A 600 FT ALTDEV BY AN SF340 FLC DURING AN UNEXPECTED ENCOUNTER WITH SEVERE TURB 35 NM SE OF PIT; PA.

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I WAS THE PF AND THE AUTOPLT WAS ENGAGED. AFTER WE SWITCHED OVER TO PITTSBURGH APCH; WE HEARD RPTS OF AREAS OF MODERATE TURB AROUND THE ARPT AND ALONG THE APCH COURSE. WE WERE APPROX 35 MI FROM THE ARPT LEVEL AT 7000 FT AND RECEIVING VECTORS FOR AN ILS APCH. I HAD SLOWED THE AIRPLANE TO OUR ROUGH AIR PENETRATION SPD ANTICIPATING THE AREAS OF MODERATE TURB. WE THEN ASKED THE APCH CTLR FOR A 30 DEG TURN TO THE L FOR 10 MI TO AVOID A CELL DEPICTED ON OUR RADAR THAT WAS RPTED BY ANOTHER PLT TO BE HVY RAIN. AS WE BEGAN TO PASS THE AREA OF HVY RAIN THE AIRPLANE BEGAN TO SHAKE VIOLENTLY. THE AUTOPLT BECAME DISENGAGED ON ITS OWN AND I BEGAN TO HAND FLY IT SINCE THE AUTOPLT WOULD NOT ENGAGE AGAIN. I WAS ABLE TO RIDE THE AIRPLANE OUT OF A BANK AND THEN THE AIRPLANE BEGAN TO DSND UNCONTROLLABLY. I ADDED ALL AVAILABLE PWR AND PITCHED UP BUT THE AIRPLANE KEPT DSNDING. FINALLY AT AROUND 6400 FT; THE AIRPLANE BEGAN TO CLB AGAIN. I LEVELED THE AIRPLANE AT 7000 FT AND THE CAPT RPTED THAT THE MODERATE TURB IN THAT AREA WAS ACTUALLY SEVERE AND DUE TO THE TURB WE HAD LOST 500-600 FT. I HAND FLEW THE REMAINDER OF THE APCH AND WE LANDED UNEVENTFULLY. I BELIEVE THAT A POWERFUL DOWNDRAFT WAS THE MAIN FACTOR IN THIS OCCURRENCE. I ALSO FEEL THAT MANY PLTS ARE RELUCTANT TO RPT AREAS OF SEVERE TURB AROUND ARPTS WITH A LARGE NUMBER OF COMMERCIAL AIRLINE TFC SINCE A RPT OF SEVERE TURB WOULD DISRUPT THE FLOW OF TFC INTO THE LARGE ARPTS. DURING THIS OCCURRENCE THERE WAS NOT AN ACFT WITHIN AT LEAST 10 MI OF US ACCORDING TO OUR TCASII SYS.