Narrative:

Shortly after given assigned altitude of 3000 ft (from 4000 ft), an unexplained drop in RPM was noticed. I became distracted as I employed carburetor heat, set mixture to full rich, checked the primer and adjusted the throttle friction. I did not stop the descent at 3000 ft, but continued to 2300 ft. The improper altitude was noticed at 2300 ft and I began an ascent to 3000 ft. Detroit approach brought the altitude to my attention as well, and reminded me of the correct altitude. The major factor on this incident was the distraction and my failure to monitor the altimeter as the RPM problem was diagnosed. The flight was uneventful after the return to 3000 ft. No additional RPM problems were noted, and we proceeded to flint after doing a missed approach at pontiac (ceiling below directional gyroscope). Callback conversation with reporter revealed the following information: reporter stated that he was operating a cessna 172 during this incident. He believed that the power loss was definitely due to engine intake icing since the power was regained after a brief period of carburetor heat application. He realizes that operation of a sel engine aircraft in low WX ceiling sits could be hazardous since there would be no opportunity to select the best possible landing site. There was a safety pilot on board.

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Original NASA ASRS Text

Title: PVT INST RATED PLT OF AN SMA SET OVERSHOT DSCNT ALT ASSIGNMENT DUE TO A DISTR OF A PARTIAL ENG PWR LOSS.

Narrative: SHORTLY AFTER GIVEN ASSIGNED ALT OF 3000 FT (FROM 4000 FT), AN UNEXPLAINED DROP IN RPM WAS NOTICED. I BECAME DISTRACTED AS I EMPLOYED CARB HEAT, SET MIXTURE TO FULL RICH, CHKED THE PRIMER AND ADJUSTED THE THROTTLE FRICTION. I DID NOT STOP THE DSCNT AT 3000 FT, BUT CONTINUED TO 2300 FT. THE IMPROPER ALT WAS NOTICED AT 2300 FT AND I BEGAN AN ASCENT TO 3000 FT. DETROIT APCH BROUGHT THE ALT TO MY ATTN AS WELL, AND REMINDED ME OF THE CORRECT ALT. THE MAJOR FACTOR ON THIS INCIDENT WAS THE DISTR AND MY FAILURE TO MONITOR THE ALTIMETER AS THE RPM PROB WAS DIAGNOSED. THE FLT WAS UNEVENTFUL AFTER THE RETURN TO 3000 FT. NO ADDITIONAL RPM PROBS WERE NOTED, AND WE PROCEEDED TO FLINT AFTER DOING A MISSED APCH AT PONTIAC (CEILING BELOW DIRECTIONAL GYROSCOPE). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE WAS OPERATING A CESSNA 172 DURING THIS INCIDENT. HE BELIEVED THAT THE PWR LOSS WAS DEFINITELY DUE TO ENG INTAKE ICING SINCE THE PWR WAS REGAINED AFTER A BRIEF PERIOD OF CARB HEAT APPLICATION. HE REALIZES THAT OP OF A SEL ENG ACFT IN LOW WX CEILING SITS COULD BE HAZARDOUS SINCE THERE WOULD BE NO OPPORTUNITY TO SELECT THE BEST POSSIBLE LNDG SITE. THERE WAS A SAFETY PLT ON BOARD.

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.