INST RATED PVT PLT OF A C172 EXPERIENCED ACFT ICING DURING IFR FLT AT CRUISE CAUSING A DECREASE IN ENG AND ACFT SPD PERFORMANCE. PLT CONTINUED ON TO DEST WHICH WAS CLOSER THAN TURNING BACK TO ORIGINATING ARPT. ICING CONDITIONS WERE FORECASTED; BUT NO PIREPS WERE AVAILABLE. PLT HAD SPRAYED PROP WITH SILICONE IN ANTICIPATION OF ICING CONDITIONS.

Narrative

WX FORECAST WAS FOR VFR CONDITIONS ON TKOF AT Y47 ARPT AND IFR CONDITIONS ENRTE AND AT THE DEST ARPT JXN. LIGHT OCCASIONALLY MODERATE RIME OR MIXED ICING FORECAST FROM FREEZING LEVEL TO 8000 FT. SIMILAR ICING FORECAST IS STANDARD DAILY FROM OCTOBER TO APRIL IN MICHIGAN. NO PIREPS HAD YET INDICATED ICING. HAD FLOWN AT 3000 FT MSL THE PREVIOUS NIGHT IFR AND NOT HAD ANY ICE. LIGHT CLR ICE STARTED IN THE CLB PASSING 3000 FT FOR 4000 FT. DEFROST CLRED THE L-HAND SIDE OF THE WINDSHIELD ADEQUATELY. LOST 5 KTS IN CRUISE DUE TO LIGHT ICE ACCUMULATION. NO VIBRATION FROM PROP WAS NOTICED. DECIDED TO CONTINUE ON SHORTEST RTE WHICH WAS THE DEST. WX AT JACKSON HAD WORSENED TO TWR SPECIAL OF 500 FT AND 1 1/2 MI MIST. SHOT ILS 24 APCH WITH ADDITIONAL ENG PWR AND LANDED EASILY AT JACKSON. INSPECTED PLANE ON THE RAMP AND MEASURED AN ACCUMULATION OF 3/8 INCH ICE ON NARROW BAND ON WING IN 2 DEGS C ABOVE FREEZING TEMPS. ILS APCH FLOWN WITH NO FLAPS (AS USUAL) AND 10 DEGS ON FINAL. NO ICE ON PROP WHICH WAS TREATED BEFORE TKOF WITH SILICONE. SINCE 'KNOWN' ICING CONDITIONS EXISTED NOW; THE PLANE WAS SECURED ON THE RAMP AND LEFT THERE AT JACKSON. RETURNED ON SUNDAY AND FLEW BACK IN VFR CONDITIONS. THE SMALL ICE ACCUMULATION AFFECTED THE PERFORMANCE; AS DID THE NEED TO RUN CONTINUOUS CARB HEAT DURING CRUISE ON THESE EARLY C172'S. HAD I NOT BEEN BTWN LAYERS AT 4000 FT TOWARDS JACKSON I WOULD HAVE GOTTEN MORE ICE DURING THIS SHORT FLT. IT WAS WARM ENOUGH AT 3000 FT (WITH SOME RAM HEATING 1-2 DEGS C) TO NEGATE ICE. AM RE-EVALUATING WINTER FLYING IFR IN MICHIGAN WITHOUT TRADING UP TO A KNOWN ICE AIRPLANE. WON'T FLY IN DENSE CLOUD LAYER IN TEMP OF 0 DEGS C TO -15 DEGS C AND IN AREAS OF FRONTAL ACTIVITY.