Narrative:

While crossing lake michigan at 7500 ft MSL, on a lake reporting flight plan with the green bay FSS, the engine began running rough, then lost almost all power, occasionally surging. I pushed the mixture to full rich (it had been leaned to peak +50-100 degrees rich), I checked fuel quantities, switched tanks and checked the magneto switch, but there was no change. I pulled carburetor heat momentarily, but when there was no change, I pushed it back in. I then set up for best glide speed of 80 mph, and turned back toward the western shore. I informed green bay FSS that I was having an engine problem, and I was headed back. When asked the nature of the problem, I told him. He asked if I was declaring an emergency, and given the fact that unless the engine regained power I would not make it back to the shoreline, I said yes. He told me to squawk 7700. Shortly after that, the engine started to recover. Green bay suggested that I apply carburetor heat, which I did, but by that time the engine was already running at full power. I was instructed to contact ZAU, I informed them that the problem seemed to be under control, but that we would continue back and land at the nearest airport (sheboygan, wi) to check out the engine. After landing, I called green bay FSS to tell them I landed safely, and to close my flight plan. A thorough check of the engine, including a ground run-up, did not reveal any problems. The most probable cause was carburetor icing, which descending into warmer air had alleviated. In retrospect, if I had left the carburetor heat on instead of only pulling it out momentarily, it probably would have solved the problem. The thought of ice did not occur to me because of the conditions. The sky was perfectly clear, and the visibility unrestr. Had the same situation occurred over land, I probably would have taken more time to evaluate the problem before declaring an emergency.

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Original NASA ASRS Text

Title: C172 PLT HAD ENG FAILURE.

Narrative: WHILE XING LAKE MICHIGAN AT 7500 FT MSL, ON A LAKE RPTING FLT PLAN WITH THE GREEN BAY FSS, THE ENG BEGAN RUNNING ROUGH, THEN LOST ALMOST ALL PWR, OCCASIONALLY SURGING. I PUSHED THE MIXTURE TO FULL RICH (IT HAD BEEN LEANED TO PEAK +50-100 DEGS RICH), I CHKED FUEL QUANTITIES, SWITCHED TANKS AND CHKED THE MAGNETO SWITCH, BUT THERE WAS NO CHANGE. I PULLED CARB HEAT MOMENTARILY, BUT WHEN THERE WAS NO CHANGE, I PUSHED IT BACK IN. I THEN SET UP FOR BEST GLIDE SPD OF 80 MPH, AND TURNED BACK TOWARD THE WESTERN SHORE. I INFORMED GREEN BAY FSS THAT I WAS HAVING AN ENG PROB, AND I WAS HEADED BACK. WHEN ASKED THE NATURE OF THE PROB, I TOLD HIM. HE ASKED IF I WAS DECLARING AN EMER, AND GIVEN THE FACT THAT UNLESS THE ENG REGAINED PWR I WOULD NOT MAKE IT BACK TO THE SHORELINE, I SAID YES. HE TOLD ME TO SQUAWK 7700. SHORTLY AFTER THAT, THE ENG STARTED TO RECOVER. GREEN BAY SUGGESTED THAT I APPLY CARB HEAT, WHICH I DID, BUT BY THAT TIME THE ENG WAS ALREADY RUNNING AT FULL PWR. I WAS INSTRUCTED TO CONTACT ZAU, I INFORMED THEM THAT THE PROB SEEMED TO BE UNDER CTL, BUT THAT WE WOULD CONTINUE BACK AND LAND AT THE NEAREST ARPT (SHEBOYGAN, WI) TO CHK OUT THE ENG. AFTER LNDG, I CALLED GREEN BAY FSS TO TELL THEM I LANDED SAFELY, AND TO CLOSE MY FLT PLAN. A THOROUGH CHK OF THE ENG, INCLUDING A GND RUN-UP, DID NOT REVEAL ANY PROBS. THE MOST PROBABLE CAUSE WAS CARB ICING, WHICH DSNDING INTO WARMER AIR HAD ALLEVIATED. IN RETROSPECT, IF I HAD LEFT THE CARB HEAT ON INSTEAD OF ONLY PULLING IT OUT MOMENTARILY, IT PROBABLY WOULD HAVE SOLVED THE PROB. THE THOUGHT OF ICE DID NOT OCCUR TO ME BECAUSE OF THE CONDITIONS. THE SKY WAS PERFECTLY CLR, AND THE VISIBILITY UNRESTR. HAD THE SAME SIT OCCURRED OVER LAND, I PROBABLY WOULD HAVE TAKEN MORE TIME TO EVALUATE THE PROB BEFORE DECLARING AN EMER.

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.