Accident Sequence
During a training flight intended to prepare a student for a private pilot certification check ride, the helicopter experienced a main rotor drive belt failure. The instructor and student were on the base leg to runway 21R, cruising at about 60 knots at 300 feet above ground level, when the helicopter's nose suddenly yawed left and then right. Concurrently, the helicopter shook, and the clutch light illuminated. The instructor took control and entered an autorotative descent. About 20 feet above ground level, he commenced a cyclic flare for landing. The underlying terrain consisted of soft sand, and the helicopter rolled over during touchdown.
Post-Accident Observations
Upon exiting the helicopter, the instructor observed that the V-belts were shredded. The instructor stated that he had monitored the student during the preflight inspection, and no evidence of any problem was detected. He also reported that there were no outstanding airworthiness issues with the helicopter.
Maintenance History
A company mechanic had performed a 100-hour inspection about 4.3 hours prior to the accident flight. During that inspection, the mechanic had loosened the self-locking nut on the belt actuator housing assembly and had rotated the down-limit stop screw upward until the screw bottomed out in the actuator housing in the full up position. The mechanic forgot to reposition the stop screw and tighten the nut. This action resulted in the main rotor drive V-belts becoming excessively loose. The belts then exited their seated position on the lower pulley sheave and were severed.