What was found
§135.415(A)(16) EMERGENCY ACTIONS EXC ENGINE SHUTDOWN THIS IS NOT A CHARTER AIRCRAFT. IN LEVEL FLIGHT DURING VFR CONDITIONS, DURING CROSS COUNTRY FLIGHT TRAINING WHILE IN LEVEL FLIGHT, A SUDDEN VIRATION AND NOISE WAS HEARD FROM ENGINE AREA. INSTRUCTOR AND STUDENT MADE PRECAUTIONARY POWERED LANDING TO SUITABLE SITE. FOUND AFT MAIN ENGINE TO TRANSMISSION DRIVE SYSTEM BELT HAD DISINTEGRATED, CAUSING FORWARD BELT TO SKIP A GROOVE FORWARD SO ONLY ONE OF FOUR DRIVE BELT SECTIONS WAS POWERING ROTOR DRIVE SYSTEM. DISSASEMBLY, INSPECTION OF SYSTEMS IN AREA, AND NEW PART REPLACEMENT TO BE PERFORMED.