Narrative:

In cruise flight at altitude I experienced a slowly increasing surging/roughness of engine; I promptly went full rich on mixture; checked gauges and everything looked fine; except for the fact that the engine was now fluctuating about 200-300 RPM. I declared an urgency situation and asked for vectors to the nearest field which where given at 11 miles on my current heading. I opted for a small airport approximately 5 miles to my 6 o'clock and informed ATC. At this point the fluctuations began to worsen; now fluctuating between 500-600 rpm. It became apparent that I would have difficulty maintaining altitude with the current engine situation. I did not have the airport in sight yet and I was unsure how much longer I would have power as the engine situation continued to degrade. With what little I knew of the situation and unsure of the engine's integrity I declared an emergency. Still continuing to descend I switched tanks from both to left and then right; shortly after switching to the right tank the roughness cleared up and I continued my descent to landing. After consultation with mechanic the final consensus was water in the fuel that ran out shortly after switching tanks. At the time I still had over 12 gallons each tank and had sumped both the carb and fuel tanks on preflight. I continued on to my destination airport and back to my departure uneventfully. One thing that I was able to take away from this was my unpreparedness for the emergency. Once I realized what was going on adrenaline hit and my mind seemed to cloud over. I don't even remember applying carb heat at all and much of what I did really made no sense thinking back on it. My flying went down the tubes as soon as the emergency presented itself. My approach to my alternate field was horrible; I nearly overshot the runway. My action steps for addressing the issue where scattered and fragmented at best. I allowed fear to completely control me and without what little training I applied it could have ended in a disaster. The only thing I could have done better was train myself better for emergencies such as this. There is no getting around it whatsoever; the only way to survive emergencies in the air is to be prepared for the worst and train for them.

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Original NASA ASRS Text

Title: C172 pilot experiences a rough running engine and diverts to the nearest suitable airport. Switching fuel tanks restores normal engine operation. Water is found in one wing tank after landing.

Narrative: In cruise flight at altitude I experienced a slowly increasing surging/roughness of engine; I promptly went full rich on mixture; checked gauges and everything looked fine; except for the fact that the engine was now fluctuating about 200-300 RPM. I declared an urgency situation and asked for vectors to the nearest field which where given at 11 miles on my current heading. I opted for a small airport approximately 5 miles to my 6 o'clock and informed ATC. At this point the fluctuations began to worsen; now fluctuating between 500-600 rpm. It became apparent that I would have difficulty maintaining altitude with the current engine situation. I did not have the airport in sight yet and I was unsure how much longer I would have power as the engine situation continued to degrade. With what little I knew of the situation and unsure of the engine's integrity I declared an emergency. Still continuing to descend I switched tanks from both to left and then right; shortly after switching to the right tank the roughness cleared up and I continued my descent to landing. After consultation with mechanic the final consensus was water in the fuel that ran out shortly after switching tanks. At the time I still had over 12 gallons each tank and had sumped both the carb and fuel tanks on preflight. I continued on to my destination airport and back to my departure uneventfully. One thing that I was able to take away from this was my unpreparedness for the emergency. Once I realized what was going on adrenaline hit and my mind seemed to cloud over. I don't even remember applying carb heat at all and much of what I did really made no sense thinking back on it. My flying went down the tubes as soon as the emergency presented itself. My approach to my alternate field was horrible; I nearly overshot the runway. My action steps for addressing the issue where scattered and fragmented at best. I allowed fear to completely control me and without what little training I applied it could have ended in a disaster. The only thing I could have done better was train myself better for emergencies such as this. There is no getting around it whatsoever; the only way to survive emergencies in the air is to be prepared for the worst and train for them.

Data retrieved from NASA's ASRS site as of April 2012 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.