THE HELICOPTER DSCNDD UNCONTROLLED & CRASHED IN A RESIDENTIAL AREA. AFTER THE ACDNT, THE PLT SAID HE AGREED TO ORBIT THE RESIDENCE OF HIS PAX. AT THE APRX 135 DEG POINT OF A LEFT ORBIT, HE FELT 'A BANG & SEVERE VIBRATION IN THE TAIL.' THE PLT LOST DRCTNL CTL & THE HELICOPTER BEGAN SPINNING TO THE RIGHT. AN EXAM OF THE TAIL ROTOR GEAR BOX (T/R GB) ASSY REVEALED 3 OF 4 T/R GB MOUNTING STUDS HAD FAILED FROM CYCLIC FATIGUE DUE TO BENDING WHICH HAD RESULTED FROM A LOSS OF CLAMP-UP TORQUE. SUBSEQUENTLY, THE LOWER RIGHT EAR OF THE T/R GB (AT THE LCTN OF THE 4TH STUD) FAILED FROM OVERLOAD & THE T/R GB SEPD. FURTHER INVESTIGATION REVEALED THAT 3 WASHERS HAD BEEN INSTALLED ON EACH OF THE MOUNTING STUDS, BUT ONLY ONE WAS PERMITTED. THE WASHERS WERE INSTALLED DURING COMPLIANCE WITH AD 86-20-07, 78 FLT HRS BEFORE THE ACDNT.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

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