In-Flight Encounter with Mountain Wave and Windshear over Central Wyoming (N1815U)

Incident Description

While flying in good weather conditions over central Wyoming, the captain observed a gentle wave motion that prompted the autothrottles to react. In response, the captain disengaged both the autothrottles and autopilot. The aircraft had lost 250 feet of altitude during the wave motion. The captain then activated the seat belt sign and initiated a climb back to 39,000 feet. Approximately 30 seconds later—about four minutes after the initial wave action—the aircraft encountered a sharp jolt. Vertical G-forces fluctuated from minus 0.6 G to positive 1.6 G over roughly 1.25 seconds. Following the jolt, the air remained smooth, and no aircraft systems were affected. A doctor on board treated injured individuals while the flight continued to its destination.

Weather Findings

A weather summary indicated that the aircraft had encountered a mountain wave and/or turbulence windshear near the tropopause. Indications normally used to forecast such waves were absent until around the time of the accident.