PVT INST RATED PLT OF A PIPER PA28-180 (PARO) WAS NOTIFIED BY ATC THAT HE WAS CRUISING 300 FT ABOVE ASSIGNED. THE RPTR BELIEVED THAT HE WAS AT ASSIGNED ALT EVEN THOUGH HE HAD GONE TO ALTERNATE AIR STATIC SOURCE AFTER THE NORMAL STATIC SOURCE HAD FROZEN OVER IN THE CLOUDS.

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IT WAS MY FIRST EXPERIENCE IN ACTUAL IFR CONDITIONS WITHOUT AN INSTRUCTOR. I DIDN'T EXPECT IMC SO I DIDN'T FILE AN IFR FLT PLAN. ABOUT 1 HR INTO MY 3 HR FLT; I ENCOUNTERED CLOUDS THAT I COULDN'T CLB OVER. I AIR-FILED AN IFR FLT PLAN AND WAS GIVEN AN IFR CLRNC. AS I WAS FLYING ALONG; CTLRS KEPT GIVING ME ALTIMETER SETTINGS APPROX EVERY 5 MINS. ABOUT 15 MINS AFTER I ENTERED IMC; MY STATIC PORT FROZE OVER (IN SPITE OF THE HEAT BEING TURNED ON) SO I ACTIVATED THE ALTERNATE STATIC SOURCE. I DECIDED I SHOULD CHK WITH ATC TO ENSURE ACCURACY GIVEN THE ALTERNATE AIR. THEY INFORMED ME THAT I HAD CONSISTENTLY BEEN SHOWING 300 FT HIGH EVER SINCE I CAME ON-LINE WITH THEM. I DON'T KNOW IF ANYTHING WILL COME OF THIS INCIDENT IN TERMS OF AN ENFORCEMENT ACTION; BUT I CAN'T UNDERSTAND WHY THEY DIDN'T JUST SAY SOMETHING ABOUT MY ALT. I WAS PATTING MYSELF ON THE BACK AS I WAS FLYING ALONG BECAUSE ACCORDING TO MY ALTIMETER; I WAS HOLDING ALT +/-75 FT -- NOT BAD FOR MY FIRST TIME; I THOUGHT. THE CHEROKEE I WAS FLYING BELONGS TO A FRIEND OF MINE WHO REGULARLY FLIES IFR WITH IT. I'VE NOTIFIED HIM OF THE DISCREPANCY AND I'M SURE HE'LL HAVE THE PROB ANALYZED AND CORRECTED; IF NECESSARY.