraulic fluid under pressure, which can be released as needed to provide immediate hydraulic power. By absorbing and releasing hydraulic fluid, accumulators help maintain stable pressure within the hydraulic system, dampening pressure spikes and reducing pulsations caused by pump operations or sudden changes in demand.
In the event of a hydraulic pump failure, accumulators can provide a temporary source of hydraulic pressure, allowing for the operation of critical systems and giving the crew time to address the issue or switch to an alternative power source.
Priority Valves, Fire Shutoff Valves and Leak Measurement Valves
Priority Valves
Priority valves control the distribution of hydraulic fluid to various aircraft systems based on their importance. These valves ensure that critical systems receive hydraulic pressure first, especially during periods of low hydraulic pressure or high demand. Under normal operating conditions, priority valves remain open, allowing hydraulic fluid to flow to all systems. If the hydraulic system pressure drops below a certain threshold, the priority valves close off less critical systems, redirecting fluid to essential components such as flight controls and landing gear.
Fire Shutoff Valves
Fire shutoff valves are designed to isolate sections of the hydraulic system in the event of a fire, preventing the spread of flames and stopping the flow of hydraulic fluid to the affected area.
Each of the green and yellow hydraulic systems has a fire shutoff valve located in the line upstream of its respective engine-driven pump. The flight crew can activate these valves by pressing the ENG 1 FIRE or ENG 2 FIRE pushbutton in the cockpit.
Leak Measurement Valves
Leak measurement valves help in identifying and isolating hydraulic leaks within the system. Each hydraulic system has a leak measurement valve upstream of the primary flight controls.
They can be operated via LEAK MEASUREMENT VALVES pushbutton switch on the maintenance panel.
Distribution of Hydraulic System
Green System | Blue System | Yellow System |
Slats and Flaps | Slats | Flaps |
Rudder | Rudder | Rudder |
Aileron | Aileron | No Aileron Input |
Left Elevator | Left and Right Elevator | Right Elevator |
Stab | No Stab Input | Stab |
Spoiler 1 & 5 | Spoiler 3 | Spoiler 2 & 4 |
Yaw Damper 1 | Yaw Damper 2 | |
Reverser 1 | Reverser 2 | |
Normal Braking + Anti-Skid | Alternate/Parking Brake + Anti-Skid | |
Landing Gear | Nose Wheel Steering |
Controls and Indicators – Hydraulic Panel
- In the default mode, the hydraulic pump pressurizes the system when the engine is running. The overhead panel is equipped with ENG 1 Pump and ENG 2 Pump pushbutton switches, which can be used to turn the pumps off as needed. Conversely, the yellow electric pump is off in its default setting and can be activated using a pushbutton switch located on the overhead panel. This setup provides flexibility and control over the hydraulic system’s operation, allowing the flight crew to manage pump activity as necessary.
- The blue electric pump is on by default, ensuring continuous operation of the hydraulic system. The overhead panel features a guarded switch that allows the pump to be turned off when necessary.
- FAULT lt (amber) comes on in case of reservoir low level, reservoir overheat, reservoir low pressure, pump pressure low or pump overheat. Light stays on as long as the overheat lasts.
Controls and Indicators – Maintenance Panel
- The BLUE PUMP OVRD switch is used during certain operational procedures outlined in the Minimum Equipment List (MEL). It allows ground personnel to manually switch on the blue electric pump while the aircraft is on the ground. This functionality is essential for specific maintenance tasks and system checks that require hydraulic pressure without engine operation.
- With LEAK MEASUREMENT VALVES pushbutton switches for green, blue and yellow system, the corresponding valves closes and shuts off hydraulic supply to primary flight controls.
Additional Info: PTU Logic