PARTIAL ENG FAILURE AND EMER LNDG.

Narrative

ON DEC/WED/94 AT APPROX XA30 LCL; MY COPLT AND I WERE FLYING OUR PIPER CHEROKEE WARRIOR ON AN IFR FLT PLAN ABOUT 12 NM NNE OF RDU ON A VECTOR TO INTERCEPT V-155 NBOUND AT 5000 FT MSL; WHEN WE EXPERIENCED A PARTIAL ENG PWR FAILURE. THE ENG SUDDENLY LOST ABOUT 400 RPM (FIXED PITCH PROP) AND STARTED RUNNING QUITE ROUGH. WE IMMEDIATELY TURNED BACK (S) TO RDU AND NOTIFIED RDU APCH CTL OF THE PROB. APCH ASKED IF WE WANTED TO DECLARE AN EMER; AND I RESPONDED IN THE AFFIRMATIVE. WE WERE GIVEN A VECTOR DIRECT TOWARD RDU AND CLRED FOR A VISUAL APCH TO RWY 23L. WE WERE ALLOWED TO REMAIN AS HIGH AS WE WANTED ON THE APCH. I SET THE FUEL MIXTURE TO FULL RICH WITH THE THROTTLE WIDE OPEN AND TURNED ON THE ELECTRIC FUEL PUMP. I CHKED THE OIL PRESSURE AND TEMP (OK); THE MAGNETOS (OK); CARB HEAT (OK); FUEL PRESSURE AND QUANTITY (OK) AND CONCLUDED THE ENG HAD SUFFERED A MECHANICAL PROB -- PROBABLY INVOLVING 1 CYLINDER. WE WERE DSNDING AT APPROX 200 FPM WHILE MAINTAINING NEAR NORMAL CRUISE AIRSPD. WE WERE HANDED OFF TO RDU TWR AND GIVEN AN IMMEDIATE CLRNC TO LAND ON RWY 23L. I EXECUTED A HIGHER THAN NORMAL APCH TO THE RWY (7500 FT IN LENGTH) AND LANDED UNEVENTFULLY. AS WE TURNED OFF THE RWY; WE WERE MET BY THE FIRE AND RESCUE CREW WHO FOLLOWED US TO THE FBO. WE WERE INFORMED THAT WE WERE TRAILING SMOKE; ESPECIALLY WHEN WE CHANGED PWR SETTINGS. WHEN WE OPENED THE ENG COWLING; WE NOTICED OIL DRIPPING ON THE EXHAUST PIPE OF CYLINDER #3. AN AVIATION MECH REMOVED THE 2 SPARK PLUGS FROM CYLINDER #3. THEY SHOWED OBVIOUS PHYSICAL DAMAGE. THE PRELIMINARY ANALYSIS IS THAT THE EXHAUST VALVE IN CYLINDER #3 BROKE. IT IS CLR THAT THE ENG WAS OPERATING ON 3 OUT OF 4 CYLINDERS DURING THE APCH. THE ENG WAS APCHING ITS RECOMMENDED TIME BTWN OVERHAUL (APPROX 1960 HRS OUT OF 2000 HRS TIME BTWN OVERHAUL). UP UNTIL THIS INCIDENT; EXCEPT FOR USING OIL AT A RATE COMMENSURATE WITH THE TIME ON THE ENG; THE ENG OPERATED AND PERFORMED NORMALLY. A MAJOR ENG OVERHAUL WILL BE PERFORMED BEFORE THE ACFT IS FLOWN AGAIN. THE PRIORITY TREATMENT WE RECEIVED FROM ALL ATC PERSONNEL WAS VERY WELCOME AND APPRECIATED. WE BELIEVE WE DID NOT UNDULY DISRUPT OPS AT RDU ARPT; AND WE HAVE NOTHING BUT PRAISE FOR THE TREATMENT WE RECEIVED FROM ATC AND THE FIRE/RESCUE CREW. WE ARE NOT AWARE OF ANY ABNORMALITIES WITH THE ENG OR ITS OP WHICH WOULD HAVE CAUSED THIS PROB. WE AWAIT AN ANALYSIS OF THE ENG AFTER TEARDOWN.