Narrative:

We were on an instrument flight in a C172. During the course of the flight; we climbed to 12000 ft (per ATC) in order to stay on top of the haze layer and reduce turbulence. We were over the a regional airport at 12;000 ft; and we were in communication with approach. At that point; the controller directed us to 'descend and maintain 9000 ft.' flight conditions were IMC; -4 degrees C; and no airframe icing was being encountered. We reduced throttle in order to descend; within a few seconds of reducing throttle; the engine stopped running. After quickly completing the 'engine failure checklist;' with no success; we declared an emergency with approach and received expedited handling. The controller directed us to squawk 7700; which we did. We continued on our present heading with the intent of making an emergency landing at a nearby CTAF airport; which was very close and below us. Upon further discussion with the controller; however; we elected to instead head for a nearby class D airport because of the fact that the controller had weather information there; and not at the first airport; and also because better emergency equipment was available. At our altitude of 12;000 ft; a glide to the airport (-15NM) was within reach at that point; per the poh. The weather was reported to us by the controller as overcast 3500 ft; with light rain and 7 miles visibility. As we descended through 9000 ft (still in IMC) we were able to restart the engine; although it was still running roughly at that point. We continued to descend towards the airport and broke out of the clouds into good VMC at approximately 4800 ft. At that point the engine was running smoothly and at full power. We cancelled our emergency status with approach; and he once again cleared us to our original destination; the controller returned us to our original transponder code. Upon further analysis; it is clear that this engine failure incident was caused by severe carburetor ice -- just below the freezing level; in cloud; with visible ice crystals in the cloud. Although the ice crystals were not of the type that created airframe ice (no airframe ice was reported in our area); it was ideal for causing carburetor ice; which built up more rapidly than we were able to clear using carburetor heat. As we descended into warmer air; the carburetor ice cleared and the engine ran normally. No other anomalies were found with the airframe or the engine.

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Original NASA ASRS Text

Title: A C172 pilot at 12;000 FT descending in IMC with no visible ice; experienced an engine failure because of carburetor ice. An emergency that was declared with ATC was later cancelled after the engine was restarted at 4;880 FT in VMC.

Narrative: We were on an instrument flight in a C172. During the course of the flight; we climbed to 12000 FT (per ATC) in order to stay on top of the haze layer and reduce turbulence. We were over the a regional airport at 12;000 FT; and we were in communication with Approach. At that point; the Controller directed us to 'descend and maintain 9000 FT.' Flight conditions were IMC; -4 degrees C; and no airframe icing was being encountered. We reduced throttle in order to descend; within a few seconds of reducing throttle; the engine stopped running. After quickly completing the 'engine failure checklist;' with no success; we declared an emergency with Approach and received expedited handling. The Controller directed us to Squawk 7700; which we did. We continued on our present heading with the intent of making an emergency landing at a nearby CTAF airport; which was very close and below us. Upon further discussion with the Controller; however; we elected to instead head for a nearby Class D airport because of the fact that the Controller had weather information there; and not at the first airport; and also because better emergency equipment was available. At our altitude of 12;000 FT; a glide to the airport (-15NM) was within reach at that point; per the POH. The weather was reported to us by the Controller as overcast 3500 FT; with light rain and 7 miles visibility. As we descended through 9000 FT (still in IMC) we were able to restart the engine; although it was still running roughly at that point. We continued to descend towards the airport and broke out of the clouds into good VMC at approximately 4800 FT. At that point the engine was running smoothly and at full power. We cancelled our emergency status with Approach; and he once again cleared us to our original destination; the Controller returned us to our original transponder code. Upon further analysis; it is clear that this engine failure incident was caused by severe carburetor ice -- just below the freezing level; in cloud; with visible ice crystals in the cloud. Although the ice crystals were not of the type that created airframe ice (no airframe ice was reported in our area); it was ideal for causing carburetor ice; which built up more rapidly than we were able to clear using carburetor heat. As we descended into warmer air; the carburetor ice cleared and the engine ran normally. No other anomalies were found with the airframe or the engine.

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.