PA31 PLT WAS OPERATING IN ICING CONDITIONS AND HAD ATTITUDE INST PROBS. HE DID NOT HAVE A MEDICAL CERTIFICATE IN HIS POSSESSION.

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I WAS ON AN IFR FLT PLAN FROM TTN TO OWD. DURING THE FLT ON V139 FROM MANTA INTXN TO THE HTO VOR; I ENCOUNTERED ICING AT 7000 FT. I CALLED ATC TO RPT THE ICING AND HE ASKED FOR THE TYPE AND TEMP. IT WAS LIGHT RIME ICE AND IT WAS -12 DEGS C. HE CAME BACK AND SAID THAT WE WERE CLRED TO DSND TO 5000 FT. I BEGAN TO DSND TO 5000 FT AND WE WERE NOW IN SNOW CONDITIONS. I WAS KEEPING A CHK ON THE LEADING EDGE OF MY WING AND ATC INFORMED ME THAT I WAS AT 4000 FT. I CONFIRMED THAT I WAS AT 4000 FT AND TOLD HIM THAT WE WOULD CLB TO 5000 FT AND IMMEDIATELY DID SO. DURING THIS TIME; MY ATTITUDE INDICATOR WAS SHOWING ME IN A R TURN. HOWEVER; THE OTHER INSTS SHOWED THAT I WAS STRAIGHT AND LEVEL AT 5000 FT. ZNY ASKED THAT I CONTACT PROVIDENCE ATC AND AFTER CALLING PROVIDENCE ATC I RPTED THAT I WAS HAVING TROUBLE WITH AN INST; I WOULD LIKE TO DECLARE AN EMER AND HAVE A HEADING TO THE NEAREST ARPT. SHE GAVE ME A HEADING AND PERMISSION TO DSND. UPON REACHING 3500 FT; WE HAD VISUAL CONDITIONS AND LANDED AT GROTON NEW LONDON ARPT. AFTER LNDG IN GROTON; CT; I WAS MET BY A FIREMAN TO FILL OUT A RPT. I GAVE HIM MY PLT'S LICENSE TO COPY; BUT DID NOT HAVE MY MEDICAL CERTIFICATE WITH ME. THE NEXT DAY; I MET WITH A MECH AND WE WENT OVER THE PROBABLE CAUSES. I WAS ABLE TO CONTINUE MY FLT VFR AND NO PROBS OCCURRED.