Narrative:

I was flying from blairstown, nj to frederick, md at 4500' MSL. I had just passed through the allentown arsa and had been handed off to the reading ATC facility when I noticed a decrease in RPM. At first I thought the throttle had slipped so I increased the throttle. Then soon after when the RPM dropped again I presumed some carburetor ice was forming. I therefore pulled on full carburetor heat. There was quite a bit of ground fog, but at 4500' the visibility was good. The RPM decreased with the addition of carburetor heat as expected, and I continued on. Then, suddenly, the engine began sputtering. RPM fluctuated between 1500 RPM and 2000 RPM. I looked at my engine failure during course checklist and performed the following sequence: airspeed 60 KIAS, carburetor heat on, power in and locked, fuel valve on, mixture rich, ignition switch left, right, both, start. Even with a 60 KIAS airspeed I was losing altitude and the engine wa still surging. I called 'mayday' to reading ATC, who asked me the nature of my emergency. I communicated that I was having intermittent engine failure, which carburetor heat was not resolving. Another pilot advised I lean out the mixture, which I tried. Reading ATC asked me what I would like to do. I requested vectors straight in to reading, which was 10 mi west of me, directly on my route of flight. They advised I would probably not clear the ridges ahead and instead began vectoring me to pottstown municipal. I followed the vectoring by using my magnetic compass as the dg had and was continuing to precess as the vacuum fluctuated with engine RPM. At some point I was handed off to another controller (phl, I believe), who continued to give me vectors for the airport. When I finally positively idented the field I was at 2000' and they had given me airport information, elevation, runway length, unicom frequencys. Power was still intermittent, but becoming more regular. I entered the pattern for pottstown and made a power on landing. The most probable cause of the engine failure was carburetor ice, though it is improbable that this was the total cause since carburetor heat was on for fully 6 mins before the power began coming on more frequently. The plane is presently having an annual inspection, which I hope will uncover the cause.

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Original NASA ASRS Text

Title: SMA PLT DECLARES AN EMERGENCY WHEN HIS ENGINE BEGINS RUNNING VERY ROUGH. HE IS GIVEN VECTORS TO POTTSTOWN.

Narrative: I WAS FLYING FROM BLAIRSTOWN, NJ TO FREDERICK, MD AT 4500' MSL. I HAD JUST PASSED THROUGH THE ALLENTOWN ARSA AND HAD BEEN HANDED OFF TO THE READING ATC FAC WHEN I NOTICED A DECREASE IN RPM. AT FIRST I THOUGHT THE THROTTLE HAD SLIPPED SO I INCREASED THE THROTTLE. THEN SOON AFTER WHEN THE RPM DROPPED AGAIN I PRESUMED SOME CARB ICE WAS FORMING. I THEREFORE PULLED ON FULL CARB HEAT. THERE WAS QUITE A BIT OF GND FOG, BUT AT 4500' THE VISIBILITY WAS GOOD. THE RPM DECREASED WITH THE ADDITION OF CARB HEAT AS EXPECTED, AND I CONTINUED ON. THEN, SUDDENLY, THE ENG BEGAN SPUTTERING. RPM FLUCTUATED BTWN 1500 RPM AND 2000 RPM. I LOOKED AT MY ENG FAILURE DURING COURSE CHKLIST AND PERFORMED THE FOLLOWING SEQUENCE: AIRSPD 60 KIAS, CARB HEAT ON, PWR IN AND LOCKED, FUEL VALVE ON, MIXTURE RICH, IGNITION SWITCH L, R, BOTH, START. EVEN WITH A 60 KIAS AIRSPD I WAS LOSING ALT AND THE ENG WA STILL SURGING. I CALLED 'MAYDAY' TO READING ATC, WHO ASKED ME THE NATURE OF MY EMER. I COMMUNICATED THAT I WAS HAVING INTERMITTENT ENG FAILURE, WHICH CARB HEAT WAS NOT RESOLVING. ANOTHER PLT ADVISED I LEAN OUT THE MIXTURE, WHICH I TRIED. READING ATC ASKED ME WHAT I WOULD LIKE TO DO. I REQUESTED VECTORS STRAIGHT IN TO READING, WHICH WAS 10 MI W OF ME, DIRECTLY ON MY ROUTE OF FLT. THEY ADVISED I WOULD PROBABLY NOT CLR THE RIDGES AHEAD AND INSTEAD BEGAN VECTORING ME TO POTTSTOWN MUNI. I FOLLOWED THE VECTORING BY USING MY MAGNETIC COMPASS AS THE DG HAD AND WAS CONTINUING TO PRECESS AS THE VACUUM FLUCTUATED WITH ENG RPM. AT SOME POINT I WAS HANDED OFF TO ANOTHER CTLR (PHL, I BELIEVE), WHO CONTINUED TO GIVE ME VECTORS FOR THE ARPT. WHEN I FINALLY POSITIVELY IDENTED THE FIELD I WAS AT 2000' AND THEY HAD GIVEN ME ARPT INFO, ELEVATION, RWY LENGTH, UNICOM FREQS. PWR WAS STILL INTERMITTENT, BUT BECOMING MORE REGULAR. I ENTERED THE PATTERN FOR POTTSTOWN AND MADE A PWR ON LNDG. THE MOST PROBABLE CAUSE OF THE ENG FAILURE WAS CARB ICE, THOUGH IT IS IMPROBABLE THAT THIS WAS THE TOTAL CAUSE SINCE CARB HEAT WAS ON FOR FULLY 6 MINS BEFORE THE PWR BEGAN COMING ON MORE FREQUENTLY. THE PLANE IS PRESENTLY HAVING AN ANNUAL INSPECTION, WHICH I HOPE WILL UNCOVER THE CAUSE.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.