What happened
The pilot was performing a series of practice hovering autorotations. Following the third landing, the touchdown was described as acceptable and meeting check ride criteria. Approximately two seconds after the helicopter touched down, an obvious component failure occurred throughout the airframe. This failure was accompanied by severe vibrations and both audible and physical indications of structural distress.
Upon exiting the aircraft, the pilot initially expected to find a malfunction in the main rotor head. However, the inspection revealed that the tail boom had bent downward at its midpoint.
The investigation
A postaccident examination of the helicopter identified a discrepancy in the construction of the tail boom. The boom was fabricated using skins with a thickness of 0.025 inches, which did not meet the 0.032-inch skin thickness specified in the structural repair manual. Aircraft records indicated that this tail boom had been installed at the factory when the helicopter was new and had accumulated 14,053 hours in service.