AN MD88; AFTER PUSHBACK AND ENG START WITH THE TUG CONNECTED; ROLLED FORWARD. NO DAMAGE INCURRED.

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FLT XXX FROM ZZZ TO ATL ON MAY/XA/98. WE WERE DISPATCHED WITH THE APU INOP; WHICH MEANS STARTING BOTH ENGS AT THE GATE. PRIOR TO STARTING ENGS; I INSTRUCTED THE GND CREW TO LEAVE EXTERNAL AIR AND PWR HOOKED TO THE ACFT UNTIL AFTER ENG START. THE ACFT WAS ATTACHED TO A TUG; AND WAS CHOCKED AT BLOCK-IN. I VISUALLY SAW THE TUG AND CHOCKS (I STEPPED OUTSIDE ON THE JETWAY TO SEE WHAT THE DAY WAS LIKE). I ASKED FOR THE PUSHBACK/START CHKLIST; THE AGENT MOVED THE JETWAY BACK; AND THE CSC TOLD ME THE 'CABIN WAS READY FOR PUSHBACK.' WE STARTED #1 FIRST. AS WE STARTED #2; I NOTICED THAT #1 STARTED TO ROLL BACK SLIGHTLY (N2 AT 48-49%); AND THE EGT STARTED TO RISE; SO I ADDED THROTTLE TO STABILIZE THE START IN ACCORDANCE WITH ACR PROCS. I WAS AWARE OF THE CLOSE PROX OF PERSONNEL AND EQUIP BEHIND ME; SO I WAS CAREFUL ADDING PWR. WHEN THE #2 GENERATOR CAME ON LINE; I TOLD THE TUG DRIVER THE GND CREW WAS CLRED TO DISCONNECT EXTERNAL AIR AND PWR. AS #2 CAME ON LINE; IT DID THE SAME THING AS #1: N2 AT 48% AND EGT RISING. I AGAIN ADDED PWR TO STABILIZE THE START. AT THE TIME THAT #2 WAS STABILIZING; THE FO STATED IT SEEMED TO HIM THAT THE JETWAY WAS MOVING. I WAS HEAD DOWN ON THE ENG INSTS AND LOOKED UP. AS I LOOKED UP; THE FO STATED THAT THERE WAS NO ONE IN THE TUG AND I INSTANTLY GRABBED THE BRAKES. THE ACFT AND TUG HAD STARTED TO MOVE FORWARD. WE STOPPED INSTANTLY WITH LESS OF A JOLT THAN MOST PUSHBACKS. IMMEDIATELY; THE GND PERSONNEL CAME RUNNING; JUMPED INTO THE NOW STOPPED TUG; AND LOOKED TO MOVE LEVERS; SWITCHES. THE ACFT MOVED 3-5 FT. I ASKED THE GND CREW IF ANYONE WAS HURT. THE ANSWER WAS NO. I ASKED IF THE ACFT HIT ANYTHING AT ALL; THE ANSWER WAS AGAIN NO. I ASKED ARE YOU SURE; AND THE ANSWER WAS STILL NO. I ASKED THE CSC IF EVERYTHING WAS OK IN THE BACK; BOTH FOR FLT ATTENDANTS AND PAX. HER ANSWER WAS EVERYTHING WAS OK; THE FLT ATTENDANTS DID NOTICE THE STOP; BUT EVERYONE WAS FINE. I DECIDED THAT SINCE ALL PAX; PERSONNEL; AND EQUIP WERE OK; WE WOULD PROCEED. THE FLT TO ATL WAS UNEVENTFUL. I WILL NEVER (I HOPE) START THE ENGS AGAIN WITHOUT THE BRAKES SET.