Aircraft systems: the hydraulic system

Welcome to part three, the hydraulic system of the A320, chapter 29. The exam scope stops after the Green system, so we cover the architecture shared by the three systems, the controls and indications, and then follow the Green system from its reservoir to its users; the detailed Yellow and Blue pages are out.

The A320 has three independent hydraulic systems, named by colour. Green works as the main system: it has a pump driven by engine 1, the most users and the biggest reservoir. Yellow behaves as an alternate system: an engine-driven pump on engine 2, plus an electric pump and a hand pump. Blue is the emergency system, with an electric pump and the ram air turbine; Airbus does not use these labels, but compared with other aircraft those are their roles. All three work at 3000 psi, and the engine-driven pumps are of the variable displacement type. Each system has its own reservoir, pumps, pipes and users, so a leak in one cannot drain another; the only link is the power transfer unit between Green and Yellow, which is mechanical and acts at a pressure difference of 500 psi. And a detail the quiz tests: all three reservoirs are filled from the service panel of the Green system.

This deck page lists the users of each system, read upwards from each reservoir. Green, the biggest, supplies the landing gear and its doors, the nose-wheel steering, the normal brakes, the reverser of engine 1, the flap and slat motors, the trimmable horizontal stabiliser, the rudder, a yaw damper and many flight control servos. Yellow supplies the reverser of engine 2, the flaps, the cargo doors, the alternate brakes and its share of flight controls. Blue supplies the constant speed motor generator, which is the emergency electrical generator, the slats and the rest of its flight control servos. The flight controls show the redundancy philosophy: each aileron has two servo actuators, one Blue and one Green. And the elevators: on the left one servo is Green and the other Blue, on the right one Blue and one Yellow, so pitch control survives the loss of any single system; fail safe, as the notes say.

Let us look at the power sources. The engine-driven pumps, Green on engine 1 and Yellow on engine 2, sit on the accessory gearbox, so they turn whenever the engine turns, even when they are not delivering pressure. They are variable displacement pumps: pistons in a rotating barrel, like the cylinder of a revolver, rest on a plate whose tilt is set by a solenoid valve; tilted, the pistons stroke and deliver fluid at 3000 psi, and with the plate neutral there is no flow, which is the state with the pushbutton at OFF. They are fed from the reservoir at 50 psi so that they never cavitate, and the same fluid lubricates and cools the pump and returns through the case-drain filter. The alternating current electric pumps are a backup on Yellow, also used for the cargo doors, and the main pump of the Blue system; they take fluid at 50 psi and deliver 3000. The minimum equipment list treats them differently: without the Blue electric pump the aircraft cannot fly, while the Yellow one is a category C item, 10 days to repair; since they are identical, a failed Blue pump can be swapped with the Yellow one. Finally Yellow has a hand pump, to operate the cargo doors when there is no electrical power.

Three more elements complete the power side. The ram air turbine is the emergency source of the Blue system: a two-blade turbine stowed in the left ventral fuselage whose variable-pitch blades hold a constant speed whatever the airspeed, the timing and rpm figures being on a later page beyond the exam scope, driving a Blue pump and, through the constant speed motor generator, electrical power. It is deployed by a compressed spring pack, released when either of two solenoid valves is energised, automatically on a loss of electrical power, or manually with the guarded RAT MAN ON pushbutton. Accumulators hold fluid against a nitrogen pre-charge, 1885 psi at 20 degrees in the one-litre Green accumulator, so that when a user suddenly demands flow, the pressure drop is absorbed before the pump reacts. The power transfer unit is a hydraulic motor and a pump on a common shaft, one on the Green side and one on the Yellow side: with its pushbutton in AUTO, when one system is more than 500 psi below the other, the healthy system drives the motor, which turns the pump of the weak system, transferring power but never fluid. It is inhibited during engine start and when the cargo doors are operated with the Yellow electric pump, so that an aircraft on the ground does not start transferring power unexpectedly.

The reservoirs are pressurised with air, and that is where the two systems of this exam meet. Each reservoir is pressurised to 50 psi, 3.5 bar, so that the pumps are always primed and never cavitate. The air normally comes from stage 8 of the high pressure compressor of engine 1. If engine 1 is lost, the pneumatic system supplies it, and on the ground a ground supply can be used. The reservoirs are sealed, so they keep their pressure after a pressurisation failure or an engine shut-down. If the air pressure falls below 22 psi in Green or Yellow, or 30 psi in Blue, the ECAM shows a low air pressure warning.

The crew controls the hydraulics from overhead panel 40VU, next to the air conditioning and pneumatic panels. The engine pump pushbuttons, ENG 1 PUMP for Green and ENG 2 PUMP for Yellow, are dark when the pump is pressurising; a white OFF means the pump is turning with the engine but not delivering pressure, and an amber FAULT means a failure. The Yellow electric pump shows a blue ON when it has been switched on, a non-normal state that must be cancelled later, and amber FAULT on failure. The Blue electric pump pushbutton sits in AUTO, and, with electrical power available, the pump runs when an engine is running, or in flight, which the system knows from the nose-gear shock absorber being extended, or when the BLUE PUMP OVERRIDE is selected. The PTU pushbutton arms the power transfer unit in AUTO, with its own amber FAULT. The RAT MAN ON pushbutton is protected by a red guard. A second panel further aft holds four guarded pushbuttons: BLUE PUMP OVERRIDE, used by maintenance to run the Blue pump on the ground, and the three leak measurement valves, Green, Blue and Yellow.

Pressing HYD on the ECAM control panel brings up this page on the lower display. Each system has its label and its pressure from a transmitter, green when normal, around 3000 psi, and amber when low. Each pump has a symbol: a square in line with the supply line and green when delivering pressure; across the line and amber when the pump is selected off; and an amber LO when it is selected on but the pressure switch after it reads less than 1740 psi. The reservoir shows its level with an arrow, green at the box when normal, lower and amber when low, plus an overheat indication when the returning fluid is too hot, which suggests heavy internal leakage, and LO AIR PRESS when the reservoir pressurisation falls. The fire valves of Green and Yellow are shown in line and green when open and across and amber when fully closed, and the PTU and RAT are white when available, amber while changing state, and green when supplying.

And an exercise with the HYD page: picture it before each answer appears. One: normal flight. The three system labels are green at about 3000 psi, the Green and Yellow engine pumps are drawn in line and green, the fire valves are open, and the PTU and the RAT are white, meaning available. Two: the Green engine-driven pump has failed, with the PTU in AUTO. The Green pump shows an amber LO, the PTU symbol turns green with its arrow pointing towards Green, and Green holds about 2 500 psi, still green. Three: the engine 1 fire pushbutton is pushed. The Green fire valve is drawn across the line, in amber, fully closed, and the Green pump shows LO. Four: the Green reservoir is leaking. The reservoir arrow drops below the box and turns amber when the low level is reached at 3 litres, while an overheat is a different failure: heavy internal leakage heats the returning fluid without lowering the level.

In the aircraft, every pipe carries a coded band, and this slide shows the A320 hydraulic code. There are supply and return lines, and the return lines are usually of larger diameter. The colour combination identifies a hydraulic line, as opposed to fuel; pneumatic ducts are much thicker and easy to tell apart. Arrows show the direction of flow, and black dots mark a pressure line. A numbered band identifies the system: 1 is Green, 2 is Blue and 3 is Yellow. And a part number lets the mechanic find exactly that pipe in the manuals.

The leak measurement valves deserve their own slide because the quiz uses them. In each system, a leak measurement valve isolates the group of primary flight control users from the rest of the users of that system. Normally it is open, so the flight controls are permanently supplied through it; it closes for the leak test, and on Yellow also automatically when the cargo doors are operated with the electric pump, so that no surface can move. Leaks are tested because the return flow equals the demand plus the permanent internal leakage of every component, which flows even when the users are in neutral. The test uses an external hydraulic power unit with a flowmeter; the internal leakage is read on the flowmeter, and closing the valves section by section, right wing, left wing, tail and centre, locates the guilty group by comparison with the allowed values. Excess leakage heats the fluid and degrades the system; when a servo control is suspected, it is disconnected and the section is re-tested to confirm it.

Now we follow the fluid through the Green system, which is the last part of the exam scope. The reservoir holds 18 litres at most, about 14 in normal operation, with a low level warning at 3 litres; it is pressurised to 50 psi with a low air pressure warning at 22 psi. The filling equipment of this system serves all three: a selector valve chooses whether the hand pump fills the Green, Yellow or Blue reservoir. From the reservoir, the fluid passes the fire valve, located in the left pylon and driven by an electric motor; when the crew pushes the ENG 1 FIRE pushbutton, it closes and isolates the Green fluid from the burning engine. Then comes the engine-driven pump, 3000 psi, self-regulating and of variable displacement, whose case-drain flow returns to the reservoir through its own filter, with a clogging indication at a pressure difference of 87 psi, not to be confused with the 3000 psi system pressure; a pressure switch after it triggers the LO indication below 1740 psi, and the line also feeds the reverser of engine 1.

The pump delivers into the high pressure manifold, a block that carries most of the Green system's small components, all with numbers worth knowing. The high pressure filter has a clogging indicator that pops out at a pressure difference of 87 psi, and is replaced when it does. There is a check valve, and a sample port valve to take fluid samples, which the laboratory analyses for water and particles that reveal worn components. A pressure transducer sends the pressure to the ELAC flight control computers and to the ECAM display. A pressure switch informs the ELAC, SEC and FAC flight computers, the brake and steering control unit, the BSCU, and the ECAM; it trips at 1450 psi. The relief valve opens at 3500 psi to protect the system and closes again at 3200 psi. Finally the leak measurement valve, and outside the manifold the accumulator: a one-litre steel cylinder wrapped in Kevlar with a nitrogen pre-charge of 1885 psi at 20 degrees.

The last block of the Green system is the PTU manifold, with its own dedicated set of valves. The normal brake selector valve supplies the normal braking system and cuts it off, putting it to return, when there is a problem. A solenoid valve, open with the PTU pushbutton in AUTO, cuts the operation of the power transfer unit when the pushbutton is off. Two check valves complete the PTU path. The priority valve, marked P, isolates the large-flow users, the landing gear and doors, the nose-wheel steering and the slat and flap motors, when the system pressure drops, so that all the pressure available goes to the primary flight controls and the aircraft stays controllable. The fluid returns through the return manifold, with a temperature transmitter and a return filter of 29 psi, back to the reservoir.

Back on the Green schematic, the top block is the leak measurement manifold, and it is worth knowing who sits behind it. Through the leak measurement valve, it feeds the Green primary flight control users, split into three groups: tail, left wing and right wing. The tail group holds the hydraulic motor that drives the trimmable horizontal stabiliser screw, the rudder, and one of the yaw damper actuators that prevent Dutch roll. The left wing group holds one of the two servo actuators of the aileron and spoilers 1 and 5. The right wing group is the mirror image on the right. The spool valves drawn at the top are normally closed and are used only during maintenance, to check the internal leakage section by section.

Finally, where all of this lives. Almost every Green component is in the main landing gear bay: opening the gear door is enough to see the reservoir, the high pressure and PTU manifolds and the PTU. The reservoir is made of two welded metal cylinders, with a drain valve at the bottom, an air manifold carrying a pressure gauge, a pressure switch and a check valve, and a direct-reading quantity indicator on its side. Its low level sensor is a cylindrical capacitor in which the hydraulic fluid acts as the dielectric: as the quantity changes so does the capacitance, and at 3 litres it sends the low level signal.

This last Green page shows the PTU manifold on the left and the service panel on the right. On the PTU manifold you can recognise the priority valve, the two check valves and the solenoid valve that we met in the schematic. The service panel, behind the left main gear door, has the reservoir filling selector valve, to choose Green, Blue or Yellow, and a quantity indicator with a coloured range for each system. Its filling port with a filter and its filling hand pump, not to be confused with the Yellow hand pump of the cargo doors, are used to fill all three reservoirs, which is the fact behind statement b of the 2020 quiz. It also has the suction and pressure ports to connect a hydraulic ground cart, the mule, which bypasses the Green pump for maintenance and hangar tests.

To finish the Green system, let us trace failures the way an exam statement does; pause after each question. One: the Green engine-driven pump fails in flight with the PTU in AUTO; what is the first indication? The pressure switch at the pump outlet reads less than 1740 psi, and the ECAM shows the amber LO on the Green pump. And then? As soon as Green is 500 psi below Yellow, the Yellow system drives the PTU, which holds Green at about 2 500 psi, so the 1450 psi manifold switch is never reached, and the brakes and the priority valve are not affected. Three: the same failure, but with the PTU off or inhibited, or with the Green fluid lost, so the pressure falls below 1450 psi. The manifold pressure switch tells the brake and steering control unit, which loses the normal brakes and switches to the alternate brakes on the Yellow system. Four: and as the pressure keeps falling? The priority valve shuts off the heavy users, the landing gear and doors, the nose-wheel steering and the slat and flap motors, so that what is left goes to the primary flight controls. Five: the crew pushes the engine 1 fire pushbutton. The fire valve in the left pylon closes, the engine-driven pump loses its fluid and shows LO, and the PTU can still pressurise Green from Yellow, transferring power, not fluid. Six: on the ground, the cargo doors are operated with the Yellow electric pump. The PTU is inhibited, the Yellow leak measurement valve closes and the flap computer is told not to move the flaps; these happen together, which is the trap of statement b in quiz question 5, but none causes the others.

As in part one, pause after each statement, decide, then listen. One: the engine-driven pumps give 3000 psi as soon as the engines turn. False: they turn with the engine but only deliver pressure when the ENG PUMP pushbutton is not at OFF, which tilts the swashplate. Two: the PTU can move fluid from Yellow to Green. False: it transfers power through its shaft, never fluid. Three: the Green reservoir low level warning is at 3 litres. True. Four: the Green relief valve opens at 3200 psi. False: it opens at 3500 and closes again at 3200. Five: the leak measurement valves are normally closed and opened for tests. False: they are normally open, feeding the flight controls, and are closed for the leak test. Six: the accumulator is pre-charged with nitrogen at 1885 psi at 20 degrees. True. Seven: the Green pump LO indication appears below 1740 psi. True. Eight: the reservoir air normally comes from the pneumatic manifold. False: it normally comes from stage 8 of engine 1, and the manifold is the backup.

Question 4 of the 2020 quiz, the false statements. Statement a is true: the nose-wheel steering is a Green user, fed through the priority valve. Statement b is false: there is a single filling point, the Green service panel, with a selector valve for the three reservoirs. Statement c is true: the ram air turbine drives a Blue pump and, through the constant speed motor generator, provides electrical power. Statement d speaks of hydraulic transfer: the conditions are right, AUTO, 500 psi, inhibited during engine start and cargo door operation, but what the PTU transfers is power, through its shaft, never fluid, since the systems are independent; read as power transfer, which is clearly what is meant, d is true. So the false statement is b, and if your examiner reads transfer literally as fluid, d as well: write one line saying the PTU transfers power, not fluid, and you are covered either way.

Question 5 asks for the correct statements, and requires the logic of the PTU manifold. Statement a is wrong: arming the PTU has nothing to do with the leak measurement valve. Statement b is wrong too, and it is a trap: during cargo door operation with the Yellow electric pump, the PTU is inhibited and the Yellow leak measurement valve closes at the same time, but neither causes the other, and the PTU is never inhibited because a Green leak measurement valve closes. Statement c is correct: the Green pressure switch, which trips at 1450 psi, informs the brake and steering control unit, and with Green below that value the normal brakes are lost and the alternate brakes on Yellow take over. Statement d is correct: 800 psi is more than the 500 psi threshold, so the Yellow system drives the PTU motor, and the PTU pump on the Green side draws fluid from the Green reservoir and delivers it into the Green high pressure line. So the correct statements are c and d; there is no official key, so in the exam add one line of justification to each choice.

To finish, the hydraulic numbers in one table. 3000 psi in every system, 500 psi of pressure difference for the PTU, 50 psi in the reservoirs with a warning at 22, and the Green reservoir of 18 litres maximum, 14 normal, with a low level warning at 3. 1740 psi for the pump low pressure indication, 1450 psi for the manifold pressure switch, 3500 to open and 3200 to close for the relief valve, 87 and 29 psi for the filters, and 1885 psi of nitrogen in the one-litre accumulator. And the RAT, the Blue emergency source, deployed automatically on electrical loss or with its guarded pushbutton. With the three lectures you have the whole exam scope: redo the 2020 quiz without looking, explain the TLT, the TCT and the crossbleed aloud, and sketch the bleed circuit and the Green system from memory; good luck on Thursday.