PLT OF SMA ACFT DID NOT MAINTAIN ASSIGNED ALT DURING APCH VECTOR DUE ACFT'S LACK OF PERFORMANCE DUE TO INFLT ICING CONDITIONS.

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PROCEEDING EBOUND ON VICTOR 58 JUST W OF WILLIAMSPORT (IPT); I NOTICED I WAS ACCUMULATING SOME RIME ICE AT 7000 FT. I QUERIED THE ZNY CTLR FOR THE WILLIAMSPORT WX; AND BASED ON THE INFO I RECEIVED; I REQUESTED 6000 FT TO GET OUT OF THE CLOUDS AND OUT OF THE ICING. I WAS CLRED TO 6000 FT. I THEN CONTINUED TOWARD LAKE HENRY ON VICTOR 58; OCCASIONALLY ENCOUNTERING SOME MODERATE TURB. BTWN WILLIAMSPORT AND LAKE HENRY; IT BECAME DARK. WE WERE IN LIGHT SNOW. ON ARRIVING CLOSER TO LAKE HENRY; WE AGAIN BECAME IMC. ABOUT 5 MINS BEFORE ARRIVING AT LAKE HENRY; I NOTICED THE AIRSPD BEGINNING TO DROP OFF VERY SLOWLY. I ALSO NOTICED A SLIGHT INCREASE IN THE ACCUMULATION OF ICE. PITOT HEAT WAS ON. AS WE DREW CLOSER TO LAKE HENRY; I DECIDED TO EXECUTE A PRECAUTIONARY LNDG AT WILKES- BARRE; DUE TO THE INCREASED ICE AND ALSO BECAUSE THE AIRSPD WAS BEGINNING TO DROP OFF SIGNIFYING TO ME A DEGRADATION IN PWR OUTPUT AND PERHAPS AN INCREASE IN STALL SPD. I COMMUNICATED THESE INTENTIONS TO WILKES- BARRE APCH. WILKES-BARRE CLRED ME TO DSND TO 3300 FT AND TO INTERCEPT THE LOC. ONCE I WAS LEVEL AT 3300 FT; THE AIRSPD CONTINUED TO DROP. IT WAS NOW AT 60 KTS. I WAS STILL L OF THE LOC. BEFORE I INTERCEPTED THE GS I REALIZED I COULD NOT MAINTAIN AIRSPD IN LEVEL FLT. FEARFUL OF A STALL; I DEPARTED MY ASSIGNED ALT AND SIMULTANEOUSLY OBSERVED; SLIGHTLY TO THE L OF THE ACFT; A LIGHTED AREA. I ELECTED TO CIRCLE THE LIGHTED AREA BECAUSE I REALIZED THAT THIS WAS MOUNTAINOUS TERRAIN AND I HAD DEPARTED FROM MY ASSIGNED ALT. THE CTLR THEN INSTRUCTED ME TO EXECUTE A MISSED APCH; TO FLY A HDG OF 040 DEGS AND MAINTAIN 4000 FT. BASED ON MY JUDGEMENT THAT THE ACFT WAS IMMINENTLY CLOSE TO A STALL WITH ALL PWR AVAILABLE; I DECLINED TO CLB. INSTEAD; I REQUESTED A VECTOR TOWARD THE ARPT. THE CTLR ASSIGNED ME A HDG; AND I ATTEMPTED TO MAINTAIN ALT. THE CTLR INSTRUCTED ME TO CHK MY PITOT HEAT; WHICH I DID -- IT WAS ON. THE AIRSPD THEN DROPPED TO 0. AT THAT POINT; I DIAGNOSED THE PROB AS AN AIRSPD INDICATION MALFUNCTION. I DISREGARDED THE AIRSPD INDICATION. WHEN I WAS APPROX 1/2 MI FROM THE ARPT; I OBSERVED THE REIL LIGHTS AND LANDED ON RWY 28 IN LIGHT SNOW. ONCE ON THE GND; ON DEPARTING THE ACFT; I OBSERVED THE PITOT TUBE -- IT WAS ENCASED IN ICE. I THEN RETURNED TO THE COCKPIT; TURNED ON THE MASTER AND TURNED ON THE PITOT HEAT TO CONFIRM THE HEATER WAS MALFUNCTIONING.