Widebody Transport
Flight Crew Air Transport Pilot (ATP)
Distraction; Physiological - Other; Workload
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Regained Aircraft Control; Flight Crew Took Evasive Action
Weather
Severe Turbulence at FL380 crossing the Andes Mountain range.  Airspeed fluctuated both plus and minus 45 knots; into both the max speed and the lower foot causing stick shaker and overspeed warnings.  Altitude was uncontrollable during the event and fluctuated both plus and minus approximately 800 feet.  ATC was notified.  Maintenance was notified upon landing; however no Maintenance Log entry was made; specifically for the overspeed warning.  We briefed possible turbulence and queried both ATC and Dispatch prior to the area in question for flight planning purposes. Suggest better weather modeling or forecasting.
A Wide Body freighter aircraft encountered severe turbulence at FL380 crossing the Andes Mountains into Santiago; Chile which resulted in altitude deviations of plus and minus 800 feet along with high and low speed warnings caused by airspeed fluctuation of 45 knots.