What happened
The captain reported encountering gentle wave motion while cruising in clear weather over central Wyoming. The disturbance caused the autothrust system to react, prompting the pilot to disengage both the autothrust and autopilot systems. During this initial phase, the aircraft lost approximately 250 feet of altitude. In response, the captain activated the seat belt sign and initiated a climb back to the cruising altitude of 39,000 feet.
Approximately four minutes after the initial wave action, the aircraft experienced a sharp jolt. Vertical G-forces shifted rapidly from negative 0.6 to positive 1.6 G within roughly 1.25 seconds. Following this event, the air returned to smooth conditions with no apparent impact on aircraft systems. A doctor aboard treated any injured passengers while the flight continued to its destination.
The investigation
A weather summary indicated that the aircraft had encountered mountain wave activity and/or turbulence windshear near the tropopause. Forecasting indicators typically used to predict such waves were not present until approximately the time of the incident.
Findings
The crew's decision to disengage automated flight controls allowed for manual recovery from the initial altitude loss. The rapid change in vertical G-forces was consistent with severe clear-air turbulence associated with mountain wave activity.