AN MD90 PARKED ON GATE ON ARR WITH WHEELS CHOCKED ROLLED BACKWARDS WHEN THE PARKING BRAKE WAS RELEASED. CHOCKS MOVED DUE TO SLIPPERY RAMP.

Narrative

COMPLETED SCHEDULED FLT SEGMENT WITH LNDG AT ZZZ. TAXI TO GATE WAS NORMAL; WITH SLUSH AND ICE ON TXWYS AND RAMP. I STOPPED THE ACFT AT THE GATE WITH NORMAL USE OF BRAKES. EXTERNAL PWR WAS CONNECTED; ENGS SHUT DOWN; AND RAMP AGENT SIGNALED 'CHOCKS IN.' I RETURNED THE SIGNAL; VERIFIED BOTH ENGS OFF; AND RELEASED BRAKES. FO CALLED SHUTDOWN CHK COMPLETE. I LEFT MY SEAT TO OPEN THE COCKPIT DOOR. WHILE UNLOCKING THE DOOR I FELT A HARD JOLT. INITIALLY; I THOUGHT THE JETWAY HIT THE ACFT; BUT WHEN I LOOKED OUTSIDE; THE JETWAY WAS NOT NEAR US. THE FO RPTED THAT THE ACFT HAD ROLLED BACKWARDS. HE APPLIED THE BRAKES AS SOON AS HE WAS AWARE OF THE MOTION. THE JOLT WAS THE ACFT STOPPING ABRUPTLY. I TURNED THE SEATBELT SIGN ON AGAIN; AND MADE AN ANNOUNCEMENT FOR PAX TO BE SEATED SO THE ACFT COULD BE TOWED BACK INTO POS. RAMP PERSONNEL CHKED FOR DAMAGE; FLT ATTENDANTS CHKED FOR INJURIES. NO DAMAGE OR INJURIES FOUND. AN ACFT WAS TOWED BACK TO STOP POINT; CHOCKS REINSTALLED; AND BRAKES REMAINED SET. GND CREW SAID THAT THE CHOCKS WERE IN PLACE WHEN THEY SIGNALED IT. THEY SAID THE RAMP WAS SO SLIPPERY THAT THE ACFT SLID BACKWARDS WITH THE CHOCKS IN PLACE. HOWEVER; BRAKING ACTION ON THE RAMP APPEARED NORMAL; AND THE ACFT STOPPED MOVING ABRUPTLY WHEN THE FO APPLIED BRAKES. ALSO; I FOUND NO MARKS IN THE ICE OR ON THE RAMP CONSISTENT WITH TIRES OR CHOCKS SLIDING.