The A320 hydraulic system: green, blue, yellow and the PTU

The A320 has three independent hydraulic systems, green, blue and yellow, each at 3,000 PSI. No fluid can pass between them. The only link is the power transfer unit (PTU) between green and yellow, which transfers power, not fluid.

The three systems at a glance

On the ECAM HYD page a system's label and pressure turn amber below 1,450 PSI.

Why three systems?

Redundancy. Important users such as the rudder, elevators, ailerons and the trimmable horizontal stabiliser are powered by two or three systems, so losing one system does not mean losing a flight control. Because there is no fluid connection, a leak in one system cannot drain the others.

What each system powers

Summarised from the FCOM hydraulic distribution diagram. Your operator's FCOM is the reference.

Engine-driven pumps

Engine 1 drives the green pump and engine 2 drives the yellow pump. Each pressurises its system whenever the engine is running and its ENG PUMP pushbutton is on. The pushbutton's FAULT light comes on for a low reservoir level, reservoir overheat, low reservoir air pressure or low pump pressure (the last is inhibited on the ground with the engine stopped).

The electric pumps

The blue electric pump is the normal source for blue. With its guarded pushbutton in AUTO and AC power available, it runs in flight, and on the ground when one engine is running or the BLUE PUMP OVRD pushbutton on the maintenance panel is pressed.

The yellow electric pump runs when selected ON. It also starts automatically when the cargo door selector is set to OPEN or CLOSE; the other yellow functions, including the PTU, are then inhibited, except alternate braking and engine 2 reverse. If electrical power is removed, it stays off when power returns.

The PTU

The PTU is a bidirectional unit: a hydraulic motor in one system drives a pump in the other. With the PTU pushbutton in AUTO it runs automatically when the pressure difference between green and yellow is more than 500 PSI. It is inhibited during the first engine start and tested automatically during the second.

On the ground with the engines stopped, selecting the yellow electric pump ON pressurises yellow. The difference to green is now more than 500 PSI, so the PTU runs and pressurises green too. That is the barking sound you hear on the ramp. When it runs, the E/WD shows HYD PTU in green.

The RAT

The ram air turbine drops out into the airflow and drives a pump on the blue system. It deploys automatically if AC BUS 1 and AC BUS 2 are both lost, and the crew can extend it with the RAT MAN ON pushbutton. Blue then drives the emergency generator, so the RAT is also the emergency source of electrical power. It can be stowed only on the ground.

Interview questions that come up

• How do you get green pressure on the ground with the engines off? Yellow electric pump ON; the PTU then pressurises green.
• When is the PTU inhibited? During the first engine start (it is tested during the second), and while the cargo doors are being operated.
• Which system powers the nose wheel steering? Yellow.
• Which flight control has all three systems? The rudder.
• What does the ENG FIRE pushbutton do to hydraulics? It closes that engine's hydraulic fire shutoff valve: green for engine 1, yellow for engine 2.

ECAM procedures, limitations and ground-crew clearance requirements are not covered here. Always follow your operator's FCOM, QRH and SOPs. The FCOM SOP warns not to pressurise the hydraulic systems until ground personnel give clearance.