Date: 201712
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ZZZ
Aircraft component: Parking Brake
Component problem: Malfunctioning; Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Taxi
Result: General None Reported / Taken
Contributing factors: Aircraft; Procedure
Primary problem: Procedure

Narrative: This was the latest occurrence of an event I've experienced multiple times in the last year or more. On at least 4 occasions including this one; I've done a gate return for maintenance. I've also written the issue up when a gate return was not indicated. At the completion of pushback; when the parking brake was set with only one engine operating; on the triple gauge; accumulator pressure dropped out of the green arc and brake pressures dropped to approximately 1000-1500 psi. On at least 3 occasions; the 'drop' has been enough to generate an ECAM for low parking brake pressure or low accumulator pressure. On one occasion; the pressure dropped way out of the green arc; but no ECAM was generated. After using the yellow electric pump to 'recharge' the system; it registered in the green momentarily and almost immediately fell back out of the green with the parking brake set. In each case an ECAM (including this one) showed up and the one incident where no ECAM was generated; as well as a few instances where I wrote up the system; the accumulator air pressure was determined to be very low and the accumulator was serviced by maintenance after which pressure 'held' in the green. During the proper servicing of the accumulator; mechanics test the system; usually in a properly charged system at least 7-10 brake applications are possible before the pressure generates a low pressure ECAM.During preflight; we are required to check the accumulator pressure & brake pressure indicators to verify accumulator pressure in the green band and recharge it if necessary. On specific aircraft an alternate braking system check requires brake pressure to be between 2000 and 2700 psi. Prior to start and pushback; the Flight Operations Manual (FOM) again cautions that the accumulator pressure indication must be in the green band with the parking brake ON. After a flight; during parking; FOM again requires that the accumulator pressure gauge be in the green band and that the brake pressure indications on the triple gauge be checked. FOM states that the accumulator should supply at least seven full brake applications. The problem is that as soon as the second engine is started; OR if the yellow electric pump is turned on; the pressures on the accumulator pressure and brake pressure gauge WILL rise into the green arc and stay there. This is because the system is now measuring ONLY hydraulic pressure which can/will be in the green even with the accumulator air pressure is abnormally low. In the event of a failure where the accumulator was required for braking; it would potentially NOT supply the necessary brake applications/pressure. I have asked coworkers; Check Airmen; training captains and the Line Standards Captain; Maintenance/mechanics; and the Airbus Program Manager and still haven't gotten a consistent or satisfactory answer to my concerns. Multiple coworkers and a few check airmen/training Captains either haven't noticed this problem or don't believe it's an issue. I respectfully disagree and am concerned that we are not noticing OR are accepting an 'abnormal' that could become a safety issue. In many aircraft the pressure is dropping; sometimes to the point of generating an ECAM after only a single brake application. Is it correct to accept pressure dropping out of the green with only one brake application during pushback when our procedures require multiple checks that pressure to be in the green prior to pushback and/or during parking? When an accumulator should provide 7 brake applications? I do not expect the pressure to stay in the green through all 7 brake applications! Can our procedures be changed to safely ensure that the accumulator air pressure is adequate (since it will not be obvious after the 2nd engine is started or if the yellow pump is turned on)? Perhaps parking brake should be set and pressure checked during pushback with only ONE engine started. No ECAM or indication will indicate an abnormal once both enginesare started or if yellow pump is on. I've also had coworkers and check airmen state that they aren't concerned as long as the pressure doesn't drop 'too far' out of the green! How do you quantify 'not too far?' Can Airbus or someone verify whether it is normal and acceptable for the pressure to drop out of the green arc with a single brake application (only one engine running only)? If so; how far? By the time an ECAM is generated for low accumulator pressure or low parking brake pressure; how 'deficient' is the system? How many brake applications would be guaranteed?I have also asked whether the accumulator pressure is regularly checked during maintenance inspections and haven't gotten a consistent answer to that question either. Maintenance control recently told me that the accumulator pressure is checked during the 8 day checks. Several mechanics have indicated that the system may not be getting serviced properly i.e. the air charge is being filled; but the system is not being 'bled' and tested as it should be for proper servicing. Regardless; I'd like this issue addressed and would appreciate an answer from a reliable source as to what is considered acceptable brake and accumulator pressure indications during pushback. If the system is not in the green and/or brake pressure is below 1500 psi; how reliable is the system?

Callback: The reporter stated that the issue of the brake system pressure being low is not an unusual occurrence. The reporter stated that if you start the second engine prior to setting the brakes you will not see the low pressure in the brake system; but if you set the park the brake prior to starting Number 2 engine; (second engine in the start sequence); you will see a drop in the brake pressure and perhaps get an ECAM message if the accumulator pressure is low. The reporter also stated if ground personnel operate the cargo door that will deplete the yellow hydraulic system which is used for the parking brake. The reporter stated that normally you should get seven or eight pumps with a fully charged accumulator. The reporter also stated that very often if the aircraft has sat overnight; the first flight of the day you only get maybe two or three pumps before you lose all hydraulic pressure to the brakes. The reporter stated that perhaps the accumulators are losing pressure overnight.  The reporter stated that the concern is that if something should happen during pushback where you need to use the brakes (such as the tow bar coming loose); they question whether there is enough brake pressure to stop the aircraft.



Synopsis: A320 Captain reported that when the parking brake was set with only one engine operating; the brake pressure dropped out of the green arc and generated an ECAM for Low Parking Brake Pressure or Low Accumulator Pressure.