ATA 29 4th hydraulic system on A320, are you sure?

Well it depends how are you looking on it.
It is called
Auxiliary Low-Pressure Control-System and it is here as hydromechanical control of alternate brake system
But it is only on classic A320F.
On enhanced version it was replaced by electromechanical control ABCU- Alternate brake Control Unit
Here is description of components which you can find on Hydromechanical controls:
Brake-control hydraulic reservoir


The reservoir supplies the master cylinders of the Alternate system with hydraulic fluid to make allowance for:

-

the changes in the volume of the fluid,

-

possible leakages.

A spring-loaded piston pressurizes the reservoir lightly (1.5 bars absolute).

The reservoir has a filling valve and two calibrated valves:

-

one which limits the internal pressure,

-

the other which removes fluid if an overpressure occurs.

(2)

Master cylinders

There are two master cylinders (one for the left side, one for the right side).

The master cylinders are used to:

-

control the distribution dual valve of the Alternate brake system by displacement of a volume of fluid from chamber C1,

-

give artificial feel at the pedals by means of an internal spring and the load resulting from the pressure that increases in the control circuit.



When the pedals are released an internal valve opens to connect the chamber C1 to the upper chamber C2 which is then connected to the hydraulic reservoir.

The chamber C2 contains a safety reserve of fluid.

(3)

Alternate-brake distribution dual-valve


The distribution dual valve includes two independent pressure-reducing valves, one for each main gear. A master cylinder operates each valve.

The distribution dual valve is installed in the hydraulics compartment.


A transparent drain tube connected to the cover of the distribution dual valve, allows to check for possible presence of water.

A cap closes this tube.

Each pressure reducing valve includes a control piston which operates a distribution slide valve through a spring R1 and a rocker arm.

When the pedal is released, the Yellow pressure at A is shut off and the brake port C is connected to the reservoir return B.

When the pedal is pushed in, the volume of fluid moved by the master cylinder causes the displacement of the piston which then operates the rocker arm and the slide valve; B is shut off and the pressure port A is connected to the brake port C.

When line C is filled, the pressure pushes on the end of the slide valve (chamber D) which, through the rocker arm, causes the spring R1 to compress; the control piston remains in the same position.

A check valve is installed to rapidly move the fluid from the chamber D to the brake port C: thus the slide valve can be in the fully open position as soon as the pedals are pushed in and the brakes are filled without delay.

In balanced state, the pressure supplied is in proportion to the compression of the spring R1 and consequently to the displacement of the control piston.

The pressure increases in the control line as a result of the compression of the return springs of the control piston.

The delivery pressure (decreased) in relation to the control piston travel and in relation to the control pressure is shown on the graphs.

The maximum pressure supplied is 175 bars (2537 psi)
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———-Video content———————

00:00 - Intro
00:13 - Auxiliary Low-Pressure Control-System
00:28 - components
02:10 - Function of components
05:48 - Difference between classic and enhanced A320
06:45 - components of ABCU and they're function
09:18 - Outro

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