Are you curious to know about Hydraulic uses and systems working in aircraft? If Yes! then, you have come to the right place. In this post, you will learn all the aspects of an aircraft hydraulic system. Before you begin, you need to know how the hydraulic system works.
How Hydraulic System Works
The fundamental principle behind the aviation hydraulic system is to move a specific component from one position to another using pressurized fluid. The science behind the working principle of the hydraulic system – is Pascal’s Law. Simply stated, a confined hydraulic fluid exerts equal pressure at every point and in every direction in the fluid.
The Aircraft Hydraulic System is a complex system that provides a means for the operation of aircraft components. The most frequent applications for aircraft hydraulic systems are flight control surfaces, landing gear, thrust reversers, and brakes. Before going into complex systems, first, you need to understand a simple hydraulic system.
Simple Hydraulic System
Basically a hydraulic system requires:
- A source of hydraulic power (the pump),
- Pipes or hoses to carry the hydraulic fluid from one point to another,
- A valve mechanism to control the flow and direction of the hydraulic fluid,
- A device for converting the fluid power to the movement (actuating cylinder or hydraulic motor), and
- A reservoir to store the hydraulic fluid.
Advantages of Hydraulic System in Aircraft as a Power Source
When we talk about the efficiency of Hydraulic System, Operations of hydraulic systems are almost 100 percent efficient, with only negligible loss due to fluid friction. Apart from this, there are other advantages as well.
- Light-weight,
- Cost-effective,
- Ease of installation,
- Simplification of inspection, and
- Minimum maintenance requirements.
Hydraulic System of A320 Aircraft
The A320 aircraft has three independent hydraulic systems:
- Green Hydraulic System:
Normal Power Source – EDP1 (Engine Driven Pump)
Auxiliary Power Source – PTU (Power Transfer Unit) - Yellow Hydraulic System:
Normal Power Source – EDP2 (Engine Driven Pump)
Auxiliary Power Source – PTU & Electric Pump - Blue Hydraulic System:
Normal Power Source – Electric Pump
Auxiliary Power Source – RAT (Ram Air Turbine)
Normal hydraulic system operating pressure is 3000 PSI (2500 PSI when powered by the RAT). Hydraulic fluid cannot be transferred from one system to another in A320 aircraft.
Hydraulic System Components of A320 Aircraft
Below is the complete list of GREEN, BLUE, and YELLOW Hydraulic System Components of A320 Aircraft.
| GREEN SYSTEM | BLUE SYSTEM | YELLOW SYSTEM |
| ➔ Reservoir ➔ Fire valve ➔ EDP ➔ EDP pressure switch ➔ Ground service panel ➔ High Pressure (HP) manifold comprises: ◆ Two pressure switches, ◆ Pressure transducer, ◆ Leak measurement valve, ◆ HP filter, ◆ Pressure relief valve, ◆ Sampling valve. ➔ Accumulator ➔ Optional leak measurement manifold ➔ Power Transfer Unit (PTU) ➔ PTU manifold comprises: ◆ Normal braking selector valve, ◆ Solenoid valve, ◆ Priority valve. ➔ Return manifold comprises: ◆ One return filter with a clogging indicator and a by-pass device. ◆ One temperature sensor. | ➔ Reservoir ➔ Electric pump ➔ E-Pump pressure switch ➔ Acoustic filter ➔ Ram Air Turbine (RAT) ➔ Ground coupling ➔ Current transformer and phase unbalance detector ➔ High Pressure (HP) manifold comprises: ◆ Two pressure switches, ◆ Pressure transducer, ◆ Leak measurement valve, ◆ HP filter, ◆ Priority valve, ◆ Pressure relief valve, ◆ Sampling valve. ➔ Accumulator ➔ Optional leak measurement manifold ➔ Return manifold comprises: ◆ One return filter with a clogging indicator and a by-pass device. ◆ One temperature sensor. | ➔ Reservoir ➔ Fire valve ➔ EDP ➔ EDP pressure switch ➔ Ground coupling ➔ Hand pump ➔ Electric pump ➔ Current transformer and phase unbalance detector ➔ High Pressure (HP) manifold comprises: ◆ Two pressure switches, ◆ Pressure transducer, ◆ Leak measurement valve, ◆ HP filter, ◆ Pressure relief valve, ◆ Sampling valve. ➔ Accumulator ➔ Optional leak measurement manifold ➔ Power Transfer Unit (PTU) ➔ PTU manifold comprises: ◆ Quick disconnect ◆ Solenoid valve ◆ Priority valve ➔ The return manifold comprises: ◆ One return filter with a clogging indicator and a by-pass device. ◆ One temperature sensor. |
RESERVOIR
Each hydraulic system has its own reservoir and three levels in each reservoir. The reservoir is pressurized to 50 psi and is sealed by a check-valve to hold the pressure when there is no air supply to prevent cavitation.
| GREEN SYS RESERVOIR | BLUE SYS RESERVOIR | YELLOW SYS RESERVOIR |
| ➔ Normal fill level: 14L ➔ Maximum gaugeable level: 18L ➔ Low level warning: 3.0 ± 0.4L | ➔ Normal fill level: 6L ➔ Maximum gaugeable level: 8L ➔ Low level warning: between 2.0 and 2.3L | ➔ Normal fill level: 12L ➔ Maximum gaugeable level: 18L ➔ Low level warning: 3.0 ± 0.4L |
ELECTRIC PUMPS
The Yellow Electric Pump mainly used on the ground for maintenance and cargo door operation. If no electric power is available, a hand pump in the yellow system can be used to operate the cargo doors. The Blue Electric Pump can be used as an auxiliary power source for maintenance purposes on the ground.
RAT (RAM AIR TURBINE)
The RAT is an auxiliary pressure supply for the blue system and for the emergency electrical power Constant Speed Motor/Generator (CSM/G). The RAT is deployed automatically or manually depending on the failure conditions. It pressurizes the blue hydraulic system at 2500 psi. It can be retracted on the ground only, following a specific maintenance procedure.
POWER TRANSFER UNIT (PTU)
A Power Transfer Unit (PTU) enables the green system to be pressurized by the yellow system and vice versa. It transfers the hydraulic power but does not transfer the hydraulic fluid. The PTU is an auxiliary pressure supply for either the green or yellow systems without the transfer of fluid between the two systems.
Power Transfer Unit (PTU) will run automatically when the differential pressure between the green and yellow circuits is greater than 500 psi.
Purpose of Hydraulic Filters
Hydraulic filters are required to filter out any particles that may enter the hydraulic fluid. These particles may enter the system when it is being serviced or during the wear of operating components. If these contaminants were allowed to remain in the circulating fluid. they could damage the seals and cylinder walls, causing internal leakage and prevent components such as check valves from seating properly.
- HP filter – High Pressure (HP) filter installed in the pressure line.
- LP filter – Low Pressure (LP) filter installed in the return line.
- Case Drain filter – Installed in the case drain line pump cooling and lubricating.
All these filters have a red clogging indicator, which pops out when the filter is clogged (dirty) and replaceable filter elements.
FAQ on Aircraft Hydraulic System
What is the purpose of an accumulator in the hydraulic system?
The accumulator acts as a damper for small changes and it holds 1L of the total volume of fluid when it is full. The accumulator Aid or supplement the power pump when several units are operating at once by supplying extra power from its accumulated, or stored, power.
What is the purpose of a priority valve?
In the event of low hydraulic pressure, the priority valves maintain the operation of essential systems by cutting off hydraulic power to heavy load users. The threshold of the priority valve is 1842 psi in the A320 aircraft hydraulic system.
What are the conditions when PTU operation is inhibited?
PTU operation is inhibited in A320 aircraft:
1. During cargo door operation,
2. If the PTU P/B is switched OFF,
3. When the A/C is on the ground, with one engine running and if the parking brake is set on or the nose wheel steering is disconnected for towing (example: First engine start-up during push back).
