A CESSNA 172 ON APCH AT 7100 FT AND 4 MI FROM END OF RWY WHEN THE ENG STOPPED. ACFT LANDED OFF ARPT. PWR LOSS CAUSED BY FAILED OR STUCK EXHAUST VALVE.

Narrative

AT APPROX XA45; I HEARD A LOUD BANG FOLLOWED BY ROUGH ENG PERFORMANCE. I WAS IN-BTWN MUDDY MOUNTAIN AND CASPER MOUNTAIN; LEVEL AT 10500 FT. THE BANG SOUNDED LIKE IT HAD JOSTLED A PART LOOSE IN THE ENG; AND THE ROUGHNESS WAS SIMILAR TO A FOULED SPARK PLUG. I IMMEDIATELY TURNED TO THE NE TO LOOK FOR AN EMER LNDG AREA; AND TO AVOID THE MOUNTAINS. WHEN I ENTERED CRUISE I LEANED THE MIXTURE AND CHKED THE OIL GAUGES AND BOTH WERE IN NORMAL OPERATING RANGE. I USED A SETTING OF APPROX 2600 RPMS FOR CRUISE WHICH WAS GIVING ME BTWN 100-95 KTS INDICATED. I THEN LISTENED TO THE CURRENT ATIS; AND CONTACTED THE TWR. I WAS JUST GETTING READY TO CLOSE MY FLT PLAN WHEN I HEARD THE BANG IN THE ENG. I THOUGHT IT MIGHT HAVE JUST BEEN A FOULED PLUG OR A BAD MAGNETO; SO I TRIED TO LEAN THE MIXTURE A BIT TO SEE IF IT WOULD BURN ANY DEPOSITS OFF OF THE SPARK PLUG. THIS DIDN'T COMPLETELY REMEDY THE SIT BUT IT SEEMED THAT THE ENG WASN'T QUITE AS ROUGH AFTER I LEANED THE MIXTURE SLIGHTLY. AS I PASSED HARTFORD THE ENG DIDN'T SEEM TO SHOW ANY SIGNS OF GETTING WORSE SO I CONTINUED ON FOR AN EXTENDED L BASE FOR RWY 21 AT NATRONA COUNTY. I WAS APPROX 4 MI FROM THE END OF RWY 21 WHEN THE ENG POPPED ONCE MORE AND COMPLETELY STOPPED. AT THIS POINT I WAS AT 7100 FT MSL; AND I NOTICED PWR LINES AND ROLLING TERRAIN BTWN MYSELF AND THE ARPT SO I DECIDED TO TURN AWAY FROM THE ARPT; TOWARDS THE N; AND FIND THE FLATTEST FIELD TO LAND IN. AFTER LNDG I NOTICED THAT THE OIL TEMP GAUGE WAS 5/8 OF THE WAY FROM THE BOTTOM OF THE OPERATING RANGE AND THERE WAS NO INDICATION ON THE PRESSURE GAUGE. OIL WAS SPEWING FROM THE COWLING AND DRIPPING DOWN THE FRONT LNDG GEAR. IT HAD EVEN BLOWN BACK TO THE MAIN LNDG GEAR STRUTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ENG FAILURE WAS CAUSED BY A FAILED OR STUCK EXHAUST VALVE. THE RPTR SAID THE ACFT INCURRED NO DAMAGE IN THE OFF ARPT LNDG.