Narrative:

After an uneventful flight; approach and landing we proceeded to gate. The APU was up and running so upon arrival at the gate; with the airplane stopped; we shut down both engines. I glanced at the brake pressure indicator and observed brake pressure. We were waiting for the doors to be disarmed due to the jetway not being next to the airplane; therefore the first officer and I started putting our charts away. It was after putting away my charts that I realized we were moving backwards and the jetway driver was trying to get my attention. I instinctively stepped on the brake pedals and the airplane stopped moving. At first I thought I forgot to set the brakes because our procedure is to release parking brake then apply the brakes. However we both confirmed that the brake lever/switch was in the 'brake set position;' but only the accumulator pressure was showing. We recycled the brake handle after also cycling the anti-skid and nose wheel steering switch and the pressure was back up on all three needles on the brake pressure indicator. We don't know which action actually cleared the problem as we were not able to duplicate it. We had to be towed forward about 6 ft and after brake set again; with 'pressure normal' we proceeded to wait for jetway. After doors disarmed we completed the parking checklist. Thankfully; there were no personnel or equipment near the airplane as this could potentially have caused damage or injury. I will from now on check and double check that pressure indicator and be more aware of my surroundings until the chocks are set. Too many times we feel that when the brake is set we are done with our airplane. I have learned a valuable lesson today.

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Original NASA ASRS Text

Title: An A320 began rolling backwards at the gate after the parking brake was set but somehow pressure released.

Narrative: After an uneventful flight; approach and landing we proceeded to gate. The APU was up and running so upon arrival at the gate; with the airplane stopped; we shut down both engines. I glanced at the brake pressure indicator and observed brake pressure. We were waiting for the doors to be disarmed due to the jetway not being next to the airplane; therefore the First Officer and I started putting our charts away. It was after putting away my charts that I realized we were moving backwards and the jetway driver was trying to get my attention. I instinctively stepped on the brake pedals and the airplane stopped moving. At first I thought I forgot to set the brakes because our procedure is to release parking brake then apply the brakes. However we both confirmed that the brake lever/switch was in the 'Brake Set Position;' but only the accumulator pressure was showing. We recycled the brake handle after also cycling the Anti-Skid and Nose Wheel Steering switch and the pressure was back up on all three needles on the Brake Pressure indicator. We don't know which action actually cleared the problem as we were not able to duplicate it. We had to be towed forward about 6 FT and after brake set again; with 'pressure normal' we proceeded to wait for jetway. After doors disarmed we completed the parking checklist. Thankfully; there were no personnel or equipment near the airplane as this could potentially have caused damage or injury. I will from now on check and double check that pressure indicator and be more aware of my surroundings until the chocks are set. Too many times we feel that when the brake is set we are done with our airplane. I have learned a valuable lesson today.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.