Narrative:

While in cruise at FL370 with the autopilot engaged in smooth air; we encountered a sudden severe turbulence event probably associated with mountain wave activity. During the event; the airframe speed reached mach .85 momentarily; followed by airspeed excursions of +/- 20 KIAS; +/- five degrees of bank; and altitude change of +200 feet. The encounter was brief and sudden; with two sudden 'jolts' of turbulence; then mostly light chop during the mountain wave which affected the aircraft a further 2 minutes or so. The autopilot and autothrottles remained engaged throughout the event; although manual power control was necessary to remain at a safe airspeed.occasional mdt turbulence was forecast along our offshore route and we had made several altitude changes in the previous twenty minutes to find smoother air; helped out by reports from several other flights ahead of us bound for secl. Occasional mdt turbulence and occasional mdt mountain wave were also forecast in the immediate area of the encounter; however no other traffic had reported turbulence activity eastbound. Although we had been in smooth air for approximately five minutes prior to the encounter; the seat belt sign was illuminated due to the forecast.immediately prior to the event we were at 295;000 pounds gross weight and mach .815. The captain was in the cabin in the crew rest seat. The event occurred while eastbound near repev on UL560; near the crux of the andes mountain range.after the event; the cabin crew reported that service items had been displace to the floor and one flight attendant (flight attendant) had hot coffee spilled on him and possibly burned. No actual injuries were noted; however; and the flight attendant who had the spilled coffee said later he was alright. No passenger issues were reported to us in the cockpit.

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Original NASA ASRS Text

Title: An air carrier aircraft encountered un-forecasted severe turbulence and mountain wave activity over the Andes Mountains in South Central Argentina.

Narrative: While in cruise at FL370 with the autopilot engaged in smooth air; we encountered a sudden severe turbulence event probably associated with mountain wave activity. During the event; the airframe speed reached Mach .85 momentarily; followed by airspeed excursions of +/- 20 KIAS; +/- five degrees of bank; and altitude change of +200 feet. The encounter was brief and sudden; with two sudden 'jolts' of turbulence; then mostly light chop during the mountain wave which affected the aircraft a further 2 minutes or so. The autopilot and autothrottles remained engaged throughout the event; although manual power control was necessary to remain at a safe airspeed.Occasional MDT turbulence was forecast along our offshore route and we had made several altitude changes in the previous twenty minutes to find smoother air; helped out by reports from several other flights ahead of us bound for SECL. Occasional MDT turbulence and Occasional MDT mountain wave were also forecast in the immediate area of the encounter; however no other traffic had reported turbulence activity eastbound. Although we had been in smooth air for approximately five minutes prior to the encounter; the seat belt sign was illuminated due to the forecast.Immediately prior to the event we were at 295;000 LBS Gross weight and Mach .815. The captain was in the cabin in the crew rest seat. The event occurred while eastbound near REPEV on UL560; near the crux of the Andes mountain range.After the event; the cabin crew reported that service items had been displace to the floor and one Flight Attendant (FA) had hot coffee spilled on him and possibly burned. No actual injuries were noted; however; and the FA who had the spilled coffee said later he was alright. No passenger issues were reported to us in the cockpit.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.