BRAKE CHECK PRESSURE 0

What is the reason behind not requiring a call for pressure 0 during a brake check on the new A320s?

The A320 aircraft has three levels of braking: normal braking, alternate braking, and the parking brake.

In normal braking, which is the primary braking system, the green hydraulic system powers the brakes. When the top of the rudder pedals is pressed, electrical signals are sent to the Braking and Steering Control Unit (BSCU). The BSCU opens the normal brake selector valve, allowing pressure from the green hydraulic system to reach the brakes and apply them.

The alternate braking system, powered by the yellow hydraulic system or a pre-charged accumulator, is the second level of braking. It is indicated by two needles on a brake pressure indicator. The top needle shows the accumulator pressure. Unlike normal braking, there is no specific indication for normal braking in the cockpit.

The parking brake also uses the yellow hydraulic system and the accumulator.

In older A320s, the BSCU controls normal braking. When the pilot presses the rudder pedals, the BSCU opens the normal brake selector valve, allowing green hydraulic pressure to reach the brakes. This action blocks the yellow hydraulic pressure using an automatic selector valve.

Alternate braking on older A320s is achieved through a mechanical system called the Dual valve. When the pilot applies toe brakes, pressure signals are sent to the Dual valve, which controls the alternate braking. The automatic selector valve should immediately block the yellow system when the brakes are pressed, and there should be no indication of alternate braking on the brake pressure indicator. If an indication is present, it could mean a failure in the automatic selector valve or insufficient green hydraulic pressure. In such cases, maintenance action is required before the flight.

In newer A320s, the alternate braking system is controlled by the Alternate Braking Control Unit (ABCU), which is a computer. There is no longer an auxiliary distribution line. When normal braking is active, the ABCU automatically shuts off the Dual valve, preventing the alternate braking system from operating. The valve is only allowed to operate when the normal braking system fails or when the green hydraulic pressure depletes. As a result, there is no need to check for a pressure zero indication, as the valve is closed off by the computer when normal braking is active. The system is no longer mechanical.

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