SMA ENGINE FAILURE RESULTS IN OFF ARPT LNDG ON HIGHWAY. NO DAMAGE TO ACFT OR INJURIES.

Narrative

AUG 1988; WHILE RETURNING FROM LAKE TEXOMA LODGE; I WAS AT 3000' MSL; ALONG WITH A FRIEND WHO WAS IN HIS SMA. WE STARTED OUR DSCNT FOR APCH TO DALLAS NORTH AIRPORT (STRAIGHT IN LNDG ON RWY 16). THE DSCNT STARTED AT ABOUT 5 MI FROM THE ARPT. ON REACHING AND PASSING THROUGH 1900' MSL; THE ENGINE BEGAN TO BOG DOWN. MY FIRST REACTION WAS TO RECHECK THE MIXTURE CTL (WHICH WAS IN FULL RICH); CHECK CARB HEAT; AND INCREASE THE THROTTLE ABOUT THREE-EIGHTHS INCHES. THAT WAS TO NO AVAIL. THEN THROTTLE TO FULL POWER. ALSO TO NO AVAIL. AT THAT POINT; I WAS ABOUT 3 MI FROM THE RWY AND XING HWY 121. THE ENGINE MADE NO ATTEMPT TO RUN RIGHT AND BEGAN TO WINDMILL AND THEN; WITHIN A FEW SEC; QUIT. IT WAS MY DECISION TO MAKE A SHALLOW LEFT TURN AND MAKE A DEAD STICK LNDG ON HWY 121 PAST THE RED LIGHT INTXN; WHERE THERE WAS NO POWER LINE POLES OR CROSSOVER WIRES. (THAT AREA IS MY HOME BASE AND I TRAVELED THAT ROAD MANY TIMES AND HAVE NOTED THAT THERE IS NO OVERHEAD LINES FOR A FEW MI.) MY OTHER ALTERNATIVE WAS TO TRY TO GET AS CLOSE TO THE ARPT AS I POSSIBLY COULD; AND LAND IN A KNOWN FRESHLY PLOWED FIELD THAT HAS SHRUB BRUSH AND SEMI-ROUGH TERRAIN. MY ESTIMATED LNDG SITE AT THAT POINT WAS ONLY 1 TO 1 AND A QUARTER MI. STILL NEARLY 3 MI FROM THE ARPT; THE DECISION TO LAND ON HWY 121 WAS MADE AND EXECUTED W/O FLAW. THERE WAS LIGHT TFC ON THE ROADWAY; AND A TEXTBOOK DEAD STICK LNDG WAS MADE. ON EXITING THE ACFT; I IMMEDIATELY POSITIONED THE ACFT TO WHERE IT WAS NOT ENTIRELY IN THE ROADWAY THE BEST THAT I COULD. I WAS AIDED BY AN UNKNOWN MOTORIST THAT HAD STOPPED TO SEE WHAT WAS HAPPENING. I THEN BEGAN HUNTING FOR A CAUSE OF THE ENGINE QUITTING LIKE IT DID. I HAD PLENTY OF GAS; APPROX 6 GALS IN EACH WING; BTWN ONE-QUARTER AND ONE-HALF TANK INDICATED ON THE PANEL. A VISUAL INSPECTION OF THE TANK REVEALED THAT I DID INDEED HAVE MORE THAN ENOUGH GAS TO CONTINUE FLYING FOR 45 MORE MIN WITH 45 MIN RESERVE. I THOUGHT THAT THERE MIGHT BE WATER IN THE FUEL (PREFLT DID NOT REVEAL ANY AT TKOF). ON CHKING THE WING QUICK DRAINS; NO SIGN OF WATER WAS FOUND. ON INSPECTION OF THE GASCOLATOR; I FOUND THAT IT HAD NOT COMPLETELY CLOSED; THOUGH IT WAS NOT LEAKING. I AM AWARE OF PROBLEMS THAT HAVE BEEN RPTED THAT THE VACUUM CAUSED BY A QUICK DSCNT COULD CAUSE THE GASCOLATOR TO NOT FEED THE CARB PROPERLY IF NOT PROPERLY SEATED. ON CHKING THAT POINT (THE LOWEST POINT IN THE FUEL SYS) FOR POSSIBLE CONTAMINANTS; NONE WERE FOUND. I THEN DECIDED TO SEE IF IT WOULD START; EVEN THOUGH A RESTART WAS UNSUCCESSFUL ON THE WAY DOWN. THE ACFT STARTED ON THE SECOND SPIN OF THE PROP AS IT NORMALLY DOES AND RAN FINE. I DID A FULL STATIC RUN UP WITH LEANING PROCS AND MAG CHKS AT DIFFERENT RPMS. THE MACHINE RAN AS ADVERTISED W/O FLAW. I HAD GOOD OIL PRESSURE AND TEMP. THERE SEEMED TO BE NOTHING WRONG AT THIS POINT. EXTRA FUEL WAS ADDED AS A PRECAUTION AS IT WAS DARK AND LIGHTING WAS POOR AND FLASHLIGHTS WERE A HELP; BUT NO SUBSTITUTE FOR AN EXTRA MARGIN OF SAFETY IN AN EXCITING MOMENT. (THE FRIEND THAT I WAS FLYING BACK WITH LANDED AT DALLAS NORTH AND BROUGHT OUT 10 EXTRA GALS; THAT GIVING ME ALMOST A FULL TANK.) EVEN THOUGH THE MACHINE WAS RUNNING AS NORMAL AND THE STATIC RUN UP REVEALED NO PROB. I CAN ONLY SPECULATE THAT THE GASCOLATOR NOT PROPERLY SEATING WAS MY PROB. I WAS NOT ALLOWED TO TAKE OFF BY THE HWY PATROL BY DIRECTION OF THE LOCAL FAA OFFICE; AND FORCED TO TAXI SOME 4 AND ONE-HALF MI TO THE ARPT. ON REACHING THE ARPT (WHICH BY THE WAY WAS NO FUN TRIP; HAVING TO DODGE SIGN POST AND MAIL BOXES; TREES; ETC.); I TOOK THE ACFT AROUND THE PATTERN TO CHK IT OUT. THERE WERE NO FLAWS IN EITHER TKOF OR LNDG. EVERYTHING WAS WORKING PROPERLY. ON AUG 1988; I TOOK THE ACFT TO MY MECHANIC FOR A LOOK SEE; I.E.; THE GASCOLATOR AND RESET THE TRIM TAB.