Incident
A helicopter crashed during a maintenance test flight after the blue main rotor blade separated. The pilot reported that on a previous flight, the helicopter had excessive vertical vibration and required excessive collective forces. Mechanics worked on the main rotor blades to correct the problem. During the subsequent test flight, the pilot initially noted that the vertical vibration had 'gone away,' but before landing, the vibration returned. Shortly thereafter, the pilot heard a loud 'pop.' As he flared for landing, the blue main rotor blade separated, and the helicopter crashed.
Examination
Examination of the separated blade revealed that the wood in the area of the blade grip had failed, allowing the blade to separate. Evidence of a previous repair was present in this area. This repair lacked adequate adhesive between some of the mating surfaces. Additionally, an unapproved sealant had been used over the repaired area, and oil had contaminated the wood in that region. There was no evidence of dry rot in the failed area. The logbook records did not show the previous blade repair.