MLG FLC AT ENG START UP HAVE VIOLENT BUMP IN NOSE GEAR AREA. TOW BAR BROKEN DUE BRAKES SET.

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SERVING AS FO IN A 2-MAN COCKPIT; CAPT REQUESTED BEFORE START CHKLIST ALL THE WAY THROUGH TO THE CLRED FOR START PORTION. I RPTED CHKLIST COMPLETE AND ADVISED GND WE WERE STARTING ENGS ON THE RAMP. THEY TOLD US TO RPT READY TO TAXI. CAPT GAVE START HAND SIGNAL TO THE GND CREW. THEY RETURNED THE SIGNAL AND THE CAPT INITIATED THE START SEQUENCE. A COUPLE OF SECONDS LATER THERE WAS A VIOLENT BUMP ON THE NOSE GEAR AREA OF THE ACFT. THE GND CREW TRIED TO TOW THE ACFT BACK WHILE THE CAPT HAD THE BRAKES SET. THE TOW BAR BROKE AND DAMAGED THE NOSE GEAR DOORS OF THE ACFT. THE DOORS WERE REPAIRED AND SIGNED OFF IN THE LOGBOOK AND AFTER AN EXTENSIVE DELAY; WE WERE ON OUR WAY WITHOUT FURTHER INCIDENT. THE CAPT HAD INTERPHONE COMS WITH GND PERSONNEL AND NO COM EVER INDICATED THE GND PERSONNEL WERE GOING TO TOW THE ACFT. WE; AS A FLC; WERE NOT AWARE THE TOW BAR WAS CONNECTED. GND PERSONNEL SAID THEY THOUGHT WE WERE AWARE OF THE TOWING PUSHBACK EVEN THOUGH NO SIGNALS OR VERBAL COM INDICATED THIS WOULD HAPPEN. FACTORS WERE: ASSUMPTION BY GND CREW THAT CREW WAS AWARE OF TOWING. GND CREW SET ON TOWING DID NOT RECOGNIZE START SIGNAL OR THOUGHT THEY SAW BRAKE RELEASE. NOT HAVING 2 MEN MONITORING START SEQUENCE ESPECIALLY INTERPHONE COM. BOTH PLTS WERE RESERVE CREW AND DID NOT REALIZE ACFT WAS TOWED AT GDL. YOU COULD EXIT RAMP FORWARD OF ACFT WITHOUT PUSHBACK. I THINK CLOSER ATTN AND BETTER COM VERBAL BY FLC AND GND CREW COULD HAVE AVOIDED THIS INCIDENT.