Accident Sequence
During a demonstration flight, the tiltrotor aircraft experienced a loss of control. The left proprotor governor disconnected, causing the propeller to surge to its maximum pitch setting. This resulted in a roll around the longitudinal axis. Flight data recordings indicated that the aircraft rolled to the right, initially slowly, then accelerating to an inverted position in less than three seconds. The aircraft subsequently impacted the ground while inverted.
Mechanical Findings
Investigation revealed that the governor disconnected due to a missing safety on the castellated nut that secures the proprotor to the governor linkage. The absence of this safety allowed the nut to loosen, leading to the disconnection.
Emergency Egress Issues
After the accident, neither occupant could exit the aircraft without assistance. This was attributed to inconsistent instructions provided for emergency egress procedures.
Investigation Findings
Investigation determined the cause of the accident was the disconnection of the left proprotor governor caused by a missing safety on the castellated nut that secures the proprotor to the governor linkage. No other causal factors were specified in the source.