Accident Sequence
A helicopter experienced a loss of roll control during takeoff and subsequently rolled over during a precautionary landing. The certificated flight instructor (CFI) and a student pilot had departed for training at another airport. During takeoff to return to their home base, the CFI heard a clicking noise from the helicopter and decided to perform a precautionary landing. As the maneuver began, the helicopter started rolling to the right. The CFI applied left cyclic input, but there was no corresponding response. The right skid contacted the ground, and the helicopter rolled over, coming to rest on its left side.
Maintenance History
A review of the aircraft logbook revealed that the left rib at the top of the mast fairing had been installed during the last 100-hour inspection, approximately 2.5 flight hours before the accident. To replace this rib (bulkhead), the bolt and nut assembly connecting the left cyclic push-pull tube to the left lug of the non-rotating swashplate had to be disassembled and then reassembled.
Investigation Findings
Following the accident, the bolt, nut, and two spacers that connected the left cyclic push-pull tube to the swashplate were located on the runway. A Federal Aviation Administration inspector later recovered the self-locking nut during a walk of the accident area. According to the manufacturer's Illustrated Parts Catalog, the connection requires a bolt, self-locking nut, three spacers (two were found), a washer, and a locking palnut (not found).
A metallurgical examination of the swashplate assembly, left push-pull tube, left rib, nut, bolt, and associated hardware found no preexisting damage on any fracture surfaces. The absence of the bolt and thread imprints within the hole of the left lug of the non-rotating swashplate indicated that the bolt had been backing out over time. The recovered bolt and nut were in good condition, and the hole was not significantly ovalized, suggesting that the bolted connection was not subjected to excessive loads. The loss of the nut and subsequent backing out of the bolt indicated that the connection was not properly assembled following the recent maintenance that required its disassembly.