ACR LGT HITS TUG AFTER PARKING BRAKE IS SET IN RAMP OP ACFT PARKING PROC.

Narrative

WE TAXIED IN AND PARKED AT GATE. THE SIGNAL MAN GAVE THE SET PARKING BRAKE SIGNAL. THIS WAS ACCOMPLISHED AND THE RED PARKING BRAKE WAS ON (LIGHT). THIS WAS VERIFIED BY CREW AND JUMPSEATERS. I PLACED THE APCH PLATE ON MY FLT BAG. AT THAT TIME; I SENSED MOVEMENT; LOOKED UP AND STEPPED ON THE BRAKES. AT THE SAME TIME; THE ACFT CONTACTED THE TUG. THE BRAKES WERE WRITTEN UP. I INSPECTED THE ACFT AND THEN PROCEEDED TO OPS AND CONTACTED MY SUPVR AT ACR OPS. SUPPLEMENTAL INFO FROM ACN 206286: AS WE APCHED GATE AT ORD; I CALLED OUT TO THE CAPT THAT THE BRAKE PRESSURE WAS NORMAL. A GUIDEMAN SIGNALED US TO COME ON IN AND PARK. ONCE THE PLANE WAS STOPPED; THE PARKING BRAKE WAS SET; AND ENG #3 WAS SHUT DOWN. SUDDENLY; WITHOUT WARNING; THERE WAS A JOLT. IT APPEARED THAT THE PARKING BRAKE HAD RELEASED; AND WE HAD HIT SOMETHING. AT THE TIME OF THE INCIDENT; THE 'B' PUMPS WERE BOTH ON; PRESSURE WAS GOOD; AND IN THE GREEN BAND. I SINCERELY BELIEVE THAT THERE MIGHT BE SOMETHING WRONG IN THE MECHANICAL LINKAGE. I ALSO FEEL THAT IF WE HAD BEEN CHOCKED RIGHT AWAY; WHEN WE HAD PARKED AT THE GATE; THAT THIS INCIDENT COULD HAVE POSSIBLY BEEN AVERTED. THE PLANE RECEIVED ONLY MINOR DAMAGE WHEN IT HIT THE TUG AT THE GATE. EVERYTHING WAS DONE SOP!!