A B747-400 AFTER PARKING AND ENG SHUTDOWN; ROLLED BACKWARDS 25 FT BEFORE BRAKES APPLIED AND CTL REGAINED.

Narrative

THE CAPT PARKED THE ACFT NORMALLY AT LAX GATE XY; SET THE PARKING BRAKE; AND BEGAN GOING THROUGH HIS PARKING FLOWS (INCLUDING TURNING OFF THE SEAT BELT SIGN TO LET THE PAX UP). THE FO SHUT DOWN THE ENGS AND WAS GOING THROUGH HIS PARKING FLOWS. I HAD STARTED FINAL RECORD KEEPING; NOTING THE IRS LATITUDE/LONGITUDE GND SPD READINGS; AND THE 'IN' TIME ON THE ACARS. I HEARD APPROX 3 CHIMES AND LOOKED UP TO NOTE 'GND CREW CALL' ON THE INTERCOM CTL PANEL. AT THE SAME TIME I NOTED MOVEMENT IN MY PERIPHERAL VISION; AS THOUGH WE WERE MOVING BACKWARDS; BUT THOUGHT IT WAS AN ILLUSION CAUSED BY THE JETWAY BEING MOVED. AT THE SAME TIME; THE CAPT WAS ANSWERING THE GND CALL; AND NOT RECEIVING ANY REPLY. I NOTICED THE ACFT DRIFTING SLOWLY BACKWARD FROM THE TERMINAL THROUGH THE FORWARD COCKPIT WINDOW AND SAID SOMETHING TO THE EFFECT OF 'WE'RE MOVING' OR 'WE'RE BACKING UP;' AND THE CAPT IMMEDIATELY APPLIED THE BRAKES; STOPPING THE ACFT. THE ACFT HAD ROLLED BACK ABOUT 25 FT FROM THE INITIAL PARKING POS; AND ABOUT 15 FT FROM WHERE I FIRST NOTED MOTION. THE MECH CAME ON THE INTERCOM AND SAID HE HAD TRIED TO GET OUR ATTN WITH THE CALL BUTTON WHEN HE NOTICED US ROLLING BACK FROM OUR PARKING POS. WE COORDINATED WITH THE MECH TO HOOK UP A TOW TRACTOR TO THE ACFT; TURNED THE SEAT BELT SIGN BACK ON AND MADE AN ANNOUNCEMENT TO THE PAX TO BE RESEATED UNTIL THE ACFT WAS MOVED INTO A FINAL PARKING POS; AND REPARKED THE ACFT. REPARKING WAS NORMAL; AS WAS PARKING CHKLIST. WE DEBRIEFED AS A CREW PRIOR TO EXITING COCKPIT. TAXI SPD AND BRAKE PRESSURE WERE NOTED BY FO ON INITIAL PARKING APCH. BRAKE SETTING PROCS AND 'FEEL' WERE NORMAL TO CAPT. ALL FO'S NOTED NORMAL BRAKE SETTING MOTIONS BY CAPT. NO ONE REMEMBERS FOR SURE NOTING 'PARK BRAKE SET' EICAS MESSAGE; ALTHOUGH CAPT THOUGHT HE HAD SEEN IT. 'IN' TIME WAS LOGGED BY ACARS AS THOUGH PARK BRAKE HAD BEEN SET NORMALLY. 2 OF FO'S NOTED BRAKE PRESSURE RETURNING TO 3000 PSI FROM SOME UNDETERMINED NUMBER AROUND 2000 PSI AFTER CAPT STOPPED ACFT AND BEFORE PARK BRAKE WAS RESET.