B757 IN PROCESS OF PARKING AND SHUT DOWN HAS UNCOMMANDED PWR ADVANCE WHILE SWITCHING TO GND PWR.

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FLT ARRIVED AT BOSTON LOGAN INTL ARPT. ON TAXI-IN; R ENG WAS SECURED AFTER A 2-MIN COOL DOWN PERIOD FOLLOWING LNDG. ACFT WAS PARKED UNEVENTFULLY. THE CAPT ORDERED THE FO TO SAY GOODBYE TO THE PAX AFTER THE ACFT WAS CHOCKED. THE CAPT THEN WAITED FOR EXTERNAL PWR TO BE CONNECTED TO ACFT; SO THAT THE L ENG COULD BE SECURED AND THE SHUTDOWN CHKLIST COMPLETED. AN FAA PRINCIPAL AIRWORTHINESS INSPECTOR WAS IN THE COCKPIT AND OBSERVING THE OP. AS EXTERNAL PWR WAS CONNECTED TO THE ACFT AND MADE AVAILABLE; THE CAPT ATTEMPTED TO TRANSFER PWR TO THE GND PWR UNIT (SO THAT THE L ENG COULD BE SECURED); BUT THE TRANSFER DID NOT WORK ON THE FIRST ATTEMPT. THE FO STEPPED BACK INTO THE COCKPIT (STANDING ALONG SIDE THE FAA INSPECTOR) TO INFORM THE CAPT THAT THE JETWAY WAS BEING PULLED UP TO THE L FORWARD ENTRY DOOR. JUST AS THE EXTERNAL PWR 'AVAILABLE' LIGHT RE-ILLUMINATED; BOTH THROTTLES SIMULTANEOUSLY ADVANCED TO FULL PWR (BY THEMSELVES). THE FO AND FAA INSPECTOR BOTH WITNESSED THE UNCOMMANDED THROTTLE ADVANCE; FOLLOWED BY THE CAPT'S QUICK RESPONSE IN REDUCING THE THROTTLES MANUALLY TO IDLE PWR AND HIS APPLICATION OF THE BRAKES. HE THEN SELECTED EXTERNAL PWR AND SECURED THE L ENG; AND COMPLETED THE SHUTDOWN CHKLIST. THE CREW NOTED GND PERSONNEL WITH CONCERN; EXPECTING THAT THE UNCOMMANDED PWR SURGE MAY HAVE CAUSED A GND HAZARD; THE CREW WENT TO THE RAMP TO SEE. THE L ENG SURGE HAD BLOWN A TRUCK ON ITS SIDE THAT HAD PASSED BEHIND THE ACFT DESPITE THE FACT THAT AN ACFT ENG WAS RUNNING AND THE ROTATING BEACON WAS ON. THE TRUCK WAS DAMAGED; BUT IT APPEARED THAT NONE OF THE OCCUPANTS WERE INJURED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES A MAINT CHK WAS MADE AND THE MECHS COULD NOT RECREATE THE SIT. HOWEVER A CALL TO BOEING INDICATED THAT THERE ARE LIMITS SET IN NANOSECONDS FOR PWR BEING CHANGED TO GND. IT IS POSSIBLE THAT A SPIKE CAN BE SENT AND THAT WOULD CAUSE THE THROTTLES TO ADVANCE WITH AUTOTHROTTLES ON. COMPANY HAS ADOPTED A NEW PROC SINCE THIS HAPPENED A SECOND TIME ON ANOTHER ACFT. CHKLISTS INCLUDE A TURN OFF OF THE AUTOTHROTTLES ON LEAVING THE RWY AND STARTING TAXI IN. AUTOTHROTTLES ARE NOT TURNED ON AGAIN UNTIL TAXI OUT. RPTR STATES HE WAS GRATEFUL THE POI FROM FAA WAS PRESENT AS HE FEELS HE WOULD HAVE HAD A HARD TIME CONVINCING ANYONE THAT THE THROTTLES ADVANCED UNCOMMANDED.