FENIX A320 | Accumulator & Brake Pressure
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The Accumulator & Brake Pressure indicator also called the Triple Pressure Indicator, is only used to indicate the alternating mode braking system or braking used with the yellow (Y) hydraulic system.
The parking brake uses just the yellow system, and when engaged, the Parking Brake Selector Valve (PBSELV or Parking Brake Selector Valve) opens, allowing hydraulic pressure to apply the brakes and keep the aircraft in its parking position.
Note: This is the exact reason why during engine start-up, engine number 2 is turned on first if single engine cab is not required. In addition, whenever the parking brake is applied, the Triple Pressure Indicator is always checked to verify that the braking system is active.
Accumulator Pressure:
The topmost pointer indicates the pressure available in the yellow system accumulator dedicated to the brakes.
The accumulator is a 'unit that is automatically pressurized by the associated hydraulic system when the aircraft engines are running. It can also be pressurized (or recharged) manually through the Yellow electric pump.
If the Green (G) and Yellow (Y) hydraulic systems fail, alternate braking is provided by this brake accumulator alone. It is designed to provide enough pressure for at least 7 full applications.
Brake Pressure:
The last two hands on the other hand, Indicate the yellow system pressure delivered to the right and left main bogie brakes, measured upstream of the Servo-valves. Normally it is the Green (G) hydraulic system that supplies pressure to the brakes in normal mode.
Note: Brake pressure is indicated only when the Yellow hydraulic system controls brake pressure, that is, when:
Alternate mode braking system is activated.
If The parking brake is applied.
*Why is it very important to check the brake pressure?
it is important to check these two indicators after the parking brake lever is set to ON and before releasing the brake pedals. This prevents unexpected aircraft movement resulting in potential injury or significant damage in the event of a collision with obstacles on the ground.
❚ ALTERNATE BRAKES ❚
The yellow hydraulic system automatically provides braking action in alternate mode when:
● Green hydraulic pressure is insufficient
● Power supply is interrupted
● Both BSCUs are faulty
● The A/SKID & N/W STRG switch is selected to OFF.
Note: In the last three cases, alternative braking is available but without anti-skid. Based on the demand from the brake pedals, the ABCU controls the pressure passing through the DDV (Direct drive valve) Servo-Valve.
Since anti-skid is not available, to prevent wheel locking and limit the risk of tire blowout, the brake pressure is automatically limited to 1000 PSI.
Another interesting thing, is when you press the brake pedals several times, you are able to check whether the DDV distribution valve of the alternating brake is actually opening as required by the ABCU. This is to ascertain that in case of failure of the braking system in normal mode, the alternating mode will work properly with the pedals.
Switch A/SKID & N/W STRG
On ON: If green hydraulic pressure is available, anti-skid is available. If green hydraulic pressure is lost, then:
● pressure in the yellow hydraulic system automatically takes over to supply the brakes;
● anti-skidding and front wheel steering remain available.
● The brake pressure indicator shows the pressure exerted by the yellow hydraulic system.
On OFF: The yellow hydraulic system supplies pressure to the brakes through the "Alternate Brake Selector Valve" controlled by the ABCU. Therefore:
● Anti-skid is no longer available. The rider must refer to the ACC & BRK Pressure indicator to limit brake pressure and avoid locking a wheel.
● Front wheel steering is no longer available.
● Differential braking remains available through the pedals.
● The brake pressure indicator shows the pressure exerted by the yellow hydraulic system.
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