B727-100 CREW ENCOUNTERED SEVERE CLR AIR TURB IN ZDC CLASS A AIRSPACE.

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ON FLT FROM FLL TO ABE; WE WERE IN CRUISE FLT AT AROUND FL250. SUDDENLY; WE ENCOUNTERED CLR AIR TURB. IN 14 YRS OF FLYING; I HAVE NEVER EXPERIENCED SUCH VIOLENT TURB. I CLICKED OFF THE AUTOPLT; SLOWED THE AIRPLANE DOWN WHILE THE CAPT TOLD ATC THAT WE WERE EXPERIENCING SEVERE TURB AND NEEDED TO DSND. 2 OTHER AIRLINERS ALSO RPTED SEVERE TURB AND WERE ASKING FOR LOWER. AT TIMES; I COULD NOT CTL THE ACFT OR EVEN READ THE INSTS BECAUSE THE JARRING WAS SO RAPID IN ALL DIRECTIONS. LUCKILY; WE WERE A CREW ONLY FERRY FLT AND EVERYONE IN THE BACK ALSO HAD ON THEIR SEATBELTS; FASTENED. I FINALLY GOT THE AIRPLANE DSNDED BELOW 17000 FT AND IT FINALLY SMOOTHED OUT. WE WERE LITERALLY EXHAUSTED AND WERE NOT SURE WHICH DIRECTION WE WERE SUPPOSED TO BE GOING. ATC TOLD US TO TURN TO A HDG NOW OR WE WOULD PENETRATE RESTR AIRSPACE. IN THE DSCNT; I DIDN'T HEAR ANY TCASII WARNINGS AND I DON'T THINK WE CONFLICTED WITH ANY OTHER ACFT OR PENETRATED ANY RESTR AIRSPACE. WE CONTINUED ON TO ABE WITHOUT FURTHER INCIDENT.