Flight crew reported the parking brake did not hold pressure during tug disconnect during push back. The PTU system did not activate so the Captain activated Yellow Hydraulic System to pressurize the parking brakes and stop the aircraft movement. The aircraft taxied back to the gate for maintenance.
Synopsis
Flight crew reported the parking brake did not hold pressure during tug disconnect during push back. The PTU system did not activate so the Captain activated Yellow Hydraulic System to pressurize the parking brakes and stop the aircraft movement. The aircraft taxied back to the gate for maintenance.
Narrative
Pushed back from ZZZ Gate. Cleared to start during push back. Started ENG 1. Push completed and asked to set brake. Set the brake; ECAM Park Brake message visible; I am certain of this. Brake Triple indicator showed brake pressure zero (Nose Wheel Steering bypass pin still in). I am also required to check brake accumulator and I sure believe I did this. I always point to it and verbalize it. The tug was a claw type and when it was disconnecting the nose wheel dropped hard; and this was somewhat startling. I then noticed the aircraft start to move forward. I was surprised an asked to ground crew if they were pulling us forward. I looked at the ECAM; still had PRK BRK indication. I hesitated to slam on the brakes because I honestly thought the tug was pulling us. I looked at the triple indicator and the accumulator indicated zero psi. The aircraft moved several feet forward and I then applied the brakes. Holding the brake pedals; I pulled up the HYD page; Yellow system was unpressurized. Turned on Yellow electric pump and cycled the parking brake handle. Brake set normally. Because the #1 ENG was started during push back with the brake off; I thought the PTU (Power Transfer Unit) would kick in and pressurize the yellow system. When I set the brake; the PTU was not running; and I forgot that this happens with the parking brake set. Spoke to ground crew via interphone and verified no injuries or damage. After they were clear; started ENG 2 and returned to gate; called ZZZ Maintenance. I wrote up what happened in the aircraft logbook. Maintenance came and ran a full test of the PTU system.I still am uncertain exactly how this happened. It is possible that after I did the brake check at the gate; the accumulator pressure was low; and it's possible that I didn't fully repressurize with the yellow electric pump as a usually do. It's also possible that pressing the pedals prior to setting the parking brake reduced it further. It's very disconcerting that this happened. I am generally very methodical and do it the same way every time. It's disturbing to think I may have missed something. I am glad that there were no injuries or damage; but I know that there could have been; and that is weighing on me heavily. After consulting my base chief; I completed the sequence uneventfully.Closer attention to brake accumulator pressure during push back.
Second reporter narrative
Flight ZZZ to ZZZ1 during push back from gate in ZZZ in A319 (Aircraft X) ground crew disconnected tow bar from nose. Captain set brake and aircraft seemed to roll forward with brake set. Captain called Ground Crew via interphone to inquire what happened and after brief conversation with Ground Crew elected to return to gate for maintenance safety inspection. We started the number two engine after ground crew was clear and coordinated with operations to obtain new gate. After pulling into new gate and shutting down; maintenance personnel came to inspect and upon successful inspection and maintenance checks; continued flight to ZZZ1 and pushed off gate uneventfully.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.