Incident Overview
During a local instructional flight, the certified flight instructor (CFI) observed that the main rotor RPM was below the green arc. In response, the CFI entered an autorotation and attempted to restore the rotor RPM to the green arc. Despite these efforts, the helicopter collided with the ground while still experiencing low main rotor speed.
Investigation Findings
Post-accident examination revealed that the engine drive pulley was worn beyond the factory-established limit. This wear allowed the drive belt to slip, contributing to the low rotor RPM. Further, the investigation noted that the minimum serviceable wear limit for the drive pulley was not published in the factory maintenance procedures.
Technical Details
The engine drive pulley is a critical component that ensures proper tension and operation of the drive belt system. When worn beyond specified limits, the pulley cannot maintain adequate belt tension, leading to belt slippage and reduced power transmission to the main rotor. The absence of a published wear limit in maintenance procedures may have prevented timely detection of the worn pulley.
Safety Considerations
While no explicit causal factors were stated, the findings highlight the importance of clear maintenance limits for components affecting flight-critical systems. Proper adherence to maintenance guidelines is essential for preventing similar events.