Incident Description
During a practice power recovery autorotation to a hover at approximately 600 feet above ground level, the two pilots heard a loud bang followed by severe vibration. The pilot in command (PIC) lowered the collective and rolled both throttles off to continue the autorotation to the ground, landing without further incident.
Inspection Findings
Post-landing inspection of the helicopter revealed that the 90-degree gearbox with tail rotors attached had separated from the tail boom. Examination of the tail rotors showed that the tip weight in one of the blades was missing. Further analysis of that blade revealed evidence of adhesive debonding between the adhesive and the spar surface. As a secondary securing method, four countersunk screws were positioned in line chordwise and offset in pairs on each side of the blade. All four screw holes showed no significant evidence of deformation.
Cause Determination
It was determined that the four countersunk screws were ineffective due to limited clamping provided by the machining at the manufacturer. The countersunk holes were machined with a diameter too large relative to the screw head diameter, allowing the four secondary load path screws to slip behind the spar. This resulted in the tip block separating from the blade. The manufacturer reported that although the bonding adhesive is the primary securing method for the tip block and the four countersunk screws serve as the secondary method, either method independent of the other is capable of carrying the load.