A B747-400 CREW REPORTS UNEXPECTED SEVERE CLEAR AIR TURBULENCE AT FL350. THE RVSM ALT DEV LIMIT WAS EXCEEDED AS WAS VMO.

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WE WERE CLRED FROM TERMINAL AREA TO YYY. YYY IS THE FIR BOUNDARY. WE WERE TOLD 100 NM PRIOR TO YYY TO CONTACT NEXT SECTOR AT YYY. 40 NM PRIOR TO YYY; WE FELT A SLIGHT ACCELERATION (MOUNTAIN WAVE) FOLLOWED IMMEDIATELY BY SEVERE TURB. WE WERE UNABLE TO MAINTAIN ALT AND THE ACFT STARTED DSNDING AND WE EXCEEDED 400 FT ALTDEV. PFD WAS HARD TO READ; AND WE KNOW AIRSPD DEVS EXCEEDED VMO. AFTER WE REGAINED CTL; WE RETURNED TO THE PREVIOUS CTLR TO INFORM THEM WE HAD EXPERIENCED SEVERE TURB AND HAD BEEN UNABLE TO MAINTAIN ALT. THE CTLR NOTED THAT SHE HAD SEEN US EXCEED OUR RVSM ALT AND WAS GOING TO FILE A PIREP. WE THEN NOTIFIED THE NEXT SECTOR OF OUR PIREP. SHORTLY AFTER; WE HEARD ANOTHER ACR FLT ALSO RPT SEVERE TURB. WE WERE COST INDEX 200 MACH .85 660K GROSS WT RIGHT AT OPTIMUM ALT IN THE MIDDLE OF LOW AND HIGH BUFFET SPD. THIS WAS THE WORST TURB EXPERIENCE BY 5 CREW MEMBERS ABOARD. SUPPLEMENTAL INFO FROM ACN 777130: FL370 WE AGAIN EXPERIENCED SEVERE TURB. AUTOPLT DISENGAGED AND PF HAD TO TAKE OVER. WE WERE UNABLE TO MAINTAIN ALT. JUST PRIOR TO EXCEEDING LOWER LIMIT (300 FT) WE WERE ABLE TO REQUEST A 2000 FT BLOCK ALT FROM CTR. WE SAW AIRSPD GAINS OF OVER 40 KTS; EXCEEDING VMO AND DROP OF 40 KTS INTO THE LOWER BUFFET LIMIT. AUTOPLT WAS RE-ENGAGED BUT WE ALSO THINK IT WAS AGAIN DISCONNECTED EITHER BY TURB OR BY THE PLT MONITORING TRYING TO REACH THE PUSH-TO-TALK. THIS WAS OUR SECOND ENCOUNTER OF SEVERE TURB IN ONE FLT AND ALL CREWS AGREED BOTH RANKED AS WORST EXPERIENCE. THE FIRST ONE FELT WORSE ALTHOUGH INDICATION OF THE DATA RECORDED SHOWED THE SECOND BEING FAR MORE. ALL CREW MEMBERS AND JUMPSEATER HAD THEIR SEATBELTS ON AND WE HAD NO INJURIES. WINDS ON THE PFD INSTANTLY INCREASED OVER 100 KTS. SUPPLEMENTAL INFO FROM ACN 777132: CRUISING IN SMOOTH CLR AIR. NO RPTS OF TURB ON RADIO FROM OTHER ACFT. ALL OF A SUDDEN WE FLEW THROUGH THE MOST SEVERE MOUNTAIN WAVE OR CAT I HAVE EVER EXPERIENCED IN 20 YRS OF FLYING. UPON ARR; THE CAPT WROTE UP THE TURB AND RECORDED THE EVENTS IN THE ACFT LOGBOOK FOR A PROPER ACFT INSPECTION BY MAINT.