SMA IN TURB EXPERIENCES VIOLENT SHAKING AND ACFT BECOMES ALMOST UNCTLABLE. MAKES EMER LNDG.

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I WAS FLYING AN SMT ON A VFR FLT FROM GLYNCO ARPT BRUNSWICK; GA; TO LEESBURG; VA. WX CONDITIONS INCLUDED RAIN TSTMS MOVING TO THE NE OVER PARTS OF CENTRAL AND EASTERN SC AND NC. THE WX RPT INDICATED IMPROVING CONDITIONS AFTER XX00Z. SINCE I NEEDED TO MAKE A FUEL STOP; I DECIDED TO FLY AS FAR N AS POSSIBLE AND STOP FOR FUEL AND WAIT FOR IMPROVING WX IF NEEDED. CRUISING AT 7500 MSL; THE FLT WAS SMOOTH AND UNEVENTFUL UNTIL I REACHED VANCE VOR. CLOUDS BEGAN TO BUILD AHEAD; SO I STARTED TO DSND TO AN ARPT TO REFUEL AND WAIT. DURING THE DSCNT; I ENCOUNTERED MODERATE TURB AND LIGHT RAIN SHOWERS. I REDUCED PWR AND TURNED TOWARD THE S TO AVOID THE CONDITIONS WHEN THE AIRPLANE BEGAN TO SHAKE VIOLENTLY. THE YOKE WAS MOVING OUT AND IN ABOUT AN INCH AND THE RUDDER PEDALS OSCILLATED. I LOCATED THE NEAREST ARPT WHICH WAS 33 MI AWAY BUT WAS BENEATH A STORM. I THEN LOCATED THE SANTEE-COOPER REGIONAL ARPT AND CALLED 'MAYDAY' TO SHAW APCH ON 118.85 BECAUSE OF THE MECH PROBLEM WITH THE ACFT. I LOWERED THE FLAPS AND REDUCED AIRSPD TO 80 MPH WHICH MINIMIZED THE BUFFETING OF THE CTLS. THE AIR TFC CTLR CONFIRMED MY COURSE AND INFORMED ME THE ARPT WAS 10 MI. HE PROVIDED CONTINUAL GUIDANCE DURING THE REMAINDER OF THE FLT. I LANDED WITHOUT FURTHER INCIDENT APPROX XX00Z AND HAVE SINCE BEEN IN CONTACT WITH THE FAA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES WHEN ON GND HE WAS ABLE TO GET A MECH WHO DETERMINED THE HORN WHICH THE CABLE FOR THE RUDDERVATOR IS ATTACHED TO SNAPPED AND CRACKED; THE CABLE ITSELF THEN TORE UP THE INSIDE OF THE LEFT RUDDERVATOR. THE ACFT IS AN OLDER ONE AND MAY BE JUST WEAR AND STRESS CAUSED FROM TURB CAUSED FINAL STRESS THAT CRACKED IT. STATED HE'S NOT SURE WHAT HAD WHO BY THE TAIL WHILE GETTING THE ACFT DOWN.