MLG HITS TOW BAR ON INITIAL BRAKE RELEASE.

Narrative

AFTER PUSHBACK THE ACFT ROLLED ONTO THE DISCONNECTED TOW BAR. ACFT RETURNED TO JETWAY FOR MAINT INSPECTION; BOTH NOSEWHEEL/TIRE ASSEMBLIES WERE REMOVED AND REPLACED. ACFT RETURNED TO SVC. AFTER THE TUG DRIVER DISCONNECTED THE TOW BAR AND HEADSET; HE DID NOT DEPART BUT REMAINED IN FRONT OF THE ACFT UNDER THE NOSE; NOT VISIBLE FROM THE FLT DECK. WHEN THE MARSHALLER WAS GIVEN THE TAXI SIGNAL THEY DID NOT MAINTAIN THE HOLD SIGNAL; MOMENTARILY MOVING THE WANDS TO THE READY TO ASSUME CTL SIGNAL AND BACK TO THE HOLD SIGNAL. DURING THIS MOMENT THE BRAKES WERE RELEASED -- THE ACFT ROLLED ONTO THE TOW BAR; AND THEN THE BRAKES WERE REAPPLIED. CORRECTIVE ACTION: TUG DRIVERS -- AFTER CLRED TO DISCONNECT; DEPART; OR AT LEAST MOVE TO A POS VISIBLE FROM THE FLT DECK. MARSHALLERS -- BETTER TRAINING. PLTS -- WHEN THE PROCS BREAK DOWN; IT IS OUR RESPONSIBILITY TO BACK THEM UP. DILIGENTLY OBSERVE THE DEP/TOW BAR AND TUG. DELAY SETTING SWITCHES AND PERFORMING PRETAXI FLOW UNTIL THE ABOVE IS COMPLETED. AIRPLANES -- NOSE WINDOW OR CAMERA OR MIRRORS (NOT LIKELY).