ACFT EQUIP PROB -- ACFT DAMAGED -- ACR FLC SET THE PARKING BRAKE; BUT THE ACFT ROLLED AND STRUCK GND EQUIP.

Narrative

AFTER A NIGHT FLT FROM S AMERICA A ROUTINE LNDG AND PARKING AT THE TERMINAL GATE OCCURRED. OBSERVED PARKING MECH GIVE STOP SIGNAL AT WHICH TIME BRAKES WERE SET AND BRAKE HANDLE VERIFIED ENGAGED. MECH LEFT POS AT THE FRONT OF ACFT; I ASSUMED TO CONNECT GND PWR; AND I THEN LOOKED TOWARD JETWAY TO VERIFY PAX SVC PEOPLE WERE AWARE OF OUR ARR. THERE WAS NO DETECTABLE ACFT MOVEMENT. AT THIS TIME I RETURNED MY ATTN TO THE FLT DECK AND SHUTDOWN PROCS. WHILE MOVING SEAT AND HEAD POS AROUND PANELS I DETECTED NO ACFT MOVEMENT. OUT OF THE CORNER OF MY EYE I DETECTED LIGHTS AND ACTIVITY. THE PARKING MECH WAS JUMPING UP AND DOWN TO GET MY ATTN. AS SOON AS HE SAW WE HAD EYE CONTACT HE INDICATED ACFT WAS MOVING BY A ROLLING MOTION OF HIS BATONS. I STILL SENSED NO MOTION BUT QUICKLY SLID SEAT FORWARD AND APPLIED BRAKES. NO SENSATION OF STOPPING OCCURRED; BUT ACFT DID STOP. EITHER RAPID BODY MOVEMENTS; SEAT MOVEMENT; VERY SLOW ACFT MOVEMENT; OR ACFT HAD ALREADY STOPPED ON ITS OWN COULD HAVE ELIMINATED SOME OF THE SENSATION OF STOP. DARKNESS DEFINITELY AFFECTED OUTSIDE REF. WHILE PAX WERE DEPLANING A RAMP SUPVR CAME ON BOARD TO ADVISE WE HAD HIT A PIECE OF GND EQUIP WITH THE R ENG COWLING (HAD BEEN SHUT DOWN). NEITHER COCKPIT CREWMEMBER SENSED OR FELT MOTION OR IMPACT. SUSPECT A FAILURE IN BRAKE SYS OCCURRED. PREVENTING SUCH INCIDENTS COULD BE ACCOMPLISHED BY A DEFINITE CHOCKING INDICATION BEFORE ANY COCKPIT INTERNAL ACTIVITY AND NO VEHICLE ACTIVITY NEAR ACFT UNTIL ALL ENGS ARE SHUT DOWN AND BEACON TURNED OFF. THE L ENG WAS STILL RUNNING AT OCCURRENCE.