Narrative:

I briefed the possibility of needing an IFR approach into my destination. I noticed a cloud bank ahead and readied the IFR charts. ATC asked that I fly northwest; and upon complying noted the clouds ahead at my altitude. I advised approach I'd need to go IFR in ten miles; and asked for an IFR to my destination. Ten miles later I had my clearance and descended through the first batch of cumulus clouds; then level expecting vectors for the approach. They were not extensively developed; no thunderstorms in the making; although they were turbulent. My head hit the side window once; and I was glad for my durable headset. Passing through another set of build-ups; the engine suddenly ran rough. I was through the emergency flow in seconds; carburetor heat on; mixture rich; throttle full; fuel pump on; switch fuel tanks; magneto check; as it dawned on me that turning carburetor heat on made the engine run much rougher; even quitting and coming back to life. I was descending like a rock; thus declared the emergency with approach passing through 400 ft low from assigned altitude. I stated I needed an immediate descent to 4;000 ft (2;000 ft lower than MEA) and turned away from the higher terrain to the north. There were breaks in the clouds and it seemed that there was a high ceiling in my area as well as a nearby airport's ATIS reporting a high ceiling. I knew I was not yet to the mountains due east of my destination. I thought if I was getting carburetor ice; getting out of the clouds was the best option; then I could also make a better decision on where to land; if I could get to my destination in sight; or possibly another nearby airport. As I was descending; I checked the OAT; still 10 degrees C. I switched tanks again; the engine seemingly running smoother on the right tank; the one it was on initially. The engine sputtered and died several times until I cleared the clouds; then as I was leveling and trimming for best glide; I spotted my destination and an airliner landing on the runway. At the same time approach; bless them; was clearing me to land at my destination; any runway. With one last cough; the engine began to run smoothly. I tried several power settings and mixtures; all smooth. I tried a magneto check; smooth; full power; lean; smooth; idle; smooth. Both my destination and another nearby airport were in the clear; no clouds overhead or low approach required. The mechanic in me kicked in and with no further problems; and too high to land at my destination without orbiting and no small aircraft maintenance facilities available; I elected to continue to the other airport; maintaining altitude until I could glide to that airport. I advised approach to cancel the emergency; it was carburetor ice. They sent me to the other airport's tower; where I again advised them to cancel the emergency. I made a normal landing; inspected everything; even did an engine run-up and all was fine. My fuel calculations were exact; I had 1.3 hours of fuel remaining; 13 gallons. There was no abnormality found with the fuel; carburetor; carburetor heat; intake; or engine systems. The aircraft has flown two hours since; with no abnormalities. Carburetor ice is very rare in pipers even more rare with this aircraft's particular stc'd engine installation. But it does happen; and that day was my day.

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Original NASA ASRS Text

Title: PA28 pilot experienced a rough running engine on an IFR flight plan and declared an emergency. Initial corrective actions had no positive effect. After the aircraft descended out of IMC the engine returns to smooth running and the flight continued to destination.

Narrative: I briefed the possibility of needing an IFR approach into my destination. I noticed a cloud bank ahead and readied the IFR charts. ATC asked that I fly northwest; and upon complying noted the clouds ahead at my altitude. I advised Approach I'd need to go IFR in ten miles; and asked for an IFR to my destination. Ten miles later I had my clearance and descended through the first batch of cumulus clouds; then level expecting vectors for the approach. They were not extensively developed; no thunderstorms in the making; although they were turbulent. My head hit the side window once; and I was glad for my durable headset. Passing through another set of build-ups; the engine suddenly ran rough. I was through the emergency flow in seconds; carburetor heat on; mixture rich; throttle full; fuel pump on; switch fuel tanks; magneto check; as it dawned on me that turning carburetor heat on made the engine run much rougher; even quitting and coming back to life. I was descending like a rock; thus declared the emergency with Approach passing through 400 FT low from assigned altitude. I stated I needed an immediate descent to 4;000 FT (2;000 FT lower than MEA) and turned away from the higher terrain to the north. There were breaks in the clouds and it seemed that there was a high ceiling in my area as well as a nearby airport's ATIS reporting a high ceiling. I knew I was not yet to the mountains due east of my destination. I thought if I was getting carburetor ice; getting out of the clouds was the best option; then I could also make a better decision on where to land; if I could get to my destination in sight; or possibly another nearby airport. As I was descending; I checked the OAT; still 10 degrees C. I switched tanks again; the engine seemingly running smoother on the right tank; the one it was on initially. The engine sputtered and died several times until I cleared the clouds; then as I was leveling and trimming for best glide; I spotted my destination and an airliner landing on the runway. At the same time Approach; bless them; was clearing me to land at my destination; any runway. With one last cough; the engine began to run smoothly. I tried several power settings and mixtures; all smooth. I tried a magneto check; smooth; full power; lean; smooth; idle; smooth. Both my destination and another nearby airport were in the clear; no clouds overhead or low approach required. The mechanic in me kicked in and with no further problems; and too high to land at my destination without orbiting and no small aircraft maintenance facilities available; I elected to continue to the other airport; maintaining altitude until I could glide to that airport. I advised Approach to cancel the emergency; it was carburetor ice. They sent me to the other airport's Tower; where I again advised them to cancel the emergency. I made a normal landing; inspected everything; even did an engine run-up and all was fine. My fuel calculations were exact; I had 1.3 hours of fuel remaining; 13 gallons. There was no abnormality found with the fuel; carburetor; carburetor heat; intake; or engine systems. The aircraft has flown two hours since; with no abnormalities. Carburetor ice is very rare in Pipers even more rare with this aircraft's particular STC'd engine installation. But it does happen; and that day was my day.

Data retrieved from NASA's ASRS site as of April 2012 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.