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As part of the preliminary cockpit preparation in my company’s older A320s we have to perform an alternate brake system check but not in the newer ones as per FCOM Normal procedures as below. I found out that there is a system difference but didn’t fully understand could someone please explain the difference?

ALTERNATE BRAKING SYSTEM
Note: The purpose of this check is to verify, before the first flight of the day, the efficiency of the alternate braking system (absence of “spongy pedals”).
Y ELEC PUMP pb-sw....................................................................................................CHECK OFF CHOCKS..............................................................................................................CHECK IN PLACE PARK BRK handle.......................................................................................................................OFF BRAKE PEDALS.....................................................................................................................PRESS Apply maximum pressure on both pedals.
BRAKE PRESSURE (on BRAKE press indicator).................................................................CHECK Pressure must build up without delay symmetrically on left and right sides for the same application simultaneously applied on left and right pedals. With full pedal deflection, the pressure must be between 2 000 and 2 700 PSI.
BRAKE PEDALS................................................................................................................RELEASE PARK BRK handle........................................................................................................................ON The parking brake must be on during the exterior inspection to allow the flight crew to check brake wear indicators.

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  • $\begingroup$ Different braking systems, see Alternate Brake Check on A320/321 $\endgroup$ – mins Jan 11 '18 at 9:19
  • $\begingroup$ I still didn’t understand? This is the answer which my company has provided. But I don’t understand the logic. The aircraft is without electrically controlled Alternate Braking Control Unit (ABCU). Alternate braking in turn is controlled by BSCU.As a consequence of this during the preliminary cockpit preparation, it is necessary to check the alternate braking system as it is not automatically monitored. $\endgroup$ – Jai Jan 11 '18 at 9:38
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In older A320s, the alternate braking system unlike the normal braking is not computer controlled. The alternate braking is controlled by auxiliary LP pressure distribution lines and is entirely mechanical. When you press the brakes with normal braking unavailable, pressure is sent by the hydraulic lines which closes off the automatic selector valve and opens a dual valve through which the hydraulic pressure is sent for braking.

Interestingly, even when normal braking is available, the pressure is sent through the alternate pressure lines (powered by yellow hydraulic system) and it continues to supply hydraulic pressure and the dual valve also opens. But as the larger side of the automatic selector valve is placed on the side of the normal braking line, the green hydraulic pressure overcomes the yellow pressure and keeps the yellow side closed preventing the fluid reaching the dual valve. This way, the alternate braking is inhibited.

enter image description here

The alternate brake check we do in the preliminary cockpit preparation is to check whether the dual valve operates and functions as it should. As the entire system is mechanical, it is good practice that we check the working of the alternate braking system before the first flight. You do not want a situation where you lose breaking due to a sticky valve, do you?

Why this procedure is not required in newer A320 models is because, the alternate braking is no longer controlled with auxiliary distribution lines. Like the BSCU (Brake and steering control unit) computer that controls the normal braking, now there is a separate computer called an ABCU (Alternate braking control unit). With this, the dual valve which was previously controlled manually is now controlled by the ABCU. When the green hydraulics or the normal braking is not available, the ABCU orders the dual valve to open and when normal braking functions as it should the valve is kept close.

enter image description here

https://web.facebook.com/A320Theory/photos/a.207337252739678/790898894383508/

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On the old system the switching between the normal braking using the green circuit to the alternate braking using the yellow circuit is done by an automatic switching valve pressure dependant (Automatic selector).The alternate system being very little used, it happened the valve did not switch from green to yellow when loosing the green circuit

please refer to the old system here below

http://www.a320dp.com/A320_DP/landing-gear/images/braking-schematic-d.jpg

On the new system the alternate brake is activated by a specific computer(ABCU) decision and is automatically tested by the computer at power up as soon as the hydraulic pressures are available, please refer here below to the new circuit

https://m.facebook.com/A320Theory/posts/normal-brakingnormal-braking-is-obtained-when-the-green-hydraulic-pressure-is-av/790898894383508/

Hope this has been useful and explains why it was very important to do the testing on the old system

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