On June 3, 2000, a Bell 212 (registration CFHDY) was involved in an aviation accident near Eugene, OR. Investigators recorded the probable cause as: The four countersunk screws were ineffective due to the limited amount of clamping provided by the machining of too large a diameter in the countersunk hole relative to the diameter of the screw head, allowing the screws to slip behind the spar, resulting in… This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During a power recovery autorotation practice at 600 ft AGL, a helicopter experienced a loud bang and severe vibration. The pilots performed an autorotation landing without further incident. Inspection revealed the tail rotor gearbox had separated due to adhesive debonding and ineffective secondary screws.
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.
Probable cause
The four countersunk screws were ineffective due to the limited amount of clamping provided by the machining of too large a diameter in the countersunk hole relative to the diameter of the screw head, allowing the screws to slip behind the spar, resulting in tip block separation.