Narrative:

I was invited to land by the owner of a private airstrip. I made all the required preflight planning; as well as a briefing from the owner about the airstrip. I made a slow descent from 11500 ft MSL to the strip at about 5000 ft MSL. I was the sole occupant with a light fuel load; thus the aircraft was about 600 pounds below gross weight and the air was already rising somewhat as the sun began to warm the ground. A fairly low power setting was needed to get down. I'd not been to this private airstrip before so with the engine controls set for approach/landing (rich mixture and carburetor heat 'on'); I made two slow; low power; left hand patterns to inspect the landing area; the engine seemed to be running smoothly. When on short final; the sight picture of the runway changed somewhat due to some taller weeds blocking my view of the surface texture; I decided to overfly the touchdown area then do a go around. Increasing engine roughness developed as power was advanced causing me to choose a left turn toward an open field rather than proceeding straight ahead over an obstacle. I stopped advancing the throttle when I had enough power to get back to final as it seemed to get rougher the more it was advanced. This left turn insured a crop-field area for immediate forced landing if power failed. It also caused me to basically make a low; close-in left traffic pattern back to final. Back on final again and in position for normal landing; I advanced the throttle to full. The engine continued to be rough for a moment but then gave full and smooth power. I did a go around straight ahead this time up to a safe altitude to do a magneto check; where I found the engine was rough on 1 magneto but not both. I adjusted a low cruise power setting and leaned for peak egt. After a few mins the engine was smooth on either magneto as well as both. I returned to the landing strip and did a normal left pattern to a normal landing. Later I did a normal run-up and left traffic departure without further problem. I've surmised that spark plugs must have fouled during the long low power descent/approach; the pre-landing mixture setting along with the carburetor heat 'on' likely caused an overly rich fuel/air mixture condition.

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Original NASA ASRS Text

Title: A C180 PILOT REPORTS A ROUGH RUNNING ENG AFTER AN IDLE DESCENT FROM 11500 FT TO 5000 FT. A MAG CHECK LEADS TO SUSPICION OF FOULED PLUGS. AFTER POWER ADJUSTMENTS THE ENG SMOOTHED OUT.

Narrative: I WAS INVITED TO LAND BY THE OWNER OF A PVT AIRSTRIP. I MADE ALL THE REQUIRED PREFLT PLANNING; AS WELL AS A BRIEFING FROM THE OWNER ABOUT THE AIRSTRIP. I MADE A SLOW DSCNT FROM 11500 FT MSL TO THE STRIP AT ABOUT 5000 FT MSL. I WAS THE SOLE OCCUPANT WITH A LIGHT FUEL LOAD; THUS THE ACFT WAS ABOUT 600 LBS BELOW GROSS WT AND THE AIR WAS ALREADY RISING SOMEWHAT AS THE SUN BEGAN TO WARM THE GND. A FAIRLY LOW PWR SETTING WAS NEEDED TO GET DOWN. I'D NOT BEEN TO THIS PVT AIRSTRIP BEFORE SO WITH THE ENG CTLS SET FOR APCH/LNDG (RICH MIXTURE AND CARB HEAT 'ON'); I MADE TWO SLOW; LOW PWR; L HAND PATTERNS TO INSPECT THE LNDG AREA; THE ENG SEEMED TO BE RUNNING SMOOTHLY. WHEN ON SHORT FINAL; THE SIGHT PICTURE OF THE RWY CHANGED SOMEWHAT DUE TO SOME TALLER WEEDS BLOCKING MY VIEW OF THE SURFACE TEXTURE; I DECIDED TO OVERFLY THE TOUCHDOWN AREA THEN DO A GAR. INCREASING ENG ROUGHNESS DEVELOPED AS PWR WAS ADVANCED CAUSING ME TO CHOOSE A L TURN TOWARD AN OPEN FIELD RATHER THAN PROCEEDING STRAIGHT AHEAD OVER AN OBSTACLE. I STOPPED ADVANCING THE THROTTLE WHEN I HAD ENOUGH PWR TO GET BACK TO FINAL AS IT SEEMED TO GET ROUGHER THE MORE IT WAS ADVANCED. THIS L TURN INSURED A CROP-FIELD AREA FOR IMMEDIATE FORCED LNDG IF PWR FAILED. IT ALSO CAUSED ME TO BASICALLY MAKE A LOW; CLOSE-IN L TFC PATTERN BACK TO FINAL. BACK ON FINAL AGAIN AND IN POS FOR NORMAL LNDG; I ADVANCED THE THROTTLE TO FULL. THE ENG CONTINUED TO BE ROUGH FOR A MOMENT BUT THEN GAVE FULL AND SMOOTH PWR. I DID A GAR STRAIGHT AHEAD THIS TIME UP TO A SAFE ALT TO DO A MAGNETO CHK; WHERE I FOUND THE ENG WAS ROUGH ON 1 MAGNETO BUT NOT BOTH. I ADJUSTED A LOW CRUISE PWR SETTING AND LEANED FOR PEAK EGT. AFTER A FEW MINS THE ENG WAS SMOOTH ON EITHER MAGNETO AS WELL AS BOTH. I RETURNED TO THE LNDG STRIP AND DID A NORMAL L PATTERN TO A NORMAL LNDG. LATER I DID A NORMAL RUN-UP AND L TFC DEP WITHOUT FURTHER PROB. I'VE SURMISED THAT SPARK PLUGS MUST HAVE FOULED DURING THE LONG LOW PWR DSCNT/APCH; THE PRE-LNDG MIXTURE SETTING ALONG WITH THE CARB HEAT 'ON' LIKELY CAUSED AN OVERLY RICH FUEL/AIR MIXTURE CONDITION.

Data retrieved from NASA's ASRS site as of May 2009 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.