FLC OF A HVT CARGO LOST CTL OF ACFT MOMENTARILY WHEN SEVERE TURB DISCONNECTED THE AUTOPLT AND THE ACFT INVOLUNTARILY CLBED 1800 FT; WITH PWR PULLED BACK BEFORE REGAINING FULL CTL. THERE WERE NO FORECASTS FOR TURB OR PIREP RPTS TO INDICATE A POSSIBLE TURB ENCOUNTER.

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CRUISE FLT AT FL290; SMOOTH RIDE; NO WX SHOWING ON THE RADAR. THE DEP WX PACKET SHOWED SOME WX OVER JAPAN BUT WE APPEARED TO BE ABOVE IT WITH STARS ABOVE. WITH NO FOREWARNING OF LIGHT TO MODERATE TURB; WE ENCOUNTERED SEVERE TURB. THE AUTOPLT AUTOMATICALLY DISENGAGED AND WE CLBED 1800 FT TO FL308 WITHIN 15 SECONDS OR LESS. IT WAS EXTREMELY DIFFICULT TO MAINTAIN A WINGS LEVEL ATTITUDE AND A REASONABLE PITCH. WE NOTIFIED ATC OF THE ALTDEV; DSNDING BACK TO FL290 AND REQUESTED FL330. THE SEVERE TURB LASTED ABOUT 15 SECONDS THEN MODERATE FOR ABOUT 1 1/2 MINS. SMOOTH RIDE AFTER THAT. THE FE WAS RETURNING FROM THE BACK DURING THE ENCOUNTER. HE GRABBED THE SEAT AND FELL TO THE FLOOR. HE WAS UNINJURED. THE #4 GENERATOR WAS KNOCKED OFF LINE. AFTER THINGS RETURNED TO NORMAL WE GAVE A PIREP TO ATC. TO PREVENT A RECURRENCE OR CORRECT THE SIT; I REALLY CAN'T SEE WHAT WE COULD HAVE DONE DIFFERENTLY SINCE THE TURB WAS UNFORECAST; NOT ANTICIPATED AND NO WX SHOWED UPON THE RADAR. I WAS IMMEDIATELY ON THE CTLS AND CLOSED THE THROTTLES; BUT THE CLB WAS BEYOND OUR CTL. THIS IS ANOTHER REMINDER FOR ALL PAX AND CREWS TO KEEP THEIR SEAT BELTS FASTENED. (SOME PAX STILL DON'T UNDERSTAND THAT; FORTUNATELY WE ONLY HAD CARGO ON BOARD.)