An air carrier flight crew reported their parking brake failed during push back and the aircraft rolled into the towbar.
Synopsis
An air carrier flight crew reported their parking brake failed during push back and the aircraft rolled into the towbar.
Narrative
Performed an external air engine start #1 engine at the gate. Pushed back from gate to the release point on ramp. Was told to set brakes; I did so and got the correct indication on the triple indicator as well as the Parking brake on the ECAM. While waiting for the ground personnel to disconnect and show me the gear pin I noticed that the airplane was rolling forward; I immediately stepped on the brakes and brought the aircraft to a stop. Right as I stepped on the brakes I felt the aircraft impact the tow bar. I asked the ground personnel if they were still there and they responded. They said that no one was injured/hurt but we had ran over the tow bar. I then looked at the triple indicator and the pressure was at zero but the parking brake indication on the ECAM was still there. Not knowing exactly what had happened and not wanting to move the aircraft; I asked the ground personnel to get chocks while waiting for maintenance to come do an inspection. We then shut the engine down and were eventually towed back into the gate. The parking brakes accumulator pressure went to zero after setting the brakes. The parking brakes failed after setting them; not sure what I could have done differently.
Second reporter narrative
Completed air start on number 1 engine at the gate (APU MEL). Called for push back clearance. Aircraft was pushed backed from the gate to drop point. Ground crew called for parking brake. Captain set parking brake. All indications showed parking brake set. Captain than called for clear to disconnect. During the disconnect Aircraft began to roll forward with the parking brake still set. Aircraft was stopped; Maintenance came out looked at aircraft and determined we were able to be towed back to the gate. Aircraft seemed to lose hydraulic pressure in the accumulator for parking brake.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.