A LCL TV STN HAD HIRED THE OPER TO PROVIDE A HELICOPTER FOR USE IN BROADCASTING AN AIRBORNE TRAFFIC SHOW. THE PLT TOOK OFF WITH A RPRTR & A CAMERA MAN ABOARD. DRG A LIVE TRAFFIC RPRT, THE ACFT WAS HDG EASTBOUND AT SLOW SPEED OVR A DOWNTOWNAREA AT APRX 400 FT AGL. WHILE IT WAS AT OR NR A HOVER, WITNESSES HEARD A NOISE. THE ACFT THEN ENTERED A RAPID SPIN TO THE RGT & BGN AN UNCTLD DSCNT. SUBSEQUENTLY, IT STRUCK A TREE & A UTILITY POLE, THEN IMPACTED THE GND. AFTER BYSTANDERS REMOVED THE OCCUPANTS FM THE WRECKAGE, A FIRE ERUPTED. THE ENG THROTTLE GRIP WAS FND IN THE FULL OPEN (PWR ON) PSN. AN INV SHOWED THE AFT TAIL ROTOR DRIVE SHAFT FLEXIBLE (FLEX) COUPLING, PN 369H5501, HAD FAILED FM FATIGUE. AD 81-17-02 REQDTHAT ALL 369D'S WITH MAIN ROTOR BRAKES (MRB'S) BE INSPD AT 100 HR INTERVALS UNTIL THE FWD FLEX COUPLINGS WERE REPLACED WITH FAIL SAFE COUPLINGS (PN 369H92564), BUT THE ORIGINAL FWD & AFT FLEX COUPLINGS WERE IDENTICAL. ALSO, OTHER MODELS OFTHE 369 HAD NRLY IDENTICAL MRB'S, BUT THE AD DID NOT APPLY TO THESE. N9045F, A HUGHES 369HE, HAD NO MRB.