Narrative:

Prior to flight a thorough preflight was conducted and a duat WX briefing was obtained, including NOTAMS. While in cruise flight at 2500' MSL on a local flight out of W29, I noticed slight engine roughness. I immediately turned back toward W29. Engine roughness became more pronounced. I suspected a fouled spark plug and checked magnetos. Power loss was on both mags. Adjusted mixture to verify proper lean setting. I ruled out carburetor icing since previous operation was a no-go decision at run-up due to oil fouled spark plug in #5 cylinder. Leaning had no positive effect on performance. Prior to his flight my mechanic installed new spark plugs in #5 cylinder and suggested the aircraft be flown several hours and he would reinspect #5 cylinder, which he suspected may have a cracked piston ring. The engine is a factor remanufactured with 123 hours. I tried to nurse aircraft back to W29, which was 15 mi away. The engine then began to run very rough, backfiring, indicating a high manifold pressure reading and loss of compression. I recognized I had an engine failure as there was insufficient power to hold altitude. I established the best engine inoperative airspeed, and attempted to land at 3w3, which was approximately 4 mi to the west. Winds aloft were approximately 310/25. Surface winds were approximately 300/15g25. I advised W29 unicom of my problem and intentions. An off airport, soft field landing was made approximately .4 mi east of 3w3 west/O damage to aircraft, injury to pilot or third party property damage. Initial inspection of the engine revealed left intake hose was separated from engine (possibly caused by backfire pressure), and that the #5 cylinder had metal and lead in the cylinder. Total time of flight was 24 mins.

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Original NASA ASRS Text

Title: GA SMA FORCED OFF ARPT LNDG. ENGINE FAILURE.

Narrative: PRIOR TO FLT A THOROUGH PREFLT WAS CONDUCTED AND A DUAT WX BRIEFING WAS OBTAINED, INCLUDING NOTAMS. WHILE IN CRUISE FLT AT 2500' MSL ON A LCL FLT OUT OF W29, I NOTICED SLIGHT ENG ROUGHNESS. I IMMEDIATELY TURNED BACK TOWARD W29. ENG ROUGHNESS BECAME MORE PRONOUNCED. I SUSPECTED A FOULED SPARK PLUG AND CHKED MAGNETOS. PWR LOSS WAS ON BOTH MAGS. ADJUSTED MIXTURE TO VERIFY PROPER LEAN SETTING. I RULED OUT CARB ICING SINCE PREVIOUS OPERATION WAS A NO-GO DECISION AT RUN-UP DUE TO OIL FOULED SPARK PLUG IN #5 CYLINDER. LEANING HAD NO POSITIVE EFFECT ON PERFORMANCE. PRIOR TO HIS FLT MY MECH INSTALLED NEW SPARK PLUGS IN #5 CYLINDER AND SUGGESTED THE ACFT BE FLOWN SEVERAL HRS AND HE WOULD REINSPECT #5 CYLINDER, WHICH HE SUSPECTED MAY HAVE A CRACKED PISTON RING. THE ENG IS A FACTOR REMANUFACTURED WITH 123 HRS. I TRIED TO NURSE ACFT BACK TO W29, WHICH WAS 15 MI AWAY. THE ENG THEN BEGAN TO RUN VERY ROUGH, BACKFIRING, INDICATING A HIGH MANIFOLD PRESSURE READING AND LOSS OF COMPRESSION. I RECOGNIZED I HAD AN ENG FAILURE AS THERE WAS INSUFFICIENT PWR TO HOLD ALT. I ESTABLISHED THE BEST ENG INOP AIRSPD, AND ATTEMPTED TO LAND AT 3W3, WHICH WAS APPROX 4 MI TO THE W. WINDS ALOFT WERE APPROX 310/25. SURFACE WINDS WERE APPROX 300/15G25. I ADVISED W29 UNICOM OF MY PROB AND INTENTIONS. AN OFF ARPT, SOFT FIELD LNDG WAS MADE APPROX .4 MI E OF 3W3 W/O DAMAGE TO ACFT, INJURY TO PLT OR THIRD PARTY PROPERTY DAMAGE. INITIAL INSPECTION OF THE ENG REVEALED LEFT INTAKE HOSE WAS SEPARATED FROM ENG (POSSIBLY CAUSED BY BACKFIRE PRESSURE), AND THAT THE #5 CYLINDER HAD METAL AND LEAD IN THE CYLINDER. TOTAL TIME OF FLT WAS 24 MINS.

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.