Incident Overview
A flight experienced severe clear air turbulence while cruising at flight level 350. The event prompted a diversion to Reno, Nevada. Prior to the encounter, no other pilots had reported clear air turbulence in the area, and such conditions were not included in any forecasts. At the time of the turbulence, the seat belt signs were off.
Flight Data Recorder Analysis
Review of the flight data recorder (FDR) revealed that during the turbulent period, vertical acceleration (Nz) varied significantly over a 6-second span. The recorded values ranged from +1g to +1.5g, then dropped to -0.07g, followed by +1.3g, and finally +0.86g. This indicates rapid and severe changes in vertical motion. The FDR data also indicated that atmospheric stability existed at flight level 350 several minutes prior to the onset of turbulence, suggesting a sudden change in conditions.
Crew Warning and Communication
There was little or no warning given to the flight crew of the impending turbulent conditions. The absence of pilot reports and forecasts, combined with the sudden onset, contributed to the unexpected nature of the encounter.