Narrative:

While I was flying my 1941 J-3 cub eastbound over the city of boynton beach a strong vibration developed in the engine compartment. Prior to the vibration developing, I heard a bang come from the area of the engine compartment. Once the vibration developed the engine started to lose power and the aircraft started to descend. Seeing that the aircraft was losing power and altitude, I turned to lantana airport so that I could land there and find out what was causing the vibration. En route to lantana airport, I determined that the aircraft did not have enough power or altitude to make lantana airport. At this time, I circled over a field just south of a building and attempted to gain altitude with negative results. Not being able to gain altitude, and in fact losing altitude, I landed in the field south of the building without damaging the aircraft or causing personal injury. Once I had safely landed the aircraft, I shut down the engine and got out of the airplane. I then went to the engine compartment and saw that a spark plug was hanging by its plug wire for the left side top front cylinder. Seeing this I put the spark plug back into it's respective hole and then started the engine. Upon starting the engine and running it, I saw that there was no longer a vibration and that the engine was developing normal power. The aircraft had to be trucked out on a flat bed truck though because of local ordinance which prohibited aircraft from taking off or landing in the city limits. The only reason that I can determine that the spark plug came out of the cylinder is that I did not torque it enough when I had put it back into the cylinder. I had just replaced all the old spark plugs with new ones.

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Original NASA ASRS Text

Title: PARTIAL ENG FAILURE, OFF ARPT LNDG.

Narrative: WHILE I WAS FLYING MY 1941 J-3 CUB EBOUND OVER THE CITY OF BOYNTON BEACH A STRONG VIBRATION DEVELOPED IN THE ENG COMPARTMENT. PRIOR TO THE VIBRATION DEVELOPING, I HEARD A BANG COME FROM THE AREA OF THE ENG COMPARTMENT. ONCE THE VIBRATION DEVELOPED THE ENG STARTED TO LOSE PWR AND THE ACFT STARTED TO DSND. SEEING THAT THE ACFT WAS LOSING PWR AND ALT, I TURNED TO LANTANA ARPT SO THAT I COULD LAND THERE AND FIND OUT WHAT WAS CAUSING THE VIBRATION. ENRTE TO LANTANA ARPT, I DETERMINED THAT THE ACFT DID NOT HAVE ENOUGH PWR OR ALT TO MAKE LANTANA ARPT. AT THIS TIME, I CIRCLED OVER A FIELD JUST S OF A BUILDING AND ATTEMPTED TO GAIN ALT WITH NEGATIVE RESULTS. NOT BEING ABLE TO GAIN ALT, AND IN FACT LOSING ALT, I LANDED IN THE FIELD S OF THE BUILDING WITHOUT DAMAGING THE ACFT OR CAUSING PERSONAL INJURY. ONCE I HAD SAFELY LANDED THE ACFT, I SHUT DOWN THE ENG AND GOT OUT OF THE AIRPLANE. I THEN WENT TO THE ENG COMPARTMENT AND SAW THAT A SPARK PLUG WAS HANGING BY ITS PLUG WIRE FOR THE L SIDE TOP FRONT CYLINDER. SEEING THIS I PUT THE SPARK PLUG BACK INTO IT'S RESPECTIVE HOLE AND THEN STARTED THE ENG. UPON STARTING THE ENG AND RUNNING IT, I SAW THAT THERE WAS NO LONGER A VIBRATION AND THAT THE ENG WAS DEVELOPING NORMAL PWR. THE ACFT HAD TO BE TRUCKED OUT ON A FLAT BED TRUCK THOUGH BECAUSE OF LCL ORDINANCE WHICH PROHIBITED ACFT FROM TAKING OFF OR LNDG IN THE CITY LIMITS. THE ONLY REASON THAT I CAN DETERMINE THAT THE SPARK PLUG CAME OUT OF THE CYLINDER IS THAT I DID NOT TORQUE IT ENOUGH WHEN I HAD PUT IT BACK INTO THE CYLINDER. I HAD JUST REPLACED ALL THE OLD SPARK PLUGS WITH NEW ONES.

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.