A PA28 PLT; FLYING OVER THE MOUNTAINS OF VIRGINIA; ENCOUNTERED TURB AND A MOUNTAIN WAVE CONDITION; RENDERING HIM INCAPABLE OF MAINTAINING ASSIGNED ALT.

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CHEROKEE FILED FOR AND WAS CLRED TO 8000 FT ON DEP FROM BVI. AT 8000 FT WE WERE IN VMC WITH A CLOUD LAYER OF AROUND 6500 FT MSL TOPS. 1 HR INTO THE FLT ATC BROUGHT US DOWN TO 7000 FT DUE TO TFC. 1/2 HR LATER THE CLOUD TOPS WENT UP TO 7000 FT MSL. IMMEDIATELY I ASKED FOR A CLB BACK UP TO 8000 FT MSL. ZDC WAS UNABLE DUE TO TFC 2 MI BEHIND AT 8000 FT. HE ASKED MY CONDITIONS; AND I SAID A ROUGH RIDE WITH LIGHT ICE. ZDC HAD THE 2 MI TFC CLB UP TO 9000 FT BEFORE CLRING US TO CLB TO 8000 FT. AFTER THE TFC REACHED 9000 FT WE THEN WERE CLRED TO 8000 FT. OVER THE MOUNTAINS WITH A LIGHT BUILD UP OF RIME WE HAD A VERY SLOW RATE OF CLB. WE WERE ABLE TO BREAK OUT AT AROUND 7400 FT MSL; BUT WERE UNABLE TO CLB TO 8000 FT. AT 7400 FT MSL DODGING THE TOPS OF CLOUDS WE STARTED TO DSND. I THEN ASKED FOR VECTORS TO THE NEAREST ARPT. AT THAT POINT I BELIEVE DUE TO MOUNTAIN WAVE WE HAD A MASSIVE DSCNT OF 2000 FT IN A MATTER OF SECONDS. I THEN DECLARED AN EMER TO THE CTLR. ONCE BREAKING OUT OF THE CLOUDS I WAS ABLE TO LEVEL THE PLANE OFF AND MAINTAIN ALT AT AROUND 4600 FT MSL. I THEN RECEIVED VECTOR AND WAS ABLE TO LAND SAFELY AT HOT SPRINGS ARPT 30 MI N OF ROANOKE. IT ALL HAPPENED EXTREMELY FAST. IF THERE ARE ANY QUESTIONS YOU HAVE; PLEASE CONTACT ME.