MLG SUPPOSEDLY PARKED; ROLLS BACKS PRIOR TO CHOCKS INSTALLED.

Narrative

UPON ARR AT THE GATE IN ATLANTA ON FLT MEM-ATL AND DURING PARKING OF THE ACFT AND ENG SHUTDOWN; A SIT OCCURRED THAT RESULTED IN THE ACFT ROLLING BACKWARDS UNCTLED. THE ATL TERMINAL HAS A LIGHT SYS FOR PARKING. THE RED LIGHT COMING ON IS THE SIGNAL TO STOP THE ACFT. WHEN THE LIGHT GOES OFF OUR COMPANY PROCS SPECIFY THAT MEANS THE CHOCKS ARE IN. THE LIGHT IS TURNED OFF BY A RAMP AGENT WHO THEN FOLLOWS THAT ACTION WITH HAND SIGNALS INDICATING THE CHOCKS ARE IN. WHILE WAITING FOR THE LIGHT TO GO OUT; MYSELF AND THE COPLT ARE INVOLVED IN SHUTTING DOWN THE #1 ENG WHICH WE; TYPICALLY; TAXI IN ON. WE HAVE ALREADY SHUT DOWN #2 WHILE TAXIING IN; UNDER NORMAL CIRCUMSTANCES; AND THAT WAS THE CASE THAT DAY. THE REMAINING ENG IS SHUT DOWN ONCE EXTERNAL PWR IS PLUGGED IN TO THE ACFT. AFTER SHUTTING DOWN #1 AND WHILE WAITING FOR THE RED LIGHT TO GO OUT; I INADVERTENTLY RELEASED THE BRAKES (THE PARKING BRAKE WAS NOT SET). BECAUSE THE RAMP IS NOT LEVEL THE AIRPLANE BEGAN TO SLOWLY ROLL BACKWARDS. THIS MOVEMENT WAS NOT APPARENT TO ME OR THE FO; CONTRIBUTING TO OUR LACK OF RECOGNITION IS THE MOVEMENT OF THE JETWAY. EXPERIENCING THIS JETWAY MOVEMENT WHILE PARKED AT THE GATE GIVES THE SENSATION OF MOVING. THIS OPTICAL ILLUSION IS WELL KNOWN AND ROUTINELY DISREGARDED; SO IT TOOK SEVERAL SECONDS BEFORE I REALIZED WE WERE MOVING AND REAPPLIED THE BRAKES. I STOPPED THE AIRPLANE BEFORE WE CONTACTED ANY PERSONNEL OR GND EQUIP. WHILE IT'S OBVIOUS MY MENTAL LAPSE IS NOT EXCUSABLE; THERE ARE SEVERAL CONSIDERATIONS THAT COME TO MIND THAT MAY GO ALONG WAY TOWARD EXPLAINING THAT LAPSE AND PREVENTING MORE SUCH OCCURRENCES. FIRST AND FOREMOST; THE FIRST THING THAT SHOULD OCCUR ONCE THE ACFT COMES TO A STOP (RED LIGHT ON) IS THE CHOCKS SHOULD BE INSTALLED. THIS SHOULD HAPPEN BEFORE ANY VEHICLES APCH THE ACFT; BEFORE ANY CARGO DOORS ARE OPENED; BEFORE ANY PERSONNEL APCH EXCEPT THOSE DESIGNATED TO CHOCK THE WHEELS; AND EVEN BEFORE EXTERNAL PWR IS APPLIED TO THE ACFT. FINALLY; RAMP WORKERS HAVE OR DISPLAY DIFFERENT LEVELS OF COMPLIANCE WITH THE PARKING PROC DESCRIBED ABOVE. SOME FORGET TO TURN THE LIGHT OFF; SOME FORGET THE HAND SIGNAL; SOMETIMES THE LIGHT BLINKS; SOME TURN THE LIGHT OFF TOO SOON. THE EFFECT OF THESE DIFFERENCES IS THE TENDENCY TO NOT PAY ATTN TO THE VERY THINGS THAT WE SHOULD AND TO BECOME COMPLACENT.