Narrative:

At roughly 1000 AGL (due to floor of TCA in that area) I experienced engine roughness. I had left the propeller control in full and throttle full which I don't normally do (it was to be a short flight. Hadn't flown for 2 weeks). It did not seem a normal (750 hours experience) situation. Oil temperature was high, pressure seemed lower, just below green and engine was rough. I spotted a large, uninhabited pasture and initiated a precautionary landing under power (reduced, I think). After hiking out and returning with help, we did a runup, a complete check of aircraft (after adding additional fuel to ensure adequate VFR reserves and preclude fuel contamination). Walked the area, charted takeoff distance, and departed area to nearest airport. In retrospect it may have been a combination of density altitude (lack of proper cooling and thinning of 20-50 oil) and carburetor ice or a small amount of contamination in fuel. I am really not sure. Upon our return and subsequent runup, everything seemed normal. Since it was private property and neither I, the aircraft, or any property was damaged, I feel that the decision was the right one. Continued flight might have resulted in reduced choices. One of the pilots suggested carburetor ice and the symptoms seem to fit, but I guess fuel contamination (water?) might also. I did drain both wing tanks and gascolator drain. The aircraft has been fine since. Since I have not had any accidents or incidents in 8 hours of flying, I could only wish that I had been higher AGL so that I would have had more time to let carburetor heat work or the engine to process the contamination before 'commit' time. I feel the biggest cause of injury accidents and aircraft damage is the failure to make a decision. I did not declare an emergency because I did not feel that I was in mortal danger at any time.

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Original NASA ASRS Text

Title: A PVT SMA PLT LANDED OFF ARPT WITH A ROUGH RUNNING ENG. THERE WAS NO DAMAGE OR INJURY RPTED.

Narrative: AT ROUGHLY 1000 AGL (DUE TO FLOOR OF TCA IN THAT AREA) I EXPERIENCED ENG ROUGHNESS. I HAD LEFT THE PROP CTL IN FULL AND THROTTLE FULL WHICH I DON'T NORMALLY DO (IT WAS TO BE A SHORT FLT. HADN'T FLOWN FOR 2 WKS). IT DID NOT SEEM A NORMAL (750 HRS EXPERIENCE) SITUATION. OIL TEMP WAS HIGH, PRESSURE SEEMED LOWER, JUST BELOW GREEN AND ENG WAS ROUGH. I SPOTTED A LARGE, UNINHABITED PASTURE AND INITIATED A PRECAUTIONARY LNDG UNDER PWR (REDUCED, I THINK). AFTER HIKING OUT AND RETURNING WITH HELP, WE DID A RUNUP, A COMPLETE CHK OF ACFT (AFTER ADDING ADDITIONAL FUEL TO ENSURE ADEQUATE VFR RESERVES AND PRECLUDE FUEL CONTAMINATION). WALKED THE AREA, CHARTED TKOF DISTANCE, AND DEPARTED AREA TO NEAREST ARPT. IN RETROSPECT IT MAY HAVE BEEN A COMBINATION OF DENSITY ALT (LACK OF PROPER COOLING AND THINNING OF 20-50 OIL) AND CARB ICE OR A SMALL AMOUNT OF CONTAMINATION IN FUEL. I AM REALLY NOT SURE. UPON OUR RETURN AND SUBSEQUENT RUNUP, EVERYTHING SEEMED NORMAL. SINCE IT WAS PRIVATE PROPERTY AND NEITHER I, THE ACFT, OR ANY PROPERTY WAS DAMAGED, I FEEL THAT THE DECISION WAS THE RIGHT ONE. CONTINUED FLT MIGHT HAVE RESULTED IN REDUCED CHOICES. ONE OF THE PLTS SUGGESTED CARB ICE AND THE SYMPTOMS SEEM TO FIT, BUT I GUESS FUEL CONTAMINATION (WATER?) MIGHT ALSO. I DID DRAIN BOTH WING TANKS AND GASCOLATOR DRAIN. THE ACFT HAS BEEN FINE SINCE. SINCE I HAVE NOT HAD ANY ACCIDENTS OR INCIDENTS IN 8 HRS OF FLYING, I COULD ONLY WISH THAT I HAD BEEN HIGHER AGL SO THAT I WOULD HAVE HAD MORE TIME TO LET CARB HEAT WORK OR THE ENG TO PROCESS THE CONTAMINATION BEFORE 'COMMIT' TIME. I FEEL THE BIGGEST CAUSE OF INJURY ACCIDENTS AND ACFT DAMAGE IS THE FAILURE TO MAKE A DECISION. I DID NOT DECLARE AN EMER BECAUSE I DID NOT FEEL THAT I WAS IN MORTAL DANGER AT ANY TIME.

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.