Flight Preparation
The flight crew briefed and prepared the aircraft for flight in forecast severe turbulence. The aircraft tracked from waypoint HONEY to DALEY, entering the lee side of the Brindabella Ranges. The tracking direction was northeast with a strong tailwind component. The wind forecast indicated 30 knots at 5,000 feet, increasing to 45 knots at 7,000 feet, with direction within 30 degrees of perpendicular to the ridgeline at that location.
Turbulence Encounter
During the flight, the aircraft encountered sudden turbulence. According to the investigation, the abrupt encounter was probably the result of mountain wave activity. The flight crew reported that during the encounter, their movement relative to the aircraft was vertical when the aircraft dropped from underneath them.
Restraint System and Injury
The primary restraint method for negative-G forces is the lap belt supported by the crotch strap. If the lap belt has slack, the body can move upward relative to the belt. If the crotch strap is slack, the lap belt itself can move upward if pulled by the shoulder harness. The captain reported that the crotch strap can loosen with occupant movement. The aircraft experienced minor fluctuations in G before the turbulence incident. The investigation concluded that the captain's injury was probably the result of some measure of slackness in the crotch strap.