What was found
BELT WAS FOUND TO BE FRAYED ABOUT .5" IN FROM THE AFT SIDE, THE ENTIRE LENGTH OF BELT. PILOT STATED BELT WAS IN GOOD CONDITION WHEN ACFT WAS PREFLIGHTED ABOUT AN HOUR PRIOR TO THE FAILURE. HE HEARD A "POP" AND RETURNED TO THE HANGAR TO FIND RUBBER CHIPS AND FINE STRING FIBERS ALL OVER THE BELT DRIVE COMPARTMENT. THE ACFT NEVER LOST DRIVE COMPONENT. THE ACFT NEVER LOST DRIVE AND THERE WAS NO NEEDLE SPLIT. UPON INSP BOTH PULLEYS CONDITION LOOKED NORMAL AND ALIGNMENT IS GOOD. IDLER TRACK LOOKS GOOD. INSTALLED NEW BELT, RIGGING AND OPS NORMAL. NOT SURE WHAT CAUSED THE BELT TO FAIL BUT IT WAS FOUND RIDING UP ON THE AFT UPPER PULLEY LIP, BELT WAS RIDING NORMAL IN THE BOTTOM PULLEY.