BA3200 FLC ABORTS TKOF WHEN DOOR OPENS ON TKOF ROLL.

Narrative

DURING TKOF ROLL AT APPROX 90 KTS; THERE WAS A LOUD POPPING SOUND ACCOMPANIED BY A DRAMATIC INCREASE IN THE CABIN NOISE LEVEL. TKOF WAS IMMEDIATELY ABORTED. THERE WERE NO WARNING LIGHTS TO INDICATE THE NATURE OF THE PROB. INSPECTION BY THE FO SHOWED THAT THE MAIN CABIN DOOR HAD OPENED AND HAD SUSTAINED ENOUGH DAMAGE THAT IT COULD NOT BE CLOSED AGAIN. THE ACFT; HOWEVER; WAS SAFE TO TAXI; SO WE RETURNED TO THE GATE AND DEPLANED THE PAX. INSPECTION BY MAINT CREW FOUND THAT THE CABIN DOOR WARNING CIRCUIT BREAKER WAS FAULTY; WHICH EXPLAINED WHY THERE WAS NO MASTER CAUTION WARNING. AS TO WHY THE DOOR OPENED THE MOST REASONABLE EXPLANATION IS THAT THE DOOR HANDLE WAS UP BUT NOT FULLY ENGAGED. HAD THE DOOR HANDLE BEEN IN THE DOWN (OPEN) POS THE DOOR WOULD ALMOST CERTAINLY OPENED BEFORE THE TKOF ROLL SINCE I HAD TO MAKE 3 SHARP 90 DEG R TURNS TO GET TO THE RWY. THE CENTRIFUGAL FORCE ON THE DOOR DURING THOSE TURNS WOULD HAVE MADE THE PROB APPARENT. HAD THE HANDLE BEEN UP AND FULLY ENGAGED (ASSUMING THE DOOR WAS NOT FAULTY) THE DOOR WOULD NOT HAVE OPENED.