Narrative:

While enroute to cec along V27 at 11000 MSL; the engine began to run rough. The flight was in solid IMC and there was intermittent icy precipitation. Various combinations of carburetor heat and mixture provided only temporary relief; and as time progressed the roughness worsened. Suspected cause was ice buildup in the engine intake. Pilot requested a descent and was granted permission to descend to 7000 (MEA was 6400). Engine began to run more smoothly; but the need was apparent to descend to warmer conditions as soon as possible to allow suspected intake ice to melt.shortly thereafter; ATC cleared the aircraft for the cec RNAV (GPS) runway 11 approach. The wipog feeder route for the IAF shows 4000 MSL; but the descent was started immediately instead of waiting for wipog. ATC noticed the early descent and issued a terrain warning with instructions to immediately climb to 6400. Contributing factors were (1) flight in near icing conditions; (2) poorly designed engine intake; (3) fear of worsening engine problems; (4) lack of experience in the IFR system (pilot's first IMC flight after adding the instrument rating).

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Original NASA ASRS Text

Title: C172 pilot experiences a rough running engine at 11;000 feet; which is believed to be caused by intake icing. A descent to 7;000 feet is requested and granted which lead to improved power but still rough. A clearance is issued for the RNAV RWY 11 Approach at CEC; but descent is initiated prior to WIPOG resulting in ATC issuing a terrain warning. The reporter is able to climb back to 6;400 feet until WIPOG and complete the approach.

Narrative: While enroute to CEC along V27 at 11000 MSL; the engine began to run rough. The flight was in solid IMC and there was intermittent icy precipitation. Various combinations of carburetor heat and mixture provided only temporary relief; and as time progressed the roughness worsened. Suspected cause was ice buildup in the engine intake. Pilot requested a descent and was granted permission to descend to 7000 (MEA was 6400). Engine began to run more smoothly; but the need was apparent to descend to warmer conditions as soon as possible to allow suspected intake ice to melt.Shortly thereafter; ATC cleared the aircraft for the CEC RNAV (GPS) RWY 11 approach. The WIPOG feeder route for the IAF shows 4000 MSL; but the descent was started immediately instead of waiting for WIPOG. ATC noticed the early descent and issued a terrain warning with instructions to immediately climb to 6400. Contributing factors were (1) Flight in near icing conditions; (2) Poorly designed engine intake; (3) Fear of worsening engine problems; (4) Lack of experience in the IFR system (pilot's first IMC flight after adding the instrument rating).

Data retrieved from NASA's ASRS site 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.