SMA ENCOUNTERS SEVERE TURB HAS ALTDEV.

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I HAD BEEN CHKING WX AT LEAST ONCE A DAY FOR A WK PRIOR TO MAKING THIS FLT. I POSTPONED THE FLT EACH DAY BECAUSE A CHANCE OF AFTERNOON TSTMS WERE FORECAST. SEVERAL CALLS TO BRIDGEPORT FSS ON SUNDAY MORNING ALL GAVE A GREAT FORECAST FOR THE DAY WITH NO TSTM FORECAST. WE FLEW TO OUR DEST; ACK; CONDUCTED OUR BUSINESS AND CALLED FOR WX BEFORE HEADING OUT TO THE ARPT. I CALLED WX AGAIN AT THE ARPT AND WAS TOLD OF LIGHT RAIN; S AND SW OF ORH. WE DEPARTED IFR AND CALLED BOSTON RADIO ABOUT 20 MINS INTO THE FLT. THEY RPTED LEVEL 1 AND 2 IN A LINE PUTNAM TO THE S; AND A LATER REQUEST TOLD OF A LEVEL 5 W OF WORCESTER. I REQUESTED RNAV DIRECT CLY TO LAND DOWNWIND ON RWY 29 TO STAY AWAY FROM THE WX. I ASKED IF MY NEW HDG WOULD BE CLR OF ANY WX AND I WAS TOLD YES; THAT ALL THE WX WAS W AND S OF ORH. I DSNDED OUT OF 8000 FT FOR 6000 FT AND WAS CLRED TO 4000 FT. I WENT THROUGH SUCH SEVERE TURB I HAD ALL I COULD DO TO HOLD THE PLANE LEVEL. MY ORIGINAL HDG WAS APPROX 340 DEGS AND ENDED UP 060 DEGS AFTER I STABILIZED. I DON'T KNOW MY ALT AT THAT TIME. CTL WANTED TO VECTOR ME BACK TOWARDS CLY TO A HDG BACK INTO THE DARKEST PART OF THE CLOUDS. I STATED THAT IT LOOKED BRIGHTER STRAIGHT AHEAD AND WOULD LIKE TO CONTINUE ON MY PRESENT HDG UNTIL STABILIZED. I WAS ASKED BY CTL A FEW MINS LATER WHAT ALT I WAS AT AND WHAT ALT WAS I CLRED TO. I RESPONDED I WAS AROUND 3000 FT AND HAD BEEN CLRED TO 4000 FT BUT HAD JUST GONE THROUGH SOME SEVERE WX AND HAD ALL I COULD DO TO KEEP THE WINGS LEVEL. I WAS TOLD TO GO BACK TO 4000 FT; THAT I WAS CLRED FOR THE VISUAL. I HAVE SPENT MY ENTIRE FLYING CAREER AVOIDING THIS KIND OF WX BY SIMPLY NEVER FLYING WHEN IT'S FORECAST OR THERE'S A CHANCE OF IT. I BELIEVE THE FOLLOWING ACTIONS PREVENTED A POSSIBLE ACCIDENT: 1) I RESISTED THE TEMPTATION TO DO A 180 DEG TURN; KNOWING THAT THE SHORTEST RTE OUT OF THE WX WAS PROBABLY STRAIGHT AHEAD. 2) REDUCED PWR. 3) ABILITY TO HAND-FLY THE AIRPLANE UNDER THOSE CONDITIONS BY INSTS. 4) REMAINING REASONABLY CALM AND IN CTL. ALL THE ABOVE WERE POSSIBLE BECAUSE OF RECURRENT TRAINING AND LOTS OF READING. I SUBSCRIBE TO IFR AND CONSIDER IT VALUABLE AND ESSENTIAL READING. I FILED A PIREP THAT EVENING WHEN I ARRIVED HOME. I VISITED THE TWR THE NEXT DAY TO DISCUSS WHAT HAPPENED.