PARKING BRAKE FAILED; ACFT ROLLED INTO GND PWR UNIT AND DESTROYED IT.

Narrative

WE WERE MARSHALLED TO A POINT ON THE RAMP ABOUT 3 FT FROM A GND PWR UNIT. THE ACFT CAME TO A COMPLETE STOP USING THE TOE BRAKES. I SET THE PARKING BRAKE AND ANNOUNCED TO FO þCOOL DOWN (OF ENGS) COMPLETE; PARKING BRAKE IS SET;' OUR COMPANY CALLS REQUIRED TO INITIATE ENG SHUTDOWN. THE FO PUSHED ENG STOP SWITCHES. OUR COMPANY PROC REQUIRES MONITORING OF ENG RPM GAUGES BY BOTH CREW MEMBERS WITH SPECIAL CREW ACTIONS AT 50 PERCENT RPM AND 10 PERCENT RPM. WHILE WE WERE WATCHING ENG GAUGES; WE HAD A PARKING BRAKE FAILURE AND THE ACFT STARTED TO ROLL. WE DID NOT SENSE ACFT MOVEMENT. AS THE L PROP WAS STILL WINDING DOWN; IT CONTACTED THE GND PWR UNIT. THE GND PWR UNIT WAS DESTROYED; PROPS BENT; AND PIECES OF GND PWR UNIT PUT 2 HOLES IN ACFT SKIN. FACTORS CONTRIBUTING: 1) PLACEMENT OF GND PWR UNIT BEFORE ACFT IS PARKED RATHER THAN AFTER. 2) WHEELS WERE NOT CHOCKED AS SOON AS ACFT COMPLETELY STOPPED WITH TOE BRAKES. 3) COMPANY PROC REQUIRED BOTH CREW TO 'LOOK INSIDE' AT EXACT TIME INCIDENT OCCURRED. 4) SMALL CONGESTED RAMP AREA. 5) LNDG GEAR RECENTLY CHANGED ON ACFT. 6) PARKING BRAKE LOCKING MECHANISM DISCOVERED TO BE INOP AS DISCOVERED DURING INVESTIGATION POST INCIDENT. SUPPLEMENTAL INFO FROM ACN 275850: THE GND PWR UNIT WAS PREPARKED IN FRONT OF OUR L ENG AND WAS FAIRLY CLOSE TO THE ACFT. THE CAPT IS ALSO LOOKING INSIDE TO PLACE THE ENGS ON THE START LOCKS. AT THIS TIME THE PARKING BRAKE FAILED AND THE ACFT ROLLED INTO THE GND PWR UNIT.