PVT INST RATED PLT OF A SMA SEL LOST CTL OF ACFT DURING IMC AND CLBED 500 FT ABOVE ASSIGNED ALT BEFORE REGAINING ACFT CTL AND RETURNING TO ASSIGNED ALT.

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I WAS ON AN IFR FLT PLAN FROM DANBURY; CT (DXR) TO TRENTON; NJ (TTN). I DEPARTED DXR AT APPROX XA15 ON APR/XX/95 WITH AN INITIAL CLRNC TO 3000 FT EXPECTING 8000 FT IN 5 MINS. UPON RADAR CONTACT I WAS CLRED TO 4000 FT. A FEW MINS LATER I RECEIVED AN AMENDED CLRNC TO 6000 FT. APCH CTL CAME BACK WITH ANOTHER AMENDED CLRNC TO LEVEL OFF AT 5000 FT. THEY HAD BEEN RECEIVING PIREPS OF RIME ICING IN THE CLOUDS AT 6000 FT. AS I BEGAN TO LEVEL OFF AND RE-TRIM THE AIRPLANE; APCH ASKED ME MY OUTSIDE AIR TEMP (OAT). THE OAT GAUGE IN THE AMERICAN GENERAL TIGER IS A SMALL GAUGE BELOW A SERIES OF TOGGLE SWITCHES. AS I LOOKED OVER; WHICH REQUIRED ME TO TURN AND LOWER MY HEAD; I WAS CAUGHT IN A SUDDEN; UNEXPECTED UPDRAFT; WHICH CAUSED A GAIN IN ALT OF 400-500 FT. I BUSTED MY CLRNC. I IMMEDIATELY STARTED DOWN; RE-ESTABLISHED 5000 FT AND THE FLT CONTINUED UNEVENTFULLY. IN THE SINGLE PLT IFR WHILE HAND FLYING AN AIRPLANE IN THE CLOUDS; DO NOT LET INSIGNIFICANT REQUESTS FROM ATC OR OTHER DISTRS TAKE YOU AWAY FROM YOUR PRIMARY RESPONSIBILITY OF FLYING THE AIRPLANE AND KEEPING UP YOUR INST SCAN. THE UPDRAFT CAUSED THE PLANE TO ASCEND; BUT MY HEAD MOVEMENT CAUSED A MOMENTARY SPATIAL DISORIENTATION. THE RESULTING VERTIGO CAUSED ME TO LOSE CTL OF THE AIRPLANE.