FLT CREW REPORTS TOW BAR BREAKAGE DURING PUSH BACK AND ENGINE START; ALLOWING ACFT TO ROLL OVER THE TUG. COMMUNICATION FROM TUG DRIVER IS NOT TIMELY OR HELPFUL.

Narrative

WE WERE PUSHING BACK FROM A GATE FOR DEP. CANNOT SEE THE TUG FROM THE COCKPIT; BUT WE WERE IN DIRECT COM WITH THE DRIVER THROUGH THE INTERCOM. ACFT ENGS WERE STARTED DURING THE PUSHBACK. ACFT CAME TO A BRIEF STOP THEN STARTED FORWARD. THIS IS NORMAL AS THE TUG DRIVER CTRS THE ACFT ON THE TAXI CTRLINE. WE WERE NEVER TOLD TO 'SET BRAKES' WHICH IS STANDARD PROC AT THE COMPLETION OF A PUSHBACK. THROUGH THE INTERCOM COULD HEAR HVY BREATHING AND CURSING. AT THAT POINT THE ACFT SLOWLY VEERED TO THE R. THEN HEARD OVER THE INTERCOM; 'NO BRAKES.' I ASSUMED THAT THE DRIVER MEANT THE TUG HAD LOST ITS BRAKES AND WAS ROLLING FREE. DID NOT WANT TO DAMAGE THE ACFT NOSE GEAR BY HITTING THE BRAKES TOO HARD; ASSUMING THAT THE TUG WAS STILL ATTACHED TO THE ACFT. SO THE ACFT BRAKES WERE EASED ON AND THE ACFT CAME TO A STOP. IT WAS THEN THAT THE CREW WAS INFORMED THAT THE TOWBAR HAD ACTUALLY BROKEN AND WE HAD ROLLED OVER THE TOP OF THE TUG. IMMEDIATELY CALLED OVER THE VHF RADIO FOR AN AMBULANCE. HOWEVER; BEFORE IT ARRIVED; WE WERE INFORMED THAT THE DRIVER OF THE TUG WAS NOT HURT. AT THIS POINT WITH HELP ON THE WAY; WE SHUT DOWN THE ENGS. AS PEOPLE CAME TO THE ACFT THE INTERCOM WAS DISCONNECTED. WE HAD TO CALL OVER THE VHF RADIO TO FIND OUT WHAT WAS HAPPENING. WE DID NOT KNOW THE STATUS OF THE DRIVER; THE TUG; OR THE ACFT. WE WERE EVENTUALLY TOWED BACK INTO THE GATE. I FEEL THIS COULD HAVE BEEN PREVENTED IF STANDARD PHRASEOLOGY BY THE PUSHBACK CREW HAD BEEN USED. COMPANY HAS RECENTLY HIRED PEOPLE WITH LITTLE OR NO ACFT EXPERIENCE TO PUSH BACK ACFT. IF AT THE FIRST SIGN OF TROUBLE THE CREW HAD BEEN INFORMED OF THE PROB OR BEEN TOLD TO SET THE BRAKES AS IS STANDARD; THE WHOLE THING WOULD HAVE ENDED WITH JUST A BROKEN TOWBAR. ALTHOUGH IT SHOULD NOT BE NEEDED BECAUSE OF THE INTERCOM SYS; A WING WALKER THAT IS VISIBLE TO THE CREW COULD ALSO HAVE PREVENTED THIS SITUATION.