A KATANA DA-20 LOST PWR AND MADE AN EMER LNDG AT ISP.

Narrative

I WAS GIVING A NEW HIRE CFI A CHKOUT IN THE KATANA DA20-C1. WHEN APPROX 9 MI S OF ISLIP; THE ENG SUDDENLY LOST PWR (DOWN TO ABOUT 25%). THIS WAS ENOUGH TO ONLY MAINTAIN ALT (1500 FT) AND AN AIRSPD OF 75 KIAS (DOWN FROM 130 KTS). THE INSTRUCTOR/A&P AND MYSELF DIDN'T KNOW WHAT THE CAUSE COULD HAVE BEEN. WE THOUGHT SOMETHING MECHANICAL WAS BROKEN; SUCH AS A PUSH ROD; A VALVE; OR PISTON ROD. NEEDLESS TO SAY; I DECIDED FRG WOULD BE A CARELESS CHOICE. I REMEMBER WHAT MY OLD CHIEF PLT ONCE SAID; 'NEVER BYPASS A GOOD FIELD;' SO I IMMEDIATELY TURNED R TOWARD ISLIP. ENRTE; WE PULLED OUT THE POH AND DID THE CHKLIST FOR PARTIAL PWR AND NOTHING WORKED (SO MUCH FOR CHKLISTS). ON INITIAL CONTACT WITH NEW YORK APCH; WE WERE TOLD TO REMAIN CLR OF CLASS C AIRSPACE (I GUESS HE WAS TOO BUSY). I TOLD HIM WE HAD ENG ROUGHNESS AND REQUIRED A PRECAUTIONARY LNDG. HE GAVE US CLRNC AND HANDED US OVER TO ISP TWR. ISP WAS USING RWY 15R AND TWR CLRED US TO LAND; BUT SINCE WE WERE COMING FROM THE S; THIS WOULD REQUIRE US TO ENTER DOWNWIND; BASE TO FINAL. I TOLD HIM WE WERE GOING TO LAND STRAIGHT-IN RWY 33L IN ORDER TO SAVE TIME. HE THEN CLRED US TO LAND RWY 33L. AT NO TIME DID I DECLARE AN EMER; AS WE WERE MAINTAINING OUR ALT AND SLOWLY -- VERY SLOWLY -- WE WERE GETTING SOME MORE PWR (FROM 1600 RPM TO 2000 RPM). BUT THE TWR SAID 'THEY DID;' WHICH I AM STILL CONFUSED ABOUT. UPON LNDG; WE TOOK COWLING OFF AND DIDN'T SEE ANY MECHANICAL DAMAGE IN THE ENG. THE ENG WAS RUNNING FINE. AFTER NO LESS THAN 4 REAL GOOD RUNUPS; WE DECIDED TO TAKE OFF TOWARDS FRG. THE FLT WAS UNEVENTFUL. THE INSTRUCTOR/A&P SUSPECTS IT WAS CARBON DEPOSITS ON THE VALVES WHICH GAVE US LOSS OF PWR. THE PLANE STILL FLIES TODAY.