A320 flight crew reported the aircraft rolled unexpectedly after ground personnel disconnected the tow bar because the parking brake failed to engage.
Synopsis
A320 flight crew reported the aircraft rolled unexpectedly after ground personnel disconnected the tow bar because the parking brake failed to engage.
Narrative
After push/pull from the gate; we started ENG #1. We set brakes; and told the tug driver Brakes set…cleared to disconnect inter phone." Unbeknownst to us; immediately after ENG #1 completed its start cycle; the parking brakes failed. We started to very slowly drift forward. For a moment I thought it was an optical illusion. I looked at the parking brake and it was still set. The tug driver screamed "STOP; THE PLANE IS MOVING". I forcefully applied the toe brakes and we came to an unusually smooth slow stop. Right at that moment my First Officer (FO) pointed to the accumulator pressure gauge and it was reading"0"! We assumed that the smooth stop might have been that the accumulator pressure was already near zero when I applied the toe brakes. I kept pressure on the brakes until a Super Tug could be hooked up to our aircraft and tug us to a gate. We spoke to line maintenance and Maintenance Control while waiting to be tugged back in and went through steps with Maintenance control to restore braking. Recycling the nose wheel switch had no effect on accumulator pressure but recycling the parking brake did. Unfortunately; the pressure fell back to zero fairly quickly. Maintenance control mentioned some sort of Technical Standard Order (TSO) issue they were working regarding brakes but I wasn't clear as to exactly how that applied to our situation. We returned to the gate and Maintenance control took the aircraft out of service Cause: In our post event debrief I told my FO I was surprised that we didn't hit the tug and how fortunate that was! He told me that the tug appeared to be clear to aircraft right when we started drifting and that she apparently was still attached to the aircraft by the com line. I'm pretty certain that's not in keeping with SOP's but in this case; it was a fortunate mistake for us; as she was the one who alerted us to the fact that we were drifting. As for us; I obviously couldn't help but wonder if I checked the triple indicator carefully enough before engine start or if complacency had been a factor. After realizing that the inbound crew parked the aircraft without incident; the FO and I checking it during our preflight flows and the troubleshooting steps with Maintenance control; it was unlikely that we missed it and it was just a failure without any warning!"
Second reporter narrative
During pushback; the Captain set the brakes once the push was complete. We had started one engine; as the tug crew pulled away; the plane started to roll. We heard the ground crew still connected on headset yell out and scream; stop stop stop. The Captain proceeded to apply manual brake pressure. I immediately looked at the brake pressure accumulator which showed 0. No damage to the aircraft occurred. We proceeded working with Maintenance control; line maintenance; ops and ramp to get the aircraft tugged back to the gate. Cause: Mechanical issue
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.