Narrative:

I was flying my privately owned small aircraft. We had just passed over lake erie at 4000 ft in VFR conditions, but were to enter IFR conditions near toledo, oh (tol). I was aware that cloud tops were 8000-9500 ft and requested a climb to 8000 ft which was approved. At 7500 ft we were IMC, temperature minus 20 degrees C, little or no wind or turbulence, and no structural icing reports. However, within 2 mins, we lost 5 inches of manifold pressure. Carburetor heat and fuel pumps had been turned on prior to entering IMC. Exhaust gas temperature climbed then abruptly went below scale, and manifold pressure decreased to 10 inches. We assumed best glide ratio, declared emergency, and executed toledo express ILS 25 approach. We were 15 mi due east of tol, and WX at tol was 400/2500 in fog and snow. You may notice we were directly over sandusky airfield, with WX of 800 ft broken visibility 1 military. This is an uncontrolled field with no instrument approach, and I wasn't going to trust the WX report. Our calculated glide was 15 mi standard temperature and pressure, so I knew we had more than 15 mi glide in current atmospheric conditions. We touched down safely at tol, and, in the surface 0.5 degrees C temperature without airflow cooling, our engine recovered. Obviously, we experienced carburetor icing from frozen clouds thawing and refreezing. Evaluate of the carburetor heat found that the valve had been 'home-repaired' and was no longer mechanically sound. The valve, not closing fully, was entraining a mixture of outside air and exhaust air. Despite repair, a test flight found the WX conditions were too extreme for the carburetor heat, and we remained 'grounded' 2 days until carburetor icing conditions had diminished. Courageously, my wife and child reboarded the plane to return home. Since the purchase of that aircraft, I had never experienced a power loss with carburetor heat on. The previous owner and the mechanic told me this was ok. One should always experience a power loss with carburetor heat on (it's plain physics)! This aircraft carburetor heat will not tolerate extreme carburetor icing conditions and should not be flown into such conditions.

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Original NASA ASRS Text

Title: SMA HAS PWR LOSS, DECLARES EMER, LANDS.

Narrative: I WAS FLYING MY PRIVATELY OWNED SMA. WE HAD JUST PASSED OVER LAKE ERIE AT 4000 FT IN VFR CONDITIONS, BUT WERE TO ENTER IFR CONDITIONS NEAR TOLEDO, OH (TOL). I WAS AWARE THAT CLOUD TOPS WERE 8000-9500 FT AND REQUESTED A CLB TO 8000 FT WHICH WAS APPROVED. AT 7500 FT WE WERE IMC, TEMP MINUS 20 DEGS C, LITTLE OR NO WIND OR TURB, AND NO STRUCTURAL ICING RPTS. HOWEVER, WITHIN 2 MINS, WE LOST 5 INCHES OF MANIFOLD PRESSURE. CARB HEAT AND FUEL PUMPS HAD BEEN TURNED ON PRIOR TO ENTERING IMC. EXHAUST GAS TEMP CLBED THEN ABRUPTLY WENT BELOW SCALE, AND MANIFOLD PRESSURE DECREASED TO 10 INCHES. WE ASSUMED BEST GLIDE RATIO, DECLARED EMER, AND EXECUTED TOLEDO EXPRESS ILS 25 APCH. WE WERE 15 MI DUE E OF TOL, AND WX AT TOL WAS 400/2500 IN FOG AND SNOW. YOU MAY NOTICE WE WERE DIRECTLY OVER SANDUSKY AIRFIELD, WITH WX OF 800 FT BROKEN VISIBILITY 1 MIL. THIS IS AN UNCTLED FIELD WITH NO INST APCH, AND I WASN'T GOING TO TRUST THE WX RPT. OUR CALCULATED GLIDE WAS 15 MI STANDARD TEMP AND PRESSURE, SO I KNEW WE HAD MORE THAN 15 MI GLIDE IN CURRENT ATMOSPHERIC CONDITIONS. WE TOUCHED DOWN SAFELY AT TOL, AND, IN THE SURFACE 0.5 DEGS C TEMP WITHOUT AIRFLOW COOLING, OUR ENG RECOVERED. OBVIOUSLY, WE EXPERIENCED CARB ICING FROM FROZEN CLOUDS THAWING AND REFREEZING. EVAL OF THE CARB HEAT FOUND THAT THE VALVE HAD BEEN 'HOME-REPAIRED' AND WAS NO LONGER MECHANICALLY SOUND. THE VALVE, NOT CLOSING FULLY, WAS ENTRAINING A MIXTURE OF OUTSIDE AIR AND EXHAUST AIR. DESPITE REPAIR, A TEST FLT FOUND THE WX CONDITIONS WERE TOO EXTREME FOR THE CARB HEAT, AND WE REMAINED 'GNDED' 2 DAYS UNTIL CARB ICING CONDITIONS HAD DIMINISHED. COURAGEOUSLY, MY WIFE AND CHILD REBOARDED THE PLANE TO RETURN HOME. SINCE THE PURCHASE OF THAT ACFT, I HAD NEVER EXPERIENCED A PWR LOSS WITH CARB HEAT ON. THE PREVIOUS OWNER AND THE MECH TOLD ME THIS WAS OK. ONE SHOULD ALWAYS EXPERIENCE A PWR LOSS WITH CARB HEAT ON (IT'S PLAIN PHYSICS)! THIS ACFT CARB HEAT WILL NOT TOLERATE EXTREME CARB ICING CONDITIONS AND SHOULD NOT BE FLOWN INTO SUCH CONDITIONS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.