The word “hydraulics” is based on the Greek word for water and originally meant the study of the physical behaviour of water at rest and in motion. Today, the meaning has been expanded to include the physical behaviour of all liquids, including hydraulic fluid. Hydraulic Systems are not new to aviation, early aircraft had hydraulic brake systems. As aircraft became more sophisticated, newer systems with hydraulic power were developed. Now, Hydraulic Systems are used on aircraft to move and actuate landing gear, flaps, brakes, etc.
The reason to use hydraulics is because they are able to transmit a very high pressure or force with a small volume of fluid (hydraulic oil).
(Using principle of Bramah’s Press – the smaller the area under load, the greater is the pressure being generated).
Hydraulic Fluid Properties:
High Flash Point: In the event of a hydraulic leak, fluid ignition should not occur at the normal operating temperatures of the surrounding components. Special hydraulic fluids with fire resistant properties have been developed for aviation use.
Adequate Viscosity: Aircraft Hydraulic Systems must work efficiently over a broad temperature spectrum. The fluid used must flow easily at very low temperatures but must also maintain adequate viscosity at high temperatures. The ideal hydraulic fluid will have a very low freezing point and a very high boiling point.
Lubricant Properties: The hydraulic fluid acts as a lubricant for the pumps, actuators and motors within the system.
Thermal Capacity/Conductivity. Hydraulic fluid acts as a system coolant. The fluid must be able to readily absorb and release heat.
Components of the Hydraulic System
Accumulator: It supplements the pump under peak load, and gives initial head off pressure for any of the services.
Pumps: Draw oil from the reservoir and deliver supply of fluid to the system. (Constant pressure pump)
Fire Shut Off Valves: They close in cases of engine fire (when the fire push button is pressed). Only present in the systems that have an engine driven pump.
Reservoir: Provides storage space for the system fluid and sufficient air space to allow any variations in the fluid system.
RAT (Ram Air Turbine): Is used to provide emergency hydraulic power for primary services in case both engines fail, it is deployed automatically, situated in the belly of the A/C.
Hand Pump: To allow ground servicing to take place without starting the engines, to operate cargo doors.
Priority Valve: Cuts off hydraulic power from heavy load users if hydraulic pressure in the system gets low.
Leak Measurement Valves: Each system has leak measurement valves upstream of the flight controls, these valves close if a leak is detected in the system by pushing the P/B on the maintenance panel.
A320 Hydraulic System
- The A320 has three continuously operating Hydraulic Systems
- Green, Blue and Yellow
- Each system has its own hydraulic reservoir – Maintaining 3000 psi
- The system maintains 2500 psi if powered by RAT
- Engine 1 – connected to Green Hydraulic System (Engine driven pump)
- Engine 2 – connected to Yellow Hydraulic System (Engine driven, electrically driven)
- NOTE: Yellow system also consists of hand pump for cargo door operation on ground
- Blue Hydraulic System is driven by electrical pump
- NOTE: Blue Hydraulic System does not consist of a fire shut off valve
- NOTE: Blue system is pressure by a pump driven by RAT (Ram Air Turbine) in cases of emergency
- Hydraulic fluid cannot not transferred from one system to another
Power Transfer Unit ( PTU)
- A bidirectional power transfer unit enables the YELLOW system to pressurise the GREEN system and vice versa
- It automatically starts working when the pressure differential between both the systems is 500 psi
Ram Air Turbine
- Purpose: Used to give emergency hydraulic power, electric power (through the emergency generator) for primary services in case both engines fail, it is deployed automatically, situated in the belly of the A/C.
- A drop out RAT coupled to a hydraulic pump allows the blue system to function if electrical power is lost or both engines fail.
- The RAT deploys automatically if AC BUS 1 and AC BUS 2 are lost.
- It can be stored only when the aircraft is on ground.
- The distribution of the various flight controls on the three hydraulic systems are as follows: