Date: 201302
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Parked
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer
Anomaly: Ground Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate
Result: Flight Crew Became Reoriented
Contributing factors: Procedure; Company Policy; Airport; Weather
Primary problem: Ambiguous

Narrative: After pulling into the gate; I received the chocks in signal from the Marshaller and released the parking brake. After we completed the 'Parking' Checklist; I noticed in my peripheral vision outside movement and immediately concluded the aircraft was rolling backwards out of the Safety Zone. I immediately applied brakes and stopped the aircraft. I opened my window and asked Ground personnel to chock the aircraft. They replied; 'It is chocked.' Because of the abrupt application of brakes; I was worried passengers may have fallen since the seatbelt sign was off and many were standing in the aisle so I went into the cabin to check on everyone. Flight attendants informed me there were no injuries. I then determined there was no damage to the aircraft and we were then pulled back into the gate. After going down on the ramp to determine what happened; I was shown how slick it was on the ramp; and especially the line paint due to deicing fluid left over from procedures earlier in the day. The aircraft indeed was chocked and simply rolled backward about 12 inches because the ramp was so slick the chock was unable to get the friction required to work properly.


Second narrative: We pulled into the Safety Zone and stopped the aircraft when directed by the Marshaller and set the parking brake. We were already on APU power due to no conditioned air at our gate and cold temperatures outside. The Marshaller gave the signal that chocks were in place and we shutdown the remaining engine. We started the 'Shutdown Checklist' (the first step is to release the parking brake). The Captain released the brake and we continued with the checklist. Just as the checklist was completed; I looked outside and noticed what I perceived was the jet bridge moving in the wrong direction and then realized we were rolling backwards. The Captain noticed at the same moment and immediately applied the brakes stopping the aircraft.The Captain contacted the Cabin crew to verify that everyone was okay due to the abrupt stop and the fact that people were already standing in the aisle. Everyone was okay and we had them all return to their seats so we could be pulled back to the parking spot. Once we were able to talk to the Ground Crew; they indicated that the chock was sliding along the ground behind the tire due to the painted stripe and deice fluid on top of the paint. The chock would not hold the aircraft in place. Once the aircraft was secure at the gate; we verified no injuries and no aircraft damage. Notify crews to keep the parking brake set on at this gate due to the slight downhill slope and slick conditions.


Synopsis: B737 flight crew detects their aircraft moving backwards after engine shut down at the gate and parking brake release and the Captain immediately applies normal brakes. Deicing fluid on the ramp and the painted lead in line combined to allow the nose wheel to push the chock on the slight incline.