On February 25, 1997, at 1309 hours Pacific standard time, a flight attendant received a serious injury during an encounter with clear air turbulence over the Sierra mountains while the aircraft was in a descent to San Francisco, California. Visual meteorological conditions existed at the time. The aircraft, a Boeing 767-300, N661UA, was operated by United Airlines as flight 961 from Paris, France, with a total of 110 people onboard. None of the other people onboard were injured. According to United Airlines, the aircraft was descending through flight level 330 when the aircraft experienced a single, severe jolt of unforecasted moderate clear air turbulence. The aft flight attendant, working in the rear galley, fractured her right ankle that later required surgery. The fasten seat belt sign was not illuminated at the time.