Skyhawk 172/Cutlass 172
Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 41; Flight Crew Total 296; Flight Crew Type 156
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors; Weather
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).
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.