Korean Air flight KE864, a Boeing 737-9B5, encountered turbulence at about 17,000ft. One cabin crew member fell in the galley and suffered 12 fractures of the thoracic vertebrae. The Air and Rail Accident Investigation Committee, hereinafter referred to as the Committee, determines the cause of this accident Unexpected clear air turbulence during approach descent for landing I decide that it is.? Approach for landing as a contributing factor Landing ?ON? ?with seat belt? during descent It is decided due to the lack of self-protection of the cabin crew who were preparing beforehand.? Based on the results of the accident investigation, the committee issued safety advisories to Korean Air. The Aviation and Railway Accident Investigation Committee determines the cause of this accident as ?clear air turbulence during the approach descent for landing.? As a contributing factor ?It is determined by the lack of self-protection of the cabin crew who were preparing before landing with the seat belts turned ?ON? during the descent.?