United Airlines Flight 8

The air carrier flight encountered turbulence associated with mountain wave activity during cruise at flight level 390 (FL390). One flight attendant located in the aft galley sustained serious injuries. The passengers and remaining crew were not injured. The aircraft was not damaged. The captain reported that the airplane was established in cruise at FL390 with smooth air conditions. She stated that the turbulence encounter began with a "slight buffet" and "rapidly increasing" airspeed. She attempted to maintain cruise airspeed by decreasing engine power and climbing to a higher altitude. She noted that as she initiated the climb, the aircraft experienced moderate turbulence. She reported that as the airplane reached FL400, the airspeed began a "rapid decrease" and she initiated a descent back to FL390. The flight subsequently descended to FL350 before the conditions smoothed out. She noted that the turbulence was encountered "suddenly and without warning." The flight data recorder indicated that the airspeed increased from 236 knots to 273 knots during the encounter. A vertical acceleration spike of 1.81 g's, followed by 0.61 g's, was recorded 64 seconds after the initial increase in airspeed. The weather forecast provided to the crew listed a risk of moderate turbulence between 20,000 and 38,000 feet. While ATC communications were describing light turbulence and moderate chop at lower altitudes, no reports from ATC, company dispatch, or other aircraft were received regarding mountain wave activity or turbulence at 39,000 feet.

Verbatim NTSB analysis and probable cause from the NTSB dataset