Air carrier flight crew reported the aircraft parking brake disengaged for unknown reasons after parking at the gate. The aircraft rolled back about 8 to 10 feet before the Captain stopped the aircraft and reset the brakes.
Synopsis
Air carrier flight crew reported the aircraft parking brake disengaged for unknown reasons after parking at the gate. The aircraft rolled back about 8 to 10 feet before the Captain stopped the aircraft and reset the brakes.
Narrative
I parked the aircraft at the gate and set the parking brake. The FO (First Officer) and I completed our shutdown flows and completed the Parking Checklist. The Marshaller gave us the inset chocks" signal and I acknowledged the signal. I unfastened my seat belt and stood up to unlock the flight deck door. When I turned back around I noticed the aircraft moving backwards. I immediately sat down; applied the brakes; brought us to a stop and reset the parking brake. I would estimate we rolled back 8 to 10 feet. I instructed the Flight Attendants to ensure all the passengers were seated until we were parked at the gate. The Ground Crew connected the tug to the aircraft and towed us to the gate. I then reset the parking brake and communicated to the Cabin Crew it was safe to let the passengers stand and collect their belongings. The FO and I completed the Parking Checklist again; and all passengers deplaned normally through the jetbridge.I could have waited longer to open the flight deck door until I was certain the chocks were inserted and the aircraft would not move. Also re-check the Parking Brake light is illuminated; prior to leaving my seat. FO did not notice we were moving because he was gathering his belongings to leave the aircraft."
Second reporter narrative
Parking brake disengaged causing the aircraft to momentarily roll back. Aircraft was stopped by use of the foot brakes.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.