On March 4, 1998, at 1315 hours Pacific standard time, Alaska Airlines Flight 684, a Boeing 737-242C, N746AS, encountered severe clear air turbulence (CAT) at flight level (FL) 350 approximately 90 miles north-northeast of Reno, Nevada. There was no damage to the aircraft. Of the 116 personnel onboard, 3 passengers and 2 flight attendants received minor injuries, and 1 flight attendant suffered a broken ankle. Due to the injuries, the flight diverted to Reno. Visual meteorological conditions prevailed at the time. The flight was operated by Alaska Airlines, Inc., as a non-stop, regularly scheduled domestic passenger flight from Seattle to Las Vegas.

According to the operator, en route conditions were smooth up until the event. The Captain noted no turbulence, chop, etc., and there were no reports by other aircraft of any turbulence. After level off, the seat belt signs were turned off and the normal announcement made by the flight crew to keep the seat belts fastened while seated. Normal service was in progress in the cabin at the time of the accident.

A review of the flight recorder data indicated that, for the CAT period involved, vertical acceleration (Nz) varied from +1.0g to +1.51g to -0.07g to +1.3g to +0.86g over a 6-second interval. (See enclosed sketch and FDR time histories.) The recorded data also indicated stable atmospheric conditions (Nz = 1.0g) at FL 350 during the several minutes prior to CAT onset.

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