Narrative:

It was approximately XA30 CST, there wasn't any visible moisture and we were cruising approximately 20 mi east of C32 at 7500 ft MSL en route to east troy. Suddenly, without warning, there was an instantaneous RPM drop from cruise (2450) to 1500 RPM. Once in a while the RPM would jump to about 2000-2100 RPM and then fall to below 1000 RPM. We were on a VFR flight plan and receiving flight following from madison approach. I immediately made them aware of our situation and requested radar vectors to madison. I figured this was pretty much the closest airport, in addition they have emergency equipment and radar to assist me. I also made them aware that, if I lost the engine completely, I didn't know if I could make the airport. Madison informed me that they would try to raise the lights at C29 near the town of middletown which is 5-10 NM closer than madison and in front of us. I trimmed the airplane for best glide speed (assuming the worst would happen) which is 80 mph and started looking for a place to land. Being night, all I could see was madison's lights in a distance and complete blackness around me. I knew there were fields but, without being able to make out the terrain features, I couldn't tell where the power lines, houses, trees, and hills were. I started the emergency checklist by first checking the engine gauges which, except for the tachometer were all reading normal. I knew that we had plenty of fuel but I double checked anyway. Next I pulled the carburetor heat, at which time the engine began to sputter. I thought we would be coasting from that point on. Thinking that some engine power was better than none, I push the carburetor heat back in. In addition to the engine almost completely quitting several times we had now developed radio problems. Taking into consideration that I might completely lose both my engine and radio, and remembering all of the accident reports that I had ever read, I decided declaring an emergency was the best course of action. I felt that any amount of paperwork was worth possibly saving my life. I completed the rest of the checklist and was able to stabilize the RPM at approximately 1500-1700 RPM after I switched the fuel tanks from 'both' to 'left,' but never being able to fully restore power. We landed safely at madison and attempted to look over the airplane. The next day we gave the airplane a thorough preflight. We noticed the oil was down 1 1/2 qts, but that was all. We refilled the oil, ran the airplane for approximately 20 mins on the ground and returned to east troy without any further incident. Nothing was found wrong with the airplane, to my knowledge. Questions: if it was carburetor ice, why didn't the engine slowly lose RPM instead of the sudden and complete loss? Once I pulled the carburetor heat and let it run, why didn't the engine clear? If fuel contamination: nothing showed up in strainer after preflight check, after I switched tanks, the engine power only increased about 300-400 RPM from 1200 RPM to approximately 1600 RPM but the power never completely returned. Why?

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Original NASA ASRS Text

Title: EMER DECLARED FOR ROUGH RUNNING ENG.

Narrative: IT WAS APPROX XA30 CST, THERE WASN'T ANY VISIBLE MOISTURE AND WE WERE CRUISING APPROX 20 MI E OF C32 AT 7500 FT MSL ENRTE TO E TROY. SUDDENLY, WITHOUT WARNING, THERE WAS AN INSTANTANEOUS RPM DROP FROM CRUISE (2450) TO 1500 RPM. ONCE IN A WHILE THE RPM WOULD JUMP TO ABOUT 2000-2100 RPM AND THEN FALL TO BELOW 1000 RPM. WE WERE ON A VFR FLT PLAN AND RECEIVING FLT FOLLOWING FROM MADISON APCH. I IMMEDIATELY MADE THEM AWARE OF OUR SIT AND REQUESTED RADAR VECTORS TO MADISON. I FIGURED THIS WAS PRETTY MUCH THE CLOSEST ARPT, IN ADDITION THEY HAVE EMER EQUIP AND RADAR TO ASSIST ME. I ALSO MADE THEM AWARE THAT, IF I LOST THE ENG COMPLETELY, I DIDN'T KNOW IF I COULD MAKE THE ARPT. MADISON INFORMED ME THAT THEY WOULD TRY TO RAISE THE LIGHTS AT C29 NEAR THE TOWN OF MIDDLETOWN WHICH IS 5-10 NM CLOSER THAN MADISON AND IN FRONT OF US. I TRIMMED THE AIRPLANE FOR BEST GLIDE SPD (ASSUMING THE WORST WOULD HAPPEN) WHICH IS 80 MPH AND STARTED LOOKING FOR A PLACE TO LAND. BEING NIGHT, ALL I COULD SEE WAS MADISON'S LIGHTS IN A DISTANCE AND COMPLETE BLACKNESS AROUND ME. I KNEW THERE WERE FIELDS BUT, WITHOUT BEING ABLE TO MAKE OUT THE TERRAIN FEATURES, I COULDN'T TELL WHERE THE PWR LINES, HOUSES, TREES, AND HILLS WERE. I STARTED THE EMER CHKLIST BY FIRST CHKING THE ENG GAUGES WHICH, EXCEPT FOR THE TACHOMETER WERE ALL READING NORMAL. I KNEW THAT WE HAD PLENTY OF FUEL BUT I DOUBLE CHKED ANYWAY. NEXT I PULLED THE CARBURETOR HEAT, AT WHICH TIME THE ENG BEGAN TO SPUTTER. I THOUGHT WE WOULD BE COASTING FROM THAT POINT ON. THINKING THAT SOME ENG PWR WAS BETTER THAN NONE, I PUSH THE CARBURETOR HEAT BACK IN. IN ADDITION TO THE ENG ALMOST COMPLETELY QUITTING SEVERAL TIMES WE HAD NOW DEVELOPED RADIO PROBS. TAKING INTO CONSIDERATION THAT I MIGHT COMPLETELY LOSE BOTH MY ENG AND RADIO, AND REMEMBERING ALL OF THE ACCIDENT RPTS THAT I HAD EVER READ, I DECIDED DECLARING AN EMER WAS THE BEST COURSE OF ACTION. I FELT THAT ANY AMOUNT OF PAPERWORK WAS WORTH POSSIBLY SAVING MY LIFE. I COMPLETED THE REST OF THE CHKLIST AND WAS ABLE TO STABILIZE THE RPM AT APPROX 1500-1700 RPM AFTER I SWITCHED THE FUEL TANKS FROM 'BOTH' TO 'L,' BUT NEVER BEING ABLE TO FULLY RESTORE PWR. WE LANDED SAFELY AT MADISON AND ATTEMPTED TO LOOK OVER THE AIRPLANE. THE NEXT DAY WE GAVE THE AIRPLANE A THOROUGH PREFLT. WE NOTICED THE OIL WAS DOWN 1 1/2 QTS, BUT THAT WAS ALL. WE REFILLED THE OIL, RAN THE AIRPLANE FOR APPROX 20 MINS ON THE GND AND RETURNED TO E TROY WITHOUT ANY FURTHER INCIDENT. NOTHING WAS FOUND WRONG WITH THE AIRPLANE, TO MY KNOWLEDGE. QUESTIONS: IF IT WAS CARBURETOR ICE, WHY DIDN'T THE ENG SLOWLY LOSE RPM INSTEAD OF THE SUDDEN AND COMPLETE LOSS? ONCE I PULLED THE CARBURETOR HEAT AND LET IT RUN, WHY DIDN'T THE ENG CLR? IF FUEL CONTAMINATION: NOTHING SHOWED UP IN STRAINER AFTER PREFLT CHK, AFTER I SWITCHED TANKS, THE ENG PWR ONLY INCREASED ABOUT 300-400 RPM FROM 1200 RPM TO APPROX 1600 RPM BUT THE PWR NEVER COMPLETELY RETURNED. WHY?

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.