Accident Sequence
Witnesses reported that the helicopter was in cruise flight when it abruptly pitched up and rolled left to an inverted attitude. Portions of the airframe separated during this maneuver, and the helicopter then descended straight down to ground impact. The wreckage examination revealed that the main rotor blades had diverged from their normal plane of rotation, and mast bumping had occurred. Subsequently, the blades struck the tail boom, causing it to separate from the helicopter. The helicopter became uncontrollable, rolled inverted, and descended into a residence. A post-impact fire consumed parts of both the helicopter and the residence.
Aircraft and Pilot Information
The pilot, who was also the builder, held a repairman certificate for his experimental category helicopter. He had replaced the original RotorWay main rotor blades with blades manufactured by another company. During assembly, each main rotor blade was attached to its respective blade yoke with a single retention nut and bolt assembly.
Wreckage Examination Findings
Examination of the wreckage revealed that the nut securing one of the main rotor blades to its retention bolt was missing. The retention bolt itself was intact with no stripped threads. The evidence indicated that during the main rotor blade installation, the nut had likely been inadequately torqued, and subsequently worked off the bolt during flight. With the bolt loose, the main rotor blade was free to diverge from its normal plane of rotation, rendering the helicopter uncontrollable.