Narrative:

[I was] flying my cessna 150 and using flight following. I was on a flight at 5;500 feet MSL when the engine began to run rough. Although the humidity was low; I suspected it could be carburetor ice. I applied carburetor heat with negative results. Having leaned the mixture for flight at 5;500 feet I enriched the mixture to full rich again having no effect on the rough running engine. I reduced the throttle and the engine almost completely stopped running. I immediately went back to full throttle. The engine was now beginning to get worse and I was experiencing a loss in RPM's and altitude as a result. Knowing I could make it; I had decided to turn the airplane 180 degrees and go back to airport. I contacted approach and notified them of the problem which continued to get worse and that I was going back to land. They informed me they contacted the tower to let them know of my situation. I told approach that I was going to maintain as much altitude as possible to assure I could make the airport. I was then asked if I wanted to declare an emergency to which I replied yes; and 3 hours of fuel and 1 person on board. I was told by approach that I could contact the control tower; and the tower cleared me for landing. I maintained as much altitude as possible to assure making the airport. At about three miles out and about 3;000 feet the tower cleared me for any maneuver and to land on any runway. Now knowing I would make the airport I began to slip the aircraft. During this maneuver the engine began to almost run as if nothing was wrong which seemed very strange to me. After landing and taxiing off the runway I was surprised that the engine was now running like normal. I contacted ground; continued to the run-up area with emergency equipment following me. I shut down and was met by fire equipment and the police. After explaining my situation and presenting the requested documents; I started up the engine which again to my surprise; everything seemed normal. I proceeded to parking and began to evaluate the problem to the best of my ability. I called my a&P to explain the situation so we could try and analyze the situation over the phone. While talking to him I checked for water in the fuel tanks. Having already done this during my preflight inspection and not finding any; I was surprised to find that here was indeed some water in the tanks. I checked the fuel strainer; holding the cup used to check the tanks so I could measure the amount; if any; of water in the strainer. I was shocked to get about 7 ounces of water from the strainer before getting fuel. I mentioned to my a&P that he needed to get in touch with the airport manager where I last purchased fuel to check their avgas tank for water and contamination. I was hoping that no others would be in jeopardy having topped off their aircraft. They contacted me a while later and said they had pumped out about 10 gallons from the only avgas fuel tank on the airport and found no water in the fuel. I'm certain that the water in the fuel on my aircraft was the problem but am completely mystified along with others as to where it came from. I left the aircraft with the intention of checking it out more thoroughly before flying it home the following saturday. I topped off the tanks so there would be little space for condensation to develop. I was unable to retrieve the aircraft the following saturday because of weather and intend to do so this saturday; weather permitting. Knowing the aircraft is mechanically sound; I intend to check the fuel tanks and strainer for any water and then do a 15 to 20 minute run-up. Afterwards; if everything appears to be normal; I will shut down the engine and again check for any water in the system. If none is found; I intend to do another run-up and then fly it back to home base.

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Original NASA ASRS Text

Title: During cruise flight the pilot of a Cessna 150 experienced degraded engine performance bordering on failure; declared an emergency; and with assistance from ATC completed a successful emergency landing at a nearby airport. Despite his earlier through preflight and fuel sump check water was discovered in the fuel.

Narrative: [I was] flying my Cessna 150 and using Flight Following. I was on a flight at 5;500 feet MSL when the engine began to run rough. Although the humidity was low; I suspected it could be carburetor ice. I applied carburetor heat with negative results. Having leaned the mixture for flight at 5;500 feet I enriched the mixture to full rich again having no effect on the rough running engine. I reduced the throttle and the engine almost completely stopped running. I immediately went back to full throttle. The engine was now beginning to get worse and I was experiencing a loss in RPM's and altitude as a result. Knowing I could make it; I had decided to turn the airplane 180 degrees and go back to airport. I contacted Approach and notified them of the problem which continued to get worse and that I was going back to land. They informed me they contacted the Tower to let them know of my situation. I told Approach that I was going to maintain as much altitude as possible to assure I could make the airport. I was then asked if I wanted to declare an emergency to which I replied yes; and 3 hours of fuel and 1 person on board. I was told by Approach that I could contact the Control Tower; and the Tower cleared me for landing. I maintained as much altitude as possible to assure making the airport. At about three miles out and about 3;000 feet the Tower cleared me for any maneuver and to land on any runway. Now knowing I would make the airport I began to slip the aircraft. During this maneuver the engine began to almost run as if nothing was wrong which seemed very strange to me. After landing and taxiing off the runway I was surprised that the engine was now running like normal. I contacted Ground; continued to the run-up area with emergency equipment following me. I shut down and was met by fire equipment and the police. After explaining my situation and presenting the requested documents; I started up the engine which again to my surprise; everything seemed normal. I proceeded to parking and began to evaluate the problem to the best of my ability. I called my A&P to explain the situation so we could try and analyze the situation over the phone. While talking to him I checked for water in the fuel tanks. Having already done this during my preflight inspection and not finding any; I was surprised to find that here was indeed some water in the tanks. I checked the fuel strainer; holding the cup used to check the tanks so I could measure the amount; if any; of water in the strainer. I was shocked to get about 7 ounces of water from the strainer before getting fuel. I mentioned to my A&P that he needed to get in touch with the Airport Manager where I last purchased fuel to check their avgas tank for water and contamination. I was hoping that no others would be in jeopardy having topped off their aircraft. They contacted me a while later and said they had pumped out about 10 gallons from the only avgas fuel tank on the airport and found no water in the fuel. I'm certain that the water in the fuel on my aircraft was the problem but am completely mystified along with others as to where it came from. I left the aircraft with the intention of checking it out more thoroughly before flying it home the following Saturday. I topped off the tanks so there would be little space for condensation to develop. I was unable to retrieve the aircraft the following Saturday because of weather and intend to do so this Saturday; weather permitting. Knowing the aircraft is mechanically sound; I intend to check the fuel tanks and strainer for any water and then do a 15 to 20 minute run-up. Afterwards; if everything appears to be normal; I will shut down the engine and again check for any water in the system. If none is found; I intend to do another run-up and then fly it back to home base.

Data retrieved from NASA's ASRS site 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.