AN INSTRUCTOR PLT IN A PA23 MADE A PRECAUTIONARY LNDG GEAR UP. HE WAS CONCENTRATING ON MAKING THE LNDG AND FAILED TO NOTE THE GEAR POS.

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I; THE INSTRUCTOR; DEPARTED SBM IN A PIPER AZTEC PA23 WITH PLT (NEWLY CERTIFICATED PVT MULTI ENG RATED) AND 2 PAX FOR THE PURPOSE OF GAINING FLT TIME FOR INSURANCE SOLO SIGNOFF. MORNING PREFLT AND WX-MATION SHOWED VFR FOR ENTIRE RTE SBM-3WO-W147-SBM. FIRST LEG SBM-3WO UNEVENTFUL AND NORMAL. STOPPED FOR BREAKFAST. DEPARTED SBM APPROX XA30 FROM ARPT. LANDED 3WO APPROX XB00 HRS. DEPARTED ON SECOND LEG XD15. PREFLT AND WALKAROUND NORMAL BEFORE DEP FROM 3WO. CLBED TO 5500 FT MSL CRUISE ALT ENRTE TO W147; WI. ON LEVELOFF; BOTH ENGS BEGAN TO SURGE AND ROUGHNESS IN RPM NOTICED. I; PNF; INSTRUCTED THE PF TO TURN ON BOOST PUMPS AS A PRECAUTIONARY MEASURE. I ALSO APPLIED CARB HEAT TO BOTH ENGS. NO CHANGE OR IMPROVEMENT OCCURRED. I; NON FLYING INSTRUCTOR; ADVISED PF IN L SEAT I WOULD NEXT TRY LEANING MIXTURES TO SEE IF I COULD GET ENGS TO SMOOTH OUT. THIS MEASURE APPEARED TO ALLEVIATE PROB. WE STARTED DOWN OUT OF ALT FOR DSCNT INTO W147. CALLED ON CTAF TO ADVISE TFC OF OUR POS AND INTENT TO LAND. PF L SEAT COMPLETED DSCNT AND APCH CHKLIST. AS HE ADVANCED MIXTURES TO FULL RICH FOR LNDG; ENG ROUGHNESS OCCURRED. I; BEING PNF; ADVISED HIM THAT I WOULD ASSUME COMMAND AS PIC BECAUSE OF EXPERIENCE LEVEL AND THAT HE WOULD ASSIST ME IN PNF DUTIES. TURNED DOWNWIND WITH ENGS INTERMITTENTLY RUNNING ROUGH. ASKED THE L SEAT PLT TO EXTEND THE GEAR; FOLLOWED BY FLAPS FIRST POS. PNF CALLED 'COMING GEAR DOWN.' I LOOKED AND SAW 3 GREEN LIGHTS; CALLED FOR FINAL FLAPS AS I TURNED FINAL AND HAD RWY MADE AND CLOSED THROTTLES FOR FLARE. AIRPLANE CAME TO REST ON RWY 14; FAVORED RWY AT TIME; APPROX 1/4 OR 600 FT DOWN FROM APCH END ON RWY CTRLINE. FOLLOWED EMER SHUTDOWN AND EVAC PROCS. NO PERSONAL INJURIES WERE SUSTAINED. MINIMAL DAMAGE TO UNDERBELLY OF FUSELAGE. WITH ONLY VISUAL DAMAGE TO PROPS; BELIEVE INCIDENT OCCURRED BECAUSE I WAS CONSUMED WITH ENG PROB AND RELIED ON RATED PNF TO BACK ME UP WHILE I TRIED TO GET ACFT SAFELY ON GND. SEVERAL DISTRS CONTRIBUTING TO GEAR UP: 1) ENG ROUGHNESS AND SURGING. 2) INBOUND TFC. 3) BRIGHT SUNLIGHT IN COCKPIT WHICH APPEARED TO GIVE ILLUSION OF 3 GREEN LIGHT. 4) POOR SOUND-DEADING IN COCKPIT AND CABIN AND USE OF HEADSETS WHICH MASKED GEAR WARNING HORN. 5) NEWLY RATED MULTI PLT.