NTSB Identification: LAX05LA122.
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Accident occurred Thursday, March 24, 2005 in Prescott, AZ
Probable Cause Approval Date: 05/30/2006
Aircraft: Robinson R22 Beta, registration: N225G
Injuries: 2 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The helicopter's main rotor drive belts broke on the base leg and the helicopter rolled over during the subsequent autorotation and landing. The instructor said that he monitored his student during the preflight inspection of the helicopter. No evidence of any problem was detected. The instructor reported that there were no outstanding airworthiness issues with the helicopter. The accident occurred while he was preparing his student for the private pilot certification check ride. The lesson plan/maneuver that he intended the student to practice was a run-on landing. On base leg to runway 21R, while cruising about 60 knots at 300 feet above ground level, the helicopter's nose suddenly yawed left and then right. Concurrently the helicopter shook, and the clutch light illuminated. The instructor took the controls from his student and entered an autorotative descent. About 20 feet above ground level he commenced a cyclic flare for the landing. The underlying terrain was soft sand, and the helicopter rolled over during touchdown. Upon exiting the helicopter, the instructor observed that the V-belts were shredded. A company mechanic had performed a 100-hour inspection about 4.3 hours prior to the accident flight. During the inspection, he 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 exited their seated position on the lower pulley sheave and were severed.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The company mechanic's improper maintenance actions, which resulted in the total failure of the main rotor drive belt system on approach to landing. A factor was the soft terrain.

Full narrative available

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