WITNESSES STATED THE HELICOPTER ENTERED A 'HARD LEFT TURN' INTO A NEAR INVERTED POSITION BEFORE 'IT WENT DOWN HARD AND FAST.' INSPECTION DISCLOSED THE METAL CONNECTION FOR THE LEFT CYCLIC STICK WAS BROKEN IN HALF. (PLT WAS FLYING FROM LEFT SEAT.) EXAM OF THE CYCLIC STICK QUICK DISCONNECT LOCKING NUT REVEALED IT WAS HELD IN PLACE ON THE CONNECTOR BY 1 THREAD. PROPERLY INSTALLED, THE LOCKING NUT TIGHTENS DOWN OVER 6 THREADS ON THE CONNECTOR. WHEN BACKED OUT TO 1 THREAD, MOVEMENT OF THE CYCLIC WILL NOT NOT PRODUCE A CORRESPONDING CHANGE IN THE LATERAL CONTROL HYDRAULIC SERVOS. THE CYCLIC WAS LAST REMOVED APRX 80 HRS PRIOR TO THE ACCIDENT DURING MAINTENANCE. ON 7/17/84, BELL ISSUED OPERATIONS SAFETY NOTICE WHICH IN PART READ, 'FLIGHT CREWS ARE CAUTIONED TO ASSURE QUICK REMOVAL FLIGHT CONTROLS ARE PROPERLY INSTALLED PRIOR TO OPERATING THE HELICOPTER.' PRIOR TO THIS NOTICE, NO REQUIREMENTS EXISTED TO DO SO.