WDB ACFT ENCOUNTERS SEVERE TURB AT LEVEL OFF. RETURN LAND FOR STRUCTURAL INSPECTION.

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WE WERE ON CLBOUT AFTER TKOF FROM FRA; COPLT FLYING THE ACFT. ENCOUNTERING LIGHT TO MODERATE TURB AT FL280. THE CAPT HAD JUST MADE A PA ABOUT STAYING IN SEATS WITH SEAT BELT FASTENED. REQUESTED AND RECEIVED CLRNC TO CLB TO FL310. INITIALLY; TURB EASED; BUT JUST AS WE WERE LEVELING AT FL310; WE ENCOUNTERED SEVERE TURB. I WAS ON THE JUMPSEAT; THE ACFT SHOOK SO VIOLENTLY MY SUNGLASSES AND HEADSET CAME OFF. THE COPLT STARTED A DSCNT; THE CAPT INITIALLY COULD NOT MAKE A RADIO CALL BECAUSE THE AIRPLANE WAS SHAKING SO VIOLENTLY HE COULDN'T USE A MIKE SWITCH. WHILE DSNDING; THE TCASII SHOWED A TFC CONFLICT AND ISSUED AN RA; THE COPLT HAD VISUALLY ACQUIRED THE TFC AND EVADED. IN THE DSCNT WE GOT TO ABOUT 22500 FT; THEN CLBED TO LEVEL AT FL230 IN MODERATE TURB. I USED THE INTERPHONE TO CHK WITH A FLT ATTENDANT ABOUT PAX. SHE SAID EVERYONE APPEARED TO BE OK; BUT SHE NEEDED TO CHK ON ONE IN THE BACK WHO MIGHT HAVE HIT HIS HEAD. LATER; SHE CONFIRMED THAT EVERYONE WAS OK. WE REQUESTED CLRNC BACK TO FRANKFURT; WHICH WE GOT. RETURN AND LNDG WERE UNEVENTFUL; AND HAD AIRPLANE CHKED FOR STRUCTURAL DAMAGE (NONE FOUND). ALL 3 FLT DECK CREW MEMBERS AGREED THAT THIS WAS; BY FAR; THE MOST SEVERE TURB THEY HAD EVER ENCOUNTERED; AND FOR THE LONGEST DURATION. I REALIZE THERE IS A TENDENCY TO THINK EVENTS LAST LONGER THAN ACTUALLY HAPPENS; AND STILL ESTIMATE THAT WE WERE IN CONTINUOUS SEVERE TURB FOR 3-4 MINS. AFTERWARDS; IN TALKING ABOUT IT; THE CAPT SAID THE FIRST TIME HE COULD READ THE ALTIMETER AFTER THE ENCOUNTER STARTED; WE WERE DSNDING THROUGH FL270 AND THINGS REALLY DIDN'T SETTLE BACK TO MODERATE UNTIL WE REACHED FL230. WE ALL THOUGHT THERE WAS A GOOD POSSIBILITY OF STRUCTURAL DAMAGE BECAUSE THE ENCOUNTER WAS SO VIOLENT. IN CONCLUSION: 1) MANUFACTURER BUILDS ONE HELL OF AN AIRPLANE. 2) TCASII WAS USEFUL IN ACQUIRING AND AVOIDING ANOTHER ACFT DURING OUR FORCED DSCNT. 3) THE FACT THAT THE CAPT HAD JUST MADE A PA ABOUT SEAT BELTS MAY HAVE CONTRIBUTED TO OUR NOT EXPERIENCING ANY INJURIES TO PAX.