What was found
DURING PUSH BACK, STARTING NR 3 ENGINE ON THE APU AFTER FUEL WAS SELECTED ON, THE ACFT LOST ELECTRICAL. MECHANIC REPORTED FLAMES 1 METER LONG COMING FROM THE TAIL. THE ENGINE WAS SHUTDOWN, THE FIRE HANDLE PULLED, AND ACFT TOWED BACK INTO BLOCKS. APU FAILED DURING START UP OF THE NR 3 ENGINE. FOLLOWED FIM 71-02-10-1-2, FLAME FROM EXHAUST NOZZLE DURING START INSPECTION OF ENGINE INLET AND EXHAUST AREA, NO DEFECTS NOTED. PERFORMED DRY MOTOR OF THE NR 3 ENGINE TO CLEAR ANY POSSIBLE EXCESS FUEL. PERFORMED A 3 MINUTE NR 3 ENG IDLE RUN. ALL CHECKED NORMAL. ACFT OK FOR CONT SERVICE.