What happened
While operating between waypoints HONEY and DALEY, the flight crew encountered severe turbulence while tracking north-east. The aircraft was flying through the lee-side of the Brindabella Ranges under conditions involving a strong tailwind. At the time of the incident, winds at 5,000 ft were forecast at 30 kt, increasing to 45 kt at 7,000 ft, positioned within 30° of perpendicular to the ridgeline.
During this period of instability, the aircraft experienced a sudden drop. The crew reported a sensation of vertical movement relative to the cabin as the aircraft descended abruptly. This encounter resulted in one injury to the captain.
The investigation
The investigation focused on the mechanics of the injury and the atmospheric conditions present during the flight. Investigators examined the aircraft's movement and the effectiveness of the occupant restraint systems. The captain noted that the aircraft had been subject to minor G-load fluctuations prior to the primary turbulence event.
Analysis of the restraint system indicated that the primary method for preventing upward movement during negative-G loads is the lap belt supported by the crotch strap. The investigation looked into how slack in these components could allow an occupant to move upward if pulled by the shoulder harness. It was noted that the crotch strap can loosen during occupant movement.
Findings
- The severe turbulence was likely caused by mountain wave activity occurring on the lee-side of the Brindabella Ranges.
- The injury to the captain was likely caused by insufficient restraint provided by the crotch strap, which allowed the captain to strike their head during the turbulence.
- The flight crew had appropriately briefed and prepared the aircraft for flight in the forecast severe turbulence.