What was found
AT APPROXIMATELY 21:15 THE EVENING OF 31 AUG 2011, DEPARTED THE COUNTY RAMP IN FOR A NIGHT VISION GOGGLE TRAINING MISSION WITH A CONTRACT INSTRUCTOR PILOT. WEATHER AT THE TIME WAS VFR. ALL PRE-FLIGHT CHECKS WERE NORMAL, AS WAS TAKEOFF AND CLIMB HEADING SOUTHEAST FROM THE AIRPORT TO A CRUISE ALITUDE OF 700 FT. APPROX 5 TO 6 MINUTES AFTER TAKEOFF, EXPERIENCED A SEVERE VIBRATION LASTING AN EXTIMATED 10 TO 20 SECONDS. THE INSTRUCTOR INITIATED A RAPID DECENT TO AN OPEN FIELD FOR A PRECAUTIONARY LANDING. AFTER LANDING AND ENGINE SHUTDOWN IT WAS DISCOVERED THAT A COUNTER WEIGHT HAD DEPARTED 1 TAIL ROTOR BLADE FOR UNKNOWN REASONS CAUSING THE VIBRATION. IT WAS ALSO DISCOVERED AFTER LANDING AND INSPECTION OF THE TAIL ROTOR THAT THE VIBRATION HAD CAUSED 3 OF THE 4 ATTACHMENT STUDS HOLDING THE TAIL ROTOR GEAR BOX TO SHEAR.