The hydraulic system on the Airbus A320 is an essential system supplying pressure to move the flight controls, landing gear, flaps, elevator, stabilizer, cargo doors etc.
There is virtually no major system on the aircraft that doesn’t interact in some way with the hydraulics.
Let’s take a look a the hydraulic system section by section:
How The Airbus A320 Hydraulic System Works
The Airbus A320 hydraulic system is comprised of three continuously operating systems referred to as the yellow, green and blue systems. Each system has its own reservoir and fluid, and the fluid cannot be transferred from one system to another.
Normal system pressure is 3000psi when supplied by the engines (by Engine Driven Pumps, or EDPs) but the pressure is reduced to 2500psi when supplied by the Ram Air Turbine (RAT).
If a hydraulic failure or other problem occurs during flight the pilots will follow the relevant ECAM (Electronic Centralized Aircraft Monitor) procedure. A full schematic of the hydraulic system is available to the pilots in the QRH (Quick Reference Handbook).
Green Hydraulic System
Power for the green hydraulic system on the A320 comes from an engine driven pump (EDP) on the number 1 engine. EDP 1 pressurizes the system to 3000 psi.
Yellow Hydraulic System
The yellow hydraulics on the Airbus A320 are normally pressurized by an engine driven pump (EDP) on engine number 2 (EDP 2).
The A320 yellow hydraulic system can also be powered by the yellow electric pump allowing the yellow hydraulics to be pressurized on the ground even with engine number 2 shut down.
The yellow system can also be pressurized by a hand pump on the ground to allow the ground crew to operate the cargo doors, even without electrical power available to the aircraft.
Blue Hydraulic System
The blue hydraulic system on the A320 is normally pressurized by the blue electric pump.
In an emergency the A320 RAT (Ram Air Turbine) can supply pressure to the blue system.
The Ram Air Turbine is a small propellor that can be deployed into the airstream to supply hydraulic power to the blue hydraulic system in the case of a dual engine failure or serious electrical problem.
Related Reading: The Airbus A320 Ram Air Turbine (RAT)
Airbus A320 PTU
On the A320 a bidirectional Power Transfer Unit (PTU) allows the green system to pressurize the yellow, or vice versa.
The PTU on the A320 is triggered when a difference between the green and yellow systems is greater than 500psi.
On the ground, with the engines stopped, it is still possible to pressurize the green system via the PTU and yellow electric pump.
Airbus A320 PTU Sound (The Barking Dog!)
The sound of the Airbus A320 PTU operating is well known. The infamous “dog barking” is actually caused by the operation of the PTU. Due to its location its most noticeable for passengers sitting close to the wings. During the start sequence the PTU begins to operate for a short time. The PTU operates to restore pressure between the yellow and green systems. As it cycles on and off during start this leads to the famous “there’s a dog barking in the cargo hold!” situation.
FAQ Summary – A320 Hydraulic System
How does the hydraulic system work on the Airbus A320?
The Airbus A320 hydraulic system is comprised of three independent hydraulic systems: green, blue and yellow. Each system has a normal pressure of 3000 psi apart from when the blue system is powered by the RAT when it has a pressure of 2500 psi.
How are the hydraulics powered on the Airbus A320?
Each of the three Airbus A320 hydraulic systems have a separate source of hydraulic power. The green and yellow system each receive hydraulic power from pumps in engine 1 & 2 respectively. The blue hydraulic system is normally pressurized by the blue electric pump, but can be supplied the Ram Air Turbine (RAT) in an emergency.
What is the “barking dog” sound on the Airbus A320?
The famous “barking dog”, or drilling sound, on the A320 is caused by the PTU (Power Transfer Unit) operating which transfers hydraulic power between the green and yellow systems. It is usually most noticeable during engine start.
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