MD88 CREW ROLLED INTO THE PUSHBACK TUG AFTER THE CAPT DID NOT SET THE BRAKES. THE CREW STARTED BOTH ENGS ON PUSHBACK BECAUSE THE APU WAS MEL'ED INOP.

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WE HAD AN INOP APU. THE DECISION WAS MADE TO START BOTH ENGS AT GATE. COM WITH GND PERSONNEL WAS ESTABLISHED. ALL BEFORE START CHKLISTS WERE COMPLETED. DURING PUSHBACK CHKLIST; IT WAS DISCOVERED A DOOR LIGHT WAS ILLUMINATED. IT TOOK SEVERAL ATTEMPTS TO GET LIGHT TO EXTINGUISH. THE START SEQUENCE WAS BEGUN WITH THE CAPT MANIPULATING THE FUEL LEVERS. AFTER THE #2 ENG WAS STARTED; I BEGAN MY AFTER START FLOW. AT A POINT ON THE OVERHEAD PANEL; I SENSED FORWARD MOVEMENT AND EXCLAIMED 'BRAKES; BRAKES.' THE ACFT MOVED FORWARD APPROX 5 FT DISPLACING THE TUG SLIGHTLY TO THE L OF THE ACFT'S CTRLINE. AN ASSESSMENT WAS MADE TO ENSURE EVERYONE'S SAFETY. MAINT CAME TO THE ACFT TO INSPECT THE NOSEWHEEL ASSEMBLY. IT WAS DISCOVERED THE TOW BAR SHEAR PIN HAD BROKEN. NO OTHER DAMAGE WAS DISCOVERED. MINS LATER; WE COMMENCED PUSHBACK AND CONTINUED ON OUR FLT. I BELIEVE OUR SIT WAS CAUSED BY A BREAKDOWN IN COM AND NORMAL PROCS. THE ACFT WAS NOT CHOCKED FOR THE START. A 'BRAKES SET' CALL WAS NEVER MADE TO THE CAPT PRIOR TO ENG START. I BELIEVE A SAFER METHOD OF STARTING ENGS IS TO HAVE THE CO-PLT MAKE ALL STARTS (TOPS AND BOTTOMS) SO THE CAPT'S ATTN CAN BE OUTSIDE THE ACFT. HE WOULD HAVE NOTICED THE FORWARD MOVEMENT EARLIER. I BELIEVE THERE SHOULD BE AN APU INOP DISPATCH WRITTEN PROC IN ALL ACFT POM'S FOR THE CREW TO REVIEW PRIOR TO CONDUCTING THIS ABNORMAL EVENT. IT SHOULD EMPHASIZE 'BRAKES SET.' WE WERE LUCKY NO ONE WAS HURT; AND LEARNED A VALUABLE LESSON.