PLT OF A C425 TURBOPROP EXCEEDED THE MAX AIRSPD AT OR BELOW 2500 FT; WITHIN A 4 MI RADIUS OF AN ARPT; WHEN HE INCREASED THE ACFT AIRSPD TO COMPENSATE FOR LOSS OF INDICATED AIRSPD BECAUSE OF AN ICED OVER PITOT TUBE. THE PLT'S PITOT TUBE HEAT HAD FAILED.

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ASSIGNED 170 KTS ON RADAR VECTORS TO ILS RWY 27L ORD. ORD WIND 290 DEGS; 25 KTS; GUSTING TO 35 KTS. LLWS ADVISORIES. LLWS RPTED BY SEVERAL ACFT. I WAS FOLLOWING A B757; DSNDING THROUGH OVCST LAYER WITH LIGHT ICING. AS I ENTERED THE OVCST ON THE GS; THE AIRSPD BEGAN TO DROP SLOWLY. INCREASED PWR TO RECOVER AIRSPD. AIRSPD STABILIZED; THEN DROPPED AGAIN 10 KTS. ASSUMING WAKE TURB OR WINDSHEAR; I INCREASED PWR TO RECOVER AGAIN. AIRSPD STABILIZED THEN DROPPED ANOTHER 10 KTS. NOW INDICATING 30 KTS SLOW; INCREASED PWR; CHKED CONFIGN AND ICE; NO ICE ON WINGS. CHKED COPLT IAS; NOW 220 KTS. PLT'S PITOT HEAT HAD FAILED AND ICE BLOCKED. RECOVERED AIRSPD TO ASSIGNED. LNDG UNEVENTFUL. THIS OCCURRED IN ONLY 2 OR 3 MINS. I WAS EXPECTING TO SEE AIRSPD DEVS DUE TO WAKE TURB OR WINDSHEAR. ASSUMED THAT WAS THE CASE; WHEN ACTUALLY PITOT HEAT FAILED AT VERY INCONVENIENT TIME; ON ILS; IN IMC; LIGHT ICING CONDITIONS. INCLUDING COPLT INSTS IN SCAN (WE FLY SINGLE PLT) OR USING A COPLT WOULD HAVE CAUGHT THE DISCREPANCY SOONER.